To understand the cosmos we rely on models, and these models are constantly being refined using new observations and theories. Over the last few years, it has become clear that the way we see the universe, the Standard Model of Cosmology, is no longer matching up with observations, creating a complex yet fascinating problem.
The issue boils down to a single parameter known as the Hubble Constant. This gives us the value of the expansion rate of the universe. But it seems that it is not constant at all. The value we get from measurements from the early universe doesn’t match what is seen in the modern universe.
A new paper, published in the Monthly Notices of the Royal Astronomical Society, provides more evidence of the tension between measurements of the Hubble Constant. The astronomers used observations of gravitationally lensed quasars to measure this value. And how they did it is incredibly cool.
A quasar is an active galaxy releasing a huge amount of energy from the supermassive black hole at its core. Some of them are so far away that we normally wouldn’t see them, but thanks to a quirk of physics, we do. If there is a massive enough galaxy or a group of galaxies between them and us, they can warp space-time so much that the light of distant quasars becomes magnified.
These lenses can generate multiple images of the quasars, as well as arcs and rings. The peculiar shape depends on the geometry of the lensing system as the light from the quasars takes different paths through the gravitational lens. Quasars flicker so the light going through slightly different paths can lead to time delays.
To measure these time delays you need to have excellent instruments. Previous observations of this kind have been performed using the Hubble Space Telescope. The research was also conducted from the ground using a technique called Adaptive Optics at the Keck Observatory. The team approached this as a blind analysis, trying to rule out sources of error and potential bias, with the final answer hidden from them until the end.
“When we thought that we had taken care of all possible problems with the analysis, we unblind the answer with the rule that we have to publish whatever value that we find, even if it’s crazy. It’s always a tense and exciting moment,” lead author Geoff Chen, a graduate student at the University of California, Davis, said in a statement.
The value is consistent with measurements of the Hubble constant in the local universe and with the data seen in previous surveys.
“Therein lies the crisis in cosmology,” Professor Chris Fassnacht, also at UC Davis, added. “While the Hubble Constant is constant everywhere in space at a given time, it is not constant in time. So, when we are comparing the Hubble Constants that come out of various techniques, we are comparing the early universe (using distant observations) vs. the late, more modern part of the universe (using local, nearby observations).”
Many astronomers across the world are studying this thorny issue. The team plans to make many more observations of lensed quasars to improve the measurement they have obtained.