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GOODS survey of distant universe
A deep-field image from the Great Observatories Origins Deep Survey, or GOODS, shows a scattering of distant galaxies. (Credit: NASA / ESA / GOODS Team / M. Giavialisco / UMass-Amherst)

It looks as if astronomers have been way, way off on their galaxy counts: A new analysis of data from the Hubble Space Telescope suggests that the observable universe holds at least 2 trillion galaxies, which is 10 times the previous estimate.

How could scientists be so far off? The key is that the early universe appears to have had lots of relatively small, faint galaxies. As they merged to form larger galaxies, the population density dwindled.

It took Hubble’s deep-field surveys to register the smaller galaxies that existed far back in time, and it took painstaking analysis to count up a sampling of those galaxies.

The team that did the analysis, led by the University of Nottingham’s Christopher Conselice, reports their findings in a paper to be published in The Astrophysical Journal. They determined that previous estimates — which put the galaxy count at around 200 billion — were at least 10 times too low.

“These results are powerful evidence that a significant galaxy evolution has taken place throughout the universe’s history, which dramatically reduced the number of galaxies through mergers between them — thus reducing their total number,” Conselice said today in a news release. “This gives us a verification of the so-called top-down formation of structure in the universe.”


The galaxy count plays a role not only in deep questions about cosmic evolution, but in one of the most basic questions about the night sky: Why is it dark?

That question was the subject of musings by German astronomer Heinrich Wilhelm Olbers in the early 1800s. If there were an infinite number of stars in the universe, their combined light should be dazzlingly bright, even at night. The fact that it isn’t led to what was once called Olbers’ paradox.

The solution is that there’s a finite number of stars in the observable universe — and that much of the light of distant stars is either absorbed by intergalactic gas and dust, or reddened out of the visual spectrum by the expansion of the universe. (The night sky would be much brighter if our eyes were sensitive to microwave radiation.)

The fact that galaxies have merged over time introduces another factor for the calculations relating to Olbers’ paradox. But more importantly, the findings point to another frontier for astronomers to explore.

“It boggles the mind that over 90 percent of the galaxies in the universe have yet to be studied,” Conselice said. “Who knows what interesting properties we will find when we discover these galaxies with future generations of telescopes? In the near future, the James Webb Space Telescope will be able to study these ultra-faint galaxies.”

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