Discovered less than a century ago, the expansion of the universe causes galaxies to rush away from Earth, stretching their light to longer, redder wavelengths. That observation spawned the idea of the big bang—and decades of bickering over the rate at which the universe is expanding, the Hubble constant. After a brief rapprochement, cosmologists are arguing again. Working from our cosmic neighborhood outward to more distant galaxies, one group has measured a rate significantly higher than the one derived by colleagues studying the cosmos’ farthest fringe and the afterglow of the big bang, the cosmic microwave background (CMB).
Recent observations from NASA’s JWST space telescope have confirmed the disparity, or Hubble tension, and chances that it can be explained as an observational artifact are fading. But so are prospects that the puzzle has a simple physics solution, a recent spate of papers shows. That may dash hopes that solving the Hubble tension could also help cosmologists sharpen their problematic theory of the universe’s makeup and evolution.
“There’s no guarantee that there’s one effect that is causing all of this,” says Adam Riess, a cosmologist at Johns Hopkins University. Some question whether the Hubble tension will ever be explained. “I wouldn’t bet my house on it,” says Sunny Vagnozzi, a cosmologist at the University of Trento.
According to cosmologists’ prevailing theory, the universe contains 5% ordinary matter; 27% invisible dark matter, whose gravity holds galaxies together; and 68% dark energy, which stretches space like a pressure. Just after the big bang, the universe grew exponentially, hugely magnifying tiny quantum fluctuations in a dense soup of fundamental particles. Dark matter gathered in the dense spots, and hundreds of millions of years later ordinary matter settled in the clumps of dark matter to form galaxies. As the universe thinned, dark energy’s push overcame the pull of gravity, so that after slowing, the universe’s expansion is now accelerating.
The model, called Lambda–cold dark matter (Lambda-CDM), is vague. It assumes dark energy is just a property of empty space called a cosmological constant, Lambda, and dark matter is just cold invisible stuff, hence CDM. But the model fits cosmological data beautifully, especially those from the CMB. The temperature of the CMB varies ever so slightly across the sky, providing a blotchy snapshot of the universe at the moment neutral atoms formed, 380,000 years after the big bang. A theory with just six parameters—including the Hubble constant—Lambda-CDM precisely fits the distribution of fluctuation sizes measured by Europe’s Planck spacecraft, says Tanvi Karwal, a cosmologist at the University of Chicago. “It’s insane that it works.”
However, the fit yields a value of the Hubble constant that clashes with the one measured directly. Since 2009, Riess and his colleagues have used various telescopes to create an elaborate “distance ladder” of the distances and red shifts of nearby galaxies. A key rung relies on observations of variable stars called Cepheids. Each pulses at a rate that reveals its intrinsic brightness, allowing observers to deduce its distance from its apparent brightness in the sky. JWST can tease out individual Cepheids in other galaxies, and its observations confirm the Hubble constant is 8% higher than Lambda-CDM predicts, Riess and colleagues reported in September.
Some researchers say they’re still not 100% convinced the discrepancy is real. “The field really needs a sensitive third way of measuring this,” says Johannes Eskilt, a cosmologist at the University of Oslo. Others have proposed myriad models to explain it.
One solution assumes dark energy isn’t a cosmological constant, but is due to some kind of new physics. If so, its concentration could have fallen or even grown as the cosmos evolved, resulting in an expansion history that could start as the Lambda- CDM predicts but end at the higher directly measured value. In fact, Riess says he started his project in hopes of finding just such an effect.
Other data nix that idea. Sound waves called baryon acoustic oscillations (BAOs) rippled through the infant universe, and the CMB records how far they spread before atoms formed. That length is also imprinted in the distribution of galaxies. Comparing the two lengths reveals the expansion history halfway between the big bang and now. The result is a curve whose shape matches Lambda-CDM’s prediction, says Ryan Keeley, a cosmologist the University of California, Merced. That constraint makes it impossible to change the expansion history enough to resolve the Hubble tension, he and a colleague argued on 15 September in Physical Review Letters (PRL). “These most recent BAO data sets put the final nail in the coffin” of that solution, Keeley says.
