Cosmology with varying fundamental constants from hyperlight, coupled scalars

TL;DR

This paper explores how hyperlight scalar fields that modulate fundamental constants like electron mass and fine-structure constant can influence cosmology, revealing new degeneracies and constraints, and potentially reconciling Hubble constant measurements.

Contribution

It introduces a model where a hyperlight scalar affects early universe constants and demonstrates its implications for cosmological parameter degeneracies and dark matter composition.

Findings

Variations in fundamental constants are constrained at percent and permille levels.

A hyperlight scalar can constitute up to a percent of dark matter.

Models with varying electron mass may reconcile different Hubble constant measurements.

Abstract

The fundamental constants at recombination can differ from their present-day values due to degeneracies in cosmological parameters, raising the possibility of yet-undiscovered physics coupled directly to the Standard Model. We study the cosmology of theories in which a new, hyperlight scalar field modulates the electron mass and fine-structure constant at early times. We find new degeneracies in cosmologies that pair early recombination with a new contribution to the matter density arising at late times, whose predictions can be simultaneously consistent with CMB and low-redshift distance measurements. Such "late dark matter" already exists in the Standard Model in the form of massive neutrinos but is necessarily realized by the scalar responsible for shifting the early-time fundamental constants. After detailing the physical effects of varying constants and hyperlight scalar fields on cosmology, we show that variations of the electron mass and fine structure constant are constrained at the percent and permille level, respectively, and a hyperlight scalar in the mass range $10^{-32}~\mathrm{eV} \lesssim m_\phi \lesssim 10^{-28}~\mathrm{eV}$ can impact what variations are allowed while composing up to a percent of the present dark matter density. We comment on the potential for models with a varying electron mass to reconcile determinations of the Hubble constant from cosmological observations and distance-ladder methods, and we show that parameter inference varies significantly between recent baryon acoustic oscillation and type Ia supernova datasets.