‘Crisis’ as scientists admit they don’t really know how fast the universe is expanding

Problems with Hubble constant could suggest that our understanding of the universe is flawed

The universe is expanding faster than scientists thought, potentially re-writing our model of the universe.

For years, researchers have been debating just how quickly the cosmos is growing. Repeated measurements come out with a variety of different answers, puzzling the researchers who take them.

Now the latest measurement has once again suggested that our understanding of the expansion of the universe is wrong. It used new methods to prove that the speed – known as the Hubble constant – is perhaps even more unusual than we had expected.

The astronomers kept the results of the study secret even from themselves during the research, to ensure that they would not be swayed by just how strange their measurements could prove.

In order to understand the expansion rate of the universe, astronomers led by University of California, Davis used Nasa's Hubble Space Telescope alongside technologies on the ground. It was the first time that the latter equipment – the WM Keck Observatory’s Adaptive Optics (AO) system – has been used to mesure the Hubble constant.

“When I first started working on this problem more than 20 years ago, the available instrumentation limited the amount of useful data that you could get out of the observations,” said co-author Chris Fassnacht, Professor of Physics at UC Davis. “In this project, we are using Keck Observatory’s AO for the first time in the full analysis. I have felt for many years that AO observations could contribute a lot to this effort.”

The results have been published in the latest issue of the Monthly Notices of the Royal Astronomical Society.

To ensure that the results were reliable, the researchers went into the research blind: ensuring that they kept the final conclusions hidden as they worked to ensure their measurement was as accurate as possible. That meant that the researchers could avoid the worry that they would unconsciously adjust the findings to ensure that they lined up with expectations.

“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,” said lead author Geoff Chen, a graduate student at the UC Davis Physics Department.

When they finally looked at the result, they found that the numbers showed the universe is expanding more quickly than they had thought.

That in turn could complicate our understanding of the universe under the standard model of cosmology. Researchers generally think that the expansion of the universe was very quick when it first formed, then slowed down as dark matter pulled on it, and is now getting faster because of dark energy.

That was created by looking at very distant objects, looking into the cosmic microwave background or the radiation that is leftover from the Big Bang whent he universe began.

But the new study looks at more local objects. Numerous studies have found that there is an unexplained difference between the speed measured from those local objects and the results found when examining distant ones.

“Therein lies the crisis in cosmology,” says Fassnacht. “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).”

That suggests either that there is a problem with our understanding of the Cosmic Microwave Background – which researchers think is unlikely – or that there is something profound and fundamental wrong with our understanding of physics.