The search for the ‘cosmic dawn’ of first light is far from over

Scientists at the Raman Research Institute continue the decade-long quest

Picture used for representational purposes. File | Photo Credit: SARAS 3 Team

In a country of a billion phones, hungry for every bit of radio signal, is a group of scientists looking for spots where one can escape them.

This continuing decade-long quest, led by scientists at the Raman Research Institute (RRI), Bengaluru has taken them multiple times to Ladakh, and to a place aptly named the Timbuktu Collective in Andhra Pradesh, and to lakes in northern Karnataka, with their radio telescope SARAS, which hopes to catch the trace of an extremely elusive sign from space — that of the birth of the first stars or what’s called “the cosmic dawn”. Harvard astronomer Abraham Loeb has remarked that the discovery of such a signal “would be worth two Nobel Prizes” because it would throw light on the structure of the universe in its infancy.

Reverberations of the Big Bang that birthed our universe 13.8 billion years continue to linger in a swathe of radiation called the cosmic microwave background (CMB). At a very specific region in this spectrum, current cosmological models of the universe say, there’s a point where the microwave radiation is a little dim and this, these models say, is because light from the first stars may have made hydrogen extra opaque at specific radio wavelengths.

Several groups around the world have designed custom-made, highly sensitive radio telescopes and are placing them in regions as remote as deserts in Australia to an island in the Antarctic ocean and, if a proposal comes through, in the lunar orbit.

Ravi Subrahmanyan, former Director at the RRI, has led efforts since 2010 using the Shaped Antenna Measurement of the Background Radio Spectrum (SARAS), but an astounding 2018 result from an American group at the Arizona State University propelled several groups, including that of Dr. Subrahmanyan’s, to sharpen their quest.

The EDGES telescope, or the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) that was placed in an Australian desert, recorded an unusual signal that the group claims is the sign of the cosmic dawn. However the signal’s pattern wasn’t shaped in the way cosmological models predicted and since 2018, when the EDGES result was published, there’s a flurry of interpretation on whether the instrument actually detected the holy grail signal, and if it did, what explained its unusual structure.

To test this, the RRI group made an updated version of SARAS, called SARAS-3. Its chief distinguishing characteristic is that, unlike other radio telescopes, it can be deployed on water bodies. The many layers of soil were themselves a source of radio wave contamination for ground based telescopes. Given that the purpose is to detect a highly elusive signal, water — being of uniform layers — would be an ideal medium, the group reckoned, to make such a sensitive measurement.

In 2020, the radio telescope was deployed in lakes in northern Karnataka, on the Dandiganahalli lake and Sharavati backwaters, to detect the EDGES signal.

Following weeks of observations and months of statistical analysis by Saurabh Singh, research scientist at the RRI, SARAS 3 did not find any evidence of the signal claimed by the EDGES experiment. The group’s paper in the journal Nature Astronomy noted that the “profile... is not of astrophysical origin... their best-fitting profile is rejected with 95.3% confidence... Our non-detection bears out earlier concerns and suggests that the profile found... is not evidence for new astrophysics or non-standard cosmology.”

Dr. Singh told The Hindu that the quest for the signature was still on. Following the measurements on the lake, the group is planning to revisit Ladakh and place the telescope in one of the lakes there in the hope of improving their odds of detecting the signal.

In fact, it’s not just any lake but freshwater lakes that are a suitable candidate simply because salinity levels of the water in other lakes could also interfere with the readings. Ladakh’s lakes are by no means a final frontier, Dr. Singh said, as the team is open to prospect more sites — from northeastern India to the deserts of Rajasthan — in its quest. “There’s a lot unknown about how those early stars looked,” Dr. Singh said. “Actually, seeing the signal would reveal more about their composition and how the early universe looked.”

• In a country of a billion phones, hungry for every bit of radio signal, is a group of scientists looking for spots where one can escape them.
• This continuing decade-long quest, led by scientists at the Raman Research Institute (RRI), Bengaluru has taken them to multiple places with hopes to catch the trace of an extremely elusive sign from space — that of the birth of the first stars or what’s called “the cosmic dawn”. 
• Harvard astronomer Abraham Loeb has remarked that the discovery of such a signal “would be worth two Nobel Prizes” because it would throw light on the structure of the universe in its infancy.