Have Pulsar Timing Arrays detected the Hot Big Bang? Gravitational Waves from Strong First Order Phase Transitions in the Early Universe

TL;DR

This paper explores various cosmological models where the Hot Big Bang is initiated by strong first order phase transitions, producing gravitational waves that could explain signals detected by pulsar timing arrays.

Contribution

It proposes new scenarios for the Big Bang involving phase transitions at different energy scales, linking them to gravitational wave signals observed by pulsar timing experiments.

Findings

Gravitational waves from these phase transitions can match NANOGrav and PTA signals.

Reheating temperatures of 1 MeV to 100 GeV are consistent with observed signals.

Models predict detectable gravitational waves for upcoming interferometer experiments.

Abstract

The origins of matter and radiation in the universe lie in a Hot Big Bang. We present a number of well-motivated cosmologies in which the Big Bang occurs through a strong first order phase transition -- either at the end of inflation, after a period of kination ("Kination-Induced Big Bang"), or after a second period of vacuum-domination in the early universe ("Supercooled Big Bang"); we also propose a "Dark Big Bang" where only the dark matter in the Universe is created in a first-order phase transition much after inflation. In all of these scenarios, the resulting gravitational radiation can explain the tentative signals reported by the NANOGrav, Parkes and European Pulsar Timing Array experiments if the reheating temperature of the Hot Big Bang, and correspondingly the energy scale of the false vacuum, falls in the range $T_* \sim \rho_{{\rm vac}}^{1/4} $= MeV--100 GeV. All the same models at higher reheating temperatures will be of interest to upcoming ground- and space-based interferometer searches for gravitational waves at larger frequency.