KICP in the News, 2014
Joshua Frieman has been awarded an Honorary Fellowship of the Royal Astronomical Society
Royal Astronomical Society, January 10, 2014
Professor Joshua Frieman, Professor of Astronomy and Astrophysics at the University of Chicago, and member of the Theoretical Astrophysics group at Fermilab. Freiman's research centres on theoretical and observational cosmology, including studies of the nature of dark energy, the early Universe, gravitational lensing, the large-scale structure of the Universe, and supernovae as cosmological distance indicators. He is a founder of and currently serves as Director of the Dark Energy Survey, a collaboration of over 120 scientists from 20 institutions on 3 continents. Honorary Fellowship is conferred to mark his singular contributions to the study of dark energy.
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KICP Members: Joshua A. Frieman
Scientific projects: Dark Energy Survey (DES)
Faculty members recognized for research, teaching and professional service with new professorships
University of Chicago News Office, February 5, 2014
Angela Olinto, who has made important contributions to the physics of quark stars, inflationary theory, cosmic magnetic fields and particle astrophysics, has been named a Homer J. Livingston Professor in Astronomy & Astrophysics and the College.
Olinto's research interests span theoretical astrophysics, particle and nuclear astrophysics, and cosmology. She has focused much of her work on understanding the origins of the highest energy cosmic rays and the ultra-compressed core of matter in neutron stars. Ultra-high-energy cosmic rays enter the atmosphere with so much energy that they produce a giant cascade of many tens of billions of secondary particles, which can be observed by large detectors such as the Auger Observatory.
Last year, Olinto was elected a fellow of the American Association for the Advancement of Science for her distinguished contributions to the field of astrophysics, particularly exotic states of matter and extremely high-energy cosmic ray studies at the Pierre Auger Observatory in Argentina.
She also is a fellow of the American Physical Society and has received the Chaire d'Excellence Award of the French Agence Nationale de Recherche. Olinto also is a recipient of the University's highest teaching honor, the Llewellyn John and Harriet Manchester Quantrell Award for Excellence in Undergraduate Teaching.
A faculty member at UChicago since 1996, Olinto now leads the U.S. collaboration of the Japanese Experiment Module-Extreme Universe Space Observatory mission to observe these ultra-energy particles from the International Space Station.
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KICP Members: Angela V. Olinto
Scientific projects: Pierre Auger Observatory (AUGER)
Communicating Science with Alan Alda
UChicago News, February 25, 2014
Case for Dark Matter Signal Strengthens
Quanta Magazine, Simons Foundation, March 6, 2014
by Natalie Wolchover, Quanta Magazine, Simons Foundation
Dan Hooper, Tracy Slatyer, Tim Linden and Stephen Portillo (shown clockwise from top left), and collaborators claim that the annihilation of dark matter particles called WIMPs is the only plausible source for the gamma-ray excess coming from the center of the galaxy.
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KICP Members: Daniel Hooper; Tim Linden
A big-bang theory gets a big boost: Evidence that vast cosmos was created in split second
The Washington Post, March 18, 2014
by Joel Achenbach, The Washington Post
In the beginning, the universe got very big very fast, transforming itself in a fraction of an instant from something almost infinitesimally small to something imponderably vast, a cosmos so huge that no one will ever be able to see it all.
This is the premise of an idea called cosmic inflation - a powerful twist on the big-bang theory - and Monday it received a major boost from an experiment at the South Pole called BICEP2. A team of astronomers led by John Kovac of the Harvard-Smithsonian Center for Astrophysics announced that it had detected ripples from gravitational waves created in a violent inflationary event at the dawn of time.
"We're very excited to present our results because they seem to match the prediction of the theory so closely," Kovac said in an interview. "But it's the case that science can never actually prove a theory to be true. There could always be an alternative explanation that we haven't been clever enough to think of."
The reaction in the scientific community was cautiously exultant. The new result was hailed as potentially one of the biggest discoveries of the past two decades.
Cosmology, the study of the universe on the largest scales, has already been roiled by the 1998 discovery that the cosmos is not merely expanding but doing so at an accelerating rate, because of what has been called "dark energy." Just as that discovery has implications for the ultimate fate of the universe, this new one provides a stunning look back at the moment the universe was born.
"If real, it's magnificent," said Harvard astrophysicist Lisa Randall.
Lawrence Krauss, an Arizona State University theoretical physicist, said of the new result, "It gives us a new window on the universe that takes us back to almost the very beginning of time, allowing us to turn previously metaphysical questions about our origins into scientific ones."
The measurement, however, is a difficult one. The astronomers chose the South Pole for BICEP2 and earlier experiments because the air is exceedingly dry, almost devoid of water vapor and ideal for observing subtle quirks in the ancient light pouring in from the night sky. They spent four years building the telescope, and then three years observing and analyzing the data. Kovac, 43, who has been to the South Pole 23 times, said of the conditions there, "It's almost like being in space."
The BICEP2 instrument sorts through the cosmic microwave background (CMB), looking for polarization of the light in a pattern that reveals the ripples of gravitational waves. The gravitational waves distort space itself, squishing and tugging the fabric of the universe. This is the first time that anyone has announced the detection of gravitational waves from the early universe.
There are other experiments by rival groups trying to detect these waves, and those efforts will continue in an attempt to confirm the results announced Monday.
"I would say it's very likely to be correct that we are seeing a signal from inflation," said Adrian Lee, a University of California at Berkeley cosmologist who is a leader of PolarBear, an experiment based on a mountaintop in Chile that is also searching for evidence of inflation. "But it's such a hard measurement that we really would like to see it measured with different experiments, with different techniques, looking at different parts of the sky, to have confidence that this is really a signal from the beginning of the universe."
The fact that the universe is dynamic at the grandest scale, and not static as it appears to be when we gaze at the "fixed stars" in the night sky, has been known since the late 1920s, when astronomer Edwin Hubble revealed that the light from galaxies showed that they were moving away from one another.
This led to the theory that the universe, once compact, is expanding. Scientists in recent years have been able to narrow down the age of the universe to about 13.8 billion years. Multiple lines of evidence, including the detection of the CMB exactly 50 years ago, have bolstered the consensus model of modern cosmology, which shows that the universe was initially infinitely hot and dense, literally dimensionless. There was no space, no time.
Then something happened. The universe began to expand and cool. This was the big bang.
Cosmic inflation throws gasoline on that fire. It makes the big bang even bangier right at the start. Instead of a linear expansion, the universe would have undergone an exponential growth.
In 1979, theorist Alan Guth, then at Stanford, seized on a potential explanation for some of the lingering mysteries of the universe, such as the remarkable homogeneity of the whole place - the way distantly removed parts of the universe had the same temperature and texture even though they had never been in contact with each other. Perhaps the universe did not merely expand in a stately manner but went through a much more dramatic, exponential expansion, essentially going from microscopic in scale to cosmically huge in a tiny fraction of a second.
It is unclear how long this inflationary epoch lasted. Kovac calculated that in that first fraction of a second the volume of the universe increased by a factor of 10 to the 26th power, going from subatomic to cosmic.
This is obviously difficult terrain for theorists, and the question of why there is something rather than nothing creeps into realms traditionally governed by theologians. But theoretical physicists say that empty space is not empty, that the vacuum crackles with energy and that quantum physics permits such mind-boggling events as a universe popping up seemingly out of nowhere.
"Inflation - the idea of a very big burst of inflation very early on - is the most important idea in cosmology since the big bang itself," said Michael Turner, a University of Chicago cosmologist. "If correct, this burst is the dynamite behind our big bang."
Princeton University astrophysicist David Spergel said after Monday's announcement, "If true, this has revolutionary impacts for our understanding of the physics of the early universe and gives us insight into physics on really small scales."
Spergel added, "We will soon know if this result is revolutionary or due to some poorly understood systematics."
The inflationary model implies that our universe is exceedingly larger than what we currently observe, which is humbling already in its scale. Moreover, the vacuum energy that drove the inflationary process would presumably imply the existence of a larger cosmos, or "multiverse," of which our universe is but a granular element.
"These ideas about the multiverse become interesting to me only when theories come up with testable predictions based on them," Kovac said Monday. "The powerful thing about the basic inflationary paradigm is that it did offer us this clear, testable prediction: the existence of gravitational waves which are directly linked to the exponential expansion that's intrinsic to the theory."
The cosmological models favored by scientists do not permit us to have contact with other potential universes. The multiverse is, for now, conjectural, because it is not easily subject to experimental verification and is unobservable - from the South Pole or from anywhere else.
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KICP Members: Christopher D. Sheehy; Michael S. Turner; Abigail G. Vieregg
Scientific projects: BICEP2/The Keck Array/BICEP3
Space Ripples Reveal Big Bang's Smoking Gun
The New York Times, March 18, 2014
by Dennis Overbye, The New York Times
CAMBRIDGE, Mass. - One night late in 1979, an itinerant young physicist named Alan Guth, with a new son and a year's appointment at Stanford, stayed up late with his notebook and equations, venturing far beyond the world of known physics.
He was trying to understand why there was no trace of some exotic particles that should have been created in the Big Bang. Instead he discovered what might have made the universe bang to begin with. A potential hitch in the presumed course of cosmic evolution could have infused space itself with a special energy that exerted a repulsive force, causing the universe to swell faster than the speed of light for a prodigiously violent instant.
If true, the rapid engorgement would solve paradoxes like why the heavens look uniform from pole to pole and not like a jagged, warped mess. The enormous ballooning would iron out all the wrinkles and irregularities. Those particles were not missing, but would be diluted beyond detection, like spit in the ocean.
"SPECTACULAR REALIZATION," Dr. Guth wrote across the top of the page and drew a double box around it.
On Monday, Dr. Guth's starship came in. Radio astronomers reported that they had seen the beginning of the Big Bang, and that his hypothesis, known undramatically as inflation, looked right.
Reaching back across 13.8 billion years to the first sliver of cosmic time with telescopes at the South Pole, a team of astronomers led by John M. Kovac of the Harvard-Smithsonian Center for Astrophysics detected ripples in the fabric of space-time - so-called gravitational waves - the signature of a universe being wrenched violently apart when it was roughly a trillionth of a trillionth of a trillionth of a second old. They are the long-sought smoking-gun evidence of inflation, proof, Dr. Kovac and his colleagues say, that Dr. Guth was correct.
Inflation has been the workhorse of cosmology for 35 years, though many, including Dr. Guth, wondered whether it could ever be proved.
If corroborated, Dr. Kovac's work will stand as a landmark in science comparable to the recent discovery of dark energy pushing the universe apart, or of the Big Bang itself. It would open vast realms of time and space and energy to science and speculation.
