Scalar-induced gravitational wave interpretation of PTA data: the role of scalar fluctuation propagation speed

TL;DR

This paper proposes a model where a scalar field with a higher fluctuation speed during the early universe can explain the observed gravitational wave background without overproducing primordial black holes, aligning with PTA data.

Contribution

It introduces a novel scenario involving scalar fluctuation propagation speed to reconcile gravitational wave signals with primordial black hole constraints.

Findings

Scalar fluctuation speed affects gravitational wave production.

The model is consistent with PTA observations and primordial black hole limits.

Future detectors can test the proposed scalar fluctuation propagation effects.

Abstract

Pulsar timing arrays gathered evidence of the presence of a gravitational wave background around nHz frequencies. If the gravitational wave background was induced by large and Gaussian primordial fluctuations, they would then produce too many sub-solar mass primordial black holes. We show that if at the time of gravitational wave generation the universe was dominated by a canonical scalar field, with the same equation of state as standard radiation but a higher propagation speed of fluctuations, one can explain the gravitational wave background with a primordial black hole counterpart consistent with observations. Lastly, we discuss possible ways to test this model with future gravitational wave detectors.