Signatures of Modified Gravity on the 21-cm Power Spectrum at Reionisation

TL;DR

This paper investigates how certain modified gravity theories influence the 21-cm brightness temperature spectrum during reionisation, revealing potential observational signatures that could test these theories beyond traditional large-scale structure surveys.

Contribution

It demonstrates that the 21-cm power spectrum can detect signatures of modified gravity models like symmetrons, especially on small, linear scales during reionisation.

Findings

Symmetron models with phase transitions before z~12 are detectable.

Detection prospects are higher for modes parallel to the line of sight.

Small-scale 21-cm spectrum deviations from GR are significant and observable.

Abstract

Scalar modifications of gravity have an impact on the growth of structure. Baryon and Cold Dark Matter (CDM) perturbations grow anomalously for scales within the Compton wavelength of the scalar field. In the late time Universe when reionisation occurs, the spectrum of the 21cm brightness temperature is thus affected. We study this effect for chameleon-f(R) models, dilatons and symmetrons. Although the f(R) models are more tightly constrained by solar system bounds, and effects on dilaton models are negligible, we find that symmetrons where the phase transition occurs before z_* ~ 12 will be detectable for a scalar field range as low as 5 kpc. For all these models, the detection prospects of modified gravity effects are higher when considering modes parallel to the line of sight where very small scales can be probed. The study of the 21 cm spectrum thus offers a complementary approach to testing modified gravity with large scale structure surveys. Short scales, which would be highly non-linear in the very late time Universe when structure forms and where modified gravity effects are screened, appear in the linear spectrum of 21 cm physics, hence deviating from General Relativity in a maximal way.