KICP in the News
Horizon 2015 : Dancing in the Dark The End of Physics
BBC, March 30, 2015
Scientists genuinely don't know what most of our universe is made of. The atoms we're made from only make up four per cent. The rest is dark matter and dark energy (for 'dark', read 'don't know'). The Large Hadron Collider at CERN has been upgraded. When it's switched on in March 2015, its collisions will have twice the energy they did before. The hope is that scientists will discover the identity of dark matter in the debris.
The stakes are high - because if dark matter fails to show itself, it might mean that physics itself needs a rethink.
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KICP Members: Juan I. Collar; Daniel Hooper
Understanding the Dark Side of Physics
Science Friday, March 27, 2015
Neutrons, protons, and electrons - these are the basic building blocks of matter. But this kind of matter is only a tiny fraction of the entire universe. The rest, about 95 percent, in fact, is divided between dark matter and dark energy. Understanding what makes up dark matter and dark energy could help answer some of the biggest questions in physics. Physicists Jodi Cooley, Dan Hooper, and Nobel Prize winner Steven Weinberg join Ira Flatow to discuss what we do and don't know about this "darker" side of physics, and what we hope to learn.
Author, "To Explain the World: The Discovery of Modern Science" (HarperCollins, 2015)
Nobel Prize Winner, 1979, Physics
University of Texas at Austin
Staff Scientist, Fermilab
Associate Professor, Astronomy and Astrophysics
University of Chicago
Associate Professor, Physics
Southern Methodist University
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John E. Carlstrom, Thomas M. Crawford and Lloyd Knox, "Particle physics and the cosmic microwave background"
Physics Today, March 13, 2015
Temperature and polarization variations across the microwave sky include the fingerprints of quantum fluctuations in the early universe. They may soon reveal physics at unprecedented energy scales.
The detection of the CMB and the consensus that the universe had a hot and dense early phase led to a fertile relationship between cosmology and particle physics. The hot early universe was a natural particle accelerator that could reach energies well beyond what laboratories on Earth will attain in the foreseeable future. Precise measurements of both the spectrum of the CMB and its tiny variations in brightness from one point to another on the sky reflect the influences of high-energy processes in the early cosmos.
For instance, the gravitational effects of neutrinos have been detected at high significance; such measurements imply that the sum of the neutrino masses is no more than a few tenths of an eV. The CMB data also show the influence of helium produced in the early universe and thus constrain the primordial helium fraction. Moreover, the data are nearly impossible to fit without dark energy and dark matter - two ingredients missing from the standard model of particle physics.
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KICP Members: John E. Carlstrom; Thomas M. Crawford
Scientific projects: South Pole Telescope (SPT)
Scientists find rare dwarf satellite galaxy candidates in Dark Energy Survey data
Fermilab, March 11, 2015
Scientists on two continents have independently discovered a set of celestial objects that seem to belong to the rare category of dwarf satellite galaxies orbiting our home galaxy, the Milky Way.
Dwarf galaxies are the smallest known galaxies, and they could hold the key to understanding dark matter and the process by which larger galaxies form.
A team of researchers with the Dark Energy Survey, headquartered at the U.S. Department of Energy's Fermi National Accelerator Laboratory, and an independent group from the University of Cambridge jointly announced their findings today. Both teams used data taken during the first year of the Dark Energy Survey, all of which is publicly available, to carry out their analysis.
"The large dark matter content of Milky Way satellite galaxies makes this a significant result for both astronomy and physics," said Alex Drlica-Wagner of Fermilab, one of the leaders of the Dark Energy Survey analysis.
Satellite galaxies are small celestial objects that orbit larger galaxies, such as our own Milky Way. Dwarf galaxies can be found with fewer than 100 stars and are remarkably faint and difficult to spot. (By contrast, the Milky Way, an average-sized galaxy, contains billions of stars.)
These newly discovered objects are a billion times dimmer than the Milky Way and a million times less massive. The closest of them is about 100,000 light-years away.
"The discovery of so many satellites in such a small area of the sky was completely unexpected," said Cambridge's Institute of Astronomy's Sergey Koposov, the Cambridge study's lead author. "I could not believe my eyes."
