Signals from the depths of the cosmos

[Nature India Special Issue: Lighting the way in physics]

One cannot help but wonder about the origin of the twinkling dots that span the night sky. The idea of the birth of the very first stars is intriguing given that the environment they would have formed in would have been very different from today. After their formation, radiation from the stars ionised the hydrogen gas in the Universe. The period, beginning with the birth of the first stars and galaxies followed by the reionisation of the Universe is often referred to as the cosmic dawn and epoch of reionisation¹.

This period lasted roughly from 100 million to 900 million years after the Big Bang. However, it is exceptionally challenging to observe these distant epochs directly. In the absence of observations, we know truly little about this period from the nature of these first sources of radiation to the exact timeline of the processes that occurred within it.

At the Raman Research Institute, the quest to understand the cosmic dawn was initiated by Ravi Subrahmanyan and N. Udaya Shankar. They envisioned telescopes that could conduct sensitive observations to understand the nature of the first stars and galaxies. One of the best ways to do so was to exploit the fact that hydrogen gas is abundant in the Universe. It was well established and observed², that hydrogen gas had a unique radiation emitted at a frequency of 1,420.405 MHz or equivalently at a wavelength of 21 cm. The sources of radiation during the cosmic dawn would imprint their signature in the brightness of this 21 cm signal, changing its shape and strength over cosmological timescales

With this goal in mind, the Shaped Antenna measurement of the background Radio Spectrum (SARAS) experiment was initiated at RRI. It aims to detect the mean brightness of 21 cm radiation using a single antenna with calibratable receivers. SARAS, along with its contemporaries, marked the emergence of small-scale precision experiments towards studying the cosmic dawn. The beauty of such experiments lies in their iterative abilities – they can be designed, developed, deployed and upgraded in a continuous cycle. The experiment requires ingenuity in antenna and electronics design, exceptional care in construction, meticulous selection of an observing site and algorithmic development for data modelling. Such a diverse range of activities require an integrated approach in science and engineering. As a result, the SARAS team comprises scientists, engineers and students who specialise in different domains.

The reason for such a high-precision design becomes clear when one looks at the challenges involved. The signal from the cosmic dawn is expected to arrive on Earth stretched in wavelength to meters and lowered in frequency by the expansion of the Universe to lie in the radio frequency band 50–200 MHz. The celestial signal is exceptionally faint as it is buried in sky radio waves that come to us from the gas in our own Galaxy, the Milky Way, which are a million times brighter. More unfortunate for astronomers is that this cosmic signal is in a radio wavelength band used by terrestrial communications equipment and TV and FM radio stations, which makes detecting the extraterrestrial signal extremely difficult.

In 2018, soon after SARAS 2 became the first experiment to constrain the properties of the first generation of stars via 21 cm observations³, the Experiment to Detect the Global EoR Signature (EDGES) led by astronomers at Arizona State University and Massachusetts Institute of Technology claimed to have detected the global 21 cm signal⁴. The strength of the reported signal from the cosmic dawn was wildly different to theoretical predictions prompting several speculations about how the Universe might be different compared to the accepted current understanding. These speculations included exotic physics, non-standard cosmology, new populations of early galaxies during the early Universe and new models of dark matter that may have resulted in such an unusual signal⁵. However, appreciating that errors in instrument calibration might result in spurious detections in such difficult measurements, cross-verification of the claim became a priority.

SARAS took a different turn in its observations to reach the sensitivity required for such a cross-examination. To ensure a clean measurement with SARAS, its antenna was floated on a raft on water. In an expedition in early 2020, the radio telescope was deployed in lakes in northern Karnataka, on Dandiganahalli Lake and the backwaters of Sharavati, all in India. After a rigorous statistical analysis, SARAS³ did not find any evidence of the signal detected by the EDGES experiment⁶. The presence of the signal was rejected after a careful assessment of the measurement’s uncertainties. The findings implied that the detection reported by EDGES was likely a calibration error. SARAS was indeed the first experiment to reach the required sensitivity to cross-verify the claim of signal detection. This research restored confidence in our understanding of the evolving Universe, re-establishing the prevailing standard cosmological model.

However, astronomers still need to know what the actual signal looks like. Having rejected the EDGES claim, the SARAS experiment is geared towards discovering the true nature of cosmic dawn. Since the publication of these findings, SARAS has undergone a series of upgrades and has conducted observations at a few radio-quiet locations in India. In the meantime, another RRI experiment is preparing to complement this quest for the signal hunt from space with PRATUSH – Probing ReionisATion of the Universe using Signal from Hydrogen. Given the challenges from ground-based observations, PRATUSH will fly in lunar orbit and conduct cosmological observations from the far side of the moon, which is expected to be pristine with no terrestrial radio frequency interference. It is currently in the pre-project studies phase, supported by the Indian Space Research Organisation.

The faint nature of the signal requires meticulous calibration of the telescope and robust cross-verification from different experiments. Therefore, SARAS and PRATUSH conducting observations from vastly different environments with unique challenges form an ideal set-up to look for the elusive 21 cm signal. A robust detection of the signal would help us unravel this last remaining gap in the history of our Universe.