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Colloqiua & Seminars
Colloquia & Seminars, 2011
The Search for CMB B-Mode Polarization
James Bock, Jet Propulsion Laboratory
Precise CMB temperature and polarization measurements strongly support the theory of inflation, a period of exponential super-luminal expansion in the instant following the big bang. However the physical basis of inflation remains elusive, arising at energy scales beyond the reach of terrestrial particle accelerators. Inflation also generates a background of gravitational waves that imprint a distinct 'B-mode' polarization pattern on the CMB, with an amplitude that depends on inflationary physics. An active competition between a dozen current experiments is well underway to detect or constrain inflationary polarization.
We describe a CMB polarization program at the South Pole, progressing from BICEP, with the current best upper limits on inflationary polarization, to the Keck Polarimeter Array, partially deployed in the field, to POLAR, now in development with a large and sensitive bolometric focal plane array. POLAR is also designed to map the yet-undetected polarization signal from gravitational lensing of the background CMB by intervening matter, a new cosmological tool for probing the formation of large-scale structure. Advances in superconducting bolometer arrays with planar antennas are propelling this rapid improvement in capability. We describe recent results, efforts to control systematic errors, and new innovations.
This program ultimately has the capability to search to an amplitude of r ~ 0.01, below which it becomes necessary to remove Galactic and CMB lensing polarization foregrounds. These developments inform the scientific and technical case for the Inflation Probe, a future space-borne CMB successor to the ESA Planck satellite.
Where will Einstein fail? Insights into Gravity and Dark Energy
Niayesh Afshordi, Perimeter Institute/ University of Waterloo
Despite being the most successfully tested theory in physics, there are strong theoretical and observational arguments for why General Relativity should fail. It is not a question of if, but rather a question of where and when! I start by summarizing the pathologies in Einstein's theory of gravity, and then attempt to forecast where we should first observe its failure. My best bets so far are: 1) Cosmological matter-radiation transition, 2) Neutron stars, 3) Astrophysical black holes, and their potential connection to dark energy. What all these scenarios have in common is the violation of Lorentz symmetry, or a revival of "gravitational aether".
Constraining Asymmetric Dark Matter
Friday noon seminar
Matthew R Buckley, Fermilab
Asymmetric dark matter takes the coincidence between the baryon and dark matter densities as the basic principle around which to build models. In this talk, I will provide an overview of the asymmetric models currently on the market. I will then discuss experimental constraints which apply to broad classes of these models, and their implications for a successful asymmetric scenario.
Cosmological Constraints from a Combined Analysis of Clustering and Galaxy-Galaxy Lensing in the SDSS
Frank C. van den Bosch, Yale University
In recent years much progress has been made in constraining halo occupation statistics, the statistical description of how galaxies of different properties are distributed over dark matter halos of different mass. This progress is due to (i) a dramatic increase in data in the form of galaxy redshift surveys, and (ii) the development of the Halo Model, which allows for an analytical description of the non-linear dark matter mass distribution in terms of its halo building blocks. After an introduction to the Halo Model, I present the Conditional Luminosity Function, which is a statistical tool to describe the galaxy-dark matter connection, and show how it can be constrained using either galaxy clustering or galaxy-galaxy lensing. Next, I show that a combination of these two data sets allows one to put tight constraints on cosmological parameters, in a manner that is both complementary to and comparative with other probes, such as Baryon Acoustic Oscillations, Cosmic Shear and SuperNovae Ia. Finally I give some brief examples of how halo occupation statistics can be used to further our understanding of galaxy formation and evolution.
Direct dark matter Detection with Liquefied Noble Gases
Daniel McKinsey, Yale University
Astrophysical evidence on a variety of distance scales clearly shows that we cannot account for a large fraction of the mass of the universe. This matter is “dark”, not emitting or absorbing any electromagnetic radiation. A compelling explanation for this missing mass is the existence of Weakly Interacting Massive Particles (WIMPs).
Detectors that are low in radioactivity and sensitive to small energy depositions can search for the rare nuclear recoil events predicted by WIMP models. In recent years, several new efforts on direct dark matter detection have begun in which the detection material is a liquefied noble gas. Advantages include: large nuclear recoil signals in both scintillation and ionization channels, good scalability to large target masses, effective discrimination against gamma ray backgrounds, easy purification, and reasonable cost. I will review the results from recent experiments, describe some recent R&D, and present the status of two of the new detectors currently under construction, LUX and MiniCLEAN. I will also discuss the possibility of using superfluid helium as a target for relatively light WIMPs.
