Forecast Analysis of Astrophysical Stochastic Gravitational Wave Background beyond general relativity: A Case Study on Brans-Dicke Gravity

TL;DR

This paper investigates how Brans-Dicke gravity affects the astrophysical stochastic gravitational wave background, analyzing the potential to detect scalar modes and constrain deviations from general relativity using simulated data from future detectors.

Contribution

It provides the first comprehensive Bayesian analysis of the AGWB under Brans-Dicke gravity, including population uncertainties and the detectability of scalar polarization modes.

Findings

Population uncertainties significantly impact BD gravity constraints.

Scalar backgrounds from BNS and NSBH mergers may alter spectral index and oscillatory features.

Tensor and scalar modes can be separated with comparable sensitivity, but scalar signals remain weak.

Abstract

Scalar-tensor gravity, exemplified by Brans-Dicke (BD) gravity, introduces additional scalar polarization modes that contribute scalar radiation alongside tensor modes. We conduct a comprehensive analysis of how gravitational wave generation and propagation effects under Brans-Dicke gravity are encoded into the astrophysical stochastic gravitational wave background (AGWB). We perform end-to-end analyses of realistic populations of simulated coalescing binary systems to generate AGWB mock data with third-generation gravitational wave detectors and conducted a complete Bayesian analysis for the first time. We find the uncertainties in the population properties of binary black holes (BBH) significantly affect the ability to constrain BD gravity. Furthermore, we explore the detectability of potential scalar backgrounds that originates from binary neutron star (BNS) and neutron-star-black-hole (NSBH) mergers, with NSBH systems expected to modify the spectral index of the scalar background and introduce oscillatory behavior. We show that the observations of the AGWB enable the separation of mixed tensor and scalar polarization modes with comparable sensitivity to each mode. However, the scalar background is expected to remain substantially weaker than the tensor background, even in scenarios where BD gravity exhibits significant deviations from general relativity (GR), resulting only upper limits can be placed on the scalar background. We conclude that for ambiguous populations, employing waveform matching with individual sources provides a more robust approach to constrain BD gravity.