Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe

Abstract

Observations of the high-redshift Universe with the 21 cm hyperfine line of neutral hydrogen promise to open an entirely new window onto the early phases of cosmic structure formation. Here we review the physics of the 21 cm transition, focusing on processes relevant at high redshifts, and describe the insights to be gained from such observations. These include measuring the matter power spectrum at z~50, observing the formation of the cosmic web and the first luminous sources, and mapping the reionization of the intergalactic medium. The epoch of reionization is of particular interest, because large HII regions will seed substantial fluctuations in the 21 cm background. We also discuss the experimental challenges involved in detecting this signal, with an emphasis on the Galactic and extragalactic foregrounds. These increase rapidly toward low frequencies and are especially severe for the highest redshift applications. Assuming that these difficulties can be overcome, the redshifted 21 cm line will offer unique insight into the high-redshift Universe, complementing other probes but providing the only direct, three-dimensional view of structure formation from z~200 to z~6.

Figures (44)

• Figure 1: Compilation of direct observational constraints on reionization (see also fan06-review). The Ly$\alpha$ forest measurements with errors and the proximity zone point in the lower panel are taken from fan06becker06-tau, the other Ly$\alpha$ forest points from becker06-tau, the proximity zone point in the upper panel from mesinger04wyithe05-prox, the Ly$\alpha$ galaxy constraint from malhotra04furl06-lyagalmalhotra06, and the GRB point from totani06. The shaded box shows the $1\sigma$ errors on the reionization redshift from the 3-year WMAP data spergel06, assuming that it is instantaneous. Note that these are approximate limits (at best) and depend upon a number of theoretical assumptions (see text).
• Figure 2: De-excitation rate coefficients for H-H collisions (solid line) and H-e$^-$ collisions (dashed line). Note that the net rates are also proportional to the densities, so H-H collisions still dominate in a weakly-ionized medium.
• Figure 3: Level diagram illustrating the Wouthuysen-Field effect. We show the hyperfine splittings of the $1S$ and $2P$ levels. The solid lines label transitions that mix the ground state hyperfine levels, while the dashed lines label complementary transitions that do not participate in mixing. From pritchard05.
• Figure 4: Background radiation field near the Ly$\alpha$ resonance at $z=10$, assuming a Voigt line profile. The upper and lower sets are for continuous photons and photons injected at line center, respectively. (The former are normalized to $J_\infty$; the latter have arbitrary normalization.) The solid and dashed curves take $T_K=10$ and $1000 \hbox{K}$, respectively. From furl06-lyheat.
• Figure 5: Decay chains for Ly$\beta$ and Ly$\gamma$. We show Ly$n$ transitions by dashed curves, Ly$\alpha$ by the dot-dashed curve, cascades by solid curves, and the forbidden $2S \rightarrow 1S$ transition by the dotted curve. From pritchard05.