Abstract Details
| Name: A.Raghunathan Affiliation: Raman Research Institute Conference ID: ASI2015_698 Title : Investigation of Techniques to Detect Cosmological Backgrounds Authors and Co-Authors : A.Raghunathan Abstract Type : Oral Abstract Category : Thesis Abstract : Epoch of reionzation (EoR) defines a period in the history of universe during which the universe underwent a phase change from neutral to ionized state as a consequence of formation of first stars and galaxies. Hence studying the EoR enhances our scientific understanding of the evolution of the universe. Study could be carried out by probing the state of the neutral hydrogen gas during that epoch. Neutral hydrogen being the most abundant element during that epoch, is expected to undergo spin flip transition at 1420.4 MHz. These transitions are expected to appear as 20 – 30 mK features in the spectrum of the cosmic radio background. Due to the expansion of the universe, the transition frequency gets redshifted by factors 8 – 15 into octave band (87.5 – 175) MHz. Since the universe is assumed to be isotropic and homogeneous, these features ought to be visible in all sky directions. Several pathfinder experiments like the COsmological Reionization Experiment (CORE), Experiment to Detect the Global EoR Signature (EDGES),(BIGHORNS) Broadband Instrument for the Global HydrOgen ReionisatioN Signal have attempted detecting all sky features in red shifted 21cm. All these experiments used single antenna element to measure the power spectrum of the sky background radiation. The power spectrum measured by each one of them was in direct proportion to the noise contributions due to antenna and low-noise amplifier in addition to dominant Galactic and Extragalactic foreground and the EoR signal. Undesired spectral features resembling EoR signal due to the instrumental effects like i) frequency dependent antenna gain ii) frequency dependent LNA noise and iii) impedance mismatch between antenna and LNA. limited the sensitivity of each of them to detect the EoR signal. These limitations in the receiver systems motivated us to develop alternate schemes to overcome them. This thesis describes steps taken to develop instruments and techniques to detect such an all-sky feature. The work includes design and development of specialized purpose built systems for pursuing this goal using both single elements and interferometers. As part of development, i) a new frequency independent fat-dipole antenna was invented to minimize in the output spectrum features arising from the brightness variations over the sky due to antenna gain variation with frequency and ii) Interferometer technique was adopted to reduce the effect of receiver noise in the output spectrum. The inherent insensitivity of the interferometer to uniform sky background was enhanced using a semi-transparent screen in between the two antennas. The system requirements like tolerance to radio frequency interference, dynamic range, linearity were first computed for this science goal; subsequently, the analog receiver chain, calibration noise, and stable frequency reference were designed and built. After signal conditioning, the mutually coherent signal samples in the two arms of the interferometer are multiplied and integrated in a spectro-correlator to produce the power spectrum of the background radiation. A receiver system has been built based on all the stringent design requirement and demonstrated successfully various techniques developed. |