Reionization on Large Scales IV: Predictions for the 21 cm signal
  incorporating the light cone effect
  We present predictions for the 21 cm brightness temperature power spectrum
during the Epoch of Reionization (EoR). We discuss the implications of the
"light cone" effect, which incorporates evolution of the neutral hydrogen
fraction and 21 cm brightness temperature along the line of sight. Using a
novel method calibrated against radiation-hydrodynamic simulations, we model
the neutral hydrogen density field and 21 cm signal in large volumes ($L = 2$
Gpc/$h$). The inclusion of the light cone effect leads to a relative decrease
of about 50% in the 21 cm power spectrum on all scales. We also find that the
effect is more prominent at the midpoint of reionization and later. The light
cone effect also can introduce an anisotropy along the line of sight. By
decomposing the 3D power spectrum into components perpendicular to and along
the line of sight, we find that in our fiducial reionization model, there is no
significant anisotropy. However, parallel modes can contribute up to 40% more
power for shorter reionization scenarios. The scales on which the light cone
effect is relevant are comparable to scales where one measures the baryon
acoustic oscillation. We argue that due to its large comoving scale and
introduction of anisotropy, the light cone effect is important when considering
redshift space distortions and future application to the Alcock-Paczynski test
for the determination of cosmological parameters.
