The Redshifted 21cm Hydrogen line
It is generally acknowledged that the study of the 21cm emission line from neutral hydrogen may be our main hope to study the formation of the first structures during the dark ages and the Epoch of Reionization. This hyperfine transition arises due to the spin-spin interaction between the electron and the proton in hydrogen: the parallel spins (triplet) state has higher energy than the anti-parallel spins (singlet) state. This transition is highly forbidden with an extremely small transition probability of 2.6x10-15
. Despite its low probability, the 21 cm hyperfine transition is one of the main tools of observational astronomy owing to the very large amount of hydrogen in the Universe. The strength of the 21 cm emission or absorption depends on the relative occupation number of the ground state and the occupation number of the excited state. The 21 cm line can be excited through either collisions or the so-called Wouthuysen-Field effect.
The 21 cm emission line emitted during the EoR is redshifted, due to the expansion of the Universe, to a wavelength somewhere in the 2-metre waveband. For example, at a redshift of 9 (550 million years after the Big Bang) the 21 cm line appears at 2.1 m which corresponds to a frequency of ~140 MHz. Measuring the 21 cm radiation emitted from the diffuse intergalactic medium prior to and during the EoR holds great promise for studying the matter distribution at these very early stages of cosmic evolution. It will also tell us a lot about the nature of the first sources and their abundance and distribution as well as on the detailed manner by which the EoR has progressed
Figure 2 The evolution of the brightness temperature of the intergalactic medium. This figure shows a slice through time of the evolution of the reionization process. The x-axis gives the redshift or time and the y-axis gives the spatial coordinate in Mpc/h. At high redshifts the Universe is mostly neutral except for small volumes around the first ionizing sources. As time progresses the number of ionizing sources increases and the bubbles become larger and larger until they percolate to fill the whole Universe. The color scheme is given in the bar above the figure where the black areas show the ionization bubbles. (Thomas et al. 2009).
The most likely physical picture for the epoch of reionization is simple. After the first radiation-emitting objects form they start ionizing their immediate surroundings, forming ionized bubbles which expand until the ionization consumes most of their ionizing photons. As the number of these objects increases so does the number and size of these bubbles which eventually percolate to become volume filling. As said, this picture needs lots of details to be determined, for example what controls the formation of these first objects and how much ionizing radiation does each produce? How do these bubbles expand into the intergalactic medium and which areas do they ionize first, the high density or the low density regions? In order to form a reasonable picture of the process many cosmologists try to simulate the EoR by combining simulations of structure formation that track the formation of dark matter halos with radiation transport simulations that track the evolution of the ionization bubbles.
Figure 2 shows a slice through such a simulation where the scale of the vertical axis is 100 comoving Mpc/h (Megaparsecs per h, where h~0.7 is Hubble constant measured in units of 100 km/s/Mpc) and the horizontal axis shows the redshift (bottom ticks) and time since the Big Bang (upper ticks) (4). To produce this figure we begin with a cosmological structure formation simulation in which we identify possible ionization sources and then use a spherically symmetric radiative transport code to calculate the amount of ionization around each source as a function of redshift (time). Here we assumed that the ionization is driven by black holes with masses ranging from 100 to 107 solar masses at the centre of mini-quasars. Obviously, the detailed history one can get is very different depending on the assumptions of the simulation. For example, for the same amount of ionizing photons stars will produce smaller bubbles but at many more locations.