Talks & Events
KICP Colloquia: 2011
Bars, Streams, and PAndAS: Galactic Dynamics and Hierarchical Clustering in the Nearby Universe
The Local Group is a dynamic environment. Its two main constituents, M31 and the Milky Way, are continually accreting smaller galaxies. This process is beautifully captured in the Pan-Andromeda Archeological Survey, which is providing, in unprecedented detail, the structure and content of M31 and M33. Moreover, both M31 and the Milky Way are barred, which suggests that these galaxies are undergoing secular evolution. I will describe simulations that are designed to explore these and other phenomena. A numerical toolbox for generating flexible, equilibrium models of disk galaxies will also be presented.
First Result from QUIET
Cosmic microwave background (CMB) polarization is the ultimate probe of primordial gravity waves in the early universe, via the B-mode (or parity odd) signal on degree angular scales. A detection of such a signal would be strong evidence of the inflation scenario and represent indirect observation of a fundamentally new phenomenon near the grand unification energy scale. With its unique HEMT radiometer technology, QUIET is among the most competitive experiments aiming to detect such a signature in the CMB. QUIET has just completed its observation from October 2008 through December 2010, first with 43GHz receiver and then with 95GHz one, collecting over 10000 hours of data in total. In this talk, I will review the QUIET experiment and report the first result from QUIET using 3458 hours of data taken with the 43GHz receiver. The result is supported by analysis techniques such as cross correlation of the maps with different pointings and the blind analysis. Thoroughly estimated systematic errors, being the least of those reported to date, demonstrate systematic cleanness of QUIET and represent a good prospect for a future project.
A New Framework for Treating Small Scale
We provide a general framework that allows one to analyze---in a mathematically precise manner---solutions to Einstein's equation wherein, a priori, nonlinear effects of small scale inhomogeneities can produce significant effects on the background geometry. This framework is a generalization (to the case where inhomogeneous matter is present) of the ``shortwave approximation'' that has been used to analyze the back-reaction effects of gravitational radiation. We prove within this framework that, provided that matter satisfies the weak energy condition (i.e. has positive energy density in all frames), the ``effective stress energy" produced by small scale inhomogeneities also must satisfy the weak energy condition and must be traceless, so it cannot mimic the effects of dark energy. We also analyze cosmological perturbation theory within this framework and calculate the corrections produced by small scale inhomogeneities to the equations satisfied by long wavelength perturbations.
Exploring the Beginning of the Universe.
In the past few decades Cosmological observations have allowed us to explore the history of our universe and the nature of the Big Bang with greater and greater precision. They strongly suggest that our Universe started with a period of cosmic acceleration that we call Inflation. I will describe what we currently know about this initial epoch and how, thanks to the upcoming experiments, our knowledge about it might improve drastically, revealing us the dynamics that lead to it and making us extremely confident about the existence of such an epoch at the beginning of our Universe.
A New Twist on Galactic Structure
Unlike the theory of stellar structure, which has a simple and intuitive outline, that of galactic structure is piecemeal and ad hoc. In fact, it has been difficult even to determine whether or not one should expect there to be such an analog. Numerical modeling of the problem grows ever more sophisticated and detailed in its efforts to match observations, suggesting that perhaps the problem is beyond any simple description. However, I will, using simple and general arguments, demonstrate that the global structure of galaxies of all sizes, masses, and morphological types can be described to a high degree using only two observational parameters. I will then explore the nature of those two parameters.
Is the cosmic-ray positron excess evidence for Dark Matter Decay?
Recent measurements of the positron/electron ratio in the cosmic ray (CR) flux exhibits an apparent anomaly, whereby this ratio increases between 10 and 100 GeV. This has triggered significant excitement as it can be interpreted as evidence for dark matter decay. I will show instead that the standard source for cosmic rays, that of supernova remnants, explains this ''anomaly'' once one considers the actual inhomogeneous distribution of SNe in the disk. This also explains other cosmic ray characteristics, such as the energy dependence of the Boron to Carbon ratio.
Gamma-ray Burst Afterglows as Cosmic Probes
Gamma-ray bursts are among the most energetic events in the universe. Many bursts are followed by extremely luminous optical afterglows that can serve as a sensitive probe of "dark", intervening baryonic matter in space. I will review recent progress in our understanding of interstellar medium and intergalactic matter in the distant universe based on observations of long-duration gamma-ray bursts.
