January 13, 2016 | 3:30 PM | ERC 161 | Astronomy Colloquium New Frontiers in Simulating Black Hole Accretion and Jets Alexander (Sasha) Tchekhovskoy, UC Berkeley
Black holes are responsible for a wide variety of astrophysical phenomena. They devour stars, eject relativistic jets, affect star formation and galaxy evolution, and enrich the Universe with heavy elements. In the next several years, the Event Horizon Telescope will produce resolved images of infalling gas and jets on the event horizon scale that promise to revolutionize our understanding of black hole physics. However, until recently, no first-principles models to quantitatively interpret these observations existed. I will present the first such models, the simulated spectra and images, and the constraints on the near event horizon physics coming from the comparison to the observations of the supermassive black hole at the center of our galaxy. I will then use simulations to constrain black hole physics in several other astrophysical contexts. I will finish by making connections to my future research plans.
January 20, 2016 | 3:30 PM | ERC 161 | Wednesday colloquium Searching for Dark Matter With Bubble Chambers Andrew Sonnenschein, Fermilab
PDF | Video Development of bubble chamber detectors for WIMP dark matter was pioneered at KICP in the early 2000’s. In the intervening years, we scaled the technology from the initial test-tube sized detectors operated in the basement of the LASR building to a 60 kg chamber now installed 2-km underground at SNOLAB. I will review the history of these developments and the most recent results from the PICO-2L and PICO-60 experiments.
January 27, 2016 | 3:30 PM | ERC 161 | Astronomy Colloquium The Universe's most extreme star-forming galaxies Caitlin Casey, University of Texas at Austin
Dusty star-forming galaxies host the most intense stellar nurseries in the Universe. Their unusual characteristics (SFRs=200-2000Msun/yr) pose a unique challenge for cosmological simulations and galaxy formation theory, particularly at early times. Although rare today, they were factors of 1000 times more prevalent at z~2-5, contributing significantly to the buildup of the Universe's stellar mass and the formation of high-mass galaxies. However, an ongoing debate lingers as to their evolutionary origins at high-redshift, whether or not they are triggered by major mergers of gas-rich disk galaxies, or if they are solitary galaxies continually fed pristine gas from the intergalactic medium. Observational evidence has been mixed over recent years; some studies clearly point to chaotic kinematic histories and fast gas depletion times (~<100Myr), while other work may demonstrate secular (though active) disks can sustain high star-formation rates over long periods of time. Similarly, some works argue such extreme star-formers contribute very little to cosmic star-formation, while others find quite the opposite. Furthermore, their presence in early protoclusters, only revealed quite recently, pose intriguing questions regarding the collapse of large scale structure. I will discuss some of the latest observational programs dedicated to understanding their origins and frequency at early times, their context in the cosmic web, and future long-term observing campaigns that will reveal their relationship to `normal’ galaxies, thus teaching us valuable lessons on the physical mechanisms of galaxy growth and the collapse of large scale structure in an evolving Universe.
February 3, 2016 | 3:30 PM | ERC 161 | Wednesday colloquium Quantum Twists of Space: Exotic Rotational Correlations from Quantum Geometry, Their Effects on Interferometer Signals, and Their Connection with Cosmic Acceleration Craig Hogan, University of Chicago
PDF | Video The talk will review theoretical arguments that if space and time emerge from a quantum system at the Planck scale, there should be nonlocal exotic quantum correlations of positions of massive bodies, even on scales much larger than the Planck length. In relational theories with no fixed background space, these could take the form of rotational quantum fluctuations in the inertial frame. Basic quantum principles are used to derive their effect on correlations in the signals of interferometers. An experimental test is proposed, based on a reconfiguration of the Fermilab Holometer. It is conjectured that entanglement of these rotational correlations with the Standard Model vacuum could explain the value of the cosmological constant in terms of known scales of physics.
February 10, 2016 | 3:30 PM | ERC 161 | Astronomy Colloquium IceCube: The Discovery of High-Energy Cosmic Neutrinos Francis Halzen, University of Wisconsin, Madison
Francis Halzen, Principal Investigator for IceCube
The IceCube project has transformed one cubic kilometer of natural Antarctic ice into a neutrino detector. The instrument detects more than 100,000 neutrinos per year in the GeV to PeV energy range. Among these, we have recently isolated a flux of high-energy cosmic neutrinos. The high cosmic neutrino flux observed indicates that a significant fraction of the radiation in the non-thermal universe, powered by compact objects from neutron stars to supermassive black holes, is generated by proton accelerators. We will discuss the IceCube instrument, the analysis of the data, and the significance of the discovery of cosmic neutrinos.
