Light-cone anisotropy in 21-cm fluctuations during the epoch of reionization

The delay in light traveltime along the line of sight generates an anisotropy in the power spectrum of 21-cm brightness fluctuations from the epoch of reionization. We show that when the fluctuations in the neutral hydrogen fraction become non-linear at the later stages of reionization, the light-cone anisotropy becomes of the order of unity on scales ≳50 comoving Mpc. During this period the density fluctuations and the associated anisotropy generated by peculiar velocities are negligible in comparison.