Observations of 21-cm signal

The global or mean 21-cm signal refers to the absorption signal averaged over the whole sky and is shown in terms of the (negative) brightness temperature in Fig 2 and 3. Although current telescopes have the sensitivity to observe this signal the foreground removal may prove to be very challenging.

Figure 2: Brightness temperature as a function of actual redshift (z). Upper panel shows $T_b$ with different values of $\alpha$. Bottom panel shows the fractional change $(T_b\left (\alpha \right ) - T_b)/T_b$.

Figure 3: Brightness temperature as a function of observed redshift, $z_{obs}\equiv \nu _{21}^{now}/\nu _{obs} - 1$. Upper panel shows $T_b$ with different values of $\alpha$. Bottom panel shows the fractional change $(T_b\left (\alpha \right ) - T_b)/T_b$.

There are also fluctuations in the 21-cm signal due to fluctuations in the baryon density, temperature and spin temperature The ionization fraction will contribute negligibly. Thus we can measure the spatial angular power spectrum of 21-cm radiation and that too will depend on the value of $\alpha$ during the dark ages (Fig 4 and 5). Foreground removal, though still challenging, may be more feasible in this case utilizing the differences in the statistical properties of the foregrounds and the 21-cm signal.

Figure 4: Angular power spectrum compared at actual redshift (z). Upper panel shows the angular power spectrum $\sqrt {l(l+1)C_l/2\pi }$ at several redshifts. Bottom panel shows $(\sqrt {C_l\left (\delta \alpha =-2\%\right )}-\sqrt {C_l})/\sqrt {C_l}$ for the same redshifts.

Figure 5: Angular power spectrum compared at observed redshift, $z_{obs}\equiv \nu _{21}^{now}/\nu _{obs} - 1$. Upper panel shows the angular power spectrum $\sqrt {l(l+1)C_l/2\pi }$ at several redshifts. Bottom panel shows $(\sqrt {C_l\left (\delta \alpha =-2\%\right )}-\sqrt {C_l})/\sqrt {C_l}$ for the same redshifts.