Over the past 30 years, the space observatory has helped scientists discover and refine this rate of acceleration, as well as uncover a mysterious wrinkle that only brand new physics can solve.
Hubble has observed more than 40 galaxies including pulsating stars as well as exploding stars called supernovae to measure even greater cosmic distances. These two phenomena help astronomers mark astronomical distances like mile markers, which indicated the rate of expansion.
In their quest to understand how fast our universe is expanding, astronomers already made an unexpected discovery in 1998: “dark energy”. This phenomenon acts as a mysterious repulsive force that accelerates the rate of expansion.
And there is another twist: an unexplained difference between the rate of expansion of the local universe and that of the distant universe just after the big bang.
Scientists don’t understand the discrepancy, but acknowledge that it’s bizarre and might require new physics.
“You get the most accurate measure of the rate of expansion of the universe from the gold standard of telescopes and cosmic kilometer markers,” Nobel laureate Adam Riess told the Space Telescope Science Institute and eminent professor at Johns Hopkins University in Baltimore, in a report.
“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.”
Decades of observation
The telescope is named after pioneering astronomer Edwin Hubble, who discovered in the 1920s that the distant clouds of the universe were actually galaxies. (He died in 1953.)
Hubble built on the work of astronomer Henrietta Swan Leavitt, who in 1912 discovered periods of brightness in pulsating stars called Cepheid variables. Cepheids act as cosmic mile markers as they periodically brighten and darken in our galaxy and others.
Hubble’s work led to the revelation that our galaxy was one of many, forever changing our perspective and our place in the universe. The astronomer continued his work and discovered that distant galaxies seemed to be moving fast, suggesting that we live in an expanding universe that began with a big bang.
Riess continues to lead SHOES, short for Supernova, H0, for Dark Energy Equation of State, a scientific collaboration that studies the expansion rate of the universe. His team publishes a paper in The Astrophysical Journal that provides the latest update on the Hubble constant, as the rate of expansion is known.
An unresolved gap
Measuring distant objects has created a “cosmic distance scale” that can help scientists better estimate the age of the universe and understand its underpinnings.
Several teams of astronomers using the Hubble telescope have arrived at a Hubble constant value of 73 plus or minus 1 kilometer 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 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 Licia Verde, a cosmologist at the Catalan Institute for Research and Advanced Studies and the Institute of Cosmos Sciences at the University of Barcelona, in a statement.
But the actual expected expansion rate of the universe is slower than what the Hubble telescope observed, according to astronomers using the standard cosmological model of the universe (a theory suggesting the components of the big bang) and measurements taken by Planck from the European Space Agency. missions between 2009 and 2013.
Planck, another space observatory, was used to measure the cosmic microwave background, or residual radiation from the big bang 13.8 billion years ago.
Planck scientists arrived at a Hubble constant of 67.5 plus or minus 0.5 kilometers per second per megaparsec.
This poses an exciting challenge for cosmologists who were once bent on measuring the Hubble constant – and are now wondering what additional physics might help them unravel a new mystery about the universe.
“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 said.