One of the tenets of our cosmological model is that the universe is expanding. For reasons we still do not fully understand, space itself extends over time. Turning your head around is a strange idea, but the evidence for it is conclusive. It's not just that galaxies appear to be moving away from us, as their redshift shows. Distant galaxies also appear larger than they should due to cosmic expansion. They are also distributed in superclusters that are separated by large cavities. Then there is the cosmic microwave background, where even its small temperature fluctuations confirm the cosmic expansion.
We know the universe is expanding, but as we have gathered more data, it is clear that we don't know how fast it is expanding. We know the rate is around 70 (km / s) / Mpc, but we are struggling to pinpoint it. The reason for this is that if we measure the rate in different ways, the results will not match.
How a gravitational lens can measure cosmic expansion. Photo credits: Martin Millon / Swiss Federal Institute of Technology, Lausanne
You are probably familiar with the way supernovae measure the distance to distant galaxies. By comparing this measurement with the redshift of the galaxies, we can measure the cosmic expansion. This method first confirmed the cosmic expansion. Another method is to look at fluctuations in the cosmic background. You can also look at the gravitational waves created when two neutron stars merge, or even the microwave laser light that comes from material near a black hole.
Each method is based on different physical models, which means that they are independent measurements of cosmic expansion. If our understanding of the cosmos is correct, the rate of expansion that each method measures should be the same. But it turns out that they disagree a little. Not much, but enough to make it clear that there is something about cosmic expansion that we do not fully understand.
The answer probably lies in the complexity of our methods. For example, the supernova method relies on a hierarchy of measurements known as the cosmic ladder of distance. Each step in the ladder has its own methodology, and errors can add up at each step. One solution to this problem is to find a method that doesn't rely on so many assumptions. Recently, a team of astronomers put their hopes in the gravitational lens of distant quasars.
The timing of the light along each path varies slightly. Photo credit: M. Millon and F. Courbin
The team uses a time delay cosmography method. Imagine there is a distant quasar with a galaxy between it and us. Light from the quasar is gravitationally deflected or lens-shaped by the mass of the galaxy. This allows multiple images of the quasar to be created. So it looks like the galaxy is surrounded by multiple quasar images. Due to objectification, the distance that light travels to create each image is slightly different. Since the speed of light is constant, the light of each image reaches us at a slightly different time. So if the quasar is a little bright, each image will flicker at a slightly different time.
Time-delay cosmography uses the time differences to measure the actual distance between quasar images. By comparing it with the apparent size of the images, the team can determine the speed of cosmic expansion. The advantage of this method is that it does not depend on a cosmic distance conductor. It only depends on the behavior of the light in curve space as described by general relativity.
In this latest work, the team achieved a value of 64.2 – 71.5 (km / s) / Mpc. While this is not accurate enough to solve the mystery of cosmic expansion, it does show that the method can work. With more observations of lens quasars, the method could become good enough to give us an accurate measure. The solution to this problem seems to be only a matter of time.
Reference: Birrer, S. et al. "TDCOSMO IV: Hierarchical Time Delay Cosmography – Common Inference of Hubble Constant and Galaxy Density Profiles." Astronomy & Astrophysics 643.A165 (2020): 40