Peter Adshead

We study pseudoscalar inflation in the Einstein-Cartan-Palatini (first-order) formulation of gravity while allowing for torsion. We introduce two non-minimal interactions in the gravitational sector — pseudoscalar couplings to the Pontryagin density (Chern-Simons term) and the Nieh-Yan topological invariant. In the presence of these terms the rolling pseudoscalar sources non-trivial torsional fields during inflation. We show that pathological gradient and ghost instabilities limit the strength of the coupling to the Pontryagin density during inflation. Furthermore, we show that the interaction with the Nieh-Yan term induces a new contribution to the pseudoscalar kinetic term which parametrically increases its decay constant and allows for inflation on steep potentials. The torsion field generated by the background is parity violating, which is manifest in the resulting chiral gravitational wave spectrum. We find that the scalar sector is largely unaffected beyond the remapping of the axion decay constant to a larger value. Consequently, we demonstrate that Generalized Natural Inflation, D-brane models, and Hilltop Squared Inflation can satisfy current observational constraints with sub-Planckian decay constants.

Inspired by recent advances in observational astrophysics and continued explorations in the field of analog gravity, we discuss the prospect of simulating models of cosmology within the context of synthetic mechanical lattice experiments. We focus on the physics of expanding universe scenarios described by the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. Specifically, quantizing scalar fluctuations in a background FLRW spacetime leads to a quadratic bosonic Hamiltonian with temporally varying pair production terms. Here we present a mapping that provides a one-to-one correspondence between these classes of cosmology models and feedback-coupled mechanical oscillators. As proof of principle, we then perform experiments on a synthetic mechanical lattice composed of such oscillators. We simulate two different FLRW expansion scenarios with universes dominated by vacuum energy and matter and discuss our experimental results.

We study gauge preheating following pseudoscalar-driven inflation in full general relativity. We implement the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) scheme to solve the full nonlinear evolution of the metric alongside the dynamics of the pseudoscalar and gauge fields. The dynamics of the background and emission of gravitational waves are broadly consistent with simulations in a Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime. We find large, localized overdensities in the BSSN simulations of order δ = δρ/ρ ∼ 30, and the dimensionless power spectrum of δ peaks above unity. These overdense regions are seeded on length scales only slightly smaller than the horizon, and have a compactness C ∼ 0.1. The scale of peak compactness is shorter than the Jeans length, which implies that pressure of the matter fields plays an important role in the evolution of these objects.