Scientists have previously found more than two dozen of these satellite galaxies around our Milky Way. About half of them were discovered in 2005 and 2006 by the Sloan Digital Sky Survey, the precursor to the Dark Energy Survey. After that initial explosion of discoveries, the rate fell to a trickle and dropped off entirely over the past five years.
The Dark Energy Survey is looking at a new portion of the southern hemisphere, covering a different area of sky than the Sloan Digital Sky Survey. The galaxies announced today were discovered in a search of only the first of the planned five years of Dark Energy Survey data, covering roughly one-third of the portion of sky that DES will study. Scientists expect that the full Dark Energy Survey will find up to 30 of these satellite galaxies within its area of study.
Atlas image obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
While more analysis is required to confirm any of the observed celestial objects as satellite galaxies, researchers note their size, low surface brightness and significant distance from the center of the Milky Way as evidence that they are excellent candidates. Further tests are ongoing, and data collected during the second year of the Dark Energy Survey could yield more of these potential dwarf galaxies to study.
Newly discovered galaxies would also present scientists with more opportunities to search for signatures of dark matter. Dwarf satellite galaxies are dark matter-dominated, meaning they have much more mass in unseen matter than in stars. The nature of this dark matter remains unknown but might consist of particles that annihilate each other and release gamma rays. Because dwarf galaxies do not host other gamma ray sources, they make ideal laboratories to search for signs of dark matter annihilation. Scientists are confident that further study of these objects will lead to even more sensitive searches for dark matter.
In a separate result also announced today, the Large Area Telescope Collaboration for NASA's Fermi Gamma-Ray Telescope mission reported that they did not see any significant excess of gamma ray emission associated with the new Dark Energy Survey objects. This result demonstrates that new discoveries from optical telescopes can be quickly translated into tests of fundamental physics.
"We did not detect significant emission with the LAT, but the dwarf galaxies that DES has and will discover are extremely important targets for the dark matter search," said Peter Michelson, spokesperson for the LAT collaboration. "If not leading to an identification of particle dark matter, they will certainly be useful to constrain its properties."
The Dark Energy Survey is a five-year effort to photograph a large portion of the southern sky in unprecedented detail. Its primary instrument is the Dark Energy Camera, which - at 570 megapixels - is the most powerful digital camera in the world, able to see galaxies up to 8 billion light-years from Earth. Built and tested at Fermilab, the camera is now mounted on the 4-meter Victor M. Blanco telescope at the Cerro Tololo Inter-American Observatory in the Andes Mountains in Chile.
The survey's five-year mission is to discover clues about the nature of dark energy, the mysterious force that makes up about 70 percent of all matter and energy in the universe. Scientists believe that dark energy may be the key to understanding why the expansion of the universe is accelerating.
"The Dark Energy Camera is a perfect instrument for discovering small satellite galaxies," said Keith Bechtol of the Kavli Institute for Cosmological Physics at the University of Chicago, who helped lead the Dark Energy Survey analysis. "It has a very large field of view to quickly map the sky and great sensitivity, enabling us to look at very faint stars. These results show just how powerful the camera is and how significant the data it collects will be for many years to come."
The Dark Energy Survey is a collaboration of more than 300 scientists from 25 institutions in six countries.
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KICP Members: Keith Bechtol; Alex Drlica-Wagner; Joshua A. Frieman
Scientific projects: Dark Energy Survey (DES)
One Hundred Years of General Relativity
Science Friday, March 6, 2015
Albert Einstein published his theory of general relativity 100 years ago. The theory has shaped the idea of black holes, pulsars, and modern cosmology. Science historian David Kaiser guides us through the history of Einstein's insight, and physicists Michael Turner and Alex Filippenko discuss where the theory might take us in the future.
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KICP Members: Michael S. Turner
Mystery of the Universe's Gamma-Ray Glow Solved
Space.com, February 5, 2015
The steady glow of high-energy gamma-ray light that spreads across the cosmos has puzzled astronomers for decades. One team of researchers thinks it has the best explanation yet for the source of this strange emission.
