Primordial gravitational waves and the H0-tension problem

TL;DR

This paper investigates whether primordial gravitational waves can help resolve the H0-tension by analyzing their contribution to the universe's expansion and comparing models with observational data.

Contribution

The study examines the impact of primordial gravitational waves on the H0-tension, considering their contribution to Neff and testing multiple scenarios with recent cosmological data.

Findings

Neff^GW does not fully resolve the H0-tension

Primordial GWs slightly alleviate the tension

Models including GWs are consistent with current data

Abstract

We analyse the H0-tension problem in the context of models of the early universe that predict a blue tilted spectrum of primordial gravitational waves (GW's). By considering the GW's contribution, Neff^GW, to the effective number of relativistic degrees of freedom, Neff, and assuming standard particle physics, we discuss the effects of Neff^GW on the background expansion, especially the constraints on the Hubble parameter H0. We analyse three scenarios which take into account the contribution of Neff^GW and perform a statistical study using recent data of cosmic microwave background, baryon acoustic oscillation, the latest measurement of the local expansion rate, along with the LIGO constraints on the tensor to scalar ratio and the tensor index. For the models explored, we show that an additional contribution from the primordial GW's background to Neff does not solve but alleviate the current H0-tension problem.