Confirming inflation would mean that the universe we see, extending 14 billion light-years in space with its hundreds of billions of galaxies, is only an infinitesimal patch in a larger cosmos whose extent, architecture and fate are unknowable. Moreover, beyond our own universe there might be an endless number of other universes bubbling into frothy eternity, like a pot of pasta water boiling over.
'As Big as It Gets'
In our own universe, it would serve as a window into the forces operating at energies forever beyond the reach of particle accelerators on Earth and yield new insights into gravity itself. Dr. Kovac's ripples would be the first direct observation of gravitational waves, which, according to Einstein's theory of general relativity, should ruffle space-time.
Marc Kamionkowski of Johns Hopkins University, an early-universe expert who was not part of the team, said, "This is huge, as big as it gets."
He continued, "This is a signal from the very earliest universe, sending a telegram encoded in gravitational waves."
The ripples manifested themselves as faint spiral patterns in a bath of microwave radiation that permeates space and preserves a picture of the universe when it was 380,000 years old and as hot as the surface of the sun.
Dr. Kovac and his collaborators, working in an experiment known as Bicep, for Background Imaging of Cosmic Extragalactic Polarization, reported their results in a scientific briefing at the Center for Astrophysics here on Monday and in a set of papers submitted to The Astrophysical Journal.
Dr. Kovac said the chance that the results were a fluke was only one in 10 million.
Dr. Guth, now 67, pronounced himself "bowled over," saying he had not expected such a definite confirmation in his lifetime.
"With nature, you have to be lucky," he said. "Apparently we have been lucky."
The results are the closely guarded distillation of three years' worth of observations and analysis. Eschewing email for fear of a leak, Dr. Kovac personally delivered drafts of his work to a select few, meeting with Dr. Guth, who is now a professor at Massachusetts Institute of Technology (as is his son, Larry, who was sleeping that night in 1979), in his office last week.
"It was a very special moment, and one we took very seriously as scientists," said Dr. Kovac, who chose his words as carefully as he tended his radio telescopes.
Andrei Linde of Stanford, a prolific theorist who first described the most popular variant of inflation, known as chaotic inflation, in 1983, was about to go on vacation in the Caribbean last week when Chao-Lin Kuo, a Stanford colleague and a member of Dr. Kovac's team, knocked on his door with a bottle of Champagne to tell him the news.
Confused, Dr. Linde called out to his wife, asking if she had ordered anything.
"And then I told him that in the beginning we thought that this was a delivery but we did not think that we ordered anything, but I simply forgot that actually I did order it, 30 years ago," Dr. Linde wrote in an email.
Calling from Bonaire, the Dutch Caribbean island, Dr. Linde said he was still hyperventilating. "Having news like this is the best way of spoiling a vacation," he said.
By last weekend, as social media was buzzing with rumors that inflation had been seen and news spread, astrophysicists responded with a mixture of jubilation and caution.
Max Tegmark, a cosmologist at M.I.T., wrote in an email, "I think that if this stays true, it will go down as one of the greatest discoveries in the history of science."
John E. Carlstrom of the University of Chicago, Dr. Kovac's mentor and head of a competing project called the South Pole Telescope, pronounced himself deeply impressed. "I think the results are beautiful and very convincing," he said.
Paul J. Steinhardt of Princeton, author of a competitor to inflation that posits the clash of a pair of universes as the cause of genesis, said that if true, the Bicep result would eliminate his model, but he expressed reservations about inflation.
Lawrence M. Krauss of Arizona State and others also emphasized the need for confirmation, noting that the new results exceeded earlier estimates based on temperature maps of the cosmic background by the European Space Agency's Planck satellite and other assumptions about the universe.
"So we will need to wait and see before we jump up and down," Dr. Krauss said.
Corroboration might not be long in coming. The Planck spacecraft will report its own findings this year. At least a dozen other teams are trying similar measurements from balloons, mountaintops and space.
Spirals in the Sky
Gravity waves are the latest and deepest secret yet pried out of the cosmic microwaves, which were discovered accidentally by Arno Penzias and Robert Wilson at Bell Labs 50 years ago. They won the Nobel Prize.
Dr. Kovac has spent his career trying to read the secrets of these waves. He is one of four leaders of Bicep, which has operated a series of increasingly sensitive radio telescopes at the South Pole, where the thin, dry air creates ideal observing conditions. The others are Clement Pryke of the University of Minnesota, Jamie Bock of the California Institute of Technology and Dr. Kuo of Stanford.
"The South Pole is the closest you can get to space and still be on the ground," Dr. Kovac said. He has been there 23 times, he said, wintering over in 1994. "I've been hooked ever since," he said.
In 2002, he was part of a team that discovered that the microwave radiation was polarized, meaning the light waves had a slight preference to vibrate in one direction rather than another.
This was a step toward the ultimate goal of detecting the gravitational waves from inflation. Such waves, squeezing space in one direction and stretching it in another as they go by, would twist the direction of polarization of the microwaves, theorists said. As a result, maps of the polarization in the sky should have little arrows going in spirals.
Detecting those spirals required measuring infinitesimally small differences in the temperature of the microwaves. The group's telescope, Bicep2, is basically a giant superconducting thermometer.
"We had no expectations what we would see," Dr. Kovac said.
The strength of the signal surprised the researchers, and they spent a year burning up time on a Harvard supercomputer, making sure they had things right and worrying that competitors might beat them to the breakthrough.
A Special Time
The data traced the onset of inflation to a time that physicists like Dr. Guth, staying up late in his Palo Alto house 35 years ago, suspected was a special break point in the evolution of the universe.
Physicists recognize four forces at work in the world today: gravity, electromagnetism, and strong and weak nuclear forces. But they have long suspected that those are simply different manifestations of a single unified force that ruled the universe in its earliest, hottest moments.
As the universe cooled, according to this theory, there was a fall from grace, like some old folk mythology of gods or brothers falling out with each other. The laws of physics evolved, with one force after another splitting away.
That was where Dr. Guth came in.
Under some circumstances, a glass of water can stay liquid as the temperature falls below 32 degrees, until it is disturbed, at which point it will rapidly freeze, releasing latent heat.
Similarly, the universe could "supercool" and stay in a unified state too long. In that case, space itself would become imbued with a mysterious latent energy.
Inserted into Einstein's equations, the latent energy would act as a kind of antigravity, and the universe would blow itself up. Since it was space itself supplying the repulsive force, the more space was created, the harder it pushed apart.
What would become our observable universe mushroomed in size at least a trillion trillionfold - from a submicroscopic speck of primordial energy to the size of a grapefruit - in less than a cosmic eye-blink.
Almost as quickly, this pulse would subside, relaxing into ordinary particles and radiation. All of normal cosmic history was still ahead, resulting in today's observable universe, a patch of sky and stars billions of light-years across. "It's often said that there is no such thing as a free lunch," Dr. Guth likes to say, "but the universe might be the ultimate free lunch."
Make that free lunches. Most of the hundred or so models resulting from Dr. Guth's original vision suggest that inflation, once started, is eternal. Even as our own universe settled down to a comfortable homey expansion, the rest of the cosmos will continue blowing up, spinning off other bubbles endlessly, a concept known as the multiverse.
So the future of the cosmos is perhaps bright and fecund, but do not bother asking about going any deeper into the past.
We might never know what happened before inflation, at the very beginning, because inflation erases everything that came before it. All the chaos and randomness of the primordial moment are swept away, forever out of our view.
"If you trace your cosmic roots," said Abraham Loeb, a Harvard-Smithsonian astronomer who was not part of the team, "you wind up at inflation."
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KICP Members: John E. Carlstrom; Christopher D. Sheehy; Abigail G. Vieregg
Scientific projects: BICEP2/The Keck Array/BICEP3
Telescope captures view of gravitational waves: Images of the infant Universe reveal evidence for rapid inflation after the Big Bang
Nature, March 18, 2014
by Ron Cowen, Nature
Astronomers have peered back to nearly the dawn of time and found what seems to be the long-sought 'smoking gun' for the theory that the Universe underwent a spurt of wrenching, exponential growth called inflation during the first tiny fraction of a second of its existence.
Using a radio telescope at the South Pole, the US-led team has detected the first evidence of primordial gravitational waves, ripples in space that inflation generated 13.8 billion years ago when the Universe first started to expand.
The telescope captured a snapshot of the waves as they continued to ripple through the Universe some 380,000 years later, when stars had not yet formed and matter was still scattered across space as a broth of plasma. The image was seen in the cosmic microwave background (CMB), the glow that radiated from that white-hot plasma and that over billions of years of cosmic expansion has cooled to microwave energies.
The fact that inflation, a quantum phenomenon, produced gravitational waves demonstrates that gravity has a quantum nature just like the other known fundamental forces of nature, experts say. Moreover, it provides a window into interactions much more energetic than are accessible in any laboratory experiment. In addition, the way that the team confirmed inflation is itself of major significance: it is the most direct evidence yet that gravitational waves - a key but elusive prediction of Albert Einstein's general theory of relativity - exist.
"This is a totally new, independent piece of cosmological evidence that the inflationary picture fits together," says theoretical physicist Alan Guth of the Massachusetts Institute of Technology (MIT) in Cambridge, who proposed the idea of inflation in 1980. He adds that the study is "definitely" worthy of a Nobel prize.
Guth's idea was that the cosmos expanded at an exponential rate for a few tens of trillionths of trillionths of trillionths of seconds after the Big Bang, ballooning from subatomic to football size. Inflation solves several long-standing cosmic conundrums, such as why the observable Universe appears uniform from one end to the other. Although the theory has proved to be consistent with all cosmological data collected so far, conclusive evidence for it has been lacking.
Cosmologists knew, however, that inflation would have a distinctive signature: the brief but violent period of expansion would have generated gravitational waves, which compress space in one direction while stretching it along another (see 'Ripple effect'). Although the primordial waves would still be propagating across the Universe, they would now be too feeble to detect directly. But they would have left a distinctive mark in the CMB: they would have polarized the radiation in a curly, vortex-like pattern known as the B mode (see 'Cosmic curl').
Last year, another telescope in Antarctica - the South Pole Telescope (SPT) - became the first observatory to detect a B-mode polarization in the CMB (see Nature http://doi.org/rwt; 2013). That signal, however, was over angular scales of less than one degree (about twice the apparent size of the Moon in the sky), and was attributed to how galaxies in the foreground curve the space through which the CMB travels (D. Hanson et al. Phys. Rev. Lett. 111, 141301; 2013). But the signal from primordial gravitational waves is expected to peak at angular scales between one and five degrees.
And that is exactly what John Kovac of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, and his colleagues now say they have detected, using an instrument dubbed BICEP2 that is located just metres away from its competitor, the SPT.