After observing the universe with NASA's Fermi Gamma-ray Space Telescope for six years, scientists with the mission say the majority of the gamma-ray glow they have seen can be explained by objects already known to science. If there are any as-yet unknown sources out there, their contribution to the glow would be very small, scientists say.
"We have a very plausible story. We're not 100 percent confident that this is the final answer, but it really constrains what other exotic possibilities could be out there," said Keith Bechtol, a postdoctoral researcher at the University of Chicago and a member of the Fermi collaboration who worked on the analysis.
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KICP Members: Keith Bechtol
Scientific projects: Fermi Gamma-ray Space Telescope (Fermi)
Neutrino astronomy: Balloon with a view
The Economist, January 20, 2015
An experiment in Antarctica may solve the mystery of cosmic rays
MEET ANITA. Strictly, ANITA III--for she is the third iteration of the Antarctic Impulsive Transient Antenna. Her job, when she is launched sometime in the next few days, will be to float, suspended from a giant balloon, over Antarctica's ice, in order to record radio waves which that ice is giving off. These radio waves are generated by neutrinos passing through the ice, making Antarctica the biggest neutrino-detection laboratory in the world.
The particular neutrinos that ANITA seeks are of extremely high energy. Where they come from, no one knows--nor, strictly speaking, is it actually known that they exist, for ANITAs I and II, which were smaller devices, failed to find them. But theory says they should be there, generated in whatever giant explosions also create cosmic rays.
Cosmic rays are high-velocity protons, sprinkled with a smattering of heavier atomic nuclei, that fly through space until they hit something such as Earth's atmosphere, when they disintegrate into a shower of other particles. They have been known for a century, but their origin remains mysterious because, being electrically charged, their paths are bent by the galaxy's magnetic field. That means the directions they come from do not point to whatever created them.
Neutrinos, however, are electrically neutral, as their name suggests. Their paths should thus point back towards their origins. Neutrinos do not interact much with other sorts of matter, but when one of ultra-high energy does so, the result is a shower of particles travelling at speeds which exceed that of light in ice. An object travelling faster than light's speed in the medium through which it is passing will generate electromagnetic waves. These are known, after their discoverer, as Cherenkov radiation. And it is pulses of radio-frequency Cherenkov radiation, the electromagnetic equivalent of a sonic boom, which ANITA is looking for.
Once airborne under her balloon--an object made of cling-film-like plastic that, when fully inflated, will be a fifth of the size of a football stadium--ANITA will take advantage of the polar vortex, a wind in constant revolution around the pole. She will fly at an altitude of 35-40km, which will mean her antennae can see 1.5m km2 of ice. Ultra-high-energy neutrinos travelling through the ice are thought to interact with it and produce Cherenkov radiation about once per century per km^2, so an area of this size would be expected to yield about 40 bursts a day. ANITA will complete several laps of the continent, each lasting about 15 days. Then the balloon will be cut loose, and she will deploy a parachute and be guided back to the surface for re-use.
Astrophysicists are not the only people rubbing their mittens together in expectation of the results of this experiment. The neutrinos ANITA is looking for are far more energetic than anything produced by the Large Hadron Collider, the world's most powerful particle accelerator. That means they may obey hitherto unperceived extensions of the laws of physics. One possibility is that, among the Cherenkov-radiation-generating particles produced when a neutrino collides with the ice, there may be an occasional miniature black hole.
That would be particularly exciting, because such black holes might themselves disintegrate in a characteristic puff of radiation named after another physicist, Stephen Hawking. If Hawking radiation exists, it means black holes are not truly black--a discovery which would almost certainly win Dr Hawking a Nobel prize.
Though it is not designed to search for Hawking radiation, ANITA would probably see it if it were there. And, since Hawking radiation is created, quite literally, out of nothing (the particles it is made from emerge from the vacuum of space and then steal the energy needed to become real from the black hole itself), that would assist understanding of a very strange piece of physics indeed.
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KICP Members: Keith Bechtol; Abigail G. Vieregg
Scientific projects: Antarctic Impulsive Transient Antenna (ANITA)
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.
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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
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
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
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)
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
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
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
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.
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KICP Members: Wayne Hu
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
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
"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
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
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)
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
Fermi Telescope data tantalize with new clues to dark matter
The University of Chicago News Office, 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