February 17, 2016 | 3:30 PM | ERC 161 | Wednesday colloquium Improved Limits from the Large Underground Xenon Dark Matter Experiment Nicole Larsen, KICP
The Large Underground Xenon (LUX) detector
PDF | Video A wealth of astrophysical research supports the existence of dark matter in the universe, yet the exact identity and nature of this unknown particle remain elusive. The Large Underground Xenon (LUX) dark matter search is a 370-kg xenon-based time projection chamber (TPC) that operates by detecting light and ionization signals from particles incident upon a xenon target. With the 2013 report of the world’s first sub-zeptobarn spin-independent WIMP-nucleon cross section limit, the LUX (Large Underground Xenon) experiment emerged as a frontrunner in the field of dark matter direct detection. In December 2015, LUX released an updated analysis of its 2013 dataset with increased detector exposure, updates to the background model, upgraded event reconstruction algorithms, and novel calibrations leading to an overall 23% increase in sensitivity for high-mass WIMPs and even more significant improvements for low-mass WIMPs. This talk details the design of the LUX experiment and reviews the analysis and reanalysis of the 2013 dataset leading to the world’s most stringent constraints on spin-independent WIMP-nucleon scattering for WIMPs above mass 4 GeV.
February 24, 2016 | 3:30 PM | ERC 161 | Astronomy Colloquium Two studies in planetary dynamics: (i) Impact seasons on Mars, (ii) The mass function of planets in the Galaxy Renu Malhotra, University of Arizona
I will present results of new calculations of the asteroidal impact flux on Mars. Mars' orbit is significantly eccentric and the planet orbits near the inner edge of the asteroid belt where the space density of asteroids has a large radial gradient. The correlated secular dynamics of Mars and the asteroids plays a significant role in modulating the impact flux on this planet. At the present epoch, this leads to a large variation -- of about a factor of three -- in the impact flux when Mars is near aphelion versus perihelion; significantly, the integrated annual impact flux is lower than would be expected in the absence of correlated secular dynamics. The second part of the talk will describe some deductions about the planet mass function from the observational data of exoplanets and theoretical considerations of long term stability. I will describe analysis of the observational data from the Kepler space mission which indicates that planetary orbital separations have an approximately log-normal distribution. Adopting some plausible ansatzs for the dynamical stability of N-planet systems to relate orbital separations to planet masses, it appears that the planet mass function is peaked in logarithm of mass, with the most probable value of log m/M⊕ ∼ (0.6 − 1.0); a modest extrapolation indicates that Earth mass planets are about ~1000 times more common than Jupiter mass planets, and that the most common planets in the Galaxy may be of lunar-to-Mars mass.
March 2, 2016 | 3:30 PM | ERC 161 | Wednesday colloquium New Approaches to Dark Matter Justin Khoury, University of Pennsylvania
PDF | Video In this talk I will discuss a novel theory of superfluid dark matter. The scenario matches the predictions of the LambdaCDM model on cosmological scales while simultaneously reproducing the MOdified Newtonian Dynamics (MOND) empirical success on galactic scales. The dark matter and MOND components have a common origin, as different phases of a single underlying substance. This is achieved through the rich and well-studied physics of superfluidity. The framework naturally distinguishes between galaxies (where MOND is successful) and galaxy clusters (where MOND is not): due to the higher velocity dispersion in clusters, and correspondingly higher temperature, the DM in clusters is either in a mixture of superfluid and normal phases, or fully in the normal phase. The model makes various observational predictions that distinguishes it from both LambdaCDM and standard MOND. In the last part of the talk, I will discuss an on-going attempt at explaining cosmic acceleration as yet another manifestation of dark matter superfluidity.
