Direct Measurement of the Positive Acceleration of the Universe and Testing Inhomogeneous Models under Gravitational Wave Cosmology

TL;DR

This paper proposes a method using gravitational wave observations from neutron-star binaries to directly measure the universe's acceleration, enabling tests of inhomogeneous cosmological models and potentially ruling out certain LTB void models.

Contribution

It introduces a GW-based approach to measure the universe's acceleration, providing a new way to test inhomogeneous models without electromagnetic data.

Findings

DECIGO/BBO can detect positive redshift drift in 3-5 years

This method can rule out LTB void models with increasing density profiles

GW observations alone outperform Lyman α forest measurements in this context

Abstract

One possibility for explaining the apparent accelerating expansion of the universe is that we live in the center of a spherically inhomogeneous universe. Although current observations cannot fully distinguish $\Lambda$CDM and these inhomogeneous models, direct measurement of the acceleration of the universe can be a powerful tool in probing them. We have shown that, if $\Lambda$CDM is the correct model, DECIGO/BBO would be able to detect the positive redshift drift (which is the time evolution of the source redshift $z$) in 3--5 year gravitational wave (GW) observations from neutron-star binaries, which enables us to rule out any Lema\^itre-Tolman-Bondi (LTB) void model with monotonically increasing density profile. We may even be able to rule out any LTB model unless we allow unrealistically steep density profile at $z\sim 0$. This test can be performed with GW observations alone, without any reference to electromagnetic observations, and is more powerful than the redshift drift measurement using Lyman $\alpha$ forest.