Scientists have a new, more accurate measure of the expansion of the universe thanks to decades of data from the Hubble Space Telescope.
The new analysis of data from the 32-year-old Hubble Space Telescope continues the observatory’s long-running quest to better understand how fast the universe is expanding 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). Hubble’s constant is difficult to pin down since different observatories examining different areas of the universe have provided different answers. But a new study expresses confidence that Hubble’s most recent effort is accurate for the expansion it sees, although there is still a difference compared to other observatories.
The new study confirms previous estimates of the expansion rate based on Hubble observations, showing an expansion of about 45 miles (73 kilometers) per megaparsec. (A megaparsec is a measure of distance equal to one million parsecs, or 3.26 million light-years.)
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“Given Hubble’s large sample size, there’s only a one-in-a-million chance that astronomers are wrong due to an unlucky coin toss…a common threshold for taking a problem seriously. in physics,” NASA said in a statement Thursday. (May 19), paraphrasing Nobel laureate and study lead author Adam Riess.
Riess has affiliations with the Space Telescope Science Institute (STScI) which operates Hubble, as well as Johns Hopkins University in Baltimore, Maryland.
Riess and his collaborators received the Nobel Prize in 2011 after Hubble and other observatories confirmed that the universe was accelerating its expansion. Riess calls this latest Hubble effort a “magnum opus” given that it draws on virtually the telescope’s entire history, 32 years of space work, to provide an answer.
Hubble data determined its observed expansion rate under a program called SHOES (Supernova, H0, for the dark energy equation of state.) The dataset doubles a sample of previous measurements and also includes more than 1,000 Hubble orbits, NASA said. The new measurement is also eight times more accurate than expected for Hubble’s capabilities.
Efforts to measure the expansion rate of the universe generally focus on two distance markers. One of these are the Cepheid stars, variable stars that brighten and darken at a constant rate; their usefulness has been known since 1912, when astronomer Henrietta Swan Leavitt pointed out their importance in the imagery she was examining.
Cepheids are good at plotting distances within the Milky Way (our galaxy) and nearby galaxies. For greater distances, astronomers rely on type 1a supernovae. These supernovae have a constant luminosity (inherent luminosity), allowing accurate estimates of their distance based on their brightness in telescopes.
In the new study, NASA said: “The team measured 42 of the supernova milestone markers with Hubble. Because they explode at a rate of about one per year, Hubble has, for all intents and purposes, recorded as many supernovae as possible to measure the expansion of the universe.” (Again, Hubble has been in space for about 32 years, having been launched on April 24, 1990; a mirror flaw that hampered early work was resolved by astronauts in December 1993.)
But the expansion rate still does not have a complete agreement between the various efforts. The new study says Hubble’s measurements are about 45 miles (73 kilometers) per megaparsec. But taking into account observations from the deep universe, the rate slows to about 42 miles (67.5 kilometers) per megaparsec.
Observations of the deep universe rely primarily on measurements from 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 were “lost” to understand why there are two different values, but suggested we may need to rethink basic physics.
Riess said the rate of expansion is best viewed not for its exact value at its time, but for its implications. “I don’t care what the specific value of the expansion is, but I like to use it to learn more about the universe,” Riess said in the NASA statement.
More measurements are expected in the next 20 years by the James Webb Space Telescope, which is completing deep-space commissioning work before examining some of the first galaxies. Webb, NASA said, will examine Cepheids and Type 1a supernovae “at greater distances or sharper resolution than Hubble can see.” This in turn can refine Hubble’s observed rate.
An article based on the research will be published in the Astronomical Journal. A preprinted version is available at arXiv.org.
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