Elastic Inflation

TL;DR

Elastic inflation replaces the canonical scalar field with a relativistic elastic medium, enabling inflation with p ≈ -ε while admitting superhorizon evolution of tensor and scalar perturbations. The approach yields modified tensor and scalar spectra through transfer functions F_T and F_S, and introduces a simple constant-speeds realization linking c_s^2 and c_v^2 via c_s^2 − 4/3 c_v^2 ≈ -1. This framework can accommodate current observational constraints and predicts potentially positive tensor tilt, with distinctive scalar tilts and tensor-to-scalar ratios determined by the elastic parameters. The methodology provides a coherent perturbation theory for elastic inflation and a practical recipe to compute spectra and tilts, potentially offering new signatures for cosmological data analysis.

Abstract

Inflation of a universe filled by an elastic continuous medium is considered. Elastic inflation, while capable of describing current observations, is qualitatively different from standard inflationary models. The scalar and tensor modes keep evolving after crossing the horizon. Due to this superhorizon evolution, the amplitude of the tensor mode (inflationary gravitational wave) is not simply proportional to the energy density of the universe at the time of horizon crossing. The spectral index of tensor modes can be positive.