Signatures from pion condensation and lepton flavor asymmetries in the cosmological gravitational wave background

TL;DR

This paper explores how large lepton flavor asymmetries in the early Universe could lead to pion condensation, affecting gravitational wave spectra and offering new constraints on early lepton asymmetries.

Contribution

It introduces a novel mechanism linking lepton asymmetries to pion condensation and predicts distinctive gravitational wave signatures for future detection.

Findings

Large lepton asymmetries can induce pion condensation in the early Universe.

Such condensation can modify the gravitational wave spectrum, especially at low frequencies.

The predicted GW signatures could be detectable by current or future pulsar timing arrays.

Abstract

Large lepton flavor asymmetries at the QCD epoch could generate a pion condensation phase in the early Universe. For large enough tau lepton flavor asymmetries, the speed of sound can exceed the conformal value, leaving a distinctive imprint on the low-frequency gravitational wave (GW) spectrum from causal sources. Beyond probing the formation of a pion condensation phase, the detection or non-detection of this signature would provide a novel constraint on lepton asymmetries in the early Universe. We estimate the GW signal and compare it with the standard case of vanishing lepton asymmetry. Finally, we discuss the implications for the stochastic GW background reported by Pulsar Timing Arrays, using the NANOGrav 15-year dataset.