Mystery over the expansion of the universe might not be a mystery at all, scientist says

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One of the fundamental mysteries of the universe might not be quite so mysterious, according to new research.

We know that the universe is expanding, but not how fast. Or, more precisely, we seem to have different answers to exactly how fast, when the answer should be the same throughout the different ways of measuring it.

Each new measurement of the expansion of the universe, known as the Hubble constant, just adds to that confusion about what exactly the rate is. Astronomers have used a variety of different tools to get a better view of it, but those measurements have only diverged, leading to what has been referred to as the “Hubble tension”.

The current mismatch largely arises from the two main ways of measuring the expansion. One looks at the cosmic microwave background, or the remains of light from the big bang, and gives one number; another watches stars and galaxies as they move away from us, a method that has produced a slightly faster estimate of the expansion.

That in turn has led to worries among scientists. Either the measurements must be wrong, or our scientific understanding is wrong – and both present their own mysteries and problems.

But a new paper in The Astrophysical Journal suggests there might not be such a problem at all. Written by Wendy Freedman, an astronomy and astrophysics professor at the University of Chicago who has been involved in some of those measurements, it suggests that the gap is closing and the problems might be resolved by better data.

She had previously looked to measure the expansion by using stars known as Cepheids, through the Hubble Space Telescope. But in recent years she has looked at different stars, known as red giants, as a way of checking the measurements from those Cepheids.

Better understanding of the numbers from those red giants seems to put it in line with the number from the cosmic microwave background, suggesting the issue is not the consequence of some unknown physics but rather difficulties with the measurement. The Cepheids might simply be a difficult way of getting a good measurement.

“The Cepheid stars have always been a little noisier and a little more complicated to fully understand; they are young stars in the active star-forming regions of galaxies, and that means there’s potential for things like dust or contamination from other stars to throw off your measurements,” said Professor Freedman in a statement.

She and her colleagues hope that even better data will help refine those measurements even more. That is expected when Nasa launches the James Webb Space Telescope next year, and which will give scientists time to observe both Cepheids and red giants.

Professor Freedman also hopes to look back on the existing data and better understand what it shows, which might also help refine that measurement.

“There is still some room for new physics, but even if there isn’t, it would show that the standard model we have is basically correct, which is also a profound conclusion to come to,” she said. “That’s the interesting thing about science: We don’t know the answers in advance. We’re learning as we go. It is a really exciting time to be in the field.”