May 11, 2023 - Science

Tension grows in the debate over the expansion of the universe

Illustration of two microphones with moon and planet on the top

Illustration: Natalie Peeples/Axios

New observations and sharper tools are fueling the debate over a long-sought measurement of how fast the universe is expanding.

Why it matters: A resolution would determine whether scientists' model of the universe is broken or if it's missing something — and would answer key questions about the age, size and history of the universe.

Where it stands: For more than 100 years, scientists have tried to measure just how fast the universe is expanding, a key measurement known as the Hubble Constant.

  • Cosmologists who study changes in the cosmic microwave background (CMB) — an imprint of radiation left on the universe just after the Big Bang — are circling around a measurement of 67 kilometers per second per megaparsec.
  • Others who have used the brightnesses of certain stars to gauge the distance to them are finding the constant's value to be about 74 kilometers per second per megaparsec. Theirs is a measurement of events and features of a more mature universe, many of which were captured by the Hubble Space Telescope.
  • Different studies — of red giant stars, gravitational waves from colliding neutron stars and other late universe measurements — have come up with numbers in between, though many are on the higher end of the range.

The big picture: These measurements from both the early and the late universe are very precise.

  • One or the other could have a problem that is "not yet diagnosed," says Patrick Kelly, a professor of astronomy at the University of Minnesota. It could be a problem with the measurement itself or the models being used to calculate the constant.
  • If all of these measurements are found to be correct, however, new physics would be required to describe the universe.

Driving the news: In a study published Thursday in the journal Science, a team of cosmologists, including Kelly, reports a constant of about 67 kilometers per second per megaparsec.

  • The value was determined by studying how the light arriving at Earth from a supernova is bent by the gravity of a galaxy cluster in its path, what's known as gravitational lensing. These two (or more) views of light from the same source traveling different paths can be used to calculate a value for the Hubble Constant.
  • The finding is consistent with early universe measurements using the cosmic microwave background.

Yes, but: The approach comes with uncertainty.

  • The current study uses different models of the distribution of mass in clusters of galaxies to calculate the time it took for the light from the supernova to reach Earth. Those differences affect the value of the Hubble Constant.
  • The authors of the new study looked at eight different models and selected the two that best matched their observations to arrive at their value. The uncertainty in the models means the value could be as high as 70.7 kilometers per second per megaparsec.

One notable aspect of the study is that it is an independent method that relies on measurements from an astronomical object not used in other studies that look at the CMB and different types of stars.

  • "It's important to measure the Hubble Constant in a number of independent ways," University of Chicago astronomer Wendy Freedman tells Axios.
  • As researchers study gravitational lensing more, the models are likely to be refined, she adds.

Flashback: A century ago, the Hubble Constant was measured to be seven or eight times the range of values cosmologists report today.

  • Three or four decades ago, the values scientists were finding differed by a factor of two.
  • "We're doing much better," Freedman says, adding the values are now within 3% to 5% of each other.

What to watch: The James Webb Space Telescope (JWST) is providing new data about galaxies and stars that could inform the debate.

  • "We have new observations from the James Webb Space Telescope, that should come out later this year that are going to probably be 10 times better than the ones we have from the Hubble Space Telescope," says Adam Riess, an astrophysicist and professor at Johns Hopkins University who received the Nobel Prize in physics in 2011 for his work on the universe's expansion.
  • Large ground-based telescopes in the works — the Giant Magellan Telescope and the Thirty Meter Telescope — will provide even more precise and sensitive data for measuring the universe's expansion.

The bottom line: "I am optimistic this legitimate discussion will wrap up one way or another," says Roger Blandford, an astrophysicist and professor at Stanford University. "It is a knowable thing ... the universe is cooperative in this sense."

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