Cosmological Constraints on First-Order Phase Transitions

TL;DR

This paper derives cosmological constraints on the temperature and strength of first-order phase transitions, using big bang nucleosynthesis and CMB data, and discusses implications for gravitational wave signals like NANOGrav.

Contribution

It provides a model-independent analysis of phase transition parameters based on cosmological observations, constraining their temperature and strength.

Findings

Phase transition temperature should be above ~2 MeV for strong transitions.

Weak transitions below 1 MeV have strength parameter less than ~0.1.

Implications for NANOGrav gravitational wave signals are discussed.

Abstract

First-order phase transitions exist in many models beyond the Standard Model and can generate detectable stochastic gravitational waves for a strong one. Using the cosmological observables in big bang nucleosynthesis and cosmic microwave background, we derive constraints on the phase transition temperature and strength parameter in a model-independent way. For a strong phase transition, we find that the phase transition temperature should be above around 2 MeV for both reheating photon and neutrino cases. For a weak one with the temperature below 1 MeV, the phase transition strength parameter is constrained to be smaller than around 0.1. Implications for using a first-order phase transition to explain the NANOGrav observed gravitational wave signal are also discussed.