There's a mystery in our universe's expansion rate and the Hubble Space Telescope is on the case

This image collection based on Hubble Space Telescope data features galaxies hosting both Cepheid variables and supernovas. Such objects help to chart the universe's expansion.
This image collection based on Hubble Space Telescope data features galaxies hosting both Cepheid variables and supernovas. Such objects help to chart the universe's expansion. (Image credit: NASA, ESA, Adam G. Riess (STScI, JHU))

Scientists have a new, more accurate, measurement of the expansion of the universe thanks to decades worth of data from the Hubble Space Telescope.

The new analysis of data from the 32-year-old Hubble Space Telescope continues the observatory's longstanding quest to better understand how quickly the universe expands, and how much that expansion is accelerating.

The number astronomers use to measure this expansion is called the Hubble Constant (not after the telescope but after astronomer Edwin Hubble who first measured it in 1929). The Hubble Constant is a tough one to pin down given that different observatories looking at different zones of the universe have delivered different answers. But a new study expresses confidence that Hubble's most recent effort is precise for the expansion it sees, although there is still a difference from other observatories. 

The new study confirms previous expansion rate estimates based on Hubble observations, showing an expansion of roughly 45 miles (73 kilometers) per second per megaparsec. (A megaparsec is a measurement of distance equal to one million parsecs, or 3.26 million light-years.)

Related: The best Hubble Space Telescope images of all time!

"Given the large Hubble sample size, there is only a one-in-a-million chance astronomers are wrong due to an unlucky draw ... a common threshold for taking a problem seriously in physics," NASA said in a statement on Thursday (May 19), paraphrasing Nobel Laureate and study lead author Adam Riess. 

Riess has affiliations at the Space Telescope Science Institute (STScI) that manages Hubble, as well as the Johns Hopkins University in Baltimore, Maryland.

Riess and collaborators received the Nobel in 2011 after Hubble and other observatories confirmed that the universe was accelerating in its expansion. Riess calls this latest Hubble effort a "magnum opus" given that it draws upon practically the telescope's entire history, 32 years of space work, to deliver an answer.

Hubble's data nailed down its observed expansion rate under a program called SHOES (Supernova, H0, for the Equation of State of Dark Energy.) The dataset doubles a previous sample of measurements and also includes more than 1,000 Hubble orbits, NASA stated. The new measurement is also eight times more precise than expectations for Hubble's capabilities. 

Efforts to measure how fast the universe is expanding usually focus on two distance markers. One of them are the Cepheid stars, variable stars that brighten and dim at a constant rate; their utility has been known since 1912, when astronomer Henrietta Swan Leavitt marked their importance in imagery she was reviewing.

Cepheids are good for charting distances that are inside the Milky Way (our galaxy) and in nearby galaxies. For further distances, astronomers rely upon Type 1a supernovas. These supernovas have a consistent luminosity (inherent brightness), allowing for precise estimates of their distance based on how bright they appear in telescopes.

In the new study, NASA stated, "the team measured 42 of the supernova milepost markers with Hubble. Because they are seen exploding at a rate of about one per year, Hubble has, for all practical purposes, logged as many supernovae as possible for measuring the universe's expansion." (Again, Hubble has been in space for about 32 years, having launched on April 24, 1990; a mirror flaw that hindered early work was addressed by astronauts in December 1993.)

But the expansion rate still does not have full agreement across different efforts. The new study says Hubble's measurements are roughly 45 miles (73 kilometers) per second per megaparsec. But when taking into account observations of the deep universe, the rate slows down to about 42 miles (67.5 kilometers) per second per megaparsec.

Deep universe observations rely principally upon measurements by the European Space Agency's Planck mission, which observed the "echo" of the Big Bang that formed our universe. The echo is known as the cosmic microwave background. NASA said astronomers are "at a loss" to figure out why there are two different values, but suggested we may have to rethink basic physics.

Riess said it is best to see the expansion rate not for its exact value at its time, but its implications. "I don't care what the expansion value is specifically, but I like to use it to learn about the universe," Riess said in the NASA statement.

More measurements are expected to come in the forthcoming 20 years from the James Webb Space Telescope, which is completing commissioning work in deep space ahead of looking at some of the first galaxies. Webb, NASA said, will look at Cepheids and Type 1a supernovas "at greater distances or sharper resolution than what Hubble can see." That may in turn refine Hubble's observed rate.

A paper based on the research will be published in the Astronomical Journal. A preprint version is available on

This article was corrected April 11, 2023 with correct measurement data.

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: