New Distance Measurement Highlights Hubble Tension

In 1925, Edwin Hubble discovered that the Milky Way was just one among millions of galaxies in an ever-expanding universe, shattering long-held beliefs in cosmology. Now, 100 years later, astronomers are uncovering key discrepancies in the universe’s rate of expansion through measurements of the distance to one of our closest intergalactic neighbors — the Coma galaxy cluster.

The rate of the universe’s expansion is known as the Hubble constant, named for the astronomer who discovered it. But the universe is not simply expanding; its expansion is also accelerating. Astronomers have built a cosmological model of the universe that accounts for the rate of expansion at different points in time.

Measurements of the early universe align with the values predicted by astronomers, obtained by studying the cosmic microwave background radiation, or the remnant glow left over by the Big Bang. However, when astronomers compare the Hubble constant predicted by the distant universe with the rate of expansion obtained from measuring nearby objects, their measurements do not align — a phenomenon known as the Hubble tension.

Precise measurements to the Coma Cluster have now exacerbated this discrepancy, in new findings presented at the 245th meeting of the American Astronomical Society (AAS) in National Harbor, Maryland. “With this [result], the Hubble tension is now a Hubble crisis,” said Daniel Scolnic (Duke University) at an AAS press conference. He is the lead author on the study published in The Astrophysical Journal Letters,. “[The Hubble tension] is saying, to some respect, that our model of cosmology might be broken,” he added.

Rungs on a Ladder

Around 321 million light-years away, the Coma Cluster is a large, rich cluster of more than 1,000 galaxies. As one of the closest galaxy groupings to our own Local Group, astronomers have measured the distance to Coma many times in different ways.

Astronomers can use various techniques to measure distances on different scales, linking successive measurements to each other like the rungs of a ladder. These techniques often rely on standard candles, such as the white dwarf explosions of Type Ia supernovae. Astronomers know the intrinsic brightness these events actually have and can then gauge their distance by observing their apparent brightness in the sky. It’s like viewing a column of lanterns stretching out into the night — the farther lanterns appear successively dimmer. Precisely measuring distances is one way by which astronomers determine the universe’s expansion rate.

Scolnic’s team used 13 Type Ia supernovae to calculate the distance to the Coma Cluster. By linking the observations to distant galaxy data from the Dark Energy Spectroscopic Instrument, the team arrived at a distance to Coma of around 321 million light-years. That distance in turn corresponds to a Hubble constant between 74.3 and 78.7 kms/s/Mpc. For comparison, the standard model of cosmology instead predicts a Hubble constant of 67.4 km/s/Mpc, with an associated Coma Cluster distance of 365 million light-years.

The paper also incorporates many past distance measurements to the Coma Cluster, some computed “blind” to the problem of the Hubble tension. For all of these measurements, no one has ever come close to the distance that the standard cosmological model predicts. “We are in high tension . . . with the prediction of the standard model of cosmology,” Scolnic said.

A Hubble Crisis

New measurements, such as this one of the Coma Cluster distance, have shown that local measurements do not align with what astronomers expect based on measurements from the early universe.

“When people measure nearby objects, they are just closer than the standard model would predict,” Scolnic said. “Objects in the mirror are closer than they appear.”

The findings underscore the importance of a consistent distance ladder, which could demonstrate the source of the tension. “The problem comes from the disagreement between two very different classes of calibrators,” said Daniel Eisenstein (Harvard University), who was not involved in the study. Cepheid variable stars are used to calibrate nearby distances on the distance ladder, while the “well-predicted travel of sound waves in the first 400,000 years after the Big Bang” is used to measure far distances.

“If the second set is correct, then we likely will have found some intriguing bias in the measurement of the nearby calibrators,” Eisenstein said. “But if the first set is correct, then it seems that our understanding of the early universe will need some substantial change, which would be quite important for cosmology. The jury is still out!”

Further measurements may clarify these issues, according to Adam Riess (Johns Hopkins University), a coauthor on Scolnic’s study. A different kind of cosmological model could also resolve the Hubble tension, though astronomers remain far from a definitive solution. “Unfortunately there are too many options to provide a simple answer,” said Riess. He cites exotic dark energy, a new species of neutrino, primordial magnetic fields, and a decaying electron as a few possible ideas.

Ultimately, the measurement has thrown this cosmological crisis into greater relief. “This study has moved the Hubble tension closer to our backyard,” Riess said, “and demonstrates that it was already baked into measurements that preceded the recognition of the tension.”

Fresh evidence has emerged to demonstrate the Hubble tension — only a new model of cosmology or further measurements across space and time could solve this cosmological mystery.