Thermal Tachyacoustic Cosmology

TL;DR

Thermal Tachyacoustic Cosmology proposes a high-energy early universe model where rapid changes in sound speed can solve cosmological problems and produce observable gravitational waves, fitting current CMB data and predicting detectable tensor modes.

Contribution

This paper introduces Thermal Tachyacoustic Cosmology with thermal initial conditions, demonstrating its compatibility with observations and exploring implications for primordial black holes and gravitational wave detection.

Findings

A phase transition with a rapid drop in sound speed fits CMB data.

Primordial black hole production constrains the acoustic history.

Tensor modes with amplitude r > 10^{-3} are predicted, detectable by future experiments.

Abstract

An intriguing possibility that can address pathologies in both early universe cosmology (i.e. the horizon problem) and quantum gravity (i.e. non-renormalizability), is that particles at very high energies and/or temperatures could propagate arbitrarily fast. A concrete realization of this possibility for the early universe is the Tachyacoustic (or Speedy Sound) cosmology, which could also produce a scale-invariant spectrum for scalar cosmological perturbations. Here, we study Thermal Tachyacoustic Cosmology (TTC), i.e. this scenario with thermal initial conditions. We find that a phase transition in the early universe, around the scale of Grand Unified Theories (GUT scale; $T\sim 10^{15}$ GeV), during which the speed of sound drops by $25$ orders of magnitude within a Hubble time, can fit current CMB observations. We further discuss how production of primordial black holes constrains the cosmological acoustic history, while coupling TTC to Horava-Lifshitz gravity leads to a lower limit on the amplitude of tensor modes ($r \gtrsim 10^{-3}$), that are detectable by CMBPol (and might have already been seen by the BICEP-Keck collaboration).