Back in the mid-1990s, cosmologists—who study the origin, composition and structure of the universe—were beginning to worry that they were facing a crisis. For starters, two astronomers had observed that a huge swath of the cosmos, a billion light-years or so across, was moving in a direction inconsistent with the general expansion of the universe. Worse, astrophysicists using the Hubble Space Telescope, then relatively new, had determined that the cosmos was between eight billion and 12 billion years old. The problem: even the high end of that range couldn’t account for stars known to be closer to 14 billion years old, leading to the nonsensical implication that the stars existed before the universe did. “If you ask me,” astrophysicist Michael Turner told Time magazine at the time, “either we’re close to a breakthrough or we’re at our wits’ end.” But the first observation was never confirmed. And the impossibly old stars were explained a few years later with the discovery that a mysterious, and still unknown, dark energy had turbocharged the expansion of the universe, making it look younger than it actually is.

Now, however, cosmologists are facing a brand-new problem—or rather a couple of problems. The Hubble constant (named, as the telescope is, for Edwin Hubble, who discovered the expansion of the universe in the 1920s) is the number that shows how fast the cosmos is expanding; it’s been measured with greater and greater accuracy over the past few decades. Yet there’s still some uncertainty because two independent methods of calculating it have come up with different answers, giving rise to what’s called the “Hubble tension.” Although the numbers aren’t dramatically different, they’re enough at odds to worry theorists. “In particle physics,” said David Gross of the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, at a conference in 2019, “we wouldn’t call it a tension or a problem but rather a crisis.”

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