Something is seriously wrong with our understanding of the cosmos
SPACE 11 July 2019
The Hubble constant may not be that constant after all
By Leah Crane
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 a number called 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 Cepheid 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 what happens when you look 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.
Read more: Mini universes could be constantly exploding at every point in space
With this independent confirmation, it seems increasingly like both the conflicting measurements of the universe’s expansion are correct. This is because of the boost it gives to the distance ladder method, which is based on more complex and less established physics than the CMB measurement, making it easier for cosmologists to disbelieve.
The tension has now reached a confidence level of 5 sigma, meaning that if both measurements are not both correct then there’s just one chance in 3.5 million that the results could be produced by random chance – a confidence level that in some areas of physics constitutes a true “discovery” instead of just an intriguing piece of data.
That could be a problem for our standard view of cosmology. If the Hubble constant in the early universe is actually different from the Hubble constant more recently, something else about our generally-accepted cosmological model has got to give.
“It’s starting to feel like maybe there really is some new physics that’s out there, but there’s not a theory ready and waiting to explain this whole thing,” says Daniel Scolnic at Duke University in North Carolina. If we truly are misunderstanding the universe in some way, it’s not clear how. There have been many suggestions for how to make theory match the measured expansion of the universe, but none of them are widely accepted.
“The Hubble constant is the biggest problem in cosmology that we have access to right now, and the hope is that this crack in our understanding is going to lead us to some even bigger cracks like dark energy and dark matter,” says Scolnic. “We just have to chase the crack.”
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