College of Arts and Sciences

I discuss ways of constraining dark matter properties using a combination of direct and indirect dark matter measurements. The DAMA, CoGeNT, and CRESST experiments have obtained tentative evidence for low mass WIMPs. I show that the CMB is a clean probe of low mass WIMPs, and the WMAP+SPT measurements place competitive bounds on light WIMPs. I discuss how these dark matter bounds may be further improved by including other data sets, such as counts of galaxy clusters.

Continue reading… Dark matter bounds from direct and indirect searches – Aravind Natarajan

Massless particles such as photons and gravitons do not travel solely on the null cone in a generic curved spacetime. They propagate at all speeds equal to and less than c. This fact does not appear to be well appreciated in cosmology, and its consequences deserve to be worked out to ensure we are interpreting observations correctly. A rather dramatic (and hypothetical) example would be the following: suppose a significant fraction of photons from a distant supernova travels slower than c, then we may be mislead into thinking the SN is dimmer than it actually is, because some of the light has not arrived yet.

Continue reading… Light does not always travel on the light cone – Yi-Zen Chu

The holographic correspondence between non-gravitational field theories and gravitational theories in one higher dimension can be used to study non-equilibrium behavior of strongly coupled quantum field theories. One such phenomenon is that of quantum quench, where a coupling of the field theory is time dependent and typically asymptotes to constants at early and late times. In the gravity dual this can describe, under suitable circumstances, either black hole formation, or passage through a spacelilke region of high curvature similar to a cosmological singularity. On one hand this has taught us about the meaning of cosmological singularities, while on the other hand this has thrown light on the process of thermalization in strongly coupled field theories.

Continue reading… Holographic Quantum Quench – Sumit Das

I will discuss our recently-proposed model (hep-th/1106.3566) of deformations of general relativity that are consistent and potentially phenomenologically viable, since they respect cosmological backgrounds. These deformations have unique symmetries in accordance with unitarity requirements, and give rise to a curvature induced self-stabilizing mechanism. Furthermore, our findings include the possibility of consistent and potentially phenomenologically viable deformations of general relativity that are solely operative on curved spacetime geometries, reducing to Einstein’s theory on the Minkowski background. I will also comment on possible phenomenological implications.

Continue reading… A Paradise Island for Deformed Gravity – Florian Kuehnel

Since Zwicky (1933), we have known that clusters of galaxies have gravitational potentials which are too large to be explained by the amount of visible baryons under the assumption of a Newtonian gravitational force law. This has led to competing hypotheses that either the masses of clusters are dominated by a non-baryonic form of matter or that gravity departs from a 1/r^2 force law on cluster scales. By using merging clusters of galaxies, I will show that the different types of matter in the clusters can be spatially seperated and, by using gravitational lensing, I will prove, independent of any assumptions about the nature of the law of gravity,

Continue reading… Measuring the dark sector with clusters of galaxies – Douglas Clowe

Conformal Field Theories (CFTs) are theories that are symmetric under changes of distance scale, like a fractal or a Russian doll. They are basic building blocks of more general Quantum Field Theories, which can describe how nature works at its most fundamental level. Despite their importance, the range of possible behavior in CFTs is poorly understood, and often the most interesting theories resist calculation with conventional perturbative methods. However, over the last few years, new techniques have emerged for mapping out the space of these important theories. I’ll explain how to use basic mathematical consistency conditions, techniques from optimization theory (a subfield of computer science),

Continue reading… Carving Out the Space of Conformal Field Theories – David Simmons-Duffin

The late-time accelerated expansion of the universe could be caused by a scalar field that is screened on small scales, as in chameleon or symmetron scenarios. We present an analogy between such scalar fields and electrostatics, which allows calculation of the chameleon field profile for general extended bodies. Interestingly, the field demonstrates a `lightning rod’ effect, where it becomes enhanced near the ends of a pointed or elongated object. Drawing from this correspondence, we show that non-spherical test bodies immersed in a background field will experience a net torque caused by the scalar field. This effect, with no counterpart in the gravitational case,

Continue reading… Understanding Chameleon Scalar Fields via Electrostatic Analogy – Kate Jones-Smith

Numerous recent experimental results have reinforced interest in a class of models dubbed “Asymmetric Dark Matter” (ADM), in which the relic dark matter density results from a particle-antiparticle asymmetry. Early models of this sort were invoked to explain the fact that the cosmic baryon and dark matter densities are of the same order, yet in the standard cosmology, they are produced by distinct physical processes. In such models, the relic dark matter density results from an asymmetry (perhaps dark matter carries B-L charge), so there are no contemporary cosmic dark matter annihilations and no opportunity for indirect detection. Otherwise, these scenarios give essentially the same cosmological predictions as the standard weakly-interacting massive particle/cold dark matter paradigm,

Continue reading… How Asymmetric Dark Matter May Alter the Conditions of Stardom – Andrew Zentner

I describe Hi-C, a novel technology for probing the three-dimensional architecture of whole genomes by coupling proximity-based ligation with massively parallel sequencing. Working with collaborators at the Broad Institute and UMass Medical School, we used Hi-C to construct spatial proximity maps of the human genome at a resolution of 1Mb. These maps confirm the presence of chromosome territories and the spatial proximity of small, gene-rich chromosomes. We identified an additional level of genome organization that is characterized by the spatial segregation of open and closed chromatin to form two genome-wide compartments. At the megabase scale, the chromatin conformation is consistent with a fractal globule,

Continue reading… How the genome folds – Erez Lieberman Aiden

Our understanding of the universe between the end of inflation and production of light elements is incomplete. How did inflation end? What did the universe look like at the end of inflation? In this talk, I will discuss the different scenarios of (p)reheating: particle production at the end of inflation. I will then concentrate on a particular scenario: the fragmentation of the inflaton into localized, long-lived excitations of the inflaton field (oscillons), which end up dominating the energy density of the universe if couplings to other fields are weak. Oscillons are produced in a large class of inflationary models which are theoretically well motivated and observationally consistent with the cosmic microwave background anisotropies.

Continue reading… Lumps and bumps in the early universe: (p)reheating and oscillons after inflation – Mustafa Amin

The mystery of dark energy suggests that there is new gravitational physics at low energies and on long length scales. On the other hand, low mass degrees of freedom in gravity are strictly limited by observations within the solar system. A compelling way to resolve this apparent contradiction is to add a galilean-invariant scalar field to gravity. Called galileons, these scalars have strong self interactions near overdensities, like the solar system, that suppress their effects on the motion of massive particles. These non-linearities are weak on cosmological scales, permitting new physics to operate. Extending galilean invariance to the coupling of galileons to stress-energy —

Continue reading… Massive gravitons and enhanced gravitational lensing – Mark Wyman