James' World 2

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Fifteen 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.

The two of us have been deeply involved in this saga. One (Riess) is an observer and co-discoverer of dark energy, one of the last pieces of the standard cosmological model. He has also spearheaded efforts to determine the Hubble constant by observing the local universe. The other (Kamionkowski) is a theorist who helped to figure out how to calculate the Hubble constant by measuring the CMB. More recently he helped to develop one of the most promising ideas to explain the discrepancy—a notion called early dark energy.

One possibility is that the Hubble tension is telling us the baby universe was expanding faster than we think. Early dark energy posits that this extra expansion might have resulted from an additional repulsive force that was pushing against space at the time and has since died out.

This suggestion is finally facing real-world tests, as experiments are just now becoming capable of measuring the kinds of signals early dark energy might have produced. So far the results are mixed. But as new data come in over the next few years, we should learn more about whether the expansion of the cosmos is diverging from our predictions and possibly why.

The idea that the universe is expanding at all came as a surprise in 1929, when Edwin Hubble used the Mount Wilson Observatory near Pasadena, Calif., to show that galaxies are all moving apart from one another. At the time, many scientists, including Albert Einstein, favored the idea of a static universe. But the separating galaxies showed that space is swelling ever larger.

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Chris Gash

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