Constraints are essential in any scientific endeavour. If a hypothesis predicts that there should be an observable phenomenon, and no trace of it remains, that’s a pretty clear indication that the hypothesis is wrong. And even false assumptions still advance science. This is the case with astronomy and, in particular, explorations of the early universe. A paper by Cambridge researchers and colleagues now puts a particularly useful strain on the development of early galaxies, which has been a hot topic in astronomy of late.
Part of the reason for this hot topic is that, for the first time, we are beginning to have telescopes capable of looking this far. The James Webb Space Telescope raised great hopes of being able to observe the first stars. However, while this instrument is still going through its data-gathering phase, many other high-performance telescopes can also provide information.
One of them is the SARAS3 radio telescope, which is deployed over a lake in India. He monitored some of the light from early galaxies and provided data to the Cambridge team, who analyzed it, looking for a defining feature.
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Known as the 21 cm hydrogen line, it is the spectrum that hydrogen emits when its electron changes state. It has been used consistently in the cosmology of the early universe since its discovery in the 1940s, largely because it can force its way through the cloudy dust that shrouds the bulk of the early universe. This ability also makes it able to effortlessly traverse Earth’s atmosphere, allowing it to be easily detected by a ground-based telescope like SARAS3.
What was notable about the Cambridge article was the absence of the 21cm line. In fact, this common line that is so widely used in other parts of cosmology seemed to be absent from galaxies that existed only 200 million years after the Big Bang.
With this constraint, the authors were able to do two things. One was to refute a tentative finding by another research group. The other was to define some of the important characteristics of these galaxies.
Another research group led by Arizona State University and Massachusetts Institute of Technology published another paper in 2018 using data from the EDGES radio telescope in Australia. This article detailed a possible detection of part of the light of the first galaxies of the universe. It also threw a wrench into one of the earliest models of cosmological expansion, which did not expect such detection. Now, the data in this new paper casts doubt on that conclusion, although further independent confirmation of it is still needed.
When defining the characteristics of galaxies, a crucial factor is their luminosity. The absence of an 21 cm line indicates a simple fact: they were not very bright. Or at least not thousands of times brighter than galaxies are today. If they were, the 21cm line should have been powered enough to show through the haze and be detectable in the SARAS3 data. Other constraints detailed in the paper include the masses of these galaxies, as well as their efficiency in heating hydrogen.
All of this means for cosmology that more data is needed, and two experiments should soon provide it. In addition to the JWST, there is a project called the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH), which is expected to present its results next year. Until then, astronomers are gathering a clearer picture of the early universe, even if it’s because of the lack of an expected signal.
Cambridge – Failure to detect key signal allows astronomers to determine what early galaxies looked and weren’t
Bevins et al – Astrophysical constraints of SARAS 3’s non-detection of the 21 cm signal averaged by the cosmic dawn sky
UT – Understanding the early universe depends on estimating neutron lifetimes
UT – A new simulation recreates an ancient time in the universe that has yet to be seen directly
Image of some of the oldest galaxies in the universe.
Credit – NASA
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