Tachyacoustic Cosmology: An Alternative to Inflation

TL;DR

Tachyacoustic cosmology proposes an alternative to inflation where superluminal sound speeds generate scale-invariant perturbations without requiring rapid expansion, offering a new approach to early universe cosmology.

Contribution

The paper constructs explicit non-canonical Lagrangians with superluminal sound speeds, demonstrating a causally consistent model that generates perturbations without inflation.

Findings

Superluminal sound speeds can produce scale-invariant spectra.

The model solves the horizon problem without inflation.

Gravitational waves are not generated in this scenario.

Abstract

We consider an alternative to inflation for the generation of superhorizon perturbations in the universe in which the speed of sound is faster than the speed of light. We label such cosmologies, first proposed by Armendariz-Picon, {\it tachyacoustic}, and explicitly construct examples of non-canonical Lagrangians which have superluminal sound speed, but which are causally self-consistent. Such models possess two horizons, a Hubble horizon and an acoustic horizon, which have independent dynamics. Even in a decelerating (non-inflationary) background, a nearly scale-invariant spectrum of perturbations can be generated by quantum perturbations redshifted outside of a shrinking acoustic horizon. The acoustic horizon can be large or even infinite at early times, solving the cosmological horizon problem without inflation. These models do not, however, dynamically solve the cosmological flatness problem, which must be imposed as a boundary condition. Gravitational wave modes, which are produced by quantum fluctuations exiting the Hubble horizon, are not produced.