Discovery Links Early Opaque Universe to Galaxy Scarcity

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Team led by UC Riverside astronomer George Becker used the Subaru telescope to make the discovery

Discovery Links Early Opaque Universe to Galaxy Scarcity


RIVERSIDE, Calif. ( — A team of astronomers led by George D. Becker at the University of California, Riverside, has made a surprising discovery: 12.5 billion years ago, the most opaque place in the universe contained relatively little matter.

It has long been known that the universe is filled with a web-like network of dark matter and gas.  This “cosmic web” accounts for most of the matter in the universe, whereas galaxies like our own Milky Way make up only a small fraction. Today, the gas between galaxies is almost totally transparent because it is kept ionized —electrons detached from their atoms— by an energetic bath of ultraviolet radiation.

Over a decade ago, astronomers noticed that in the very distant past —roughly 12.5 billion years ago, or about 1 billion years after the Big Bang— the gas in deep space was not only highly opaque to ultraviolet light, but its transparency varied widely from place to place, obscuring much of the ultraviolet light emitted by distant galaxies.

The a few years ago, a team led by Becker, then at the University of Cambridge, found that these differences in opacity were so large that either the  amount of gas itself, or more likely the radiation in which it is immersed, must vary substantially from place to place.

“Today, we live in a fairly homogeneous universe,” said Becker, an expert on the intergalactic medium, which includes dark matter and the gas that permeates the space between galaxies. “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 find out what created these differences, the team of University of California astronomers from the Riverside, Santa Barbara, and Los Angeles campuses turned to one of the largest telescopes in the world: the Subaru telescope on the summit of Mauna Kea in Hawaii. Using its powerful camera, the team looked for galaxies in a vast region, roughly 300 million light years in size, where they knew the intergalactic gas was extremely opaque.

For the cosmic web, more opacity means more gas, and hence more galaxies. But the team found the opposite: this region contained far fewer galaxies than average, which could explain the region’s opacity. Because the gas in deep space is kept transparent by the ultraviolet light from galaxies, fewer galaxies nearby might make it murkier.

“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,” Becker said.  “At this very early time, it looks like the UV 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.”

This discovery, reported in theAugust 2018 issue of the The Astrophysical Journal, may eventually shed light on another phase in cosmic history. In the first billion years after the Big Bang, ultraviolet light from the first galaxies filled the universe and permanently transformed the gas in deep space. Astronomers believe that this ocurred earlier in regions with more galaxies, meaning the large fluctuations in intergalactic radiation inferred by Becker and his team may be a relic of this patchy process, and could offer clues to how and when it occurred.

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

By studying both galaxies and the gas in deep space, astronomers hope to get closer to understanding how this intergalactic ecosystem took shape in the early universe.

The research was funded by the National Science Foundation and NASA. Becker was joined in the research by Frederick B. Davies of UC Santa Barbara; Steven R. Furlanetto and Matthew A. Malkan of UCLA; and Elisa Boera and Craig Douglass of UCR.


The paper was published in The Astrophysical Journal and can be found here.


Computer simulation of a region of the universe wherein a low-density “void” (dark blue region at center) is surrounded by denser structures containing numerous galaxies (orange/white).  The research done by Becker and his team suggests that early in cosmic history, these void regions would have been the murkiest places in the universe even though they contained the least amount of dark matter and gas.
Credit: TNG Collaboration.