Detecting the tiny B mode required measuring the CMB with a precision of one ten-millionth of a kelvin and distinguishing the primordial effect from other possible sources, such as galactic dust.
"The key question," says Daniel Eisenstein, an astrophysicist at the CfA, "is whether there could be a foreground that masquerades like this signal". But the team has all but ruled out that possibility, he says. First, the researchers were careful to point BICEP2 - an array of 512 superconducting microwave detectors - at the Southern Hole, a patch of sky that is known to contain only tiny amounts of such emissions. They also compared their data with those taken by an earlier experiment, BICEP1, and showed that a dust-generated signal would have had a different colour and spectrum.
Furthermore, data taken with a newer, more sensitive polarization experiment, the Keck array, which the team finished installing at the South Pole in 2012 and will continue operating for two more years, showed the same characteristics. "To see this same signal emerge from two other, different telescopes was for us very convincing," says Kovac.
"The details have to be worked out, but from what I know it's highly likely this is what we've all been waiting for," says astronomer John Carlstrom of the University of Chicago, Illinois, who is the lead researcher on the SPT. "This is the discovery of inflationary gravitational waves."
Cosmologist Marc Kamionkowski adds: "To me, this looks really, really solid." He was one of the first cosmologists to calculate what the signature of primordial gravitational waves should look like in the CMB. The findings are "on a par with dark energy, or the discovery of the CMB - something that happens once every several decades", says Kamionkowski, who is at Johns Hopkins University in Baltimore, Maryland.
The strength of the signal measured by BICEP2, although entirely consistent with inflation, initially surprised the researchers because it is nearly twice as large as estimated from previous experiments. According to theory, the intensity of a B-mode signal reveals how fast the Universe expanded during inflation, and therefore suggests the energy scale of the cosmos during that epoch. The data pinpoint the time when inflation occurred - about 10-37 seconds into the Universe's life - and its temperature at the time, corresponding to energies of about 1016 gigaelectronvolts, says cosmologist Michael Turner of the University of Chicago. That is the same energy at which three of the four fundamental forces of nature - the weak, strong and electromagnetic force - are expected to become indistinguishable from one another in a model known as the grand unified theory.
Because inflation took place in the realm of quantum physics, seeing gravitational waves arise from that epoch provides "the first-ever experimental evidence for quantum gravity", says MIT cosmologist Max Tegmark - in other words, it shows that gravity is at heart a quantum phenomenon, just like the other three fundamental forces. Physicists, however, have yet to fully understand how to reconcile general relativity with quantum physics from a theory standpoint.
The researchers reported the findings on 17 March at a press briefing at the CfA, held just after they described their results to scientists in a technical talk. The team also released several papers describing the results. In so doing, it seems to have beaten the SPT and also several other groups racing to find the fingerprint of inflation using an assortment of balloon-borne and ground-based experiments and one satellite, the European Space Agency's Planck spacecraft.
More-extensive maps of the B-mode polarization, and especially a full-sky survey, which the Planck telescope may be able to obtain later this year, should provide more clues about how inflation unfolded and what drove it. In addition to looking farther back in time than ever before, the discovery "is opening a window a trillion times higher in energy than we can access with the Large Hadron Collider", the world's premiere atom smasher, notes cosmologist Avi Loeb of the CfA, who is not part of the BICEP2 team.
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KICP Members: John E. Carlstrom; Christopher D. Sheehy; Michael S. Turner; Abigail G. Vieregg
Scientific projects: BICEP2/The Keck Array/BICEP3
First direct evidence of Big Bang found
WBEZ95.1, March 19, 2014
Yesterday, scientists announced they have found the first direct evidence that right after the Big Bang, the universe expanded very far and very fast, in a fraction of a second. They call the process "cosmic inflation", and the discovery has incredible implications.
Abby Vieregg is an assistant professor at the University of Chicago, and part of the research team. She joins us to tell us more about this key scientific discovery and what it means for our understanding of the universe.
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KICP Members: Abigail G. Vieregg
Astrophysicists target mystery of powerful particles: Prof. Angela Olinto continues UChicago leadership in cosmic ray research with space station telescope
The University of Chicago News Office, March 20, 2014
Every second, our bodies and the objects around us are struck by cosmic rays that come mostly from sources deep in space. The highest-energy cosmic rays are the most energetic particles in the universe - far more powerful than anything humans could produce - but their origins are a mystery.
University of Chicago astrophysicist Angela Olinto is helping to unravel that stubborn riddle by leading the United States collaboration on an international project to deploy a cosmic ray telescope on the International Space Station later this decade. The instrument will peer back at Earth to detect the collisions of cosmic rays with the atmosphere, which could shed light on what produces the enigmatic particles.
"This first space mission for the highest-energy particles may pioneer the space exploration of the Earth's atmosphere as a giant particle detector," wrote Olinto, the Homer J. Livingston Professor in Astronomy & Astrophysics, in Il Nuovo Saggiatore, an Italian science magazine.
This first space mission for the highest-energy particles may pioneer the space exploration of the Earth's atmosphere as a giant particle detector."
Homer J. Livingston Professor in Astronomy & Astrophysics
Funded by a $4.4 million grant from NASA, Olinto and her colleagues are part of a 15-nation effort to build the 2.5-meter ultraviolet telescope, called the Extreme Universe Space Observatory.
No one knows what they will find. Ultra high-energy cosmic rays may come from supermassive black holes at the centers of nearby galaxies. A far less likely possibility is that they are decaying particles left over from the Big Bang. These subatomic particles hit the atmosphere with the energy of a tennis ball traveling at 167 miles an hour. The impact produces a giant cascade of many tens of billions of secondary particles, which to date have been observed only from Earth-based detectors.
"The mechanism behind this extreme acceleration challenges our imagination," Olinto says.
UChicago has a long history of research in cosmic rays, including more than half a century of balloon- and spacecraft-borne experiments conducted by scientists in the Enrico Fermi Institute. Austrian physicist Victor Hess discovered cosmic rays in 1912; surprisingly, he discovered that cosmic-ray intensity increased with altitude.
That chance discovery showcases "a beautiful aspect of science," says 1980 Nobel laureate James Cronin, University Professor Emeritus in Physics. "The discovery was that radiation is coming from outer space into Earth. One had no idea what this radiation was, but nevertheless it was there."
Clashing Nobel laureates
Originally called hohenstrahlung - German for "radiation from above" - the radiation has been known as cosmic rays since Robert A. Millikan coined the term in 1928. Millikan later would trade barbs with his former UChicago student and fellow Nobel laureate, Arthur Holly Compton, over their conflicting cosmic-ray data in a quarrel that made the front page of The New York Times.
"Millikan retorts hotly to Compton in cosmic ray clash," reported the Times on Dec. 31, 1932. Millikan, a UChicago faculty member from 1898 to 1921, asserted that cosmic rays consisted of gamma radiation. Compton, who was on the faculty from 1923 to 1945, believed that cosmic rays were charged particles. Compton was correct, but Millikan never did revise his opinion.
Cosmic rays pose little risk to organisms on Earth, where the atmosphere and the planet's magnetic field offer protection. But cosmic rays are a factor in the planning of extended interplanetary missions, where astronauts could be exposed to dangerous levels of radiation. The particles also can affect consumer electronics, causing subtle errors when they strike integrated circuits and other components.
The cosmic ray telescope that scientists hope to install aboard the space station will look down, to detect the giant particle cascades that high-energy cosmic rays produce when they enter Earth's atmosphere. The late Pierre Auger discovered this phenomenon in 1938. A few years later Auger continued his research during a visit to UChicago.
Cronin and his associates would spend years planning and establishing a sprawling cosmic ray observatory in Argentina that they named after Auger. The Pierre Auger Observatory began collecting data in 2004. "We have solved many open questions from the last century, but we didn't find the source of the highest-energy cosmic rays," Olinto says.
Despite its vast scale, the Auger Observatory can only detect subatomic particle interactions occurring in the atmosphere directly above its telescopes. But with the installation of a downward-looking ultraviolet telescope on the International Space Station, the entire atmosphere becomes a particle detector. The cosmic processes that produce those particles far exceed the capabilities of mankind's most powerful accelerator, the Large Hadron Collider in Switzerland. Space-based observations offer a way to overcome the constraints of man-made devices.
"In my opinion, it's the way to the future," Olinto says.
Coming full circle
With this approach, the study of cosmic rays and particle physics are coming full circle. "In the early part of the 20th century, cosmic ray research and particle physics were one and the same," notes Dietrich Muller, a professor emeritus in physics who has devoted much of his career to cosmic-ray research.
But cosmic ray research and particle physics went their separate ways in the 1950s. Scientists began building powerful particle accelerators, establishing the field of high-energy particle physics. "The other branch was to make measurements above the atmosphere, and that led to what's now called particle astrophysics, gamma-ray astronomy, and X-ray astronomy," Muller says.
In the meantime, the Telescope Array Project in Utah and the IceCube Neutrino Observatory at the South Pole have begun offering hints about the source of high-energy cosmic rays. Data from the Telescope Array has found a hotspot in the northern sky that indicates a possible source of ultra high-energy cosmic rays. IceCube also has found two neutrinos coming from that same region of the sky. Neutrinos - sometimes called ghost particles because of their ability to pass through solid matter - offer a second means of determining the source of high-energy cosmic rays.
The hotspot could be a temporary phenomenon that will disappear before the Extreme Observatory begins operating on the space station. "If it persists, then we should be able to confirm that this is the first source ever measured," Olinto says.
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KICP Members: Angela V. Olinto
Scientific projects: Pierre Auger Observatory (AUGER)
Tremors from cosmic discovery reverberate through Kavli Institute
The University of Chicago News Office, March 21, 2014
Scientists at the University of Chicago's Kavli Institute for Cosmological Physics are celebrating Monday's headline-making announcement that astronomers have acquired the first direct evidence of gravitational waves rippling through our infant universe during an explosive period of growth called inflation.
Researchers from the BICEP2 collaboration Monday announced the first direct evidence for this cosmic inflation. Their data also represent the first images of gravitational waves, or ripples in space-time. These waves have been described as the "first tremors of the Big Bang." On Wednesday afternoon, nearly 200 UChicago scientists assembled in Kersten Science Teaching Center for a special symposium presented by three BICEP2 collaborators.
"Detecting this signal is one of the most important goals in cosmology today. A lot of work by a lot of people has led up to this point," said John Kovac, PhD'04, of the Harvard-Smithsonian Center for Astrophysics and leader of the BICEP2 collaboration. The collaborations' co-leaders are Jamie Bock of the California Institute of Technology, Chao-Lin Kuo of Stanford University, and Clem Pryke of the University of Minnesota.
