ON THE DETECTION OF GLOBAL 21-cm SIGNAL FROM REIONIZATION USING INTERFEROMETERS

Abstract

Detection of the global redshifted 21-cm signal is an excellent means of deciphering the physical processes during the Dark Ages and subsequent Epoch of Reionization (EoR). However, detection of this faint monopole is challenging due to the high precision required in instrumental calibration and modeling of substantially brighter foregrounds and instrumental systematics. In particular, modeling of receiver noise with mK accuracy and its separation remains a formidable task in experiments aiming to detect the global signal using single-element spectral radiometers. Interferometers do not respond to receiver noise; therefore, here we explore the theory of the response of interferometers to global signals. In other words, we discuss the spatial coherence in the electric field arising from the monopole component of the 21-cm signal and methods for its detection using sensor arrays. We proceed by first deriving the response to uniform sky of two-element interferometers made of unit dipole and resonant loop antennas, then extend the analysis to interferometers made of one-dimensional arrays and also consider two-dimensional aperture antennas. Finally, we describe methods by which the coherence might be enhanced so that the interferometer measurements yield improved sensitivity to the monopole component. We conclude (a) that it is indeed possible to measure the global 21-cm from EoR using interferometers, (b) that a practically useful configuration is with omnidirectional antennas as interferometer elements, and (c) that the spatial coherence may be enhanced using, for example, a space beam splitter between the interferometer elements.

Improved measurement of the spectral index of the diffuse radio background between 90 and 190 MHz

“…This approach is being pursued by the Experiment to Detect the Global EoR Signature (EDGES; Bernardi, McQuinn, & Greenhill 2015), Shaped Antenna measurement of background Radio Spectrum (SARAS Patra et al 2013;Singh et al 2015), and the Dark Ages Radio Explorer (DARE; Burns et al 2012). Both methods require careful and accurate subtraction of the sky foreground since the 21 cm signal is three to five orders of magnitude below the foreground.…”

Measuring the deviation from the superposition principle in interference experiments

“…The experimental result reported in this manuscript has several implications in optics as well as related areas of research like Radio Astronomy. In the latter, the community is divided in the sense that while some work in available literature seems to take into account boundary condition effects [22,23], there are others which seem to ignore them [24,25]. We have explored this application in further detail and found that by taking relevant parameters from such experiments [25], we get κ to be of the order of 10 −2 which is definitely not ignorable any more considering that here we are reporting an experiment where we have successfully measured the quantity much above the error bound.…”

Limits on foreground subtraction from chromatic beam effects in global redshifted 21 cm measurements

“…The major efforts include the Experiment to Detect the Global EoR Signature (EDGES) (Bowman & Rogers 2010), Dark Ages Radio Explorer (DARE) (Burns et al 2012), Broadband Instrument for Global Hydrogen Reionisation Signal (BIGHORNS) (Sokolowski et al 2015), Shaped Antenna measurement of background Radio Spectrum (SARAS) (Patra et al 2013;Singh et al 2015), Large-aperture Experiment to detect the Dark Age (LEDA) (Bernardi, McQuinn, & Greenhill 2015), and Sonda Cosmológica de las Islas para la Detección de Hidrógeno Neutro (SCI-HI) (Voytek et al 2014). Several studies have addressed experimental calibration issues (Rogers & Bowman 2012;Patra et al 2013;Bernardi, McQuinn, & Greenhill 2015) as well as the frequency dependence of the antenna beams Vedantham et al 2014;Bernardi, McQuinn, & Greenhill 2015).…”