Case I: Spin temperature is much higher than the CMB temperature

In order to obtain the spherically averaged 3D power spectrum of HI 21-cm signal one needs to average over the 3D volume produced by the observations. The HI 21-cm signal could evolve substantially along the line of- sight (LOS) direction of an observed 3D volume, since the received signal from different planes transverses to the LOS originated from different look-back times and could therefore be statistically different. Almost all previous studies calculate the 3D 21-cm power spectrum without taking this effect into account. Using numerical simulations we investigate this so-called light-cone effect on the spherically averaged 3D 21-cm power spectrum.

The following figures show our results.

Major Results

• LC effect is most dramatic at neutral fraction ~0.8 and ~0.2
• The effect is stronger at large scales ( 100 cMpc)
• A factor of ~1.5 and ~5 enhancement in the power spectrum for volume of the line of sight extents 241 Mpc and 607 Mpc respectively.
• No significant anisotropy is observed due to the LC effect.
• We find that the optimum bandwidth for k = 0.056 Mpc^−1 is ~7.5 MHz. For k = 0.15 and 0.41 Mpc^−1, the optimum bandwidth is ~11 and ~16 MHz, respectively, if we can neglect a 10 per cent change in the power spectra.