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What concluded the ‘dark ages’ in the early universe? New Webb data has just gotten us nearer to solving the mystery

About 400,000 years post the Big Bang, the universe was a very dark place. The glow of the cosmos’s explosive birth had cooled, and space was packed with dense gas – mostly hydrogen – with no sources of light. Gradually, over hundreds of millions of years, the gas was drawn into clumps by gravity, and finally the clumps grew big enough to ignite. These were the earliest stars. Initially their light didn’t travel too far, as most of it was absorbed by a fog of hydrogen gas. Nevertheless, as more and more stars formed, they produced sufficient light to burn away the fog by “reionising” the gas – fashioning the transparent universe dotted with wonderful points of light we see today.

But precisely which stars produced the light that concluded the dark ages and prompted this so-called “epoch of reionisation”? Researchers have used a gargantuan cluster of galaxies as a magnifying glass to stare at faint relics of the time – and learnt that stars in small, faint dwarf galaxies were likely behind this cosmic-scale transformation.

What concluded the dark ages?

Most astronomers were already in agreement that galaxies were the main force in reionising the cosmos, but it wasn’t known how exactly they did it. We are aware that stars in galaxies should make a lot of ionising photons, but these photons need to escape the gas and dust inside their own galaxy to ionise hydrogen between galaxies. It hasn’t been known what kind of galaxies would be capable of producing and emitting enough photons to get the job done. There are two separate camps among supporters of the galaxy theory.

  • The first camp thinks huge, colossal galaxies produced the ionising photons. There were not many of these galaxies present in the early universe, but each one produced quite a bit of light. So if a certain fraction of that light succeeded to escape, it might have been adequate to reionise the universe.
  • The second camp beleives we are better off ignoring the massive galaxies and concentrating on the huge number of much smaller dwarf galaxies in the early universe. Each one of these would have produced considerably less ionising light, but with the sheer weight of their numbers they could have driven the epoch of reionisation.

A magnifying glass 4 million lightyears wide

Attempting to look at anything in the early universe is very tough. The colossal galaxies are rare, so they are tough to find. Smaller galaxies are far more common but they are too faint, which makes it hard (and expensive) to get high-quality data. To fulfil the desire to look at some of the faintest galaxies around, researchers used a huge group of galaxies termed Pandora’s Cluster as a magnifying glass. The gargantuan mass of the cluster distorts space and time, magnifying the light from objects behind it.

The bright glow of hydrogen

Researchers chose some sources which were about 0.5% of the brightness of our Milky Way and checked them for the telltale glow of ionised hydrogen. These galaxies are so very faint they were only visible thanks to the magnifying effect of Pandora’s Cluster. Observations of the researchers confirmed that these small dwarf galaxies did in fact exist in the very early universe. What’s more, it was confirmed they produced around four times as much ionising light as was considered “normal”. As these galaxies produced so much ionising light, only a small fraction of it would have needed to escape to reionise the universe.

Dwarf galaxies could have played a very large role in reionisation

Previously, researchers believed that around 20% of all ionising photons would need to escape from these smaller dwarf galaxies if they are to be the overriding contributor to reionisation. New data suggests even 5% would be adequate. So now we can assuredly say these smaller dwarf galaxies could have played a very dominant role in the epoch of reionisation.


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