Closing the window on fuzzy dark matter with the 21cm signal

TL;DR

This paper investigates how the 21cm signal can detect or constrain fuzzy dark matter fractions below 100% within the allowed parameter space, using simulations and forecasts for HERA's capabilities.

Contribution

It introduces the first analysis of FDM fractional signatures in the 21cm signal, combining modified CAMB and 21cmFAST codes to forecast detectability.

Findings

HERA can detect FDM fractions as low as 1% within the FDM window.

The method improves existing bounds on FDM mass by up to an order of magnitude.

The study accounts for degeneracies with astrophysical and cosmological parameters.

Abstract

Fuzzy dark matter (FDM) is a well motivated candidate for dark matter (DM) as its tiny mass and large de-Broglie wavelength suppress small-scale matter fluctuations, thereby solving some of the small-scale discrepancies in $\Lambda$CDM. Although it has been ruled out as the single component of DM by several observables, there is still a region in the FDM parameter space (the "FDM window", $10^{-25}\,\mathrm{eV}\lesssim m_\mathrm{FDM}\lesssim10^{-23}\,\mathrm{eV}$) where FDM is allowed to comprise a large portion of the total DM. In this work, for the first time, we study the signature of FDM (comprised of ultra-light axions) in fractions less than unity on the 21cm signal and its detectability by 21cm interferometers such as HERA, taking into account the degeneracy with both astrophysical and cosmological parameters, using a new pipeline that combines modified versions of the CAMB and 21cmFAST codes. Our forecasts imply that HERA in its design performance will be sensitive to FDM fractions as small as 1% in the FDM window, and improve over existing bounds for other masses by up to an order of magnitude.