Cosmological tensions in Proca-Nuevo theory

TL;DR

This paper investigates the cosmological implications of Proca-Nuevo theory, a vector-tensor model, demonstrating its potential to alleviate the Hubble tension and providing observational constraints while addressing perturbation effects.

Contribution

It extends previous work by analyzing a general class of Proca-Nuevo models, identifying a special case, and incorporating comprehensive observational data including perturbations.

Findings

Proca-Nuevo model is preferred over ΛCDM at 1.5σ to 2.4σ levels.

The Hubble tension is significantly reduced in this model.

The vector field affects the effective Newton constant, requiring tuning to match matter power spectrum.

Abstract

We study the cosmological predictions of (extended) Proca-Nuevo theory. This vector-tensor theory enjoys stable homogeneous and isotropic solutions characterized by an effective dark energy fluid, with behavior that ranges from freezing quintessential to thawing phantom-like, serving as a motivated framework to scrutinize the cosmological tensions that affect the standard $\Lambda$CDM model. While the model we consider is sufficiently generic to encompass a large class of field theories, it distinguishes itself from scalar dark energy models (quintessential ones, kinetic ones and non-minimally coupled ones) by the presence of what would be classed as a vector degree of freedom which can be for instance inherited from more generic theories of gravity. We improve on previous work in several directions: we consider a general one-parameter class of background models; identify a so-called 'special' model and analyze observational constraints taking also into account perturbations and making use of wide up-to-date catalogs of datasets including recently released ones. We find that the one-parameter Proca-Nuevo model is preferred over $\Lambda$CDM at $1.5\sigma$ when fitting CMB and BAO data, and at $2.4\sigma$ when further adding low-redshift data. The Hubble tension is alleviated, dropping from $5.8\sigma$ to $2.3\sigma$ (resp. $1.5\sigma$) between CMB with (and resp. without) BAO data and local measurements. On the other hand, we find that the vector field generically introduces a significant enhancement of the effective Newton constant, so that matching the observed matter power spectrum requires a mild amount of tuning to suppress the impact of perturbations. Since, at the background level, Proca-Nuevo is degenerate with other classes of theories, our results are also relevant to a wider range of set-ups including and beyond vector-tensor models.