The Hubble constant may not be that constant after all.
Something is wrong with the expansion of the universe. Nearby galaxies seem to be moving away from one another too fast, we don’t know why, and every new set of data just seems to make the problem worse.
We have two basic ways to measure the expansion of the universe, which is described by the Hubble constant. The two methods have always returned clashing results, and many astronomers and cosmologists hoped that one of them was simply wrong. Now, a third independent method has solidified their disagreement. It seems more and more that both methods are correct – which could require a major reworking of our understanding of the universe.
One of the ways we measure the Hubble constant is by using the cosmic microwave background (CMB), the remains of the first light to stream across the cosmos after the big bang. Patterns in that light can tell us how fast the universe was expanding then, and researchers then use models of how it has evolved to tell us how fast it ought to be expanding now.
The other main way is using what astronomers call the “distance ladder”, in which we measure the distance to stars called Chid variables, link those distances to nearby supernovae, and use those supernovae to determine how fast relatively nearby galaxies are moving away from us. The distance ladder method has consistently resulted in an expansion rate more than 9 per cent higher than the CMB method, causing much consternation among astronomers.
“If you have two measurements that don’t agree, there is always a chance that one of them or both of them are wrong,” says team member Simon Birrer at the University of California Los Angeles. “But if you bring in a third independent measurement that comes close to one of the previous ones, then people start believing that this tension is really there.”
Now, an international team of astronomers has made that third measurement of the Hubble constant using gravitational lensing, a phenomenon where light from a distant object is bent by the gravity of a closer galaxy on its way to our telescopes. When the light arrives, it often forms several smeared images of the farther object, like looking at a light through the bottom of a water glass.
The light that forms each image travels a different path around the closer galaxy, so, as the distant object changes in brightness, there is a time delay between when that change shows up in each image. That time delay is based on the distance the light has traveled, so we can use it to measure the distance to the original object. When that is combined with the rate at which it’s moving away from us, we end up with a measurement of the Hubble constant.
Birrer and his colleagues went through this process for three quasars, some of the brightest objects in the universe which reside at the centres of some galaxies. Their measurements matched the results from the distance ladder method. (Leah Crane, New Scientist 11 July 2019)
If you don’t fully understand all this, then you are in good company! But, that aside, I think it is wonderful and reassuring that human beings are still devoting their lives to untangling the secrets of the universe, and that so far the know-nothings and moneymen have yet to stop them. The human race has a long way to go, if it survives. Good luck to the scientists!