Astronomers sharpen universe’s expansion rate, deepening cosmic mystery

A team of astronomers using a variety of ground and space-based telescopes — including W. M. Keck Observatory atop Mauna Kea on the Big Island — made one of the most precise independent measurements yet of how fast the universe is expanding.

The new measurement further deepens the divide on one of the biggest mysteries in modern cosmology.

Researchers using data gathered from Keck Observatory’s Cosmic Web Imager as well as NASA’s James Webb Space Telescope, the Hubble Space Telescope, the Very Large Telescope and European Organisation for Astronomical Research in the Southern Hemisphere independently confirmed that the universe’s current rate of expansion — known as the Hubble constant — does not match values predicted by measurements from the universe when it was much younger.

The finding strengthens what scientists call the “Hubble tension,” a cosmic disagreement that could point to new physics governing the universe.

“What many scientists are hoping is that this may be the beginning of a new cosmological model,” said distinguished professor of physics and astronomy at University of California, Los Angeles and study co-author Tommaso Treu in a release about the new measurements.

The study is published in Astronomy and Astrophysics.

“This is the dream of every physicist. Find something wrong in our understanding so we can discover something new and profound,” added assistant professor of physics at the Stony Brook University and one of the study’s corresponding authors Simon Birrer.

A constant in question questioned constantly

Coined by astronomer Edwin Hubble — who first calculated it in 1929 — the Hubble Constant is the rate at which the universe expands. This number reveals not only the universe’s current speed of growth, but also its age and history.

Nearly a century later, scientists still can’t agree on its exact value.

The Hubble Constant can be measured in two ways — one probing the universe at early times and another probing the universe at times near today.

The early universe probe, which uses cosmological models to indirectly provide the current expansion rate of the universe, favors an expansion rate of ~67 km/s/Mpc; and the late (nearby) universe probe, which measures the local universe as it exists today favors an expansion rate of 73 km/s/Mpc.

Measurements based on the nearby universe differ from predictions drawn from the early universe, resulting in what is famously known as the Hubble Tension.

Confirming this tension would force scientists to rethink the very makeup of the cosmos; perhaps revealing new particles, or evidence for an “early dark energy” phase that briefly accelerated expansion after the Big Bang.

Astronomers stress the importance of multiple independent methods to crosscheck the result because the implications are so profound.

“This is significant in that cosmology as we know it may be broken,” said Keck Observatory Chief Scientist and Deputy Director John O’Meara in the release. “If it is true that the Hubble Tension isn’t a mistake in the measurements, we will have to come up with new physics.”

A new way to measure the universe

The team used a method called time-delay cosmography to make this precise measurement.

Much like a funhouse mirror bends and distorts reflections, massive galaxies bend the light of more distant galaxies and quasars, producing multiple images of the same object.

Animation of the findings by researchers who used time-delay cosmography to independently confirm that the universe’s current rate of expansion, known as the Hubble constant, does not match values predicted from measurements from the universe when it was much younger. (Credit: W. M. Keck Observatory/Adam Makarenko)

When the distant object’s brightness changes, astronomers can measure how long it takes those changes to appear in each image. Those “time delays” act like cosmic yardsticks — allowing scientists to calculate distances across the universe and, ultimately, determine how fast it’s expanding.

Keck Observatory’s Cosmic Web Imager’s powerful spectroscopy was essential to the measurement. By observing the motion of stars within the lensing galaxies, the instrument revealed how massive those galaxies are and how strongly they bend light, critical information for pinning down the Hubble Constant.

“The key breakthrough relied on the motion of stars in the lens galaxies as measured via Keck/JWST/VLT spectroscopy to address the main source of uncertainty, known as the mass-sheet degeneracy,” said postdoctoral fellow at University of Chicago and study corresponding author Anowar Shajib in the release. “The result also relies on long-term collaborative work between observatories including time delay measurements from 20 years of photometric data obtained at [European Southern Observatory] in Chile.”

The quest continues

The team’s measurement currently achieves 4.5% precision — an extraordinary feat, but not yet enough to confirm the discrepancy beyond doubt.

The next goal is to refine that precision to better than 1.5%, a level of certainty “probably more precise than most people know how tall they are,” noted postdoctoral fellow at ETH Zurich and the third corresponding author of the study Martin Millon in the release.

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