Maximum amplitude of the high-redshift 21-cm absorption feature
  We examine the maximum possible strength of the global 21-cm absorption dip
on the Cosmic Background Radiation at high-redshift caused by the atomic
intergalactic medium, when the Lyman-$\alpha$ coupling is maximum, assuming no
exotic cooling mechanisms from interactions with dark matter. This maximum
absorption is limited by three inevitable factors that need to be accounted
for: $(a)$ heating by energy transferred from the Cosmic Background Radiation
to the hydrogen atoms via 21-cm transitions, dubbed as 21-cm heating; $(b)$
Ly$\alpha$ heating by scatterings of Ly$\alpha$ photons from the first stars;
$(c)$ the impact of the expected density fluctuations in the intergalactic gas
in standard Cold Dark Matter theory, which reduces the mean 21-cm absorption
signal. Inclusion of this third novel effect reduces the maximum global 21-cm
absorption by $\sim 10\%$. Overall, the three effects studied here reduce the
21-cm global absorption by $\sim 20\%$ at $z \simeq 17$.
