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Something doesn’t add upESA/Hubble & NASA
By Leah CraneThe question of how fast the universe is expanding just keeps getting more confusing. Our two main methods of measuring the acceleration of the universe’s expansion disagree by nine per cent. Astronomers have now come up with another independent method that should have helped us decide which of the original two methods is correct – but this third way has suggested a new figure for the acceleration rate, further muddying the waters.
There are two primary methods of measuring the rate of the universe’s expansion, which is described by a number called the Hubble constant. The first uses the cosmic microwave background (CMB), a relic of the universe’s first light, and tracks the expansion rate in the early universe to what it ought to be now according to our standard model of cosmology.
The second uses what astronomers call the “distance ladder”. This relies on stars called Cepheid variables, which fluctuate in brightness at a rate related to their absolute luminosity. Knowing their brightness allows us to measure their distance, which we use to determine the distances to nearby galaxies and then to well-studied supernovae. These distances are compared with how fast the objects are moving away from us to derive the Hubble constant.
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Something doesn’t add up
The two methods have always disagreed, and the easiest way to explain that disagreement has been to argue for a problem with our Cepheid measurements. This is because if the distance ladder is correct then the only alternative is that our standard model of cosmology is wrong and there must be some kind of exotic physics we don’t yet appreciate.
However, recent independent measurements using gravitational lensing instead of Cepheids have confirmed the distance ladder numbers. This makes exotic physics seem more likely.
Now, a group led by Wendy Freedman at the University of Chicago have released yet another independent measurement of the Hubble constant. This group used a different type of star, called the tip of the red giant branch, to build a new distance ladder.
Surprising finding
As they age, red giant stars eventually reach a plateau where they get no brighter and all shine similarly. By comparing their observed brightness to the standard brightness of these stars, we can tell very precisely how far away they are. Freedman and her colleagues used this approach – instead of Cepheid variables – to measure the distances to nearby galaxies and supernovae.
“The advantage to this method is its simplicity: these stars are well-understood, they’re in all galaxies, and they’re very bright,” says Freedman. They can also be found in the outer areas of galaxies, rather than the bright inner zones where other stars can crowd the measurements.
What they found was surprising: a value of the Hubble constant right between the CMB measurement and the Cepheid distance ladder. This is more evidence that we really don’t understand what the value of the Hubble constant is, and that we need much more precise studies before we can claim that there must be some new type of exotic physics, Freedman says.
“Honestly I would’ve liked it to come out one way or the other, but it didn’t,” she says. “The mystery heightens.”
Reference: arxiv.org/abs/1907.05922
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