March 9, 2016 | 3:30 PM | ERC 161 | Astronomy Colloquium Stellar rotation and exoplanets obliquity Tsevi Mazeh, Tel Aviv University
Spots on the stellar rotating surface can produce periodic brightness modulation that varies with the stellar rotational period. The Kepler space mission observed more than 150,000 stars continuously for more than four years, producing unprecedentedly precise light curves. We used this data gold mine to derive the rotational periods of more than 30,000 stars. About a thousand of the Kepler stars with derived rotational periods have been found to have planets orbiting around them. We use this sample and the derived amplitudes of the stellar modulation in particular, to infer some evidence for the star-planet obliquity - the angle between the stellar spin axis and the orbital planetary angular momentum of a planet. Previous studies found two populations of hot Jupiters - one around cool stars with orbits well-aligned with the stellar rotational axes, and the other around hot stars with isotropic distribution of obliquities, including planets with retrograde motion. We will show surprising statistical evidence that supports these findings. We will also discuss whether the amplitudes of the cool stars with planets depend on the planetary orbital periods, a dependence that might have some implications for the formation of short-period planetary formation.
January 15, 2016 | 12:00 PM | ERC 401 | Friday noon seminar Helical Magnetic Fields in the Cosmos Andrew Long, The University of Chicago
It is often necessary to study cosmological phase transitions, such as inflation and thermal symmetry-breaking phenomena, through the relics that they leave behind. In this sense, the cosmological magnetic field may be a powerful probe of the early universe, which has yet to be tapped. In this talk, I will survey various topics related to primordial magnetic fields (PMFs) with helicity. We will ask: how might a PMF have been generated in the early universe, how would it have evolved in the cosmological medium, and how could it be detected today? In the first part, we discuss the interesting connection between baryogenesis and magnetogenesis, which may link the sign of the cosmological baryon asymmetry to the sign of the magnetic helicity. In the second part, we discuss the interplay between the PMF and other magnetically-active objects, such as monopoles and axions. Finally we turn to detection prospects, with an emphasis on the frontier field of magnetically-broadened TeV blazar halos.
January 22, 2016 | 12:00 PM | ERC 401 | Friday noon seminar Modified gravity inside astrophysical bodies: breaking of the Vainshtein mechanism Ryo Saito, AstroParticle and Cosmology (APC) laboratory
Any infrared modification of gravity, which explains the current cosmic acceleration, should not spoil the successes of general relativity in solar-system observations. In many theories of modified gravity, it is ensured by the Vainshtein mechanism that works near dense sources. Recently, it has been found that the Vainshtein mechanism can be broken inside a dense source, although not outside, in a general class of scalar-tensor theories. In this talk, after reviewing how the Vainshtein mechanism can be broken, I will discuss its impact on the density profile of a star, modeling simply it as a polytropic sphere. I also show the existence of a universal upper bound on the amplitude of this type of modification, independently of the details of the equation of state.
January 29, 2016 | 12:00 PM | ERC 401 | Friday noon seminar New tests of not-so-dark matter Yacine Ali-Haimoud, Johns Hopkins University
The nature of dark matter remains one of the major unsolved problems in physics, and one must leave no stone unturned when exploring empirical probes. In this talk I will discuss two different test of dark matter properties. First, I will show that dark matter interactions with standard model particles at high redshift can be probed through spectral distortions of the cosmic microwave background. In particular, I will show that upper limits from FIRAS measurements allow to constrain dark matter interactions for masses below 0.1 MeV. In the second part, I will discuss an interesting possible consequence of clumpy dark matter: flares following tidal compression events by supermassive black holes.
February 5, 2016 | 12:00 PM | ERC 401 | Friday noon seminar A 3D view of the Dark Universe: illuminating intergalactic gas at high redshift with fluorescent Lyman-alpha emission Sebastiano Cantalupo, ETH Zurich
Combining cosmological simulation and the deepest Lyman alpha emission observations to constrain the physical properties of diffuse cosmic gas in the Dark Universe
Gravitational collapse during the Universe's first billion years transformed a nearly homogeneous matter distribution into a network of filaments - the Cosmic Web - where galaxies form and evolve. Because most of this material is too diffuse to form stars, its study has been limited so far to absorption probes against background sources. In this talk, I will present the results of a new program to directly detect and study high-redshift cosmic gas in emission using bright quasars and galaxies as external "source of illumination’’. In particular, I will show results from ultra-deep narrow-band imaging and recent integral-field-spectroscopy as a part of the MUSE Guaranteed Time of Observation program that revealed numerous giant Lyman-alpha emitting filaments around quasars and bright galaxies. Finally, I will discuss how the unexpectedly high luminosities of the giant Lyman-alpha filaments, together with the constraints from Helium and metal extended emission, present a serious challenge for our current understanding of the Intergalactic and Circumgalactic media based on hydrodynamical cosmological simulations.
