What are we missing?
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.