Collaborators :
Ravi Subrahmanyan, N Udaya Shankar, Jens Chluba, Frank Briggs
Photons emitted during the epochs of Hydrogen and Helium recombination are predicted to appear as additive distortions to the Cosmic Microwave Background spectrum. Detecting and measuring these cosmological recombination lines would help confront our understanding of the thermal and ionization histories of the Universe, serve as a unique probe to measure pre-stellar Helium abundance and provide an independent method to constrain certain cosmological parameters. More details can be found in the review paper by Sunyaev & Chluba (2009) and references there in. The aim of my thesis is to explore the detectability of these weak and broad cosmological signals and develop a prototype element to detect the same. An array of 128 such elements would then comprise APSERa : An Array of Precision Spectrometers for the Epoch of RecombinAtion, an experimental venture at the Raman Research Institute with the science goal of detecting the ripple like weak signals arising from the epoch of recombination. I am currently involved in simutaions to investigate the feasibility of detecting these weak cosmological signals and from the promising initial results I am looking into methods to experimentally detect them. The feasibility study includes calculations of the best frequency for a ground-based detection, calculation of signal-to-noise ratios with receivers of different noise temperatures and developing algorithms for foreground modelin and signal extraction. Along with colleagues at RRI I am looking into various aspects such as antenna desgin, analog front-end architecture, digital back-end design and system integration, calibration towards a fully operational precision spectromter which would form the individual element of APSERa.
There are many challenges to the detection of these cosmological recombination lines. Firstly, they are extremely weak. They are predicted to be about 9 orders of magnitude weaker than the sky (galactic and extragalactic contribution + the Cosmic Microwave Background) making detection a challenging problem. Adding to the challenge is that these lines are extremely broad owing to the fact that recombination was not an instantaneous process but had a (wide) finite span in redshift. So these lines do not 'stick-out' of the continuum like other spectral lines might in typical observations. A third crucial challenge, also affecting other cosmological problems like detecting the redshifted 21 cm line from the epoch of reionization (EoR), is that we do not understand the foreground precisely enough. This is because one would expect that to detect the cosmological recombination lines, one would simply have to fit to the CMB, the foreground (galactic and extraglactic) and subtract them out, leaving behind the desired signal. However if we do not know a-priori the function to fit the foreground with (CMB has a black-body spectrum), we cannot fit the measured sky-spectrum (sky temperature vs. frequency) precisely enough to reveal the buried signal of interest. This poses a great challenge to a detection experiment. In our recent paper we discuss a method for effective foreground subtraction and signal recovery.
Collaborators :
Ravi Subrahmanyan, Juergen Ott, Frank Briggs
The 2p3/2 to 2s1/2 fine structure line of atomic Hydrogen at 9.9 GHz could potentailly serve as an important diagnostic of radiative transfer in star-forming regions (see Dennison et.al (2005)). More importantly, the line transition may be developed to be a probe of cosmological reionization during the epoch of reoinization (EoR), complementary to the redshifted 21-cm line (see Sethi et.al (2005)). Some problems that face the detection of the 2p-2s line are similar to those that face the detection of the weak ripple like signals from the recombination epcoch (see above). The fine-structure transition itself is a blend of hyperfine lines and is expected to be a weak and extremely broad feature, making detection challenging. This line transition has never been detected outside the solar system! Predictions of the line strength and distribution in HII regions and in the cosmological context are uncertain owing primarily to poor knowledge of the relevant radiation background, providing furhter motivation to detect and study this line in the local Universe. My collaborators and I are currently involved in observing carefully selected variety of high emission measure Galactic HII regions for a first detection with moderate spatial resolution. The strength and nature of the line feature is inherently dependent on the Ly-alpha; field and physical parameters of the HII regions, such as its emission measure and ionization fraction. Detection and imaging would be a probe to better understand the physics of the radiative transfer, ionization equilibrium and dust absorption.