The gravitational and lensing-ISW bispectrum of 21cm radiation
  Cosmic Microwave Background experiments from COBE to Planck, have launched
cosmology into an era of precision science, where many cosmological parameters
are now determined to the percent level. Next generation telescopes, focussing
on the cosmological 21cm signal from neutral hydrogen, will probe enormous
volumes in the low-redshift Universe, and have the potential to determine dark
energy properties and test modifications of Einstein's gravity. We study the
21cm bispectrum due to gravitational collapse as well as the contribution by
line of sight perturbations in the form of the lensing-ISW bispectrum at
low-redshifts ($z \sim 0.35-3$), targeted by upcoming neutral hydrogen
intensity mapping experiments. We compute the expected bispectrum amplitudes
and use a Fisher forecast model to compare power spectrum and bispectrum
observations of intensity mapping surveys by CHIME, MeerKAT and SKA-mid. We
find that combined power spectrum and bispectrum observations have the
potential to decrease errors on the cosmological parameters by an order of
magnitude compared to Planck. Finally, we compute the contribution of the
lensing-ISW bispectrum, and find that, unlike for the cosmic microwave
background analyses, it can safely be ignored for 21cm bispectrum observations.
