Astrophysics

If dark matter consists of a multiplet with small mass splittings, it is possible to simultaneously account for DAMA/CoGeNT hints of direct detection and the INTEGRAL 511 keV gamma ray excess from the galactic center; such dark matter must be in the 4-12 GeV mass range. I present scenarios where the DM transforms under a hidden SU(2) that can account for these observations. These models can be tested in low-energy beam dump experiments, like APEX. To explain PAMELA/Fermi excess electrons from dark matter annihilations, heavier TeV scale DM is required. I will present new more stringent constraints from Fermi gamma ray data that tend to rule out such models. However we find a loophole: DM annihilations in a nearby DM subhalo, between us and the galactic center, could provide the excess leptons while respecting gamma ray constraints.

The quantum spin Hall effect relates seemingly unrelated degrees of freedom, i.e., charge and spin degrees of freedom. We will discuss such "duality" can be extended to much wider class of quantum numbers, and the corresponding order parameters. In particular, two valleys in graphene can be viewed as an SU(2) pseudo spin degree of freedom, which turns out to be "dual" to the charge degree of freedom, pretty much in the same way as spin in the quantum spin Hall effect is closely tied with charge. I.e., graphene can host "the quantum valley Hall effect" (QVHE). We will show that one of the best venues to observe the QVHE in graphene is actually superconductivity that can be induced in graphene by proximity effect, say, where passing supercurrent in one direction induces accumulation of pseudo spin ("valley spin") at the boundary of graphene sample. We will also discuss the "inverse QVHE" as a possible scenario to explain the highly resistive state found in N=0 Landau level in graphene in a high magnetic field.

I will describe recent work by Cutler&Holz and Hirata, Holz, & Cutler showing that a highly sensitive, deci-Hz gravitational-wave mission like BBO or Decigo could measure cosmological parameters, such as the Hubble constant H_0 and the dark energy parameters w_0 and w_a, far more accurately than other proposed dark-energy missions. The basic point is that BBO’s angular resolution is so good that it will provide us with hundreds of thousands of “standard sirens.” These standard sirens are inspiraling neutron star and black hole binaries, with gravitationally-determined distances and optically determinable redshifts. I explain why a BBO-like mission would also be a powerful weak lensing mission, and I briefly describe some further astrophysics that would flow from such a mission.

I'll discuss some work done in collaboration with Cliff Burgess, Louis Leblond and Sarah Shandera on the significance of the IR divergences in de Sitter space. First, I'll talk about how large fluctuations at long distances can induce the failure of the loop expansion for interacting field theories with massless degrees of freedom in de Sitter space, much in the same manner as happens in thermal field theories. Then I'll shift gears slightly and describe work involving the use of the dynamical renormalization group in resumming the secularly growing perturbative corrections to correlation functions for massless, minimally coupled scalar fields in de Sitter.

Constraints on the formation of primordial black holes - especially the ones which are small enough to evaporate - provide a unique probe of the early universe, high energy physics and extra dimensions. For evaporating black holes, the dominant constraints are associated with big bang nucleosynthesis and the extragalactic photon background, but there are also other limits associated with the cosmic microwave background, cosmic rays and various types of relic particles. For larger non-evaporating black holes, important constraints come from their gravitational and astrophysical effects. Small non-primordial evaporating black holes may be produced in the LHC if there are large extra dimensions and this would also have important implications for the early universe.

I consider some of the issues we face in trying to understand dark energy. Huge fluctuations in the unknown dark energy equation of state can be hidden in distance data, so I argue that model-independent tests which signal if the cosmological constant is wrong are valuable. These can be constructed to remove degeneracies with the cosmological parameters. Gravitational effects can play an important role. Even small inhomogeneity clouds our ability to say something definite about dark energy. I discuss how the averaging problem confuses our potential understanding of dark energy by considering the backreaction from density perturbations to second-order in the concordance model: this effect leads to at least a 10\% increase in the dynamical value of the deceleration parameter, and could be significantly higher owing to a UV divergence. Large Hubble-scale inhomogeneity has not been investigated in detail, and could conceivably be the cause of apparent cosmic acceleration. I discuss void models which defy the Copernican principle in our Hubble patch can explain acceleration through inhomogeneous cosmic curvature. These can fit the small scale CMB, and can explain the observed primordial lithium abundances - a niggling 4 or 5 sigma discrepancy in the concordance model. I describe how we can potentially rule out these models, and so provide an important test for the existence of dark energy.

Although inflation is, by far, the best known mechanism to explain the observed properties of our Universe, there is still some room for alternative models, most of which implying a contracting phase preceding the current expanding one. Both phases are connected by a bounce at which the expansion rate must vanish. General relativity can only produce such a phase provided the spatial curvature is positive, in contradiction with the current observations. I will discuss the lines along which one can modify either the matter or the gravity sector (or both) in order to implement a bounce, and show the generic observable cosmological consequences it can induce, in particular in the microwave background.