We rely on models to understand the universe, and these models are constantly being refined in response to new observations and theories. In recent years, it has become clear that the way we see the universe, the Standard Model of Cosmology, is no longer matching up with observations, resulting in a complex yet fascinating problem.
The problem is summarized by a single parameter known as the Hubble Constant. This gives us the value of the universe’s expansion rate. However, it appears that it is not consistent at all. The value obtained from measurements taken in the early universe does not correspond to what is observed in the modern universe.
A new paper published in the Monthly Notices of the Royal Astronomical Society adds to the evidence of the tension between Hubble Constant measurements. The astronomers determined this value by observing gravitationally lensed quasars. And the way they did it is incredible.
A quasar is an active galaxy that emits enormous amounts of energy from its supermassive black hole at its center. Some of them are so far away that we wouldn’t normally notice them, but thanks to a physics quirk, we do. If there is a large enough galaxy or group of galaxies between them and us, they can warp space-time enough to magnify the light of distant quasars.
These lenses are capable of producing multiple images of quasars, as well as arcs and rings. Because the light from the quasars takes different paths through the gravitational lens, the peculiar shape is determined by the geometry of the lensing system. Because quasars flicker, light traveling through slightly different paths can cause time delays.
Excellent instruments are required to measure these time delays. Previous observations of this type were made with the Hubble Space Telescope. The research was also conducted from the ground at the Keck Observatory using a technique known as Adaptive Optics. The team approached this as a blind analysis, attempting to rule out potential sources of error and bias while keeping the final answer hidden from them until the end.
“When we thought we had addressed all potential issues with the analysis, we unblinded the answer with the rule that we have to publish whatever value we find, even if it’s insane.” In a statement, lead author Geoff Chen, a graduate student at the University of California, Davis, said, “It’s always a tense and exciting moment.”
The value is consistent with Hubble constant measurements in the local universe and data from previous surveys.
“Therein lies the crisis in cosmology,” UC Davis Professor Chris Fassnacht added. “The Hubble Constant is constant everywhere in space at any given time, but not in time.” So, when we compare Hubble Constants derived from different techniques, we are comparing the early universe (using distant observations) to the late, more modern part of the universe (using local, nearby observations).”
Many astronomers around the world are working on this difficult problem. The team intends to conduct numerous additional observations of lensed quasars in order to improve the measurement they have obtained.
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