BICEP2's many collaborators include three members of the Kavli Institute: Abigail Vieregg, assistant professor in physics; Christopher Sheely, PhD'13, Kavli Institute fellow; and Erik Leitch, senior research associate.
"What an amazing discovery. The BICEP2 results are truly fantastic," said John Carlstrom, the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics, who leads a competing project, the South Pole Telescope. "It is a fantastic day for cosmology and indeed for all of physics."
Carlstrom served as Kovac's graduate-school mentor. In 2002, Kovac was lead author of a Nature paper announcing the detection of a minute polarization of the cosmic microwave background using a radio telescope called the Degree Angular Scale Interferometer. The discovery verified the framework that supported modern cosmological theory, including cosmic inflation, which improbably proposed that the universe underwent a gigantic growth spurt in a fraction of a second after the Big Bang.
Tiny fluctuations, big clues
The dramatic new results came from observations by the BICEP2 telescope of the cosmic microwave background - afterglow from the Big Bang. Tiny fluctuations in this afterglow provide clues to conditions in the early universe. For example, small differences in temperature across the sky show where parts of the universe were denser, eventually condensing into galaxies and galactic clusters.
Since the cosmic microwave background is a form of light, it exhibits all the properties of light, including polarization. On Earth, sunlight is scattered by the atmosphere and becomes polarized, which is why polarized sunglasses help reduce glare. In space, the cosmic microwave background was scattered by atoms and electrons and became polarized, too.
Gravitational waves leave behind characteristic twisting patterns on the cosmic microwave background known as B-mode polarization. Researchers took an important first step toward measuring inflationary B modes last year when they detected B modes from gravitational lensing for the first time. Gravitational lensing is a phenomenon that occurs when the trajectory of light is bent by massive objects in space, much like a lens focuses light.
The detection of gravitational lensing B modes was published last September in Physical Review Letters by a multi-institutional collaboration of researchers led by Carlstrom. They used data from SPTpol, a polarization-sensitive camera installed on the South Pole Telescope in January 2012. Physics World magazine named this finding as named one of the top 10 physics breakthroughs of 2013.
Journalists and members of the public alike have displayed enthusiastic interest in Monday's inflationary B modes announcement. The story made the front page of Tuesday's New York Times, which quoted Carlstrom in a story headlined "Space Ripples Reveal Big Bang's Smoking Gun."
The Washington Post's coverage, meanwhile, included quote from Kavli Institute Director Michael Turner, the Bruce and Diana Rauner Distinguished Service Professor in Astronomy & Astrophysics. "Inflation - the idea of a very big burst of inflation very early on - is the most important idea in cosmology since the big bang itself," Turner hold the Post. "If correct, this burst is the dynamite behind our big bang."
Other coverage included a live interview with BICEP2 collaborator Vieregg on WBEZ's Afternoon Shift program. "It's great to watch the reaction of our community," Vieregg said during the interview. "Our website that has our data and papers on it has actually gotten three and a half million hits as of last night."
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KICP Members: John E. Carlstrom; Christopher D. Sheehy; Michael S. Turner; Abigail G. Vieregg
Scientific projects: BICEP2/The Keck Array/BICEP3
Fermi Telescope data tantalize with new clues to dark matter
NASA, April 3, 2014
A new study of gamma-ray light from the center of the galaxy makes the strongest case to date that some of this emission may arise from dark matter, an unknown substance making up most of the material universe.
Using publicly available data from NASA's Fermi Gamma-ray Space Telescope, independent scientists at the Fermi National Accelerator Laboratory, the Harvard-Smithsonian Center for Astrophysics (CfA), the Massachusetts Institute of Technology and the University of Chicago have developed new maps showing that the galactic center produces more high-energy gamma rays than can be explained by known sources and that this excess emission is consistent with some forms of dark matter.
"The new maps allow us to analyze the excess and test whether more conventional explanations, such as the presence of undiscovered pulsars or cosmic-ray collisions on gas clouds, can account for it," said Dan Hooper, University of Chicago associate professor in astronomy and astrophysics. A lead author of the study, Hooper also is an astrophysicist at Fermilab. "The signal we find cannot be explained by currently proposed alternatives and is in close agreement with the predictions of very simple dark matter models," he said.
The galactic center teems with gamma-ray sources, from interacting binary systems and isolated pulsars to supernova remnants and particles colliding with interstellar gas. It's also where astronomers expect to find the galaxy's highest density of dark matter, which only affects normal matter and radiation through its gravity. Large amounts of dark matter attract normal matter, forming a foundation upon which visible structures, like galaxies, are built.
No one knows the true nature of dark matter, but WIMPs, or Weakly Interacting Massive Particles, represent a leading class of candidates. Theorists have envisioned a wide range of WIMP types, some of which may either mutually annihilate or produce an intermediate, quickly decaying particle when they collide. Both of these pathways end with the production of gamma rays - the most energetic form of light - at energies within the detection range of Fermi's Large Area Telescope (LAT).
When astronomers carefully subtract all known gamma-ray sources from LAT observations of the galactic center, a patch of leftover emission remains. This excess appears most prominent at energies between 1 and 3 billion electron volts (GeV) - roughly a billion times greater than that of visible light - and extends outward at least 5,000 light-years from the galactic center.
Hooper and his colleagues conclude that annihilations of dark matter particles with a mass between 31 and 40 GeV provide a remarkable fit for the excess based on its gamma-ray spectrum, its symmetry around the galactic center and its overall brightness. Writing in a paper submitted to the journal Physical Review D, the researchers say that these features are difficult to reconcile with other explanations proposed so far, although they note that plausible alternatives not requiring dark matter may yet materialize.
"Dark matter in this mass range can be probed by direct detection and by the Large Hadron Collider (LHC), so if this is dark matter, we're already learning about its interactions from the lack of detection so far," said co-author Tracy Slatyer, theoretical physicist at MIT. "This is a very exciting signal, and while the case is not yet closed, in the future we might well look back and say this was where we saw dark matter annihilation for the first time."
The researchers caution that it will take multiple sightings - in other astronomical objects, the LHC or in some of the direct-detection experiments now being conducted around the world - to validate their dark matter interpretation.
"Our case is very much a process-of-elimination argument. We made a list, scratched off things that didn't work, and ended up with dark matter," said co-author Douglas Finkbeiner, professor of astronomy and physics at the CfA.
"This study is an example of innovative techniques applied to Fermi data by the science community," said Peter Michelson, professor of physics at Stanford University and the LAT principal investigator. "The Fermi LAT Collaboration continues to examine the extraordinarily complex central region of the galaxy, but until this study is complete we can neither confirm nor refute this interesting analysis."
While the great amount of dark matter expected at the galactic center should produce a strong signal, competition from many other gamma-ray sources complicates any case for detection. But turning the problem on its head provides another way to attack it. Instead of looking at the largest nearby collection of dark matter, look where the signal has fewer challenges.
Dwarf galaxies orbiting the Milky Way lack other types of gamma-ray emitters and contain large amounts of dark matter for their size - in fact, they're the most dark-matter-dominated sources known. But there's a tradeoff. Because they lie much farther away and contain much less total dark matter than the center of the Milky Way, dwarf galaxies produce a much weaker signal and require many years of observations to establish secure detection.
For the past four years, the LAT team has been searching dwarf galaxies for hints of dark matter. The published results of these studies has set stringent limits on the mass ranges and interaction rates for many proposed WIMPs, even eliminating some models. In the study's most recent results, published in Physical Review D on Feb. 11, the Fermi team took note of a small but provocative gamma-ray excess.
"There's about a one-in-12 chance that what we're seeing in the dwarf galaxies is not even a signal at all, just a fluctuation in the gamma-ray background," explained Elliott Bloom, a member of the LAT Collaboration at the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the SLAC National Accelerator Laboratory and Stanford University. If it's real, the signal should grow stronger as Fermi acquires additional years of observations and as wide-field astronomical surveys discover new dwarfs. "If we ultimately see a significant signal," he said, "it could be a very strong confirmation of the dark matter signal claimed in the galactic center."
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KICP Members: Daniel Hooper; Tim Linden
Searching High & Low for Dark Matter
The Kavli Foundation, April 7, 2014
An annual conference at the University of California, Los Angeles had researchers discussing the latest progress and challenges in the hunt for dark matter.
THERE'S MORE TO THE COSMOS THAN MEETS THE EYE. In late February, dark matter hunters from around the world gathered at the University of California, Los Angeles for "Dark Matter 2014." The annual conference is one of the largest of its kind aimed at discussing the latest progress in the quest to identify dark matter, the unknown stuff that makes up more than a quarter of the universe yet remains a mystery. Nearly 160 people attended, including renowned physicists from institutions across the United States and Europe, as well as from Japan, China and Canada.
So where does the hunt stand? Between sessions, three leading physicists at the conference spent an hour discussing its biggest highlights and prospects for future progress. Joining the special conversation:
* Blas Cabrera - Professor of Physics at Stanford University, and Member of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford. Spokesperson for the SuperCDMS dark matter experiment.
* Dan Hooper - Scientist in the Theoretical Astrophysics Group at the Fermi National Accelerator Laboratory, Associate Professor in the Department of Astronomy and Astrophysics at the University of Chicago, and Senior Member of the Kavli Institute for Cosmological Physics (KICP) at UChicago.
* Tim Tait - Professor of Physics and Astronomy at the University of California Irvine, and Member of UC Irvine's Theoretical Particle Physics Group.
The following is an edited transcript of the discussion.
THE KAVLI FOUNDATION: Almost everyone at the conference seems to think we're finally on the path toward figuring out what dark matter is. After 80 years of being in the "dark," what are we hearing at this meeting to explain the optimism?
BLAS CABRERA: This conference has highlighted the progression of larger and larger experiments with remarkable advances in sensitivity. What we're looking for is evidence of a dark matter particle, and the leading idea for what it might be is something called a weakly interacting massive particle, or WIMP. We believe the WIMP interacts with ordinary matter only very rarely, but we have hints from a few experiments that might be evidence for WIMPs.
Separately at this conference, we heard about improved calibrations of last fall's results from LUX, the Large Underground Xenon detector that now leads the world in sensitivity for WIMPs above the mass of six protons - a proton being the nucleus of a single hydrogen atom. Under a standard interpretation of the data, the LUX team has ruled out a range of low-end masses for the dark matter particle, another major advance because it does not see potential detections reported by other experiments and further narrows the possibilities for how massive the WIMP might be.
Finally, Dan [Hooper] also gave a remarkable presentation here about another effort: to indirectly detect dark matter by studying radiation coming from the center of the Milky Way galaxy. He reported the possibility of a strong dark matter signal, and I would say that was also one of the highlights of the conference because it provides us with some of the strongest evidence so far of a dark matter detection in space. Dan can explain.
