Cosmology

I present a one-function family of solutions to 4D vacuum Einstein equations. While all diffeomorphic to the extremal Kerr black hole, they are labeled by well-defined conserved charges and are hence distinct geometries. Out of the appropriate combination of these charges, we construct a Virasoro algebra and consistency conditions lead us to a proposal for identifying extreme Kerr black hole microstates, dubbed as extreme Kerr fluff. Counting these microstates, we correctly reproduce the extreme Kerr black hole entropy.

The emergence of classical behavior from an out-of-equilibrium quantum wavefunction is determined by its entanglement structure, in the form of redundant information shared between many local subsystems. We show how this structure can be generated via cosmological dynamics from the vacuum state of a massless field, causing the wavefunction to branch into classical field configurations.  An accelerating epoch first excites the vacuum into a superposition of classical fields alongside a highly sensitive bath of super-horizon particles. Gravitational interactions allow these quanta to collect information about the long-wavelength field. During a subsequent decelerating epoch, this information propagates spatially, creating long-range redundant correlations in the wavefunction. The resulting classical observables (field configurations), preferred basis for decoherence, and system/environment decomposition emerge from the translation invariant wavefunction and Hamiltonian, rather than being fixed by hand. We discuss the implications for the classicality of cosmological perturbations.

I will discuss related aspects of field theories with higher-derivative Lagrangians but second-order equations of motion, with a focus on the Lovelock and Horndeski classes that have found use in modifications to general relativity. In the first half I will investigate when restricting to such terms is and is not well-justified from an effective field theory perspective. In the second half I will discuss how non-perturbative effects, like domain walls and quantum tunneling, are modified in the presence of these kinetic terms

I will describe a tidal effect whereby the decay of primordial gravity waves leaves a permanent shear in the large-scale structure of the Universe. Future large-scale structure surveys - especially radio surveys of high-redshift hydrogen gas - could measure this shear and its spatial dependence to form a map of the initial gravity-wave field. The three dimensional nature of this probe makes it sensitive to the helicity of the gravity waves, allowing for searches for early-Universe gravitational parity violation. Due to the large number of measurable modes in the high-redshift large-scale structure, these tidal imprints could ultimately be more sensitive than searches for CMB B-modes.