Dark matter microhalos: messengers from the early universe
Adrienne L Erickcek, CITA/Perimeter Institute
The abundance of dark matter microhalos depends on the primordial power spectrum and the thermal history of the Universe. First, I will show how an effectively matter-dominated era between the end of inflation and the onset of radiation domination leaves an imprint on the small-scale matter power spectrum. This imprint depends on the origin of dark matter and can trigger the formation of numerous dark matter microhalos during the cosmic dark ages. Second, I will describe how large primordial perturbations on small scales can similarly lead to early-forming microhalos. These microhalos may be detected through their astrometric microlensing signatures, and I will show how a high-precision astrometry survey like Gaia can be used to probe the primordial power spectrum on very small scales.
Equivalence principle and cosmic acceleration
Lam Hui, Columbia University
Theories that attempt to explain the observed acceleration of the universe by modifying gravity typically introduces a new long range force, which must be screened on small scales to satisfy solar system tests. I will discuss when and how such screening mechanisms lead to O(1) violations of the equivalence principle. Astronomical tests involving large scale structure, rotation curves and red giants will be discussed.
A new view of the CMB from the ATACAMA cosmology telescope
Sudeep Das, UC Berkeley
Over the coming decade, tiny fluctuations in temperature and polarization of the Cosmic Microwave Background (CMB) will be mapped with unprecedented resolution. The Planck Surveyor, the Atacama Cosmology Telescope (ACT), and the South Pole Telescope (SPT) are already making great advances. In a few years, high resolution polarization experiments, such as PolarBear, ACTPol, and SPTPol will be in full swing. While these new arc-minute resolution observations will continue to help constrain the physics of the early universe and possible deviations from the Standard Model, they will also be unique in a new way - they will allow us to measure the gravitational lensing of the CMB. This lensing is the deflection of CMB photons by intervening large scale structure. CMB lensing will probe the growth of structure over cosmic time, helping constrain the total mass of neutrinos and the behavior of dark energy. In the first part of the talk, I will review the recent progress made with ACT. In the second part, I will discuss the scientific potential of the CMB lensing signal, its first detection, a new way to constrain dark energy, and its prospects for cross-correlation with other datasets. Finally, I will discuss the upcoming polarized counterpart of ACT --- the ACTPol project, which will have greater sensitivity than ACT, and will be a premier CMB lensing experiment. I will describe our plans to extract different flavors of science from the ACTPol data, including the cross-correlations with optical lensing and galaxy surveys, such as SDSS, BOSS, DES and LSST.
Measuring Cosmic Acceleration
David H. Weinberg, Ohio State University
The discovery of accelerating cosmic expansion has inspired ambitious programs to measure the expansion history and growth of structure with perecent-level precision over a wide range of redshift. These programs include some of the largest cosmological surveys currently underway and some of the highest priority projects recommended by the Astro2010 decadal survey. I will summarize highlights from a nearly completed, book-length review article on "Observational Probes of Cosmic Acceleration" (Weinberg, Mortonson, Eisenstein, Hirata, Riess, Rozo, in prep.).
I will pay particular attention to the complementarity of baryon acoustic oscillations (BAO) and supernovae as distance indicators, to the potential of galaxy clusters calibrated by stacked weak lensing as a probe of structure growth, and to the power of a balanced, multi-pronged observational program that combines supernovae, BAO, weak lensing, and additional methods enabled by the same data sets.
The dark energy community is now searching for subtle quantitative anomalies that would have profound physical implications, distinguishing among fundamentally different theories of the energy content of the universe, the nature of gravity, and the origin of cosmic acceleration.
The road from 5-percent measurements to 1-percent or sub-percent measurements is a challenging one, but we are well equipped for the journey.
Constraining the Complete Star Formation History of Observable Galaxies from z=0 to z=8
Peter Behroozi, Stanford University
We present a comprehensive approach for deriving the star formation history of the universe self-consistently with a wide array of observations (stellar mass functions, stellar mass clustering, specific star formation rates, and the cosmic star formation rate) and with merger rates from N-body simulations in the context of LCDM cosmology. Our approach explores a wide parameter space of systematic uncertainties, allowing a broader range of possible star formation scenarios at high redshifts; these uncertainties include aspects (e.g., a steeper faint-end slope for the stellar mass function) which help resolve tensions between the cosmic star formation rate and stellar mass functions for LBGs at z>1. We present derived constraints on the star formation rates and histories for galaxies as a function of stellar and halo mass from z=0 to z=8. We show that constraints from observations favor a clear change in the star formation rates of massive galaxies around z=2, consistent with a transition from cold-mode to hot-mode accretion at that redshift and very old stellar populations at the present day. We find, on the other hand, that low-mass galaxies do not have a similar break and that many have star formation histories which have been increasing ever since the galaxies first formed. We also discuss the largest uncertainties on current constraints and the direction that future surveys should take to best increase our understanding. In summary, we provide for the first time a complete, consistent picture of the evolutionary path of galaxies over 96% of the history of the universe, with profound implications for galaxy simulations, semi-analytic models, and our understanding of star formation in the cosmos.
