Measuring the expansion rate of the universe was one of the Hubble Space Telescope's main goals when it was launched in 1990.
Over the past 30 years, the space observatory has helped scientists discover and refine that accelerating rate -- as well as uncover a mysterious wrinkle that only brand-new physics may solve.
Hubble has observed more than 40 galaxies that include pulsating stars as well as exploding stars called supernovae to measure even greater cosmic distances. Both of these phenomena help astronomers to mark astronomical distances like mile markers, which have pointed to the expansion rate.
In the quest to understand how quickly our universe expands, astronomers already made one unexpected discovery in 1998: "dark energy." This phenomenon acts as a mysterious repulsive force that accelerates the expansion rate.
And there is another twist: an unexplained difference between the expansion rate of the local universe versus that of the distant universe right after the big bang.
Scientists don't understand the discrepancy, but acknowledge that it's weird and could require new physics.
"You are getting the most precise measure of the expansion rate for the universe from the gold standard of telescopes and cosmic mile markers," said Nobel Laureate Adam Riess at the Space Telescope Science Institute and a distinguished professor at the Johns Hopkins University in Baltimore, in a statement.
"This is what the Hubble Space Telescope was built to do, using the best techniques we know to do it. This is likely Hubble's magnum opus, because it would take another 30 years of Hubble's life to even double this sample size."
The telescope was named for pioneering astronomer Edwin Hubble, who discovered in the 1920s that distant clouds in the universe were actually galaxies. (He died in 1953.)
Hubble relied on the work of astronomer Henrietta Swan Leavitt's 1912 discovery of the periods of brightness in pulsating stars called Cepheid variables. Cepheids act like cosmic mile markers as they brighten and dim periodically within our galaxy and others.
Hubble's work led to the revelation that our galaxy was one of many, forever changing our perspective and place in the universe. The astronomer continued his work and discovered that distant galaxies appeared to be moving rapidly, suggesting that we live in an expanding universe that started with a big bang.
The detection of the universe's expansion rate helped lead to the , awarded to Saul Perlmutter, Brian P. Schmidt and Riess "for the discovery of the accelerating expansion of the universe through observations of distant supernovae."
Riess continues to lead SHOES, short for Supernova, H0, for the Equation of State of Dark Energy, a scientific collaboration investigating the universe's expansion rate. His team is publishing a paper in The Astrophysical Journal that provides the latest update on the Hubble constant, as the expansion rate is known.
Measuring distant objects has created a "cosmic distance ladder" that can help scientists better estimate the age of the universe and understand its foundations.
Multiple teams of astronomers using the Hubble telescope have arrived at a Hubble constant value that equals 73 plus or minus 1 kilometre per second per megaparsec. (A megaparsec is one million parsecs, or 3.26 million light years.)
"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 Licia Verde, a cosmologist at the Catalan Institution for Research and Advanced Studies and the Institute of Cosmos Science at the University of Barcelona, in a statement.
But the actual predicted expansion rate of the universe is slower than what the Hubble telescope has observed, according to astronomers utilizing the standard cosmological model of the universe (a theory suggesting the components of the big bang) and measurements taken by the European Space Agency's Planck mission between 2009 and 2013.
Planck, another space observatory, was used to measure the cosmic microwave background, or the leftover radiation from the big bang 13.8 billion years ago.
Planck mission scientists arrived at a Hubble constant of 67.5 plus or minus 0.5 kilometers per second per megaparsec.
The , will be able to observe Hubble's mile markers in sharper resolution and at greater distances, which could contribute to understanding the discrepancy between the two numbers.
It makes for an exciting challenge to cosmologists who were once determined to measure the Hubble constant -- and now find themselves questioning what additional physics may help them unlock a new mystery about the universe.
"Actually, I don't care what the expansion value is specifically, but I like to use it to learn about the universe," Riess said.
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