Extra-tensor-induced origin for the PTA signal: No primordial black hole production

TL;DR

This paper proposes a new cosmological explanation for the PTA gravitational wave signal, attributing it to an extra tensor field that avoids primordial black hole overproduction, unlike scalar-induced models.

Contribution

It introduces a novel primordial tensor field model for the PTA signal that circumvents the PBH overproduction problem inherent in scalar-induced scenarios.

Findings

The extra tensor field can produce the PTA GW signal without PBH overproduction.

The model provides a viable alternative to scalar-induced GW explanations.

It maintains consistency with current observational constraints.

Abstract

Recently, pulsar timing array (PTA) collaborations announced evidence for an isotropic stochastic gravitational wave (GW) background. The origin of the PTA signal can be astrophysical or cosmological. In the latter case, the so-called secondary scalar-induced GW scenario is one of the viable explanations, but it has a potentially serious issue of the overproduction of primordial black holes (PBHs) due to the enhanced curvature perturbation. In this letter, we present a new interpretation of the PTA signal. Namely, it is originated from an extra spectator tensor field that exists on top of the metric tensor perturbation. As the energy density of the extra tensor field is always subdominant, it cannot lead to the formation of PBHs. Thus our primordial-tensor-induced scenario is free from the PBH overproduction issue.