Cosmological Impacts of Black Hole Mergers: No Relief in Sight for the Hubble Tension

TL;DR

This paper investigates whether black hole mergers could resolve the Hubble tension but finds that they cannot, due to unrealistic merger rates and negligible effects on cosmic measurements.

Contribution

It introduces a detailed analysis of black hole mergers' impact on the Hubble tension, showing these effects are insufficient to resolve the discrepancy.

Findings

Black hole mergers do not significantly affect the Hubble tension.

Unrealistically high merger rates are required to have any impact.

The Integrated Sachs-Wolfe effect constraints are too weak to be useful.

Abstract

The values of the Hubble constant inferred from local measurements and the cosmic microwave background (CMB) exhibit an approximately 5 sigma tension. Some have suggested this tension is alleviated if matter is converted to dark radiation via dark matter decay. As it is not clear that dark matter decays, we instead examine the effects of converting matter to gravitational radiation via black hole mergers. We consider mergers of supermassive black holes (SMBHs), mergers of stellar-mass black holes, and the formation of SMBHs from mergers of smaller black holes. We find that these processes cannot alleviate the tension, as an unrealistically large merger rate, or an overproduction of SMBHs is required. We also consider whether one can use the Integrated Sachs-Wolfe effect to constrain mechanisms that form SMBHs from mergers of smaller black holes. We find that this is also too small to be viable.