DAN HOOPER: Four and a half years ago, I wrote my first paper on searching for evidence of dark matter at the center of the Milky Way galaxy. And now we think we have the most compelling results to date. What we're looking at is actually gamma rays - the most energetic form of light - radiating from the center of the galaxy. I think that this is very likely a signal of annihilating dark matter particles. As Blas explained, we believe dark matter is made of particles, and these particles, by themselves, are expected to be stable - meaning that they don't readily decay into other particles or forms of radiation. But at the dense core of the Milky Way galaxy, we think they collide and annihilate one another, in the process releasing huge amounts of energy in the form of gamma rays.
TIM TAIT: We expect that the density of dark matter particles, and therefore the intensity of the gamma-ray radiation released when they collide, should both fall as you move away from the galactic center. So, you sort of know what the profile of the signal should be, moving from the center of the galaxy outward.
TKF: So Dan, in this case the gamma rays that we observe radiating from the center of the Milky Way match our predictions for the mass of dark matter particles?
HOOPER: That's right. We predicted what the energy level of the gamma rays should be, based on established theories for how massive the WIMP should be, and what we've seen matches the simplest theoretical model for the WIMP. Our paper is based on more data, and we found more sophisticated ways of analyzing that data. We threw every test we could think of at it. We found that not only is the signal there and very statistically significant, its characteristics really look like what we would expect dark matter to produce - in the way that the gamma-ray radiation maps on the sky, in its general brightness, and in other features.
"A discovery of dark matter ...[means] we would have identified the dominant form of matter in the universe that seeded structure and led to galaxies, solar systems and planets, and ultimately to our Earth with intelligent life." - Blas Cabrera
TKF: Tell me a bit more about this prediction.
HOOPER: We think that all the particles that make up dark matter were all produced in the Big Bang nearly 14 billion years ago, and eventually as the universe cooled a small fraction survived to make up the dark matter we have today. The amount that has survived depends on how much the dark matter particles have interacted with one other over cosmic time. The more they collided and became annihilated, the less dark matter survives today. So, I can basically calculate the rate at which dark matter particles have collided over cosmic history - based on how much dark matter we estimate exists in the universe today. And once I have the rate of dark matter annihilation today, I can estimate how bright the gamma-ray signal from the galactic center should be - if it's made of WIMPS of a certain mass. And lo and behold, the observed gamma-ray signal is as bright as we predict it should be.
TKF: What else caught everyone's attention at the conference?
TAIT: A really striking result was from Super Cryogenic Dark Matter Search, or SuperCDMS, the direct detection experiment that Blas works on. They didn't find any evidence for dark matter, and that contradicts several other direct detection experiments that have claimed a detection in the same mass range.
CABRERA: What we're looking for is an exceedingly rare collision between an incoming WIMP and the nucleus of a single atom in our detector, which in SuperCDMS is made from germanium crystal. The collision causes the nucleus of a germanium atom to recoil, and that recoil generates a small amount of energy that we can measure.
Direct detection experiments are situated underground to minimize background noise from a variety of known sources of radiation, from space and on Earth. The new detectors that we built in SuperCDMS have allowed us to reject the dominant background noise that in the past clouded our ability to detect a dark matter signal. This noise was from electrons hitting the surface of the germanium crystal in the detector. The new design allows us to clearly identify and throw out these surface events. So, rather than saying, "Okay, maybe this background could be partly a signal," we can say with confidence now, "There is no background" and you have a very clean result. What this means is we have much more confidence in our data if we do make a potential detection. And if we don't, we're more confident that we're coming up empty. Eliminating background noise vastly reduces uncertainties in our analysis - whether we find something or not.
TKF: What caught everyone's attention on the theoretical side?
CABRERA: What struck me at this meeting is that nuclear physicists have recently written papers describing a generalized framework for all possible interactions between a dark matter particle and the nucleus of a single atom of the material that researchers use in their detectors; in the case of SuperCDMS, as I've explained, it's germanium and silicon crystals. These nuclear physicists have pointed out that roughly half of all possible interactions are not even being considered now. We are trying to digest what that means, but it suggests there are many more possibilities and a lot we still don't know.
TKF: Tim, with accelerators like the Large Hadron Collider in Europe, researchers are looking for evidence of supersymmetry, which could reveal the nature of dark matter. Tell me about this idea. Also, was anything new discussed at the meeting?
TIM TAIT: Supersymmetry proposes there are mirror particles that shadow all the known fundamental particles, and in this shadow world may lurk the dark matter particle. So, by smashing together protons in the LHC, we've tried to reveal these theoretical supersymmetric particles. So far, though, the LHC hasn't found any evidence for supersymmetry. It may be that our vision of supersymmetry isn't the only vision for physics beyond the Standard Model. Or maybe our vision for supersymmetry isn't a complete one.
TKF: The LHC is going to collide protons at much higher energy levels next year, so could that reveal something we just can't see right now?
TAIT: We hope so. We have very good reason to think that the lightest of the mirror particles in this shadow family is probably stable, so higher energy collisions could very well reveal them. If dark matter was formed early in the universe as a supersymmetric particle and it's still around - which we think it is - it could show up in the next round of LHC experiments.
TKF: When you think about the different approaches to identifying dark matter, has anything discussed at this meeting convinced you that one of them will be first?
TAIT: When you look at all the different ways of looking for dark matter, what you find is that they all have incredible strengths and they all have blind spots. And so you can't really say one is doing better than the other. You can say, though, they are answering different questions and doing very important things. Because even if you end up discovering dark matter in one place - let's say in the direct detection search - the fact that you do not see it at the LHC, for example, is already telling you something amazing about the theory. A negative result is actually just as important as a positive result.
"When you look at all the different ways of looking for dark matter, what you find is that they all have incredible strengths and they all have blind spots." - Tim Tait
HOOPER: The same goes with the direct detection experiments. I'm remarkably surprised that they haven't seen anything. We have this idea of where these supersymmetric particles and WIMP particles should show up in these experiments - at the LHC and in direct detection experiments - and yet lo and behold we got there and they are not there. But that doesn't mean they're not right around the corner, or maybe several corners away.
CABRERA: Given the remarkable progress over the past few years with many direct detection experiments, we would not have been surprised to have something rear its head that looks like a true WIMP.
HOOPER: Similarly, I think if you had done a survey of particle physicists five years ago, I don't think many of them would have said that in 2014 we've only discovered the Higgs - the fundamental particle that imparts mass to fundamental particles - and not anything else.
CABRERA: Now that the Higgs has been pretty convincingly seen, the next big questions for the accelerator community are: "What is dark matter? What is it telling us that we do not see dark matter at the LHC? What does that leave open?" These questions are being asked broadly, which wasn't the case in past years.
TKF: Was finding the Higgs, in a sense, an easier quest than identifying dark matter?
HOOPER: We knew what the Higgs should look like, and we knew what we would have to do to observe it. Although we didn't know exactly how heavy it would be.
CABRERA: We knew it had to be there.
HOOPER: If it weren't there it would have been weird. Now, with dark matter, there are hundreds and hundreds of different WIMP candidates that people have written down, and they all behave differently. So the Higgs is a singular idea, more or less, while the WIMP is a whole class of ideas.
TKF: What would a confirmed detection of dark matter really mean for what we know about the universe? And where would we go from there?
CABRERA: A discovery of dark matter with direct detection experiments would not be the end of the journey, but rather the beginning of a very exciting set of follow-up experiments. We would want to determine the mass and other properties of the particle with more precision, and we'd also want to better understand how dark matter is distributed in and around our galaxy. Follow-up experiments with detectors would use different materials, and we'd also try to map which direction the WIMPs are coming from through our detectors, which would help us better understand the nature of dark matter that surrounds the Earth.
Overall, a discovery would be huge for astrophysics and cosmology, and for elementary particle physics. For astrophysics we would have identified the dominant form of matter in the universe that seeded structure and led to galaxies, solar systems and planets, and ultimately to our Earth with intelligent life. On the particle physics side, this new particle would require physics beyond the Standard Model such as supersymmetry, and would allow us to probe this new sector with particle accelerators like the LHC.
"The history of science is full of discoveries opening up whole new avenues for exploration that were not foreseen." - Dan Hooper
TAIT: I think there's a lot of different ways you could look at it. From a particle physicist's point of view, we would now have a new particle that we'd have to put into our fundamental table of particles. We know that we see lots of structure in this table, but we don't really understand where the structure comes from.
From a practical point of view, and this is very speculative, dark matter is a frozen form of energy, right? Its mass is energy, and it's all around us. Personally, if I understood how dark matter interacts with ordinary matter, I would try to figure out how to build a reactor. And I'm sure that such a thing is not at all practical today, but someday we might be able to do it. Right now, dark matter just goes right through us, and we don't know how to stop it and communicate with it.
HOOPER: That was awesome, Tim. You blow my mind. I'm picturing a 25th century culture in which we harness dark matter to make an entirely new form of energy.
TAIT: By the way, Dan, I'm toying with the idea of writing a paper so we should keep talking.
HOOPER: I would love to hear more about it. That sounds great. So, to kind of echo some of what Tim said, the dark matter particle, once we identify it, has to fit into a bigger theory that connects it to the Standard Model. We don't really have any idea what that might look like. We have a lot of guesses, but we really don't know so there's a lot of work to do. Maybe this will help us build a grand unified theory - a single mathematical explanation for the universe - and help us, for example, understand things like gravity, which frankly we don't understand at all in a particle physics context. Maybe it will just open our eyes to entirely new possibilities that we just never considered until now. The history of science is full of discoveries opening up whole new avenues for exploration that were not foreseen. And I have every reason to think that that's not unlikely in this case.
- Writer: Bruce Lieberman, 2014
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KICP Members: Daniel Hooper
Physics Today' cover page: The Dark Energy Survey
Physics Today, April 10, 2014
The Dark Energy Survey will conduct a five-year census of galaxies and stars over a full eighth of the night sky in an effort to understand what is driving the accelerating expansion of the cosmos. The survey will be carried out by the 570-megapixel Dark Energy Camera, photographed here in its black housing on the Victor M. Blanco Telescope in Chile. Josh Frieman's article on page 28 describes the science underlying the survey and some of the camera's advanced technology.
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KICP Members: Joshua A. Frieman
Scientific projects: Dark Energy Survey (DES)
Wayne Hu was elected to the American Academy of Arts and Sciences
The University of Chicago News Office, May 1, 2014
American Academy of Arts and Sciences elects 26 members with UChicago ties
New class of inductees includes eight faculty members, three trustees
Wayne Hu's research focuses on understanding structure formation in the universe as revealed in temperature differences found in the cosmic microwave background radiation (the afterglow of the Big Bang), gravitational lensing (an effect that distorts images of galaxies), and how galaxies and clusters of galaxies were seeded at the Big Bang. A professor in astronomy and astrophysics, Hu also develops and tests theories for dark energy and cosmic acceleration. Hu's honors include a Packard Fellowship, an Alfred P. Sloan Fellowship, the Warner Prize from the American Astronomical Society, and the Outstanding Young Researcher Award from the Overseas Chinese Physics Association.
