The heat death of the universe is coming for us, but we don’t know when. The cosmos is constantly expanding, and the speed of that inflation is measured by a value called the Hubble constant. We have two ways to determine this rate, and they have always returned different values, leaving researchers at an impasse. A new study of the stars we use to measure the distance to other galaxies has deepened the divide.
One way we search for the Hubble constant is to start at the beginning. We can look at the cosmic microwave background (CMB) – a relic of the first light to cross the cosmos after the big bang – and see how fast the universe was expanding back then. Models of how the cosmos has evolved since then can predict how the universe should be expanding today.
The other method is more direct. We track a spot in our galaxy’s neighbourhood to see how quickly it is moving away from us. To do this, astronomers monitor two things – a type of star called a Cepheid variable, which cyclically dims and brightens, and a supernova in the same galaxy. Then, they mathematically remove other sources of movement to figure out how much of that motion is due to the expansion of the cosmos.
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If you think of the universe as a person, the Cepheid method is like measuring the adult person’s height with a measuring tape, whereas the CMB method is like taking a picture of the person as a baby and running it through a model of how humans grow to predict the person’s current height.
Each time the two measurements are updated, they conflict. By 2016, Cepheid measurements done by at the Space Telescope Science Institute in Baltimore, Maryland, and his colleagues resulted in a value 9 per cent higher than the CMB method.
Cosmic horseplay
If this wasn’t caused by a measurement error, it is a big deal: it could mean that our understanding of the fundamental components of our universe is incorrect. “A discrepancy may mean that there’s something serious that we don’t understand,” says , a retired astrophysicist from the Harvard-Smithsonian Center for Astrophysics.
Now, Riess and his colleagues have used the Hubble Space Telescope to measure the distances to seven Cepheid variable stars with greater precision than before. Even with a better measuring tape, the problem persists.
These new measurements push the Hubble constant higher by just under 0.3 per cent. Refining the value in this way isn’t just beating a dead horse.
“It depends how tender you want your dead horse, but we really need to do this due diligence if we are going to say that resolving this tension requires new physics,” says Riess. “We need a really tender dead horse.”
A conspiracy of errors?
If this mismatch persists, it could mean that our guesses about the natures of dark matter and dark energy are wrong, or that there’s a particle out there that we’ve never detected. Figuring out just how big the discrepancy is could let us rule out some theories about new, exotic physics that are too strange to cause such a relatively small error.
“It’s starting to become that either there’s new physics or there’s a conspiracy of errors across many different ways of measuring that have nothing to do with each other,” says Riess.
The uncertainties in Cepheid variable measurements will take another beating in upcoming months and years as the Gaia space observatory continues to measure the distances to many stars, including Cepheid variables, in and around our galaxy.
“This paper has seven more measurements – Gaia is going to release one billion,” says at the Carnegie Institution for Science in Washington DC.
The Gaia data may resolve the tension or worsen it. But in terms of measuring the universe’s growth like a person’s, it won’t just improve the measuring tape; it will give us a billion different heights to compare.
Reference: arXiv,
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