The Epoch of Cosmological Recombination
The Epoch of Recombination refers to that period in cosmological evolution when the hot plasma content of the early Universe gradually transitioned to the atomic state as the Universe expands and cools. Understood to have occured somewhat above redshift z = 1100 for hydrogen, z = 2500 for HeII --> HeI and z = 6000 for HeIII --> HeII, the recombination of Hydrogen and Helium is via capture of free electrons by the nuclei. The electrons trickle down the bound quantum states of the atom proceeding over multiple quasi-equilibrium states. As the captured electrons in the atoms make bound-bound transitions, the atoms exchange energy with the radiation content of the Universe in the recombination-line transition frequencies. The atoms also undergo multiple disssociations and recaptures until finally the atom arrives at the ground state. In the case of hydrogen, the step from the first excited state to the ground state is via either the 2s two photon decay or because of emission of Lyman-alpha photons along with their removal from the background radiation via Hubble expansion. All of the bound-bound transitions result in spectral features in the spectrum of the relict radiation and these redshift with cosmological expansion to appear at cm and mm wavelengths today. As shown in the figure below, these additive photons ought to appear as 'tiny' spectral structures in the CMB spectrum, as predicted from our understanding of the physics of recombination. For a review of cosmological recombination and the emergence of the lines see Sunyaev & Chluba 2009.
With the physical environment of the Universe during the Epoch of Recombination being simple, the physics of the processes involved are believed to be fairly well understood. Firm predictions of the signatures of these processes in the CMB spectrum exist, which are limited only by our understanding of atomic processes and associated transition rates. Recent progress in the fields of atomic physics and the recent availability of superior receiver technology have brought the investigation of these predictions to the threshold of experimental astronomy. APSERa is a pioneering attempt to detect the signature of the Epoch of Recombination in the CMB spectrum at cm wavelengths.
Observing the spectral variations due to the recombination of hydrogen and helium would help verify our theoretical understanding of the given period in cosmology. With implications ranging from bettering our understanding of the thermal history of the Universe to providing a path towards improving the determination of some key parameters of the Universe, APSERa will open up unexplored possibilities in Experimental Cosomology!
The Array of Precision Spectrometers for the Epoch of RecombinAtion - APSERa - is a venture to detect recombination lines from the Epoch of Cosmological Recombination. These are predicted to manifest as 'ripples' in wideband spectra of the cosmic radio background (CRB) since recombination of the primeval plasma in the early Universe adds broad spectral lines to the relic Cosmic Radiation. The lines are extremely wide because recombination is stalled and extended over redshift space. The spectral features are expected to be the same over the whole sky.
The project will comprise of an array of 128+ small telescopes that are purpose built to detect a set of adjacent lines from cosmological recombination in the spectrum of the radio sky in the 2-6 GHz range. The radio receivers are being designed and built at the Raman Research Institute, tested in nearby radio-quiet locations and relocated to a remote site for long duration exposures to detect the subtle features in the cosmic radio background arising from recombination. The observing site would be appropriately chosen to minimize RFI in the observing band from geostationary satellites and to be able to observe towards sky regions relatively low in foreground brightness.