Estimation of cosmological parameters from neutral hydrogen observations of the post-reionization epoch

The emission from neutral hydrogen (HI) clouds in the post-reionization era ($z\le 6$), too faint to be individually detected, is present as a diffuse background in all low frequency radio observations below 1420 MHz. The angular and frequency fluctuations of this radiation ($\sim 1\mathrm{mK}$) are an important future probe of the large-scale structures in the Universe. We show that such observations are a very effective probe of the background cosmological model and the perturbed Universe. In our study we focus on the possibility of determining the redshift-space distortion parameter $\beta$, coordinate distance $r_{\nu }$, and its derivative with redshift $r_{\nu }^{\prime }$. Using reasonable estimates for the observational uncertainties and configurations representative of the ongoing and upcoming radio interferometers, we predict parameter estimation at a precision comparable with supernova Ia observations and galaxy redshift surveys, across a wide range in redshift that is only partially accessed by other probes. Future HI observations of the post-reionization era present a new technique, complementing several existing ones, to probe the expansion history and to elucidate the nature of the dark energy.

Figure 1

Here we plot $C_l(0)$ at redshifts $z=\{1.5,3.0,4.5\}$. The signal decreases monotonically with increasing redshift, so the lowest plot is for the highest redshift. We assume the bias to be $b=1$ throughout.

Figure 2

Here we plot the frequency decorrelation function ${\kappa }_{\ell }(\Delta \nu )$ as a function of $\Delta \nu$, for a fixed redshift $z=3.0$ and $\ell =\{100,1000,10000\}$. The signal declines more sharply for higher value of $\ell$.

Figure 3

Here we plot the parameters $(\beta ,r,p)$ as a function of redshift $z$ for the concordance LCDM model. The parameter $r=r_{\nu }/(6000\mathrm{Mpc})$.

Figure 4

Expected one-sigma fractional errors for parameter estimation at different redshifts for the LCDM model. The curves in each panel correspond, from top to bottom, to the cases A, B, and C, respectively.

Figure 5

Expected one-sigma confidence regions for the parameters ${\Omega }_{m0}$ and ${\Omega }_{k0}$, based on estimated errors for observations of $p$, corresponding to Fig. 4, at $z=1$ and $z=3$.