Search Talks

The network of tests shows the LCDM cosmology is a good approximation to what actually happened, but that need not mean it has all the physics relevant to cosmology back to light element formation. I will review properties of galaxies that seem to be particularly difficult to understand within LCDM and might be pointing to still better physics.

Constraining primordial non-Gaussianity can offer a window into the early universe, and into testing the inflationary paradigm, which is fully complementary to the approach offered by Cosmic Microwave Background polarization. Large-scale structure and galaxy surveys have recently received renewed attention as a way to constrain primordial non-Gaussianity. I will review the potential and the limitations of this approach and highlight its complementarity to Cosmic Microwave Background observations.

We describe recent progress on the quantum description of the Kerr black hole. Previous descriptions of black hole microstates have relied on the existence of near-horizon regions with conformal symmetry, and hence have only worked for extremal or supersymmetric black holes. We argue that the states of non-extremal black holes can also be understood in terms of a conformal symmetry, the difference being that this symmetry is not geometrically realized. Thus a Kerr black hole is an excited state of a conformal field theory. By making certain (natural) assumptions about the nature of this dual CFT we can compute its density of states. This gives a microscopic computation of the Bekenstein-Hawking entropy of a Kerr black hole with arbitrary mass and angular momentum.

It has been suggested, by Kallosh and Linde, that a generic bound on inflation in string theory keeps the Hubble scale of inflation $H$ smaller than the gravitino mass, $m_{3/2}$. Given that models with low-energy supersymmetry have $m_{3/2}$ smaller than a TeV, this is a severe constraint, and would suggest that one is forced to choose between high-scale inflation and low-scale supersymmetry. The bound arises by considering possible decompactification instabilities of the extra (compactified) dimensions of string theory, during the inflationary epoch. I explain the arguments that give rise to such a bound, and describe recent work with T. He and A. Westphal exhibiting large-field chaotic inflation models in string-inspired supergravities that have $H >> m_{3/2}$ but avoid decompactification. I conclude that even within the framework of string theory, high-scale inflation and low-energy supersymmetry may well be compatible.

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.

Magnetic fields are ubiquitous in our Universe. The are observed in galaxies and clusters in our vicinity and at high redshifts. In my talk I outline the possibilities to generate magnetic fields in the early Universe during inflation or during a first order phase transition. I explain the form of the magnetic field spectrum obtained in the different cases. I also discuss the subsequent evolution of helical and non-helical magnetic fields in the cosmological plasma and argue that fields generated at the electroweak phase transition do not have enough large scale power to represent the magnetic fields observed in galaxies and clusters, even if they are maximally helical.

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.