Talks

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.

We discuss holographic models for the inflationary epoch. We show how cosmological observables such as the primordial spectrum and non-Gaussianities can be computed via computations of correlation functions of a dual three dimensional QFT (without gravity!) We present a general class of models that have the following universal features: (i) they have a nearly scale invariant spectrum of small amplitude primordial fluctuations, (ii) the scalar spectral index runs as alpha_s = - (n_s-1), (iii) the three point function of density perturbations is exactly equal to the sum of the local and equilateral form with f_{NL}^{local} = 6 f_{NL}^{equil} = 20/3.

It has long been recognized that there are two distinct laws that go by the name of the Second Law of Thermodynamics. The original says that there can be no process resulting in a net decrease in the total entropy of all bodies involved. A consequence of the kinetic theory of heat is that this law will not be strictly true; statistical fluctuations will result in small spontaneous transfers of heat from a cooler to a warmer body. The currently accepted version of the Second Law is probabilistic: tiny spontaneous transfers of heat from a cooler to a warmer body will be occurring all the time, while a larger transfer is not impossible, merely improbable. There can be no process whose expected result is a net decrease in total entropy. According to Maxwell, the Second Law has only statistical validity, and this statement is easily read as an endorsement of the probabilistic version. I argue that a close reading of Maxwell, with attention to his use of "statistical," shows that the version of the second law endorsed by Maxwell is strictly weaker than our probabilistic version. According to Maxwell, even the probable truth of the second law is limited to situations in which we deal with matter only in bulk and are unable to observe or manipulate individual molecules. Maxwell's version does not rule out a device that could, predictably and reliably, transfer heat from a cooler to a warmer body without a compensating increase in entropy. I will discuss the evidence we have for these two laws, Maxwell's and ours.

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.