Unveiling dark fifth forces with linear cosmology

TL;DR

This paper investigates the cosmological effects of a hypothetical dark fifth force acting only on Dark Matter, deriving constraints from current data and exploring implications for the Hubble tension and dark energy.

Contribution

It introduces the concept of a long-range dark fifth force, analyzes its impact on cosmology, and provides the strongest current constraints on its strength from observational data.

Findings

Dark fifth force modifies Dark Matter evolution and structure growth.

Current CMB and BAO data constrain the force to less than 1% of gravity.

Potential links to the Hubble tension and dark energy are discussed.

Abstract

We initiate the exploration of the cosmology of dark fifth forces: new forces acting solely on Dark Matter. We focus on long range interactions which lead to an effective violation of the Equivalence Principle on cosmological scales today. At the microscopic level, the dark fifth force can be realized by a light scalar with mass smaller than the Hubble constant today ($\lesssim 10^{-33}\,\text{eV}$) coupled to Dark Matter. We study the behavior of the background cosmology and linear perturbations in such a Universe. At the background level, the new force modifies the evolution of the Dark Matter energy density and thus the Hubble flow. At linear order, it modifies the growth of matter perturbations and generates relative density and velocity perturbations between Dark Matter and baryons that grow over time. We derive constraints from current CMB and BAO data, bounding the strength of the dark fifth force to be less than a percent of gravity. These are the strongest constraints to date. We present potential implications of this scenario for the Hubble tension and discuss how our results are modified if the light scalar mediator accounts for the observed density of the Dark Energy. Finally, we comment on the interplay between our constraints and searches for violations of the Equivalence Principle in the visible sector.