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KICP Members: Wayne Hu
"The Power of Curiosity", by Michael S. Turner, Science 2 May 2014: Vol. 344 no. 6183 p. 449
Science, May 2, 2014
Michael S. Turner is the Rauner Distinguished Service Professor and director of the Kavli Institute of Cosmological Physics at the University of Chicago, Chicago, IL.
In March of 2014, 47 scientists from 15 institutions (including my own) announced that a South Pole - based microwave telescope had taken us back to a time when the universe was 10-38 seconds old - when everything that we can see today occupied a space much smaller than that occupied by a proton, and when the energy level of the universe was a trillion times greater than that produced by the world's most powerful accelerator, the Large Hadron Collider in Switzerland. Assuming that this amazing discovery by the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) collaboration is confirmed, this cosmic remnant beats the previous record holder for earliest fossil of our cosmic birth (the helium and deuterium made when the universe was seconds old) by 38 orders of magnitude. Rarely has science advanced by such a giant leap, and it will take us years if not decades to fully comprehend all the implications of this incredible moment in science. Although we are used to cosmology stunning us with beautiful images and mind-stretching discoveries such as dark energy and dark matter, even this cosmologist with almost 40 years of experience was awed and shocked by this big, big find.
According to the standard cosmological model, during its earliest moments, the universe underwent a tremendous growth spurt known as inflation, which created the seeds of galaxies and all other cosmic structures from subatomic quantum fluctuations. Since 1992, measurements of the cosmic microwave background have amassed evidence for this theory, but the BICEP2 telescope may have found the smoking gun: gravitational waves that began as quantum fluctuations in spacetime and left an imprint on the cosmic microwave background in the tiny signal (about 100 nanokelvins) detected by the BICEP2 telescope.
Because of deep and remarkable connections between quarks and the cosmos, this cosmic discovery is related to another big discovery, the Higgs boson. The Higgs, the first of a new class of elementary particles (scalar bosons), accounts for why some elementary particles have mass. The potential instigator of cosmic inflation is a hypothetical scalar boson called the inflaton, and the Higgs discovery boosted its credibility and may even explain how it fits in. Conversely, the BICEP2 discovery has given us a window on the highest energies that particle theorists can imagine, and in doing so, will provide insights into how the fundamental forces and particles are unified.
There are differences between the BICEP2 and Higgs discoveries to be sure, in technique and scale of effort, but both are exemplars of the kind of curiosity-driven science that gets scientists out of bed in the morning and inspires young people to careers in science by asking some of the deepest questions about how the universe began and the events that have shaped our existence. BICEP2 and the Higgs will launch the careers of thousands of new scientists around the world, just as quarks and quasars sparked my career. But we should not forget that great discoveries can have unforeseen practical benefits as well. Some 100 years ago, the discovery of strange new phenomena began the esoteric study of quantum mechanics, with no hint of a practical benefit. The exploitation of these quantum phenomena has enabled the information age that now underpins our economy and way of life.
Whether the fruits of our curiosity are bettering our existence on Earth or our understanding of our place in the cosmos, it all begins with a burning desire to know. The BICEP2 and Higgs discoveries remind us never to underestimate the power of this curiosity, one of humankind's greatest assets.
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KICP Members: Michael S. Turner
Scientific projects: BICEP2/The Keck Array/BICEP3
Kavli Foundation discussion: "Instantaneous Cosmic Growth: Have We Found the Smoking Gun?"
The Kavli Foundation, May 19, 2014
We may have "smoking gun" evidence the universe expanded with unmatchable speed in its earliest moments. So what does this mean? Three theoretical physicists -- Daniel Baumann, Michael S. Turner and Paul Steinhardt -- consider the evidence, its implications, and the next steps.
For decades, theorists have speculated that in its very first moments, our universe underwent a mind-bogglingly fast expansion that took it from the diminutive size of a proton to a vast expanse. Earlier this year, scientists announced a stunning development: what may be the first "smoking gun" evidence in support of this theory.
How certain is this result and, if it's corroborated, what does it mean for our theories of how the universe works? Three leading theorists spoke recently with The Kavli Foundation about the implications of these results on our understanding of the early universe.
"To have the signal come in basically as big as it could be - bigger even - was just amazing," said theorist Michael S. Turner, Director of the Kavli Institute for Cosmological Physics (KICP) and the Bruce V. and Diana M. Rauner Distinguished Service Professor at the University of Chicago. "We're used to cosmology awing us, but this time it shocked us as well."
Daniel Baumann, a lecturer in theoretical physics at Cambridge University whose research focuses on inflation and string theory, agreed: "My initial reaction was also shock and awe. I was intellectually prepared for these experiments, ...but somehow in my gut I wasn't prepared to have a signal that was as big as it actually was."
"My concern at the moment is that it's not yet clear whether or not they got it right," said Paul Steinhardt, the Albert Einstein Professor in Science and Director of the Princeton Center for Theoretical Science at Princeton University. "They've definitely seen something. But deciding whether it's due to gravitational waves produced in the early universe or due to some source in the foreground that's between us and where the microwave background was emitted, that's a key issue."
More than half a dozen experiments around the world are now seeking to confirm BICEP2's result in other frequencies and in other regions of the sky. The participants agreed that if these experiments find a similar signal and its shape matches what's expected, that will be solid proof of cosmic inflation. In addition, the opportunity would exist to see subtle surprises in the signal that could lead to the discovery of new physics.
The complete discussion.
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KICP Members: Michael S. Turner
Scientific projects: BICEP2/The Keck Array/BICEP3
Fermi Institute announces recipients of Nathan Sugarman research awards
The University of Chicago News Office, June 5, 2014
by Steve Koppes, The University of Chicago News Office
Two fourth-year students in the College and two graduate students have received the 23rd annual Nathan Sugarman Awards for Excellence in Undergraduate and Graduate Research.
The undergraduate recipients are Jane Huang, a fourth-year in chemistry and 2013 Goldwater Scholar; and Samantha Dixon, a fourth-year in physics and mathematics.
The graduate recipients are Vinicius Miranda, a doctoral student in astronomy and astrophysics; and Eric Oberla, a doctoral student in physics.
Huang was cited "for her in-depth analysis of λ5797, the Rosetta Stone of diffuse interstellar bands." She was nominated by Takeshi Oka, professor emeritus of chemistry and astronomy and astrophysics; and Donald York, the Horace B. Horton Professor of Astronomy and Astrophysics.
Dixon was cited "for her outstanding work in the calibration set-up for the DAMIC dark matter experiment, and in the measurement of radioactive contamination of the CCD detectors." Dixon was nominated by Paolo Privitera, professor of astronomy and astrophysics.
Miranda was cited "for his thorough and careful work in elucidating the effect of inflationary features on cosmic microwave background anisotropy and non-Gaussianity." Miranda was nominated by Wayne Hu, professor of astronomy and astrophysics.
Nathan Sugarman, SB'37, PhD'41, was a charter member of the Enrico Fermi Institute and a longtime professor in chemistry.
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KICP Members: Wayne Hu; Paolo Privitera; Donald G. York
KICP Students: Samantha Dixon; Vinicius Miranda
Chuan He and Wayne Hu receive named professorships
The University of Chicago News Office, June 12, 2014
The Division of Physical Sciences is pleased to announce that Professors Chuan He and Wayne Hu have received named and distinguished service professorships in recognition of their outstanding contribution to scholarship, teaching and the intellectual community of the University of Chicago.
Wayne Hu, Professor in Astronomy and Astrophysics and the Enrico Fermi Institute will receive the Horace B. Horton Professorship on November 1, 2014.
Hu's research focuses on the theory and phenomenology of structure formation in the Universe as revealed in Cosmic Microwave Background anisotropies, gravitational lensing, galaxy clustering and galaxy clusters. His work has been published in Physics Review D, The Journal of Cosmology and Astroparticle Physics, The New Journal of Physics, and other publications.
Dr. Hu has received a number of awards, including the American Astronomical Society Warner Prize, an Alfred P. Sloan Fellowship, and Packard Fellowship. He joined the University of Chicago faculty in 2000.
KICP Members: Wayne Hu
Merging galaxies and droplets of starbirth: Hubble snaps a violent galactic merger and chain of star formation
Spacetelescope.org News, July 10, 2014
The Universe is filled with objects springing to life, evolving and dying explosive deaths. This new image from the NASA/ESA Hubble Space Telescope captures a snapshot of some of this cosmic movement. Embedded within the egg-shaped blue ring at the centre of the frame are two galaxies. These galaxies have been found to be merging into one and a "chain" of young stellar superclusters are seen winding around the galaxies' nuclei.
At the centre of this image lie two elliptical galaxies, part of a galaxy cluster known as[HGO2008]SDSS J1531+3414, which have strayed into each other's paths. While this region has been observed before, this new Hubble picture shows clearly for the first time that the pair are two separate objects. However, they will not be able to hold on to their separate identities much longer, as they are in the process of merging into one. 
Finding two elliptical galaxies merging is rare, but it is even rarer to find a merger between ellipticals rich enough in gas to induce star formation. Galaxies in clusters are generally thought to have been deprived of their gaseous contents; a process that Hubble has recently seen in action. Yet, in this image, not only have two elliptical galaxies been caught merging but their newborn stellar population is also a rare breed.
The stellar infants - thought to be a result of the merger - are part of what is known as "beads on a string" star formation. This type of formation appears as a knotted rope of gaseous filaments with bright patches of new stars and the process stems from the same fundamental physics which causes rain to fall in droplets, rather than as a continuous column. 
Nineteen compact clumps of young stars make up the length of this "string", woven together with narrow filaments of hydrogen gas. The star formation spans 100,000 light years, which is about the size of our galaxy, the Milky Way. The strand is dwarfed, however, by the ancient, giant merging galaxies that it inhabits. They are about 330,000 light years across, nearly three times larger than our own galaxy. This is typical for galaxies at the centre of massive clusters, as they tend to be the largest galaxies in the Universe.
The electric blue arcs making up the spectacular egg-like shape framing these objects are a result of the galaxy cluster's immense gravity. The gravity warps the space around it and creates bizarre patterns using light from more distant galaxies.
