High-redshift test of gravity using enhanced growth of small structures probed by the neutral hydrogen distribution
Future 21-cm intensity mapping surveys such as SKA can provide precise information on the spatial 
distribution of the neutral hydrogen (HI) in the postreionization epoch. This information will allow us to 
test the standard Λ cold dark matter paradigm and with that the nature of gravity. In this work, we employ 
the SHYBONE simulations, which model galaxy formation in fðRÞ modified gravity using the IllustrisTNG 
model, to study the effects of modified gravity on HI abundance and power spectra. We find that the 
enhanced growth low-mass dark matter halos experience in fðRÞ gravity at high redshifts alters the HI 
power spectrum and can be observable through 21-cm intensity mapping. Our results suggest that the HI 
power spectrum is suppressed by ∼13% on scales k ≲ 2h Mpc−1 at z ¼ 2 for F6, a fðRÞ model which 
passes most observational constraints. We show that this suppression can be detectable by SKA1-MID with 
1000 hours of exposure time, making HI clustering a novel test of gravity at high redshift.