Forecast on $f(R)$ Gravity with HI 21cm Intensity Mapping Surveys

TL;DR

Future HI 21cm intensity mapping surveys, especially SKA1-MID, combined with CMB data, can tightly constrain $f(R)$ gravity parameter $B_0$, testing deviations from General Relativity at low redshifts.

Contribution

This work forecasts the constraints on $f(R)$ gravity parameter $B_0$ achievable with upcoming 21cm IM experiments and CMB data, highlighting their potential to test gravity theories.

Findings

BINGO can constrain $B_0$ to $\simeq 2.27 imes 10^{-6}$.

SKA1-MID Band 2 provides the tightest constraint, $\simeq 6.37 imes 10^{-8}$.

Combining 21cm IM with CMB data significantly improves $B_0$ constraints.

Abstract

Modified gravity theories offer a well-motivated extension of General Relativity and provide a possible explanation for the late-time accelerated expansion of the Universe. Among them, $f(R)$ gravity represents a minimal and theoretically appealing class, characterized by the Compton wavelength parameter $B_0$, which quantifies deviations from General Relativity. In this work, we explore the capability of future neutral hydrogen (HI) 21 cm intensity mapping (IM) observations to constrain $f(R)$ gravity at low redshifts. We perform Fisher-matrix forecasts for $B_0$ and standard cosmological parameters using upcoming 21 cm IM experiments, including BINGO and SKA1-MID (Band 1 and Band 2), both individually and in combination with Planck cosmic microwave background (CMB) priors. We find that even near-term experiments such as BINGO are able to place nontrivial bounds on $B_0$, $\sigma(B_0)\simeq 2.27\times 10^{-6}$, while SKA1-MID yields substantially tighter constraints, with SKA Band 2 providing the strongest sensitivity among the considered configurations, $\sigma(B_0)\simeq 6.37\times 10^{-8}$. We further demonstrate that the combination of low-redshift 21 cm IM data with CMB observations efficiently breaks degeneracies with background cosmological parameters and leads to a significant improvement in the constraints on $B_0$. These results highlight the potential of future HI intensity mapping surveys, in combination with CMB measurements, to provide stringent tests of General Relativity on cosmological scales.