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The Hubble tension is one of the biggest mysteries in cosmology.
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It centers around the Hubble constant—the measurement of how fast our universe is expanding—which comes out as two different numbers when calculated two different ways, even though those numbers should always match.
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Now, the James Webb Space Telescope has confirmed that this tension is real, rather than a miscalculation or a problem with our equipment. That means there’s something in the underlying physics of our universe that remains a mystery.
The Hubble tension is one of the most hotly debated discrepancies in all of astronomy. It centers around a number called the Hubble constant, which is functionally the rate at which our universe is expanding. Yup, our infinite universe is expanding. Trippy.
We’ve known that this expansion is a fact of our cosmos for a while now (we also know that expansion is speeding up, but that’s another story). But we still don’t know exactly how fast this expansion is happening—we don’t know what the Hubble constant is. And it’s not for lack of trying, either.
There’s a few ways you can measure the Hubble Constant, but there are two major ones. The first, which is how the Hubble Telescope itself does it, is to figure out how fast the things around us are moving away from us. This is done by targeting specific, reliable objects in the sky, calculating the distances to them, and combining those distance with measurements of redshift (a phenomenon that happens when an object moves away from our vantage point, stretching out the light its sending our way and making it appear redder). This is called a local measurement, and it results in a Hubble constant of around 74 kilometers per second per megaparsec (km/s/Mpc).
The second way is to look at the Big Bang. Well, not directly—that’s currently impossible. But we can look at the cosmic microwave background, which is the radiation from the Big Bang that permeates every ‘corner’ of our universe. By studying this ancient radiation, we can get a reading of what the Hubble constant would have been back when the universe began, and then use our knowledge of the laws of physics to functionally fast-forward and find what that Hubble constant should be now. Using this method, we get a predicted Hubble constant of around 68 km/s/Mpc.
And that is the Hubble tension—those numbers should be the same, but they’re not. And when expectation doesn’t match observation in astronomy, it means one of two things: there’s something wrong with our measurement techniques, or there’s something wrong with our understanding of physics.
Now, more and more, it’s looking like the latter. A recent paper—led by Adam Riess, who won the Nobel Prize in Physics in 2011 for his discovery that the expansion of the universe was accelerating and, by extension, his discovery of dark energy—announced that observations from the James Webb Space Telescope further confirm Hubble’s measurements of the local Hubble constant. According to our absolute best space telescope, the Hubble constant is around 73 km/s/Mpc (which is within the margin of error of the original Hubble measurement), not the ~68 km/s/Mpc it should be.
It might be fair to have a little bit of a “so what?” reaction to this news. After all, when you boil it down, the only thing we’ve discovered is that there is in fact something we still don’t understand about the expansion of the universe.
But the excitement behind this announcement comes from the fact that this new measurement confirms that the Hubble tension is real. It’s not a quirk of an old telescope—it’s actually a problem. And that means this isn’t some trivial squabble. There’s something we don’t know about the basic physics that underpin our entire cosmos.
Scientists are going to keep pushing to understand exactly what we’re missing that is leading to this discrepancy. And if our scientific history is anything to go by, someday, they’ll figure it out. And when they do, we’re going to have entirely new physics on our hands. How exciting.
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