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NASA’s Hubble Space Telescope has completed a nearly 30-year marathon and has calibrated more than 40 space-time “milestone markers” to help scientists accurately measure the universe’s expansion rate — a quest with a plot twist.
The pursuit of the universe’s expansion rate began in the 1920s with measurements by astronomers Edwin P. Hubble and Georges Lemaître. In 1998, this led to the discovery of ‘dark energy’, a mysterious repulsive force that is accelerating the expansion of the universe. In recent years, thanks to data from Hubble and other telescopes, astronomers have found another twist: a discrepancy between the expansion rate as measured in the local universe compared to independent observations from just after the Big Bang, which predict a different expansion value.
The cause of this discrepancy remains a mystery. But Hubble data, which includes a variety of cosmic objects that serve as distance markers, supports the idea that something strange is going on, possibly with brand new physics.
“You get the most accurate measure of the universe’s expansion rate using the gold standard of telescopes and cosmic mile markers,” said Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University in Baltimore, Maryland.
Riess leads a scientific collaboration investigating the expansion rate of the universe called SHOES, which stands for Supernova, H0, for the comparison of the dark energy state. “This is what the Hubble Space Telescope was built for, using the best techniques we know to do it. This is probably Hubble’s magnum opus, because it would take another 30 years of Hubble’s life to even double this sample size, ” said Riss .
The paper from Riess’s team, to be published in the Special Focus issue of The Astrophysical Journal, reports on the completion of the largest and likely last major update on the Hubble constant. The new results are more than double the previous sample of cosmic distance markers. His team also re-analyzed all previous data, with the entire data set now spanning more than 1,000 Hubble orbits.
When NASA devised a large space telescope in the 1970s, one of the main justifications for the cost and extraordinary engineering effort was being able to resolve Cepheids, stars that periodically brighten and dim and are seen in our Milky Way and external galaxies. Cepheids have long been the gold standard for cosmic mile markers since their usefulness was discovered by astronomer Henrietta Swan Leavitt in 1912. To calculate much greater distances, astronomers use exploding stars called Type Ia supernovae.
Combined, these objects built a “cosmic distance ladder” across the universe and are essential for measuring the universe’s rate of expansion, named the Hubble constant after Edwin Hubble. That value is crucial for estimating the age of the universe and is a basic test for our understanding of the universe.
Immediately after the launch of Hubble in 1990, the first series of observations of Cepheid stars to refine the Hubble constant was conducted by two teams: the HST Key Project led by Wendy Freedman, Robert Kennicutt and Jeremy Mould, Marc Aaronson and a others by Allan Sandage and collaborators, who used Cepheid variables as milestones to refine distance measurements to nearby galaxies. By the early 2000s, the teams declared “mission accomplished” by achieving an accuracy of 10 percent for the Hubble constant, 72 plus or minus 8 kilometers per second per megaparsec.
In 2005 and again in 2009, the addition of powerful new cameras aboard the Hubble telescope launched “Generation 2” of Hubble’s constant research, as teams set out to refine the value to an accuracy of just one percent. This was inaugurated by the SHOES program. Several teams of astronomers using Hubble, including SHOES, have converged on a Hubble constant value of 73 plus or minus 1 kilometer per second per megaparsec. While other approaches have been used to examine the Hubble constant question, several teams have come up with values close to the same number.
The SHOES team consists of longtime leaders Dr. Wenlong Yuan of Johns Hopkins University, Dr. Lucas Macri of Texas A&M University, Dr. Stefano Casertano from STScI and Dr. Dan Scolnic of Duke University. The project was designed to support the universe by matching the precision of the Hubble constant derived from studying the cosmic microwave background radiation left over after the universe’s dawn.
“The Hubble constant is a very special number. It can be used to thread a needle from the past to the present for an end-to-end test of our understanding of the universe. This took a phenomenal amount of detailed work said Dr. Licia Verde, a cosmologist at ICREA and the ICC University of Barcelona, speaks about the work of the SHOES team.
The team measured 42 of the supernova milestone markers using Hubble. Since they are seen exploding at the rate of about one a year, Hubble has, for all practical purposes, recorded as many supernovas as possible to measure the expansion of the universe. Riess said, “We have a complete sample of all the supernovae that have been accessible to the Hubble telescope over the past 40 years.” Like the lyrics to the song “Kansas City” from the Broadway musical Oklahomahas made Hubble “as furry as can be!”
Strange physics?
It was predicted that the universe’s expansion rate would be slower than what Hubble actually sees. Combining the standard cosmological model of the Universe with measurements from the European Space Agency’s Planck mission (which observed the cosmic microwave background from 13.8 billion years ago), astronomers predict a lower value for the Hubble constant: 67, 5 plus or minus 0.5 kilometers per second per megaparsec, compared to the SHOES team’s estimate of 73.
Given Hubble’s large sample size, there’s only a one-in-a-million chance that astronomers are wrong because of an unfortunate draw, Riess said, a common barrier to taking a problem seriously in physics. This finding untangles what was becoming a nice and tidy picture of the dynamic evolution of the universe. Astronomers are unable to explain the discrepancy between the expansion rate of the local universe and the primordial universe, but the answer may involve additional universe physics.
Such confusing findings have made life more exciting for cosmologists like Riess. Thirty years ago they started measuring the Hubble constant to benchmark the universe, but now it’s gotten something even more interesting. “Actually, I don’t care what the expansion value is specifically, but I like to use it to learn more about the universe,” Riess added.
NASA’s new Webb Space Telescope will expand Hubble’s work by showing these cosmic landmark markers at greater distances or sharper resolution than what Hubble can see.
The Hubble Space Telescope is an international collaboration project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, operates the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is administered for NASA by the Association of Universities for Research in Astronomy in Washington, DC
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