Model-Independent Test of Prerecombination New Physics: Measuring the Sound Horizon with Gravitational Wave Standard Sirens and the Baryon Acoustic Oscillation Angular Scale

TL;DR

This paper proposes a model-independent method to measure the sound horizon at the baryon-drag epoch using upcoming gravitational wave and BAO measurements, providing a new way to test cosmological models and address the Hubble tension.

Contribution

It introduces a novel approach combining GW standard sirens and BAO angular scale measurements for model-independent estimation of the sound horizon.

Findings

Forecasts a 1.5% precision in measuring the sound horizon with upcoming surveys.

Provides a method to test the consistency of the $ m extLambda$CDM model.

Potential to clarify the Hubble tension and detect new physics before recombination.

Abstract

In a broad class of cosmological models where spacetime is described by a pseudo-Riemannian manifold, photons propagate along null geodesics, and their number is conserved, upcoming Gravitational Wave (GW) observations can be combined with measurements of the Baryon Acoustic Oscillation (BAO) angular scale to provide model-independent estimates of the sound horizon at the baryon-drag epoch. By focusing on the accuracy expected from forthcoming surveys such as LISA GW standard sirens and DESI or Euclid angular BAO measurements, we forecast a relative precision of $\sigma_{r_{\rm d}} /r_{\rm d} \sim 1.5\%$ within the redshift range $z \lesssim 1$. This approach will offer a unique model-independent measure of a fundamental scale characterizing the early universe, which is competitive with model-dependent values inferred within specific theoretical frameworks. These measurements can serve as a consistency test for $\Lambda$CDM, potentially clarifying the nature of the Hubble tension and confirming or ruling out new physics prior to recombination with a statistical significance of $\sim 4\sigma$.