The Effects of Small Scale Inhomogeneities on Large Scale Dynamics in Cosmology
Nonlinear gravitational phenomena associated with small scale (as compared with the Hubble radius) density inhomogeneities are prevalent throughout our universe. Could the nonlinear effects of these small scale inhomogeneities produce significant effects on the large scale dynamics that are not properly taken into account by standard FLRW models, which treat the matter distribution as homogeneous? I will describe recent work with S. Green that provides a framework within the context of general relativity that allows one to analyze this issue in a mathematically precise manner. Within this framework we prove quite generally that, provided that matter satisfies the weak energy condition (i.e. has positive energy density in all frames), the “effective gravitational stress energy” associated with small scale inhomogeneities also must satisfy the weak energy condition and must be traceless---corresponding to gravitational radiation. In particular, nonlinear effects of small scale inhomogeneities cannot be responsible for the acceleration of our universe. We also analyze cosmological perturbation theory within this framework and calculate the corrections produced by small scale inhomogeneities to the equations satisfied by long wavelength perturbations.
Unveiling the Cosmic Far-IR Background with Herschel: The Nature of Dusty Star-Forming Galaxies
The talk will summarize latest results from HerMES and H-ATLAS extragalactic surveys at 250, 350 and 500 microns with the SPIRE instrument on the Herschel Space Observatory. HerMES is the multi-tiered SPIRE instrument GTO survey covering about 70 square degrees on a variety of extragalactic fields with existing ancillary data. H-ATLAS is currently the largest open-time program in Herschel with 600 hours of observations to map 550 square degrees. I will discuss a newly discovered population of lensed sub-mm galaxies, the halo properties of bright and faint sub-mm galaxies as revealed by clustering and fluctuation studies, evolution of dust and molecular properties, the role of sub-mm sources in galaxy formation and evolution models, sub-mm source follow-up effort, and the future of sub-mm astronomy. I will also summarize the scientific goals of the Herschel-SPIRE Legacy Survey, a program now under discussions with ESA to cover 1500 sq. degrees with SPIRE in a fast-scan mode with the ultimate goal of recovering a catalog of 1.5 million bright sub-mm sources for future studies with ALMA, CCAT, and SPICA.
Inflation, infinity, equilibrium and the observable Universe
Cosmic inflation has given us a remarkably successful cosmological phenomenology. But the original goal of explaining why the cosmos is *likely* to take the form we observe has proven very difficult to realize. I first review the popular idea of "eternal inflation" with an eye on the roles various infinities have (both helpful and unhelpful) in our current understanding. I then discuss attempts to construct an alternative cosmological framework that is truly finite using ideas about equilibrium and dark energy. I report some recent results that point to observable signatures.
The Multiple Crossroads of Clusters of Galaxies
In this talk, I will present galaxy clusters as systems spanning multiple spaces: the traditional crossroads of astrophysics and cosmology as well as the fast developing crossroads of theory, computation and observation. While the current paradigm for large-scale structure formation - a hierarchical web of quasi-equilibrium halos emerging from a noise field imposed by inflation - is largely secure, important details ranging from the profound (nature of dark energy) to the practical (rate of star formation as a function of galactic environment) remain poorly understood. Clusters of galaxies present opportunities for detailed testing of cosmological and astrophysical models, and I will discuss challenges posed by survey selection and cluster characterization. What's clear is that precise analysis of the cluster population requires that theory/computation meet observation on a common ground. The angle-wavelength space of the sky offers a natural crossroads for such a meeting, and this choice imposes new requirements on the community to build an efficient "cosmic sky machine". I will sketch ideas for developing community-wide cyberinfrastructure that could accelerate the pace of understanding cosmic structure formation.
Where will Einstein fail? Insights into Gravity and Dark Energy
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".
Direct dark matter Detection with Liquefied Noble Gases
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.
Equivalence principle and cosmic acceleration
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.
Measuring Cosmic Acceleration
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.
The Status of SUSY and SUSY Dark Matter
Given the absence of signals at the LHC so far, some have begun mourning the death of weak-scale supersymmetry. Here I present a different view. After reviewing the brittle concept of naturalness, I will present some attractive and viable SUSY models and explain their implications for particle physics and dark matter.