Appel reported on December 12 2024 that in summary the Hubble tension is one of the biggest mysteries in cosmology. It centers around the Hubble constant—the measurement of how fast the universe is expanding—which comes out as two different numbers when calculated two different ways, even though those numbers should always match.
Now, the James Webb Space Telescope has confirmed that this tension is real, rather than a miscalculation or a problem with the equipment. That means there’s something in the underlying physics of the universe that remains a mystery. So, with the Hubble tension being 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 the universe is expanding. So even the infinite universe is expanding.
Scientists known that this expansion is a fact of our cosmos for a while now (scientists also know that expansion is speeding up, but that’s another story). But Scientists still don’t know exactly how fast this expansion is happening—they don’t know what the Hubble constant is. And it’s not for lack of trying, either.
There’s a few ways the Hubble Constant can be measured, 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 scientists' 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 scientists look at the cosmic microwave background, which is the radiation from the Big Bang that permeates every ‘corner’ of the universe. By studying this ancient radiation, scientists can get a reading of what the Hubble constant would have been back when the universe began, and then use scientists' knowledge of the laws of physics to functionally fast-forward and find what that Hubble constant should be now. Using this method, scientists 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 scientists' measurement techniques, or there’s something wrong with scientists' 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 scientists boil it all down, the only thing they have discovered is that there is in fact something scientists 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 scientists don’t know about the basic physics that underpin the entire cosmos.
Scientists are going to keep pushing to understand exactly what they are missing that is leading to this discrepancy. And if scientific history is anything to go by, someday, they’ll figure it out. And when they do, they will have entirely new physics on their hands.
Managed & Built by I2M Consulting, LLC | © 2010-
