Hubble takes another step in the mystery of the expansion rate of the universe
After a nearly 30-year marathon, NASA’s Hubble Space Telescope has calibrated more than 40 space and time “milestone markers” to help scientists accurately measure the rate of expansion of the universe – a quest with a plot twist.
Tracking the rate of expansion of the universe 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 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 rate of expansion as measured in the local universe compared to independent observations just after the big bang, which predict a different expansion value.
The cause of this discrepancy remains a mystery. But the Hubble data, encompassing a variety of cosmic objects that serve as distance markers, supports the idea that something bizarre is going on, possibly involving entirely new physics.
“You get the most accurate measure of the rate of expansion of the universe from 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 that studies the expansion rate of the universe called SH0ES, which stands for supernova, H0, for the dark energy equation of state. “That’s what the Hubble Space Telescope was built for, using the best techniques we know to do it. It’s probably Hubble’s magnum opus, because it would take another 30 years of Hubble’s life to even double that sample size,” Riess said. .
The article by the Riess team, to appear in the Special Focus issue of The Astrophysical Journal reports the completion of the biggest and probably the last major update on the Hubble Constant. The new results more than double the previous sample of cosmic distance markers. His team also reanalyzed all previous data, with the dataset now including more than 1,000 Hubble orbits.
When NASA designed a large space telescope in the 1970s, one of the primary justifications for the extraordinary expense and engineering effort was to be able to resolve Cepheids, stars that periodically brighten and darken, seen at interior of our Milky Way and outer galaxies. Cepheids have long been the gold standard of cosmic mile markers since their usefulness was discovered by astronomer Henrietta Swan Leavitt in 1912. To calculate much greater distances, astronomers use explosive stars called supernovae of type there.
Combined, these objects have built a “cosmic distance scale” across the universe and are essential for measuring the rate of expansion of the universe, called the Hubble constant after Edwin Hubble. This value is essential for estimating the age of the universe and provides a basic test of our understanding of the universe.
Beginning just after the launch of Hubble in 1990, the first series of Cepheid star observations to refine the Hubble constant were undertaken by two teams: the HST Key Project led by Wendy Freedman, Robert Kennicutt, Jeremy Mold and Marc Aaronson , and another by Allan Sandage and collaborators, who used Cepheids as milestone markers to refine distance measurement to nearby galaxies. In the early 2000s, teams declared “mission accomplished” by achieving 10% accuracy 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 on board the Hubble Telescope launched “Generation 2” of Hubble’s constant research as teams worked to refine the value to an accuracy of barely one percent. This was inaugurated by the SH0ES program. Several teams of astronomers using Hubble, including SH0ES, have converged on a constant Hubble value of 73 plus or minus 1 kilometer per second per megaparsec. While other approaches have been used to investigate the question of the Hubble constant, different teams have found values close to the same number.
The SH0ES team includes longtime leaders Dr. Wenlong Yuan from Johns Hopkins University, Dr. Lucas Macri from Texas A&M University, Dr. Stefano Casertano from STScI and Dr. Dan Scolnic from Duke University. The project was designed to put the universe on hold by matching the accuracy of the Hubble constant deduced from studying the cosmic microwave background radiation remaining from the dawn of the universe.
“The Hubble constant is a very special number. It can be used to thread a needle from past to present for an end-to-end test of our understanding of the universe. It took a phenomenal amount of detailed work,” said said Dr Licia Verde, cosmologist at ICREA and ICC-University of Barcelona, talks about the work of the SH0ES team.
The team measured 42 of the supernova markers with Hubble. Because they are seen exploding 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. Riess said: “We have a complete sample of all supernovae accessible to the Hubble Telescope seen over the past 40 years.” Like the lyrics to the song “Kansas City,” from the Broadway musical Oklahoma, Hubble “made as much fur as possible!”
The expansion rate of the universe was predicted to be slower than what Hubble actually sees. By combining the Cosmological Standard Model of the Universe and measurements from the European Space Agency’s Planck mission (which observed the relic 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 SH0ES team estimate of 73.
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, Riess said, a common threshold for taking a serious problem in physics. This discovery unravels what was becoming a nice, crisp picture of the dynamic evolution of the universe. Astronomers are at a loss for an explanation of the disconnect between the expansion rate of the local universe and that of the early universe, but the answer may involve additional physics of the universe.
Such puzzling discoveries have made life more exciting for cosmologists like Riess. Thirty years ago they started measuring the Hubble constant to compare the universe, but now it’s become something even more interesting. “I actually don’t care what the specific value of the expansion is, but I like to use it to learn more about the universe,” Riess added.
NASA’s new Webb Space Telescope will extend Hubble’s work by showing these cosmic markers at greater distances or sharper resolution than Hubble can see.
The Hubble Space Telescope is an international cooperation 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 operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.