The Hubble Rate Trouble: An Effective Field Theory of Dark Matter

TL;DR

This paper proposes an effective field theory involving dark matter particles that increases relativistic degrees of freedom in the early universe, helping reconcile the discrepancy between local and CMB measurements of the Hubble constant.

Contribution

It introduces a new mechanism involving effective operators for dark matter that can raise the Hubble constant inferred from CMB data, compatible with observational constraints.

Findings

Mechanism increases H_0 up to 70 km/s/Mpc.

With Phantom-like cosmology, H_0 reaches 71-73 km/s/Mpc.

Parameter space identified that matches H_0 and satisfies constraints.

Abstract

The Hubble constant inferred from the 6-parameter fit to the CMB power spectrum conflicts with the value obtained from direct measurements via type Ia supernova and Cepheids observations. We write down effective operators involving spin-0, spin-1/2, and spin-1 dark matter that lead to the relativistic production of dark matter particles at early times, and consequently lead to an increase in the number of relativistic degrees of freedom. This mechanism which is amenable to CMB, BBN, and structure formation observables can sufficiently raise the value of the Hubble constant derived from CMB and reconcile local and CMB probes of the Hubble constant. This mechanism alone increases $H_0$ up to $70\, {\rm km s^{-1} Mpc^{-1}}$, and with the help of a Phantom-like cosmology, reach $H_0 \simeq 71-73\, {\rm km s^{-1} Mpc^{-1}}$. Lastly, we outline the region of parameter space which reproduces $H_0 \simeq 71-73\, {\rm km s^{-1} Mpc^{-1}}$ while obeying all relevant constraints.