One of modern cosmology’s biggest puzzles is the “Hubble tension”—the growing discrepancy between increasingly precise measurements of the universe’s expansion rate. Experts still have little idea why it exists.
A new study pushes Hubble’s expansion to a new maximum. It suggests the universe’s observed acceleration is greater than today’s physical models can explain.
Theory and practice. There are two ways to measure the universe’s expansion. The first relies on the cosmic microwave background (CMB), radiation left over from the Big Bang. Fluctuations in this background suggest an expansion rate of about 41 miles per second per megaparsec (miles/s/Mpc), consistent with widely accepted cosmological models.
Observations of the nearer universe, however, tell a different story. Cepheids, a type of variable star, pulsate at frequencies that correlate with their brightness.
This relationship allows astronomers to build a “cosmic ladder,” calibrating distance measurements step by step to more distant objects. The problem: This method estimates a significantly higher expansion rate—about 46 miles/s/Mpc.
From tension to crisis. The new study worsens this discrepancy. Its measurement, based on the “cosmic staircase,” suggests an even higher expansion rate than previous estimates: 47.53 miles/s/Mpc. The study’s authors conclude that “the tension now turns into a crisis.”
The missing step. The team built its own “cosmic ladder” using data from the Dark Energy Spectroscopic Instrument (DESI) collaboration. This instrument tracks about 100,000 distant galaxies using 5,000 robots with fiber-optic sensors.
To anchor these data, they needed a reference point—an initial rung on the “cosmic ladder” that would help measure DESI’s observed changes. They turned to the Coma Cluster, one of the galaxy clusters closest to the Milky Way.
Supernovae. To measure the cluster’s distance, the team analyzed the light curves of 12 type Ia supernovae. These supernovae shine in a highly predictable way, allowing astronomers to determine their actual distance based on apparent brightness.
The study was published in The Astrophysical Journal Letters.
The search continues. Discrepancies like this intrigue the scientific community because they often lead to new theories. “It’s exciting,” the researchers say. But for now, few clues suggest whether resolving this issue will require refining current models or a major paradigm shift.
Image | NASA
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