Cosmology

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.

Classical transition is one of the simplest consequences of cosmic bubble collisions. In quite a few simple toy model landscapes, collisions always result in classical transitions. Can it be generalized to the "real" string theory landscape? If so, does it imply some sort of hidden structure of the landscape?

In this talk, I explain how twistors can be used to provide a covariant UV cut-off for 4-D gauge theory. I'll then motivate the conjecture that the cut-off gauge theory automatically contains 4-D Einstein gravity. As evidence, I describe how the theory reproduces the gravitational MHV amplitudes.

Present treatments of eternal inflation regulate infinities by imposing a geometric cutoff. We point out that some matter systems reach the cutoff in finite time. This implies a nonzero probability for a novel type of catastrophe. According to the most successful measure proposals, our galaxy is likely to encounter the cutoff within the next 5 billion years.

By combining insights from black holes and string theory we argue for the existence of a hidden phase space associated with an underlying fast dynamical system, which is largely invisible from a macroscopic point of view. The dynamical system is influenced by slow macroscopic observables, such as positions of objects. This leads to a collection of reaction forces, whose leading order Born Oppenheimer force is determined by the general principle that the phase space volume of the underlying system is preserved. We propose that this adiabatic force is responsible for inertia and gravity. This fact allows us to calculate the hidden phase space volume from the known laws of inertia and gravity. We find that in a cosmological setting the appearance of dark energy is naturally explained by the finite temperature of the underlying system. The adiabatic approximation that leads to the usual laws of inertia and gravity breaks down in the neighborhood of horizons. In this regime the reaction force degenerates into an entropic force, and the laws of inertia and gravity receive corrections due to thermal effects. A simple estimate of these effects leads to the conclusion that they coincide with observed phenomena attributed to dark matter.

We make some remarks about the semiclassical wavefunction of the universe around de-Sitter space. In five dimensional gravity with a positive cosmological constant it is possible to compute the full semiclassical measure for arbitrary geometries at superhorizon scales. In four dimensions, the same computation can be reformulated as a problem in conformal gravity.

Six dimensional (1, 0) supergravity theories have received recent attention due to the fact that the strong constraints coming from anomalies severely restrict the theory. These constraints are restrictive enough that it is possible to get a rather good handle on the space of theories that do not exhibit any (known) inconsistencies. Many useful observations can be made by studying this space of theories. In particular, the process of comparing these theories with six-dimensional string vacua turns out to be fruitful in many aspects. I will be presenting the lessons learned from research in this direction.