Astronomers are used to finding that signals suspected to come from alien civilizations actually have more Earth-bound origins. Now the same might have happened with patterns in radio waves originally thought to mark the birth of the first stars, a time known as the “cosmic dawn”.
After the Big Bang, it took the universe a substantial amount of time to sort itself out until circumstances were suitable for the first stars to appear. Determining just how long this was, and finding traces of those first stars, is one of the great quests of astronomy. Consequently, there was considerable excitement in 2018 when researchers reported finding radiation indicative of ultraviolet radiation from stars very early in the universe's development.
More importantly still, the signal detected was surprisingly large, potentially requiring a reworking of the standard model of cosmology to explain it. Now, however, a paper in Nature Astronomy claims, with 95 percent confidence, the signal was a mistake and therefore; “Is not evidence for new astrophysics or non-standard cosmology.”
One of the ways astronomers have conducted the quest for the first stars has been seeking the distinctive spectra produced by hyperfine splitting of neutral hydrogen. This is what we anticipate finding if UV light shone on the primordial hydrogen, causing it to absorb the cosmic background radiation at a wavelength of 21 centimeters.
However, there is plenty of neutral hydrogen in the universe today, interfering with efforts to peer far back in time to close to the Big Bang and find the same thing. Despite this, four years ago, a team led by Dr Judd Bowman of the Arizona State University claimed to have found 21 cm radiation from the cosmic dawn, and, by their admission, that it was at least twice as bright as anyone expected.
Such a large quantity of radiation from so early in time would require us to rethink our picture of the early universe or adjusting our understanding of the cosmic background. Several options have been proposed that would make the early universe unexpectedly cool.
However, Dr Saurabh Singh of Raman Research Institute and co-authors have now attempted to replicate Bowman et al.'s result using the SARAS 3 radiometer. SARAS 3 is a radio antenna of a type that must be floated on a large body of water – in this case, southern Indian lakes, to avoid interference. The authors do not find a match for the peak Bowman found.
If Singh's team is right, it's possible the reported peak was an instrumental error or an illusion created out of the attempts to cancel out the radiation from closer objects. Alternatively, it could have had been caused by something in the telescope's surroundings, addressed by siting SARAS 3 in the middle of a large lake.
Complicating matters further, early neutral hydrogen radiation started out at 21 cm, but would have been so redshifted by the expansion of the universe it would appear to have a much longer wavelength to us. How much it would be shifted depends on when the starlight began. The peak reported in 2018 – and missing in the new observations – is at 78 MHz, or 3.8 meters, matching a start date of 180 million years after the Big Bang.
If we want to find the true 21 cm peak, the authors suggest further work using waterborne instruments like SARAS 3 or getting telescopes even further away from Earthly interference by putting them on the far side of the Moon. They're planning to try the first in Himalayan lakes.
Meanwhile, physicists can stop trying to explain Bowman's results, and get on with all the other things about the universe we don't understand.