February 12, 2016 | 12:00 PM | ERC 401 | Friday noon seminar Putting the Cosmology in "21 cm Cosmology" Jonathan Pober, Brown University
21 cm cosmology -- the concept of using radio telescopes to observe the highly redshifted 21 cm line of neutral hydrogen on cosmological scales -- is a field on the verge of a breakthrough. The first generation of 21 cm cosmology experiments (LOFAR, MWA, and PAPER, among others) have been operating for several years, and first results at the level of design sensitivity are potentially forthcoming. In this talk, I will focus on the work needed to establish the reliability of any putative detection of the cosmological signal and the paths forward for bringing 21 cm experiments into the "precision cosmology" fold, alongside the CMB and galaxy surveys. Recent results from the MWA and PAPER will be presented, including the first limits on the z = 8.4 IGM temperature from PAPER.
February 26, 2016 | 12:00 PM | ERC 401 | Friday noon seminar Quantifying discordance in the 2015 Planck CMB spectrum Graeme Addison, Johns Hopkins University
In this talk I will discuss the internal consistency of the Planck 2015 cosmic microwave background (CMB) temperature anisotropy power spectrum and show that tension exists between the determination of some cosmological parameters from multipoles l<1000 (roughly the scales accessible to WMAP) and l>=1000. I will show that the l>=1000 constraints are also in tension with low-redshift data sets, including Planck’s own measurement of the CMB lensing power spectrum (2.4 sigma), and the most precise determinations of the baryon acoustic oscillation scale (2.5 sigma), and Hubble constant (3.0 sigma). Finally, I will discuss some possible explanations for these disagreements.
March 4, 2016 | 12:00 PM | ERC 401 | Friday noon seminar Modeling the Outskirts of Galaxy Clusters Camille Avestruz, The University of Chicago
The observational study of galaxy cluster outskirts is a new territory to probe the thermodynamic and chemical structure of the X-ray emitting intracluster medium (ICM). Cluster outskirts are particularly important for modeling the Sunyaev-Zel'dovich effect, which is sensitive to hot electrons at all radii and has been used to detect hundreds of galaxy clusters with recent microwave cluster surveys. In cluster-based cosmology, measurements of cluster outskirts are an important avenue for estimating the cluster mass, as the outskirts are less sensitive to baryonic processes that dominate the cluster core. However, recent observations of cluster outskirts deviate from theoretical expectations, indicating that cluster outskirts are more complicated than previously thought. Computational modeling of cluster outskirts is necessary to interpret these observations. I will present cosmological simulations of galaxy cluster formation that follow the thermodynamic and chemical structures in the virialization regions of the ICM and transition to the IGM. Specifically, I will discuss how observational signatures of galaxy clusters are affected by gas flows, inhomogeneities in the ICM, and non-equilibrium physics.
January 12, 2016 | 12:00 PM | ERC 161 | Tuesday Seminar Toward Understanding the Formation of Proto-planetary Disks and Multiple Star Systems John J. Tobin, Leiden Observatory
Proto-planetary disks are thought to form early in the star formation process due to conservation of angular momentum. These disks are the future sites of planet formation, but may also be the sites of binary/multiple star formation if the disk is gravitationally unstable. However, theory and simulations have suggested that the formation of large, massive disks may be difficult due to the removal of angular momentum by magnetic fields. Observations are now sensitive enough to test these predictions, and we identified the first Keplerian disk around the youngest class of protostar. Moreover, ALMA observations are have uncovered new Keplerian disks, in addition to enabling their molecular content to be examined. At the same time, we are using data from a large VLA survey to transform our knowledge of protostellar disks and multiple star systems at spatial resolutions of ~15 AU. We have identified new protostellar disk candidates that exhibit evidence for dust growth to ~cm-sizes and radial drift already having occurred in the protostellar phase. Thus, magnetic fields may not strongly suppress the formation of disks > 10 AU in many cases. We have also discovered closer multiple systems than ever before, finding strong evidence for a bimodal distribution of protostar companion separations. This is suggestive of distinct mechanisms for multiple star formation acting on different scales. Finally, we also see indications of evolution in the separation distribution for younger protostars relative to those that are more-evolved. These results open the door to expanded multiplicity and disk surveys to determine if the initial trends we find are robust.
