Detecting the relativistic bispectrum in 21cm intensity maps
  We investigate the detectability of leading-order relativistic effects in the
bispectrum of future 21cm intensity mapping surveys. The relativistic signal
arises from Doppler and other line-of-sight effects in redshift space. In the
power spectrum of a single tracer, these effects are suppressed by a factor
$\cH^2/k^2$. By contrast, in the bispectrum the relativistic signal couples to
short-scale modes, leading to an imaginary contribution that scales as $\cH/k$,
thus increasing the possibility of detection. Previous work has shown that this
relativistic signal is detectable in a Stage IV H$\alpha$ galaxy survey. {We
show that the signal is also detectable by next-generation 21cm intensity maps,
but typically with a lower signal-to-noise, due to foreground and telescope
beam effects.
