Early Universe emptier than expected

The early Universe is more opaque than today's because it is had far fewer galaxies than expected. Without the stars from galaxies, there is no ultraviolet radiation to make the interstellar gas transparent.

Today the Universe forms a 'cosmic web' (simulation above), where the gas in galaxies makes them opaque, while the voids between them are transparent. The same was not true of the early Universe. Image Credit: TNG Collaboration
Published: August 15, 2018 at 12:00 pm
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One of the least transparent places in the Universe could contain very little matter. The counterintuitive discovery could help astronomers understand how the first galaxies interacted with the early Universe.


Astronomers know that about 12.5 billion years ago (around one billion years after the Big Bang), the Universe was more opaque than it is now, and that this opacity varied wildly from place to place.

“Today, we live in a fairly homogeneous Universe,” says George Becker an assistant processor from the University of California, Riverside.

“If you look in any direction you find, on average, roughly the same number of galaxies and similar properties for the gas between galaxies, the so-called intergalactic gas.

At that early time, however, the gas in deep space looked very different from one region of the Universe to another.”

To track down the cause of the difference, Becker led a team to study a 500 million-lightyear wide region of the early Universe which was particularly opaque and observe it using the Subaru telescope on Mauna Kea in Hawaii.

If the region behaved like the current Universe, then Becker’s team would expect to find a great number of galaxies as these are filled with gas which absorbs light.

However, they found the opposite to be true – there were very few galaxies.

But this might not be as counterintuitive as it first sounds.

Usually gas is kept transparent by its interaction with the ultraviolet in starlight.

If there are fewer galaxies, there’s less starlight and so the gas would be more opaque.

“Normally it doesn’t matter how many galaxies are nearby; the ultraviolet light that keeps the gas in deep space transparent often comes from galaxies that are extremely far away.

That’s true for most of cosmic history, anyway,” says Becker.

“At this very early time, it looks like the ultraviolet light can’t travel very far, and so a patch of the Universe with few galaxies in it will look much darker than one with plenty of galaxies around.”

Astronomers will continue to study the region and others like it to better understand this important time in cosmic history, when the galaxies first began to shine and make the Universe transparent.


“There is still a lot we don’t know about when the first galaxies formed and how they altered their surroundings,” says Becker.


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.


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