January 19, 2016 | 12:00 PM | ERC 161 | Tuesday Seminar New Insights on the Origin of Cosmic Rays Damiano Caprioli, Princeton University
I present the results of large kinetic (particle-in-cells) plasma simulations of particle acceleration at non-relativistic collisionless shocks, which in particular allow a first-principles investigation of diffusive acceleration at the blast waves of supernova remnants, the most prominent sources of Galactic cosmic rays (CRs). Ion acceleration efficiency and magnetic field amplification are obtained as a function of the shock properties and compared with theoretical predictions, multi-wavelength observations of individual remnants, and in-situ measurements at heliospheric shocks. Moreover, I outline an original mechanism (the "espresso mechanism") for the acceleration of nuclei up to ~10^20eV in the relativistic jets of powerful active galactic nuclei. The combination of the “supernova-remnant paradigm” for the origin of Galactic CRs and the "espresso" mechanism provides a unified description of the spectrum and the chemical composition of CRs over more than 11 orders of magnitude in energy.
January 26, 2016 | 12:00 PM | ERC 161 | Tuesday Seminar Continuing the Legacy of Supernova Cosmology Ryan Foley, University of Illinois at Urbana-Champaign
Type Ia supernovae (SNe Ia) are superb distance indicators and are used to map the expansion history of the Universe. In the last millennium, astronomers used observations of SNe Ia to find that the Universe's expansion is currently accelerating. This discovery resulted in the Nobel Prize for Physics in 2011. Since this initial discovery, we have used SNe Ia to loosely constrain the nature of "dark energy," which drives the accelerated expansion. To improve our dark energy constraints beyond our current basic understanding, we must design new and better SN surveys. I will present the Foundation Supernova survey, a new high-fidelity, low-redshift (z < 0.1) SN survey started in 2015 that will replace the current heterogenous low-redshift sample and reduce the (currently) largest uncertainties for SN cosmology. I will describe the survey, our implementation, and first results. I will also discuss the next major leap in SN cosmology, WFIRST, which will launch in about a decade. I will present the first simulations of the WFIRST SN survey and make some suggestions for how to further improve this mission. Between the Foundation survey, WFIRST, and other surveys, SNe Ia will continue to be a premier cosmological probe, continuing the legacy started decades earlier.
February 2, 2016 | 12:00 PM | ERC 161 | Tuesday Seminar Rethinking Galactic Architecture: Clues from Satellites and Destroyed Dwarfs Alis Deason, Stanford University
The cannibalistic nature of the Milky Way galaxy leads to the continuous capture and destruction of lower mass, dwarf galaxies. The remains of destroyed dwarfs are splayed out in a diffuse stellar halo, while the "survivors" comprise the satellite population that orbits the Milky Way. These halo populations provide a unique opportunity to decipher the accretion history of the Milky Way with a level of detail that cannot be achieved in any other galaxy. I will discuss current and future projects that aim to decipher the nature of the halo's building blocks. Observational studies of number counts and chemical abundances of halo stars suggest that the halo is dominated by relatively massive accretion event(s), in qualitative agreement with the predictions of numerical simulations. I will discuss an ongoing project utilizing multi-epoch HST photometry and Keck spectroscopy that will provide a more quantitative analysis using distant halo stars viewed in “7-dimensions”. I will also discuss the significance of group infall onto Milky Way mass halos, and the latest observational evidence for associations between satellite dwarfs and/or substructures in our Galaxy.
January 21, 2016 | 1:00 PM | ERC 161 | Special Seminar Probing Gravity: Galaxies, CMB Lensing, and Intensity Mapping Anthony Pullen, Carnegie Mellon University
We discuss recent work exploring the use of large-scale structure to probe gravity. We first consider using CMB lensing and galaxy surveys to probe E_G, the ratio between curvature and velocity perturbations. This quantity is independent of galaxy clustering bias and is distinct for various gravity models, breaking the degeneracy in current cosmological probes of gravity and dark energy. We present our constraints to E_G using CMB data from Planck and galaxy data from the SDSS BOSS survey, which are in tension with general relativity (GR). We also forecast gravity constraints for upcoming galaxy and CMB surveys. Finally, we consider intensity mapping (IM) as a gravity probe. Specifically, we discuss our recent work probing intensities of star formation lines, i.e. CO, Ly-a, at high redshifts, and how IM lines can be used to measure E_G. Image credit: ESA