Astronomers have ruled out the possibility that the blue strand is also just a lensed mirage from distant galaxies and now their challenge is to understand the origin of the cold gas that is fuelling the growth of the stellar superclusters. Was the gas already in the merging galaxies? Did it condense like rain from the rapidly cooling X-ray plasma surrounding the two galaxies? Or, did it cool out of a shock in the X-ray gas as the ten-million-degree gaseous halos surrounding the galaxies collided together? Future observations with both space- and ground-based observatories are needed to unravel this mystery.
 Mergers occur when two or more galaxies stray too close to one another, causing them to coalesce into one large body (heic0912). The violent process strips gas, dust and stars away from the galaxies involved and can alter their appearances dramatically, forming large gaseous tails, glowing rings, and warped galactic discs (heic0810).
 The merging system is forming stellar superclusters in equally spaced beads just like evenly spaced drops from a tap. The only real difference is that surface tension in the falling water is analogous to gravity in the context of the star-forming chain. This is a wonderful demonstration that the fundamental laws of physics really are scale-invariant - we see the same physics in rain drops that we do on 100 000 light-year scales.
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
Image credit: NASA, ESA/Hubble and G. Tremblay (European Southern Observatory)
Acknowledgement: M. Gladders & M. Florian (University of Chicago, USA), S. Baum, C. O'Dea & K. Cooke (Rochester Institute of Technology, USA), M. Bayliss (Harvard-Smithsonian Center for Astrophysics, USA), H. Dahle (University of Oslo, Norway), T. Davis (European SouthernObservatory), J. Rigby (NASA Goddard Space Flight Center, USA), K. Sharon (University of Michigan, USA), E. Soto (The Catholic University of America, USA) and E. Wuyts (Max-Planck-Institute for Extraterrestrial Physics, Germany).
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KICP Members: Michael D. Gladders
Do we live in a 2-D hologram?
The University of Chicago News Office, August 28, 2014
A unique experiment at the U.S. Department of Energy's Fermi National Accelerator Laboratory called the Holometer has started collecting data that will answer some mind-bending questions about our universe - including whether we live in a hologram.
Much like characters on a television show would not know that their seemingly 3-D world exists only on a 2-D screen, we could be clueless that our 3-D space is just an illusion. The information about everything in our universe could actually be encoded in tiny packets in two dimensions.
Get close enough to your TV screen and you'll see pixels, small points of data that make a seamless image if you stand back. Scientists think that the universe's information may be contained in the same way, and that the natural "pixel size" of space is roughly 10 trillion trillion times smaller than an atom, a distance that physicists refer to as the Planck scale.
"We want to find out whether spacetime is a quantum system just like matter is," said Craig Hogan, director of Fermilab's Center for Particle Astrophysics and the developer of the holographic noise theory. "If we see something, it will completely change ideas about space we've used for thousands of years."
The Holometer team comprises 21 scientists and students from Fermilab, Massachusetts Institute of Technology, University of Chicago and University of Michigan. The science team includes Hogan and Stephan Meyer, who are both professors in astronomy & astrophysics at UChicago.
Quantum theory suggests that it is impossible to know both the exact location and the exact speed of subatomic particles. If space comes in 2-D bits with limited information about the precise location of objects, then space itself would fall under the same theory of uncertainty. The same way that matter continues to jiggle, as quantum waves, even when cooled to absolute zero, this digitized space should have built-in vibrations even in its lowest energy state.
Essentially, the experiment probes the limits of the universe's ability to store information. If there are a set number of bits that tell you where something is, it eventually becomes impossible to find more specific information about the location - even in principle. The instrument testing these limits is Fermilab's Holometer, or holographic interferometer, the most sensitive device ever created to measure the quantum jitter of space itself.
Now operating at full power, the Holometer uses a pair of interferometers placed close to one another. Each one sends a one-kilowatt laser beam, the equivalent of 200,000 laser pointers, at a beam splitter and down two perpendicular 40-meter arms. The light is then reflected back to the beam splitter where the two beams recombine, creating fluctuations in brightness if there is motion. Researchers analyze these fluctuations in the returning light to see if the beam splitter is moving in a certain way - being carried along on a jitter of space itself.
"Holographic noise" is expected to be present at all frequencies, but the scientists' challenge is not to be fooled by other sources of vibrations. The Holometer is testing a frequency so high - millions of cycles per second - that motions of normal matter are not likely to cause problems. Rather, the dominant background noise is more often due to radio waves emitted by nearby electronics. The Holometer experiment is designed to identify and eliminate noise from such conventional sources.
"If we find a noise we can't get rid of, we might be detecting something fundamental about nature - a noise that is intrinsic to spacetime," said Fermilab physicist Aaron Chou, lead scientist and project manager for the Holometer. "It's an exciting moment for physics. A positive result will open a whole new avenue of questioning about how space works."
The Holometer experiment, funded by the U.S. Department of Energy and other sources, is expected to gather data over the coming year. For more information about the experiment, visit http://holometer.fnal.gov/.
Fermilab is America's premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance, LLC - a partnership of Universities Research Association and the University of Chicago. Visit Fermilab's website at www.fnal.gov.
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KICP Members: Craig J. Hogan; Stephan S. Meyer
Keith Bechtol races to victories in Chicago Half Marathon
Chicago Sun-Times, September 8, 2014
You didn't have to be an astrophysicist to tell conditions were nearly perfect Sunday morning for the start of the 18th Chicago Half Marathon on the South Side.
But apparently it helped, at least for Keith Bechtol. The astrophysicist at the University of Chicago won the men's side in 1 hour, 6 minutes, 52 seconds. Jeff Jonatis was second, 50 seconds back.
Kristen Heckert, a Plainfield South math teacher and coach from Bolingbrook, won the women's race in 1:17:35. Pamela Staton was second (1:20:45).
"Fantastic: Not much wind and right around 60 degrees," Bechtol said.
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KICP Members: Keith Bechtol
Finding Dark Energy in the Details
Quanta Magazine, September 23, 2014
The astrophysicist Joshua Frieman seeks to pinpoint the mysterious substance driving the accelerating expansion of the universe.
Like most theoretical cosmologists, Joshua Frieman was thrilled when astronomers announced in 1998 that the expansion of the universe appeared to be speeding up, driven by an invisible agent that they called "dark energy."
Frieman and his fellow theorists imagined two possible causes for the cosmic acceleration: Dark energy could be the quantum jitter of empty space, a "cosmological constant" that continues to accrue as space expands, pushing outward ever more forcefully. Alternately, a yet-undetected force field could pervade the cosmos, one akin to the field that scientists believe powered the exponential expansion of the universe during the Big Bang.
But the scientists also realized that the two options would have nearly identical observational consequences, and either theory could fit the crude measurements to date.
To distinguish between them, Frieman, a professor of astronomy and astrophysics at the University of Chicago and a senior staff scientist at the Fermi National Accelerator Laboratory (Fermilab) in nearby Batavia, Ill., co-founded the Dark Energy Survey (DES), a $50 million, 300-person experiment. The centerpiece of the project is the Dark Energy Camera, or DECam, a 570-megapixel, optical and near-infrared CCD detector built at Fermilab and installed on the four-meter Blanco Telescope in Chile two years ago. By observing 300 million galaxies spanning 10 billion light-years, DES aims to track the cosmic acceleration more precisely than ever before in hopes of favoring one hypothesis over the other. Frieman and his team are now reporting their first results.
Quanta Magazine caught up with Frieman in late August during COSMO 2014, a conference he helped organize. With his closely clipped gray beard, tortoiseshell glasses and organic cotton shirt, the scientist fit right in with the other gourmands lunching at Eataly Chicago down the street. Between bites of tagliatelle, he explained just what is and isn't known about dark energy, and how DES will help impel theorists toward one of the two disparate descriptions of its nature. An edited and condensed version of the interview follows.
QUANTA MAGAZINE: Why did you start the Dark Energy Survey?
JOSHUA FRIEMAN: As a theorist in the 1990s working on theoretical ideas for what could be causing the universe to speed up, I came to the conclusion that we could make different models and do a lot of theoretical speculation, but that we wouldn't know which of those paths to go down until we had much better data.
So a handful of us in Illinois started discussing possibilities for getting that data. And it just so happened that, around that time, the National Optical Astronomy Observatory announced an opportunity, saying, more or less, "If someone can build a really cool instrument for the telescope we operate in Chile, we'll give you a bunch of telescope time." That's when we formed the Dark Energy Survey collaboration and came up with the design for our camera.
Isn't it unusual for a theorist to lead a major astrophysics experiment?
It's somewhat unusual, but the boundaries between theory and observation in cosmology are getting blurred, which I think is a healthy development. It used to be that theorists like me would work with a pen and paper, and then observers would go out and take the data and analyze it. But we now have a new model in which teams are trained to analyze and interpret large data sets, and that isn't purely theory or purely observational work; it combines the two.
How do you picture an invisible unknown like dark energy?
One way to think about dark energy is as a fluid, in the sense that it can be described by its density and its pressure. Those two properties tell you its effects on the expansion of the universe. The more dark energy there is - that is, the greater its density - the stronger its effects are. But the thing that's really crucial about dark energy is that unlike anything else we know about, it has negative pressure, and that's what makes it gravitationally repulsive.
Why does negative pressure make it repulsive?
Einstein's theory says the force of gravity is proportional to the energy density plus three times the pressure, so pressure itself actually gravitates. That's something we're not familiar with, because for ordinary matter, the pressure is just a tiny fraction of the density. But if something has a pressure that's a sizable fraction of the energy density, and if that pressure is negative, then I can flip the sign of gravity. Gravity's no longer attractive - it's repulsive.
By far the leading candidate for dark energy is the "cosmological constant." What's that?
Albert Einstein introduced the cosmological constant in 1917 as an additional term in the equation of gravity. In Einstein's theory, gravity is the curvature of space-time: You have some source of energy and pressure that curves space-time, and then other matter moves within this curved space. Einstein's equations relate the curvature of space-time to the energy and pressure of whatever's in space.
Einstein originally put the cosmological constant on the curvature side of the equation because he wanted to get a certain solution, which turned out to be wrong. But soon after that, the Belgian physicist Georges Lemaitre realized that the cosmological constant naturally lives with the pressures and energy densities, and that it could be interpreted as the energy density and pressure of something. Already on the energy density and pressure side of the equation was everything in the universe: dark matter, atoms, whatever. If I remove all that stuff, then the cosmological constant must be the energy density and pressure of empty space.
How could empty space possess energy and pressure?
In classical physics, empty space would have no energy or pressure. But quantum effects can create energy and pressure even if there are no real particles there. In quantum theory, you can imagine virtual particles zipping in and out of the vacuum, and those virtual particles - which are always being produced and then annihilating - have energy. So if dark energy is the cosmological constant, then it could be the energy associated with these virtual particles.
How do you measure dark energy?
