CMB and energy conservation limits on nanohertz gravitational waves

TL;DR

This paper uses energy conservation principles and CMB constraints to set strict bounds on models explaining the nanohertz gravitational wave background, which could originate from supermassive black hole binaries or early universe phenomena.

Contribution

It introduces a method to constrain gravitational wave models using energy conservation and CMB limits, providing a priori bounds on model parameters.

Findings

Energy conservation imposes strong limits on GW source models.

CMB bounds on radiation energy density further restrict model parameters.

The approach narrows down viable explanations for the nanohertz GWB.

Abstract

The recent evidence for a stochastic gravitational wave background (GWB) in the nanohertz band, announced by pulsar timing array (PTA) collaborations around the world, has been posited to be sourced by either a population of supermassive black holes binaries or perturbations of spacetime near the inflationary era, generated by a zoo of various new physical phenomena. Gravitational waves (GWs) from these latter models would be explained by extensions to the standard model of cosmology and possibly to the standard model of particle physics. While PTA datasets can be used to characterize the parameter spaces of these models, energy conservation and limits from the cosmic microwave background (CMB) can be used $\textit{a priori}$ to bound those parameter spaces. Here we demonstrate that taking a simple rule for energy conservation and using CMB bounds on the radiation energy density can set stringent limits on the parameters for these models.