A Weird Form of Dark Energy Might Solve a Cosmic Conundrum

F ifteen years ago cosmologists were flying high. The simple but wildly successful “standard model of cosmology” could, with just a few ingredients, account for a lot of what we see in the universe. It seemed to explain the distribution of galaxies in space today, the accelerated expansion of the universe and the fluctuations in the brightness of the relic glow from the big bang—called the cosmic microwave background (CMB)—based on a handful of numbers fed into the model. Sure, it contained some unexplained exotic features, such as dark matter and dark energy, but otherwise everything held together. Cosmologists were (relatively) happy.

Over the past decade, though, a pesky inconsistency has arisen, one that defies easy explanation and may portend significant breaks from the standard model. The problem lies with the question of how fast space is growing. When astronomers measure this expansion rate, known as the Hubble constant, by observing supernovae in the nearby universe, their result disagrees with the rate given by the standard model.

This “Hubble tension” was first noted more than 10 years ago, but it was not clear then whether the discrepancy was real or the result of measurement error. With time, however, the inconsistency has become more firmly entrenched, and it now represents a major thorn in the side of an otherwise capable model. The latest data, from the James Webb Space Telescope (JWST), have made the problem worse.

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