There are two things we're trying to do that can give us constraints on dark energy: One is to measure distances, which tells us the history of cosmic expansion. The second is to measure the growth of structure in the universe.
The galaxy cluster Abell 1689, imaged by the Hubble Space Telescope, bends light around its center, distorting the shapes of more distant galaxies in an effect known as gravitational lensing.
For the latter, we're using a technique called "weak gravitational lensing," which involves measuring, very precisely, the shapes of hundreds of millions of galaxies, and then inferring how those shapes have been distorted because the light rays from those galaxies get bent by gravity as they travel to us. This lensing effect is really tiny, so in 99 cases out of 100, you can't tell just by looking at a galaxy if it has been lensed. So we have to tease out the signal statistically.
If we look at the shapes of galaxies that are not so far versus ones that are farther, part of the difference in the shapes will be due to the fact that the light has passed through different amounts of clumpy structure. Measuring the lensing signal will give us a measure of how the clumpiness of the universe has evolved over cosmic time, and that clumpiness is impacted by dark energy. Gravity pulls stuff in, making the universe become more and more clumpy over time, but dark energy does the opposite. It makes things push away from each other. So if we can measure how the clumpiness of the universe has changed over cosmic time, we can infer something about dark energy: how much of it there was, and what its properties were at different points in time.
DES will try to calculate the dark energy "equation of state" parameter, w. What does w represent?
The parameter w tells us the ratio of the pressure of the dark energy to its density. If dark energy is the cosmological constant, then you can show that the only w that's consistent for empty space is the one where the pressure is exactly equal to minus the energy density. So w has a very specific value: minus one.
If dark energy isn't the cosmological constant, what else might it be?
The simplest alternatives, and the ones that I worked on in the 1990s, are inspired by "inflation." Before we knew that the expansion of the universe is currently speeding up, we had this idea that the universe was speeding up in the very earliest fraction of a second after the Big Bang. That idea of very early cosmic acceleration is called inflation. So the simplest thing to do was to borrow the theory that explains this other epoch of sped-up expansion, and that involves scalar fields.
A scalar field is an entity that has a value everywhere in space. As the field evolves, it can act like dark energy: If it evolves really slowly, it will have negative pressure, which will cause the universe to accelerate. The simplest models of primordial inflation say that, for some period, the universe was dominated by one of these scalar fields, and it eventually decayed and disappeared. And if that's our best idea for what happened when the universe was speeding up almost 14 billion years ago, we should consider that maybe we have something like that going on now.
If you look at these models, they tend to predict that w, the ratio of the pressure to energy density, would be slightly different from minus one. We would like to test that idea.
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KICP Members: Joshua A. Frieman
Scientific projects: Dark Energy Survey (DES)
Cosmic-Ray Telescope Flies High
Scientific American, November 4, 2014
Cosmic rays, traveling nearly at the speed of light, bombard Earth from all directions. The electrically charged particles are the most energetic component of cosmic radiation -- yet no one knows where they come from.
Astrophysicists speculate that high-energy cosmic rays may have emerged from supermassive black holes in faraway galaxies or possibly from decaying particles from the big bang.
Whatever their origin, these rays crash into Earth's atmosphere about once per square kilometer per century. The impact produces an air shower of tens of billions of secondary, lower-energy particles that in turn excite nitrogen molecules in the atmosphere. The interactions produce ultraviolet fluorescence that lights up the air shower's path. Scientists are trying to use such paths to measure the direction and energy of cosmic rays and reconstruct their trajectories back millions of light-years into space to pinpoint their source.
Seeing these extreme events is rare. Earth-based observatories can spot cosmic- ray collisions only if they occur directly above the detectors. The Pierre Auger Observatory in Argentina, which houses the world's largest cosmic-ray detector and covers an area roughly the size of Rhode Island, records about 20 extreme-energy particle showers a year.
Hoping to improve the odds of observing the rays, a team of scientists from 15 nations came together more than a decade ago and designed a cosmic- ray telescope for the International Space Station (ISS). On the Japanese Experimental Module, the Extreme Universe Space Observatory (JEM- EUSO) will record ultraviolet emissions with a wide-angle, high-speed video camera that points toward Earth. With such a large observation area, the camera will see more air showers. The team originally hoped to launch EUSO in 2006. But troubles on Earth -- first the space shuttle Columbia disaster in 2003, then the Fukushima nuclear meltdown in 2011 and now the turmoil in Ukraine -- have delayed its deployment until at least 2018.
The science, however, marches onward. In August the team launched a prototype of the telescope 38 kilometers into the stratosphere onboard a helium filled balloon. For two hours, researchers followed below in a helicopter, shooting a pulsed UV laser and LED into the telescope's field of view. The test was a success: the prototype detected the UV traces, which are similar to the fluorescence generated by extreme energy cosmic-ray air showers. In 2016 astronauts will transport a shoebox-size prototype called Mini-EUSO to the ISS and see how it fares at the altitude of the full mission.
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KICP Members: Angela V. Olinto
Follow Wired Twitter Facebook RSS Mysterious Missing Pulsars May Have Gotten Wrapped in Dark Matter and Turned Into Black Holes
Wired, November 13, 2014
The center of the galaxy should be chock-full of rapidly spinning, dense stellar corpses known as pulsars. The problem is, astronomers can't seem to find them.
The galactic center is a bustling place. Lots of gas, dust, and stars zip about, orbiting a supermassive black hole about three million times more massive than the sun. With so many stars, astronomers estimate that there should be hundreds of dead ones, says astrophysicist Joseph Bramante of Notre Dame University. Scientists have found only a single young pulsar at the galactic center, where there should be as many as 50 such youngsters.
Bramante and astrophysicist Tim Linden of the University of Chicago have a possible solution to this missing-pulsar problem, which they describe in a paper accepted for publication in the journal Physical Review Letters. Maybe those pulsars are absent because dark matter, which is plentiful in the galactic center, gloms onto the pulsars, accumulating until the pulsars become so dense they collapse into a black hole. Poof. No more pulsars.
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KICP Members: Tim Linden
Is Quantum Entanglement Real?
The New York Times, November 17, 2014
FIFTY years ago this month, the Irish physicist John Stewart Bell submitted a short, quirky article to a fly-by-night journal titled Physics, Physique, Fizika. He had been too shy to ask his American hosts, whom he was visiting during a sabbatical, to cover the steep page charges at a mainstream journal, the Physical Review. Though the journal he selected folded a few years later, his paper became a blockbuster. Today it is among the most frequently cited physics articles of all time.
Bell's paper made important claims about quantum entanglement, one of those captivating features of quantum theory that depart strongly from our common sense. Entanglement concerns the behavior of tiny particles, such as electrons, that have interacted in the past and then moved apart. Tickle one particle here, by measuring one of its properties - its position, momentum or "spin" - and its partner should dance, instantaneously, no matter how far away the second particle has traveled.
The key word is "instantaneously." The entangled particles could be separated across the galaxy, and somehow, according to quantum theory, measurements on one particle should affect the behavior of the far-off twin faster than light could have traveled between them.
Entanglement insults our intuitions about how the world could possibly work. Albert Einstein sneered that if the equations of quantum theory predicted such nonsense, so much the worse for quantum theory. "Spooky actions at a distance," he huffed to a colleague in 1948.
In his article, Bell demonstrated that quantum theory requires entanglement; the strange connectedness is an inescapable feature of the equations. But Bell's proof didn't show that nature behaved that way, only that physicists' equations did. The question remained: Does quantum entanglement occur in the world?
Starting in the early 1970s, a few intrepid physicists - in the face of critics who felt such "philosophical" research was fit only for crackpots - found that the answer appeared to be yes.
John F. Clauser, then a young postdoctoral researcher at the Lawrence Berkeley National Laboratory, was the first. Using duct tape and spare parts, he fashioned a contraption to measure quantum entanglement. Together with a graduate student named Stuart Freedman, he fired thousands of pairs of little particles of light known as photons in opposite directions, from the middle of the device, toward each of its two ends. At each end was a detector that measured a property of the photon known as polarization.
As Bell had shown, quantum theory predicted certain strange correlations between the measurements of polarization as you changed the angle between the detectors - correlations that could not be explained if the two photons behaved independently of each other. Dr. Clauser and Mr. Freedman found precisely these correlations.
Other successful experiments followed. One, led by the French physicist Alain Aspect, tested the instantaneousness of entanglement. Another, led by the Austrian physicist Anton Zeilinger, considered entanglement among three or more particles.
Even with these great successes, work remains to be done. Every experimental test of entanglement has been subject to one or more loopholes, which hold out the possibility, however slim, that some alternative theory, distinct from quantum theory and more in line with Einstein's intuitions, may still be salvageable. For example, one potential loophole - addressed by Dr. Aspect's experiment - was that the measurement device itself was somehow transmitting information about one particle to the other particle, which would explain the coordination between them.
The most stubborn remaining loophole is known as "setting independence." Dr. Zeilinger and I, working with several colleagues - including the physicists Alan H. Guth, Andrew S. Friedman and Jason Gallicchio - aim to close this loophole, a project that several of us described in an article in Physical Review Letters.
HERE'S the problem. In any test of entanglement, the researcher must select the settings on each of the detectors of the experimental apparatus (choosing to measure, for example, a particle's spin along one direction or another). The setting-independence loophole suggests that, though the researcher appears to be free to select any setting for the detectors, it is possible that he is not completely free: Some unnoticed causal mechanism in the past may have fixed the detectors' settings in advance, or nudged the likelihood that one setting would be chosen over another.
Bizarre as it may sound, even a minuscule amount of such coordination of the detectors' settings would enable certain alternative theories to mimic the famous predictions from quantum theory. In such a case, entanglement would be merely a chimera.
How to close this loophole? Well, obviously, we aren't going to try to prove that humans have free will. But we can try something else. In our proposed experiment, the detector setting that is selected (say, measuring a particle's spin along this direction rather than that one) would be determined not by us - but by an observed property of some of the oldest light in the universe (say, whether light from distant quasars arrives at Earth at an even- or odd-numbered microsecond). These sources of light are so far away from us and from one another that they would not have been able to receive a single light signal from one another, or from the position of the Earth, before the moment, billions of years ago, when they emitted the light that we detect here on Earth today.
That is, we would guarantee that any strange "nudging" or conspiracy among the detector settings - if it does exist - would have to have occurred all the way back at the Hot Big Bang itself, nearly 14 billion years ago.
If, as we expect, the usual predictions from quantum theory are borne out in this experiment, we will have constrained various alternative theories as much as physically possible in our universe. If not, that would point toward a profoundly new physics.
Either way, the experiment promises to be exciting - a fitting way, we hope, to mark Bell's paper's 50th anniversary.
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KICP Members: Jason Gallicchio