Lyman-alpha Constraints on Ultralight Scalar Dark Matter: Implications for the Early and Late Universe

TL;DR

This paper uses Lyman-alpha forest data to set constraints on ultralight scalar dark matter, limiting its mass, initial field displacement, and impact on cosmic inflation, thereby informing theories with light scalar fields.

Contribution

It provides new bounds on scalar dark matter properties and their cosmological implications based on Lyman-alpha forest observations.

Findings

Scalar DM mass >~ 10^{-21} eV for >30% DM fraction

Upper limit on inflationary tensor-to-scalar ratio r < 10^{-3}

Limited role for ultralight scalar DM in solving small-scale crisis

Abstract

We investigate constraints on scalar dark matter (DM) by analyzing the Lyman-alpha forest, which probes structure formation at medium and small scales, and also by studying its cosmological consequences at high and low redshift. For scalar DM that constitutes more than 30% of the total DM density, we obtain a lower limit m >~ 10^{-21} eV for the mass of scalar DM. This implies an upper limit on the initial field displacement (or the decay constant for an axion-like field) of phi <~ 10^{16} GeV. We also derive limits on the energy scale of cosmic inflation and establish an upper bound on the tensor-to-scalar ratio of r < 10^{-3} in the presence of scalar DM. Furthermore, we show that there is very little room for ultralight scalar DM to solve the "small-scale crisis" of cold DM without spoiling the Lyman-alpha forest results. The constraints presented in this paper can be used for testing generic theories that contain light scalar fields.