Detecting Cosmic Reionization using Bi-Spectrum Phase

TL;DR

This paper introduces a method using bi-spectrum phase measurements to detect the 21cm hydrogen line from the epoch of reionization, reducing calibration challenges and effectively separating the cosmological signal from foreground emissions.

Contribution

The novel approach leverages bi-spectrum phase to bypass the need for precise instrumental calibration in detecting the 21cm signal from cosmic reionization.

Findings

Bi-spectrum phase is unaffected by antenna calibration errors.

Spectral differences allow separation of foregrounds from the cosmological signal.

Detectability of the H I signal in bi-spectrum phase spectra with current instruments.

Abstract

Detecting neutral Hydrogen (H\,{\sc i}) via the 21~cm line emission from the intergalactic medium at $z\gtrsim 6$ has been identified as one of the most promising probes of the epoch of cosmic reionization -- a major phase transition of the Universe. However, these studies face severe challenges imposed by the bright foreground emission from cosmic objects. Current techniques require precise instrumental calibration to separate the weak H\,{\sc i} line signal from the foreground continuum emission. We propose to mitigate this calibration requirement by using measurements of the interferometric bi-spectrum phase. Bi-spectrum phase is unaffected by antenna-based direction-independent calibration errors and hence for a compact array it depends on the sky brightness distribution only (subject to the usual thermal-like noise). We show that the bi-spectrum phase of foreground synchrotron continuum has a characteristically smooth spectrum relative to the cosmological line signal. The two can be separated effectively by exploiting this spectral difference using Fourier techniques, while eliminating the need for precise antenna-based calibration of phases introduced by the instrument, and the ionosphere, inherent in existing approaches. Using fiducial models for continuum foregrounds, and for the cosmological H\,{\sc i} signal, we show the latter should be detectable in bi-spectrum phase spectra, with reasonable significance at $|k_\parallel| \gtrsim 0.5\,h$~Mpc$^{-1}$, using existing instruments. Our approach will also benefit other H\,{\sc i} intensity mapping experiments that face similar challenges, such as those measuring Baryon Acoustic Oscillations (BAO).