Searching for coupled, hyperlight scalars across cosmic history

TL;DR

This paper investigates hyperlight scalar fields coupled to the Standard Model across cosmic history, deriving constraints from cosmological and laboratory data, and explores their potential as a subcomponent of dark matter affecting fundamental constants.

Contribution

It provides a comprehensive analysis of coupled hyperlight scalars' dynamics, extending bounds to the hyperlight regime, and combines early and late Universe observations to constrain their properties.

Findings

Most stringent constraints on scalars with masses $10^{-28.5}$ to $10^{-31}$ eV.

Limits on scalars affecting electron mass or electromagnetism strength.

Hyperlight scalars can comprise a few percent of dark matter with near- or subgravitational couplings.

Abstract

Cosmological scalar fields coupled to the Standard Model drive temporal variations in the fundamental constants that grow with redshift, positioning the early Universe as a powerful tool to study such models. We investigate the dynamics and phenomenology of coupled scalars from the early Universe to the present to consistently leverage the myriad searches for time-varying constants and the cosmological signatures of scalars' gravitational effects. We compute the in-medium contribution from Standard Model particles to the scalar's dynamics and identify only a limited range of couplings for which the scalar has an observable impact on the fundamental constants without either evolving before recombination or gravitating nonnegligibly. We then extend existing laboratory and astrophysical bounds to the hyperlight scalar regime. We present joint limits from the early and late Universe, specializing to hyperlight, quadratically coupled scalars that modulate the mass of the electron or the strength of electromagnetism and make up a subcomponent of the dark matter today. Our dedicated analysis of observations of the cosmic microwave background, baryon acoustic oscillations, and type Ia supernovae provides the most stringent constraints on quadratically coupled scalars with masses from $10^{-28.5}$ to $\sim 10^{-31}~\mathrm{eV}$, below which quasar absorption spectra yield stronger bounds. These results jointly limit hyperlight scalars that comprise a few percent of the current dark matter density to near- or subgravitational couplings to electrons or photons.