Cosmology

In this talk, I attempt to gain insight into the description of quantum gravity on cosmological spacetimes by considering the physics of families of accelerating observers in spacetimes which admit non-perturbative descriptions vis AdS/CFT.

I will explain how Liouville theory with complex values of its parameters arises naturally in speculative holographic cosmologies. We will encounter Liouville theory of both the ``spacelike'' and ``timelike'' variety. I will then use this as motivation to present some new results on the analytic continuation of Liouville theory recently obtained with Maltz and Witten.

I discuss bubble collisions from the perspective of an observer in a hat. In particular, I emphasize the breaking and restoration of conformal symmetry, as well as (independence of) initial conditions and rate equations. A cartoon version of the problem, Mandelbrot percolation, makes computations tractable. Enjoyable, even.

Motivated by the consistency of black hole complementarity, Sekino and Susskind have conjectured that no physical system can "scramble" its internal degrees of freedom in time faster than (1/T) log S, where T is temperature and S the system's entropy. By considering a number of toy examples and general Lieb-Robinson-type causality bounds, I'll explore the range of validity of the conjecture. Some of these examples suggest that nonlocal Hamiltonians can delocalize information at rates exceeding the fast scrambling bound, but the physical relevance of these examples is unclear. Joint work with Nima Lashkari and Douglas Stanford.

We investigate a simple FRW spacetime realized by a brane construction. This also comes from a Coleman-de Luccia decay from a metastable de Sitter. We motivate a dual description in terms of a low energy effective field theory (EFT) on FRW in one lower dimensions. This EFT is coupled to gravity with a time-dependent Planck mass that grows to infinity at late times. We investigate the entropy bound, correlation functions, and various particle/brane probes as first steps to understand the degrees of freedom building up the EFT. This is work in collaboration with B. Horn, S. Matsuura, E. Silverstein, and G. Torroba.

I will discuss recent work engineering "semi-holographic" constructions of de Sitter space in string theory, using elliptic fibrations and orientifolds to uplift known Freund-Rubin compactifications. The dual brane construction is compact and provides a microscopic realization of the dS/dS correspondence of Alishahiha et al., realizing de Sitter space in d dimensions as a warped compactification down to d-1 dimensional de Sitter space coupled to a pair of large N matter sectors. This provides a parametric microscopic accounting of the Gibbons-Hawking entropy. I will discuss an explicit example in three dimensions as well as ongoing work in four dimensions.

Starting from AdS/CFT, one can introduce ingredients which produce cosmological solutions, including metastable de Sitter and its decay to FRW. In the de Sitter case, this produces a compact brane construction which mirrors the dS/dS correspondence realizing de Sitter as a pair of warped throats coupled to gravity. In the FRW case, I will present simple solutions sourced by magnetic flavor branes and explore their holographic description. The basic strategy is to exhibit a time-dependent warped metric on the solution and test the interpretation of the resulting region of gravitational redshift as a low energy effective field theory (EFT) by analyzing particle dynamics and correlation functions. At finite times, the EFT has a finite cutoff since system has a propagating lower dimensional graviton and a finite covariant entropy bound, but the graviton decouples at late times as the Planck mass goes off to infinity along with the entropy. This is work in collaboration with X. Dong, B. Horn, S. Matsuura, and G. Torroba. Along the way I will make some comments on the role of microscopic (UV complete) physics in cosmological holography and mention several different approaches to deriving landscape duals.

Gauge/gravity duality is our most complete construction of quantum gravity, but it gives in a simple way only the observations of an observer at the AdS boundary. I discuss various issues regarding the representation of the bulk physics.

I'll discuss some recent insights regarding the complexity of simulating highly entangled quantum systems using classical and quantum computers, and what these advances might imply about the quantum state of the early universe.