Induced Gravitational Waves with Kination Era for Recent Pulsar Timing Array Signals

TL;DR

This paper investigates gravitational waves generated during a kination era, demonstrating they can explain recent PTA signals without leading to primordial black hole overproduction.

Contribution

It introduces a model of induced gravitational waves during a kination epoch that aligns with PTA observations and avoids PBH overproduction, a challenge in previous models.

Findings

Kination era can produce gravitational waves matching PTA data.

The model avoids overproduction of primordial black holes.

The induced gravitational waves follow an $ ext{Omega}_ ext{GW} o f^2$ scaling.

Abstract

The evidence of the stochastic gravitational-wave background around the nano-hertz frequency range was recently found by worldwide pulsar timing array (PTA) collaborations. One of the cosmological explanations is the gravitational waves induced by enhanced curvature perturbations, but the issue of primordial black hole (PBH) overproduction in this scenario was pointed out in the literature. Motivated by this issue and the $\Omega_\text{GW} \sim f^2$ scaling suggested by the data, we study the gravitational waves induced in a cosmological epoch dominated by a stiff fluid ($w=1$) and find that they can safely explain the PTA data well without PBH overproduction.