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The Expansion of the Universe Appears Faster Than We Thought

The results of a new study are close, but slightly faster than, the generally accepted rate.
Image: Sherry Suyu, ESA/Hubble, NASA

An international astronomy collaboration, aptly named H0LiCOW, has revealed that the universe might be expanding faster than we thought.

The research was conducted by cosmologists from nine countries including Germany, the United States and Switzerland. They re-examined the Hubble Constant, which calculates the expansion rate of the universe, by looking at a few key galaxies through the Hubble Space Telescope and other telescopes.


Their findings are detailed in a five-part series of papers, the last two of which were released in the Monthly Notices of the Royal Astronomical Society this week. While the group hasn't confirmed why there's a discrepancy, their results show that the expansion rate itself could have changed.

One of the lead researchers, Sherry Suyu (at Germany's Max Planck Institute for Astrophysics) has been involved in the project since 2011. She said that earlier observations about the early universe (for example, from the 2009 Planck experiment that told us much of what we known about the cosmic structure, like dark matter and dark energy) showed an expansion rate that was lower than what H0LiCOW measured.

Christopher Fassnacht, physics professor at UC Davis in California, collected a lot of the data and imaging used in the research. This collaboration dates back to the 90s and 2000s, he said, when small teams debating theories around this "specialized area of astronomy" became slowly aware of each other. In the interest of saving time, they decided to join forces.

"Rather than competing against each other and not getting time on telescopes, for example, we thought that we'd have a more compelling story if…we all worked together towards a common goal," he told Motherboard.

Fassnacht said he is proud the research was done independently, through a "blind analysis" where researchers didn't know the answer to their question until the very end—and still got numbers close to the largely accepted Hubble Constant.


"The danger in doing this is always that you sort of know what the other groups are getting and if you get close to that you say, 'OK, we're done,'" he said. It's like putting your thumbs on the scale. So the exciting part about H0LiCOW was getting numbers different from the Planck team's, which were based on certain assumptions."

Suyu added that further observation is needed for more precise measurements—don't throw everything you know out the window quite yet—but "we might need new physics to explain this."

At this point, I very politely asked Suyu: what the hell do you mean new physics?

When the Planck derived its expansion rate, it needed to assume a certain model, so it assumed the standard cosmological model, she said. It also assumed that our universe was spatially flat, or that the density of dark matter was unchanging.

"There's a standard cosmological model that has emerged in the past decade or so, that says our universe is spatially flat and is composed mostly of dark energy … and cold dark matter," she said. "But if dark energy density actually changes over time, then the value of the expansion rate from the Hubble Constant … could be higher" and agree with H0LiCOW's numbers.

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