Impact of inhomogeneous CMB heating of gas on the HI 21-cm signal during
  dark ages
  Observations of redshifted 21-cm signal from neutral hydrogen (HI) appear to
be the most promising probe of the cosmic dark ages. The signal carries
information about the thermal state along with density distribution of the
intergalactic medium (IGM). The cosmic microwave background radiation (CMBR),
through its interaction with charged particles, plays a major role in
determining the kinetic and spin temperature of HI gas in the IGM during dark
ages. A Spatially fluctuating ionization fraction, which is caused by
inhomogeneous recombinations, causes heat transfer from the CMBR to the IGM gas
inhomogeneous. We revisit the impact of this inhomogeneous heat transfer on
spatial fluctuations in the observed HI 21-cm signal over a large redshift
range during dark ages. Our study shows that the effect negatively impacts
fluctuations in the HI spin temperature and results in an enhanced HI 21-cm
power spectrum. We find that the effect is particularly important during the
transition of the gas kinetic temperature being coupled to the CMBR to fully
decoupled from it, i.e., in the redshift range $30 \lesssim z \lesssim 300$. It
is found that, on the average the HI power spectrum, $P_{T_b}(k, z)$ is
enhanced by $\sim 4\%$, $\sim10 \%$ , $\sim 20\%$, and $\sim 30 \%$ at
redshifts $60$, $90$, $140$, and $200$ respectively at $k=0.1 \, {\rm
Mpc}^{-1}$. The effect becomes even more significant at lower values of
$k_{\parallel}^2/k^2$ due to the reduced dominance of the peculiar velocity. It
is observed that the power spectrum is enhanced by $\sim 49\%$ and $\sim 93\%$
at redshifts $140$ and $200$ respectively at $k=0.1 \, {\rm Mpc}^{-1}$ for
$k_{\parallel}^2/k^2=0$. This enhancement has a weak $k$-mode dependence.
