Probing the small-scale matter power spectrum with large-scale 21-cm data

Abstract

The distribution of matter fluctuations in our universe is key for understanding the nature of dark matter and the physics of the early cosmos. Different observables have been able to map this distribution at large scales, corresponding to wavenumbers k 10 Mpc −1 , but smaller scales remain much less constrained. In this work we study the sensitivity of upcoming measurements of the 21-cm line of neutral hydrogen to the small-scale matter power spectrum. The 21-cm line is a promising tracer of early stellar formation, which took place in small haloes (with masses M ∼ 10 6 − 10 8 M ), formed out of matter overdensities with wavenumbers as large as k ≈ 100 Mpc −1 . Here we forecast how well both the 21-cm global signal, and its fluctuations, could probe the matter power spectrum during cosmic dawn (z = 12−25). In both cases we find that the long-wavelength modes (with k 40 Mpc −1 ) are highly degenerate with astrophysical parameters, whereas the modes with k = (40 − 80) Mpc −1 are more readily observable. This is further illustrated in terms of the principal components of the matter power spectrum, which peak at k ∼ 50 Mpc −1 both for a typical experiment measuring the 21-cm global signal and its fluctuations. We find that, imposing broad priors on astrophysical parameters, a global-signal experiment can measure the amplitude of the matter power spectrum integrated over k = (40−80) Mpc −1 with a precision of tens of percent. A fluctuation experiment, on the other hand, can constrain the power spectrum to a similar accuracy over both the k = (40 − 60) Mpc −1 and (60 − 80) Mpc −1 ranges even without astrophysical priors. The constraints outlined in this work would be able to test the behavior of dark matter at the smallest scales yet measured, for instance probing warm-dark matter masses up to mWDM = 8 keV for the global signal and 14 keV for the 21-cm fluctuations. This could shed light on the nature of dark matter beyond the reach of other cosmic probes.

Observability of dark matter substructure with pulsar timing correlations

“…Smaller structures have instead been observed with strong lensing of quasars [13] and with fluctuations in stellar streams [14], both confirming subhalos down to about 10 7 M . 21 cm cosmology [15] for masses in the range 10 6 − 10 8 M , strong gravitational lensing [16] and stellar wakes [17] for M > 10 5 M , astrometric lensing [18][19][20] for M > 1 M , and disruption of compact stellar systems [21] for M > 5 M , have all been proposed to extend constraints on the HMF to lower masses.…”

Sterile neutrino dark matter in left-right theories

“…Future 21cm cosmology experiments, which can trace early star and galaxy formation at cosmic dawn, are anticipated to probe the matter power spectrum on scales k 50 Mpc −1 . If no suppression on such scales is observed, searches would constrain M 1 14 keV [58], which we show with the dashed green region of figure 2.…”

Ultra-light dark matter

“…To obtain cosmological information from these measurements is a challenge. Some recent studies (Muñoz et al 2020) forecast that the matter power spectrum can be measured from the global 21 cm HI signal for an experiment with parameters close to EDGES (Bowman et al 2018), with a precision of O(10%) integrated over the scales k = (40 − 80) Mpc −1 , after imposing priors on the astrophysical effects like star formation rate and feedback amplitude. They also parametrize the effect of foregrounds, like the Galactic foreground, that plagues all 21 cm experiments and might represent a huge limitations for them if not well mitigated.…”