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 B0, 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 B0 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 B0, σ(B0)≃2.27×10−6, while SKA1-MID yields substantially tighter constraints, with SKA Band 2 providing the strongest sensitivity among the considered configurations, σ(B0)≃6.37×10−8. 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 B0. 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.
@article{arxiv.2602.05575,
title = {Forecast on $f(R)$ Gravity with HI 21cm Intensity Mapping Surveys},
author = {Yanling Song and Yu Sang and Linfeng Xiao and Boyu Zhang and Bin Wang},
journal= {arXiv preprint arXiv:2602.05575},
year = {2026}
}