Dark Ages, First Light and The Epoch of Cosmological Reionization
Well after comological recombination, the neutral gas is in thermal equilibrium with the radiation content of the universe and hence invisible - this are the `Dark Ages'. On the formation of first stars and galaxies `First Light' emerges. Owing to ionizing radiation from these first collapsed objects the primordial baryons once again transition from neutral to ionized state. This significant change in the state of the gas constitutes a poorly understood milestone in the thermal history of the universe. Reionization of the universe is believed to have occurred between 0.25 and 0.5 billion years after the Big Bang and has become an attractive and productive field of research due to the richness in its astrophysics and the opening of new observational opportunities and windows to this epoch. Ongoing research into this epoch addresses questions concering the nature of first sources of radiation: stars and ultra-dwarf galaxies, physical mechanisms driving the reionization, the evolution in spin-flip 21-cm excitation temperature, the thermal heating of the Inter-Galactic gas, the onset and completion of Reionization, and so on.
Current observational probes of this epoch include Gunn-Peterson absorption in spectra towards distant quasars, the distribution in anisotropy power in linear polarization and total intensity maps of Cosmic Microwave Background Radiation, and limits on kSZE anisotropy power on small angular scales and limits on redshifted 21-cm power at long wavelengths. These already provide first clues to the thermal history of the gas; however, details of formation of first sources of radiation and their interaction with the ambient gas medium at local and global scales is fairly unknown and poorly constrained.
One of the most important tools to study the thermal history of gas at high redshifts is via observations of the spin-flip transition of the Hydrogen atom. This transition occurs between two hyperfine levels of Hydrogen within the 1s ground state, which is split by the coupling interaction between the electron spin and the nuclear spin. This splitting of the hydrogen ground state is of extremely small energy difference and hence results in emission/absorption of photons at the radio wavelength of 21 cm (1420 MHz). Since Hydrogen is the most abundant element in the Universe, observing in redshifted 21 cm constitutes an important means for us to look back at EoR and beyond into the Dark Ages. Further, since various astrophysical parameters go into shaping this signal over cosmic time and redshift, detection of 21 cm from EoR promises to address many of the unresolved questions of these early times. A review of the physics of these epochs and the prospects for 21-cm cosmology is in Pritchard & Loeb 2012. A pictorial representation of a plausible thermal history is in the figure below.
As shown above, the global or all-sky 21 cm signal is a spectral distorsion of 10's to at most about 100 mK signal and this is present in the cosmic radio background in the frequency range 30-200 MHz as a trace additive component. At these long wavelengths the contaminating radio sky has galactic and extragalactic foreground brightness of about 100-10,000 K. Moreover, systematics from within radio-frequency spectrometers and external Radio Frequency Interference (RFI) adds to the challenges in system design and algorithm development needed in the detection of the signal. Towards this key science goal we are working on the experiment SARAS (Shaped Antenna measurement of the background RAdio Spectrum) where we are continually designing, deploying, trialling and improving a spectral radiometer system consisting of a frequency independent antenna, self-calibratable receviers and a broadband precision digital spectrometer.
SARAS is a correlation spectrometer purpose designed for precision measurements of the cosmic radio background and faint features in the sky spectrum at long wavelengths that arise from redshifted 21-cm from gas in the reionization epoch. The system measures the differential temperature between the antenna and an interanl reference, and has a complex switching scheme and observing strategy to cancel systematics and provide calibration of system parameters. Details of the architecture and data analysis procedure adopted in the first version of SARAS is in Patra et al. 2013.
The foregrounds and systematics dominate the observed spectrum, and the antenna and receiver characteristics couple the foreground and systematics to the detector with complex transfer functions that are also different for different contributors to the response. All this will confuse the reionization signal. It is therefore important to minimize instrumental signatures by paying careful attention to the system design, characterization and calibration. Consequently, our major emphasis and work is on devising methods and algorithms for properly calibrating the system, minimize systematics and model the contributors to the system response with mK accuracy so that the cosmological redshifted 21-cm signal may be discerned.
Apart from continuous improvements in the design aspects of antenna, analog and digital receivers, we also test potential observing sites to selection radio-quiet locations that are relatively free of radio frequency interference (RFI).