Invisible Hydgrogen is a Quirk of the Expanding Universe

We can see galaxies in the distant Universe because they either emit light – that is, very bright and hot galaxies – or because they absorb light from even more distant objects. These “absorbing galaxies” can tell us about the presence of cool hydrogen gas (i.e. a few degrees above absolute zero), which is pivotal to the process of star formation. Stars form from large clouds of hydrogen which collapse under their own gravity, condensing into a much denser star, igniting nuclear fusion and causing the star to shine. While generally, we can detect hydrogen through its characteristic 21cm radio-band absorption, observations over the last decade with the world’s largest radio telescopes have failed to do so in the most distant galaxies. These studies therefore suggest that the gas in them is much warmer than in present-day (i.e. near-by) galaxies, thus hindering star formation.

The detection of the 21cm signal is, however, not only subject to the temperature of the gas, but also on how well the absorbing galaxy intercepts the emitted light. There is a quirk, due to our expanding Universe, that when looking at distant objects (back to when the Universe was more than half its present age), the further an object is away, the larger it appears. This is because the Universe was much smaller in the past when the light from the distant galaxies was emitted.

Dr Stephen Curran from CAASTRO at the University of Sydney has shown that this effect can account for the low 21cm detection rate. This means that the invisibility of cool hydrogen gas in these galaxies is an illusion and not intrinsic to the galaxy. Contrary to previous consensus, this means that the gas is cool enough to form the stars which populated the younger Universe. So distant galaxies are not that hot after all!


S.J. Curran “The Geometry Effects of an Expanding Universe on the Detection of Cool Neutral Gas at High Redshifts”

The Astrophysical Journal Letters 748 L18 


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