Depending on where we look, the universe expands at different rates. Now, scientists using the James Webb and Hubble space telescopes have confirmed that the observation is not due to a measurement error. Astronomers used the James Webb and Hubble space telescopes to confirm one of the most troubling enigmas in all of physics: that the universe appears to be expanding at surprisingly different speeds depending on where we look. This problem, known as the Hubble Tension, has the potential to alter or even completely upend cosmology. In 2019, measurements from the Hubble Space Telescope confirmed that the enigma was real; in 2023, even more precise measurements from the James Webb Space Telescope (JWST) solidified the discrepancy. Now, a triple check conducted by both telescopes, working together, seems to have definitively ruled out the possibility of any measurement error. The study, published on February 6 in the Astrophysical Journal Letters, suggests that there may be something seriously wrong with our understanding of the universe. "With measurement errors ruled out, what remains is the real and exciting possibility that we have misunderstood the universe," said the study's lead author, Adam Riess, a professor of physics and astronomy at Johns Hopkins University, in a statement. Riess, Saul Perlmutter, and Brian P. Schmidt won the 2011 Nobel Prize in Physics for their 1998 discovery of dark energy, the mysterious force behind the universe's accelerated expansion. Currently, there are two "gold-standard" methods for calculating the Hubble constant, a value that describes the universe's expansion rate. The first involves analyzing small fluctuations in the cosmic microwave background radiation (CMB)—an ancient relic of the universe's first light, produced just 380,000 years after the Big Bang. Between 2009 and 2013, astronomers mapped this microwave distortion using the European Space Agency's Planck satellite to infer a Hubble constant of approximately 46,200 mph per million light-years, or roughly 67 kilometers per second per megaparsec (km/s/Mpc). The second method uses pulsating stars called Cepheid variables. Cepheid stars are dying, and their outer layers of helium gas expand and contract as they absorb and release stellar radiation, causing them to flicker periodically like distant signal lamps. As Cepheids become brighter, they pulsate more slowly, giving astronomers a way to measure their absolute brightness. By comparing this brightness with their observed brightness, astronomers can chain Cepheids into a "cosmic distance ladder" to peer deeper and deeper into the universe's past. With this ladder in place, astronomers can find a precise number for its expansion based on how the light from Cepheids has been stretched, or redshifted. But this is where the mystery begins. According to Cepheid variable measurements made by Riess and his colleagues, the universe's expansion rate is about 74 km/s/Mpc: an impossibly high value when compared to Planck's measurements. Cosmology had been thrust into uncharted territory. "We wouldn't call it a tension or a problem, but rather a crisis," said David Gross, a Nobel Prize-winning astronomer, at a 2019 conference at the Kavli Institute for Theoretical Physics (KITP) in California. Initially, some scientists thought the disparity could be the result of a measurement error caused by the blending of Cepheids with other stars in Hubble's aperture. But in 2023, researchers used the JWST, a more precise telescope, to confirm that, for the first "rungs" of the cosmic ladder, their Hubble measurements were correct. However, the possibility of crowding further back in the universe's past remained. To address this issue, Riess and his colleagues built on previous measurements, observing an additional 1,000 Cepheid stars in five host galaxies up to 130 million light-years from Earth. After comparing their data with Hubble's, the astronomers confirmed their earlier measurements of the Hubble constant. "We've now covered the full range of Hubble's observations and can rule out a measurement error as the cause of the Hubble Tension with very high confidence," said Riess. "The combination of Webb and Hubble gives us the best of both worlds. We found that Hubble's measurements remain reliable as we climb the cosmic distance ladder." In other words: the tension at the heart of cosmology is here to stay.