NuSTAR: Unveiling the Hard X-ray Universe
Varun B Bhalerao, Caltech
The Nuclear Spectroscopic Telescope Array (NuSTAR) mission will carry the first focussing Hard X-ray (6-80 keV) telescope into orbit in Feb 2012. Using grazing incidence optics and pixelated CdZnTe detectors, it will offer two orders of magnitude increase in sensitivity and an order of magnitude improvement in angular resolution over any previous instrument working in this energy range. The two-year primary science mission focuses on four key programs: studying the cosmic evolution of black holes, understanding the populations of compact objects and the nature of the central black hole in the Milky Way, constraining the explosion dynamics and nucleosynthesis in supernovae, and probing the nature of particle acceleration in active galactic nuclei. A number of additional observations will be included in the primary mission, and a guest observer program will be proposed for an extended mission to expand the range of scientific targets. I will talk about the instrument capabilities of NuSTAR and discuss the science programs.
Gravity Waves from Cosmological Phase Transitions
John T Giblin, Kenyon College/Case Western
Cosmological phase transitions occurred. I will talk about recent advances in modeling possible phase transitions when these transitions are mediated by scalar fields. I will discuss first- and second-order transitions, at various scales, and show how we can compute the background of stochastic gravitational waves produced during (and after) these transitions.
WFIRST and Euclid
Jason D Rhodes, NASA JPL
The past decade has seen tremendous progress in astronomy that has brought us to the brink of being able to answer two very fundamental questions:
What is the Universe made of?
Are we alone?
The first question can only be answered by trying to understand the mysterious 'dark energy' causing the accelerated expansion of the Universe. This dark energy, the dominant constituent of the Universe, has a number of possible theoretical explanations, ranging from a cosmological constant, to possible modifications to Einstein's General Theory of Relativity. The second question is motivated by the increasing frequency of detections of exoplanets and can be explored by seeking out the frequency of Earth-like planets in the habitable zone of stars similar to the sun. Both of these science goals can be best explored with a space-based wide-field telescope capable of imaging and spectroscopy. Such a platform, operating in optical to near infrared wavelengths would also make great strides in a myriad of ancillary astrophysical areas, including the evolution of galaxies and structures over two thirds of the age of the Universe. The European Space Agency is in the final stages of examining the Euclid mission, which is optimized to study dark matter and dark energy. NASA has begun planning for the Wide Field Infrared Survey Telescope designed to explore dark energy and perform an exoplanet survey. I'll discuss the scientific motivations of both missions and give an overview of the hardware, observing strategy and status of each mission.
Lumps and bumps in the early (and late) universe
Mustafa A Amin, Massachusetts Institute of Technology
In this talk I will address the following two (related) questions (1) What does the universe could look like at the end of inflation in the early universe ? (2) What if dark energy undergoes a phase transition in the late universe (similar to the one undergone by the inflaton at the end of inflation)? Both involve understanding some novel nonlinear scalar field dynamics which could also have implications for axionic dark matter.
(1) How did inflation end? I will discuss the different scenarios of of the end of inflation, concentrating on a particular (new) scenario: the fragmentation of the inflaton into localized, long-lived excitations of the inflaton field (oscillons), which can end up dominating the energy density of the universe. I will provide conditions for the existence, stability, emergence and domination of oscillons after inflation as well as their theoretical and possible observational consequences.
(2) In the second half, I will discuss the observational consequences of the quintessence field rolling to and oscillating near a minimum in its potential, "if" it happens close to the present epoch. The oscillations can lead to a rapid growth of the field fluctuations and the gravitational potential (times scales<< H^(-1)) on subhorizon scales. The gravitational potential power spectrum changes in a scale-dependent manner, however, the dark matter/galaxy power spectrum and the expansion history need not be significantly affected. Such a transition in the quintessence field can be constrained best by Integrated Sachs-Wolfe (ISW) and lensing. Two quoted "signatures" of modified gravity are a scale-dependent growth of the gravitational potential and a difference between the matter power spectrum inferred from measurements of lensing and galaxy clustering. Here, both effects are achieved by a minimally coupled scalar field in general relativity with a canonical kinetic term.