H. K. Vedantham, University of Groningen
The epoch of formation of the first luminous structures (stars and galaxies) called the Comic Dawn, is one of the last unexplored periods in the history of the Universe. A new generation of radio telescopes such as LOFAR aim to revolutionize our understanding of structure formation in the early Universe by directly observing the 21-cm line of hydrogen from this epoch. Due to cosmic expansion, this 21-cm signal reaches us at low radio frequencies (50 to 200 MHz). Properly accounting for and correcting the effects of propagation through an atmospheric layer called the ionosphere is an important outstanding challenge in low frequency observations. In this thesis, I develop a mathematical framework to study ionospheric scintillation (akin to stellar twinkling) and compute its effects on radio observations of the early Universe. I find that although formidable, ionospheric corruptions do not pose a fundamental limit to achieve a detection of the 21-cm signal from the early Universe. In addition, I also demonstrate a new observational technique that uses the Moon as a temperature reference to accurately measure the spectrum of the radio sky. While this technique may in future lead to a detection of the sky-averaged 21-cm signal from the cosmic dawn, I touch upon its potential in the near future to determine lunar crustal composition and temperature at unprecedented depths.
H. K. Vedantham, L. V. E. Koopmans Submitted to MNRAS
In this paper I consider random phase uctuations imposed during wave propagation through a turbulent plasma (e.g. ionosphere) as a source of additional noise in interferometric visibilities, and show that this "speckle noise" can be larger than thermal noise at low radio frequencies (< 200 MHz)
H. K. Vedantham, L. V. E. Koopmans et al. MNRAS July 2015, 450, 2291. DOI: 10.1093/mnras/stv746
We present radio observations of the Moon between 35 and 80 MHz to demonstrate a novel technique of interferometrically measuring large-scale diffuse emission extending far beyond the primary beam (global signal) for the first time.Journal Article »
H. K. Vedantham, L. V. E. Koopmans et al. MNRAS (January 11, 2014) 437 (2): 1056-1069. DOI: 10.1093/mnras/stt1878
The redshifted 21 cm brightness distribution from neutral hydrogen is a promising probe into the cosmic dark ages, cosmic dawn, and re-ionization. LOFAR's Low Band Antennas (LBA) may be used in the frequency range 45 MHz to 85 MHz (30>z>16) to measure the sky averaged redshifted 21 cm brightness temperature as a function of frequency, or equivalently, cosmic redshift.
Vedantham, Harish; Shankar, N. Udaya; Subrahmanyan, Ravi Astrophysical Journal Volume: 745 Issue: 2 Published: 2012 DOI: 176 10.1088/0004-637x/745/2/176
Tomography of redshifted 21 cm transition from neutral Hydrogen using Fourier synthesis telescopes is a promising tool to study the Epoch of Reionization (EoR). Limiting the confusion from Galactic and Extragalactic foregrounds is critical to the success of these telescopes.
H. K. Vedantham Master Thesis, 2009, University of Massachussets, Amherst
This thesis presents the design, development, detailed evaluation and first deployment of a Ka-band interferometer.