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How fast is the universe expanding?

One might assume scientists long ago settled this basic question, first explored nearly a century ago by Edwin Hubble. But right now the answer depends on who you ask. Cosmologists using the Planck satellite to study the cosmic microwave background—light from the “early” universe, only about 380,000 years after the big bang—have arrived at a high-precision value of the expansion rate, known as the Hubble constant (H0). Astronomers observing stars and galaxies closer to home—in the “late” universe—have also measured H0 with extreme precision. The two numbers, however, disagree. According to Planck, H0 should be about 67—shorthand for the universe expanding some 67 kilometers per second faster every 3.26 million light-years. The most influential measurements of the late universe, coming from a project called Supernova H0 for the Equation of State (SH0ES), peg the Hubble constant at about 74.

This discrepancy—the so-called Hubble tension—has been growing for years, increasing as study after study of both the early and late universe yield ever more precise results and leave scientists on both sides anxious and bewildered. After all, it could be that either faction is somehow just mismeasuring the universe. But the tension may be a true reflection of reality, requiring exotic new physics and a dramatic revision to our understanding of cosmic evolution.

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