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
post-reionization epoch. This information will allow to test the standard LCDM
paradigm and with that the nature of gravity. In this work, we employ the
SHYBONE simulations, which model galaxy formation in f(R) 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 f(R) 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
$k\lesssim2\,h\,\mathrm{Mpc}^{-1}$ at z=2 for F6, a f(R) 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.
