Implications of cosmologically coupled black holes for pulsar timing arrays

TL;DR

This paper explores how cosmologically coupled black holes, which grow in mass with the universe's expansion, can significantly enhance the gravitational wave background detected by pulsar timing arrays, potentially explaining observed discrepancies.

Contribution

It introduces the concept of cosmologically coupled black holes affecting the stochastic gravitational wave background, providing a novel explanation for observed amplitude discrepancies.

Findings

Enhanced SGWB amplitude due to black hole mass growth

Maintains the f^{2/3} spectral scaling in the presence of coupling

Mass growth can increase SGWB by over an order of magnitude

Abstract

It has been argued that realistic models of (singularity-free) black holes (BHs) embedded within an expanding Universe are coupled to the large-scale cosmological dynamics, with striking consequences, including pure cosmological growth of BH masses. In this pilot study, we examine the consequences of this growth for the stochastic gravitational wave background (SGWB) produced by inspiraling supermassive cosmologically coupled BHs. We show that the predicted SGWB amplitude is enhanced relative to the standard uncoupled case, while maintaining the $\Omega_{\text{gw}} \propto f^{2/3}$ frequency scaling of the spectral energy density. For the case where BH masses grow with scale factor as $M_{\text{bh}} \propto a^3$, thus contributing as a dark energy component to the cosmological dynamics, $\Omega_{\text{gw}}$ can be enhanced by more than an order of magnitude. This has important consequences for the SGWB signal detected by pulsar timing arrays, whose measured amplitude is slightly larger than most theoretical predictions for the spectrum from inspiraling binary BHs, a discrepancy which can be alleviated by the cosmological mass growth mechanism.