The following is an extract from our monthly Launchpad newsletter, in which resident space expert Leah Crane journeys through the solar system and beyond. You can sign up for Launchpad for free here.
Cosmologists have been fighting over the Hubble constant, a number that represents the rate of expansion of the universe, since before I was born. Now, it may finally be settled.
The debate about the expansion rate of the universe kicked off in the 1970s, when astronomer GĂ©rard de Vaucouleurs found it to be 100 kilometres per second per megaparsec, while astronomer Allan Sandage found it to be about 50 km/sec/Mpc. A quick break to explain what those units mean: a megaparsec is 3.26 million light years, so one km/sec/Mpc means that a galaxy 3.26 million light years away should be moving away from us by 1 kilometre per second due to the expansion of space.
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Over time, we got better at measuring the Hubble constant, and the gap between the two numbers got smaller – but it never closed. Now, there are two main ways to measure it. One is by looking at how fast the universe was expanding when it first formed – by taking detailed measurements of the light left over from the big bang, known as the cosmic microwave background (CMB) – and using our best model of cosmology to extrapolate forward in time. The other uses the “local distance ladder”, which involves repeated measurements of objects relatively near Earth to determine how space is expanding in our cosmic neighbourhood. These objects are often called standard candles because their predictable brightnesses allow us to calculate their distances accurately.
I once described these two methods like this: if you think of the universe as a human adult, the local distance ladder method is like measuring their height with a measuring tape, whereas the CMB method is like taking a picture of the personas a baby and running it through a model of how humans grow to predict their current height.
The CMB method yields a Hubble tension of about 67 km/sec/Mpc, while the local method gives a value of about 73. “The Hubble tension is certainly one of the biggest conundrums facing modern physics,” says Dan Scolnic at Duke University in North Carolina. “Importantly, though, it’s more like an evaluation zone for how good our standard model of cosmology is.” If the tension remains, the model may not be all that great, and we might need new exotic physics to explain the measurements.
The problem there is that we don’t have any clue what that new physics would look like. “There are over 1000 papers about this tension, and so far none of them has been able to explain all of the data,” says Wendy Freedman at the University of Chicago. Plus, the standard model of cosmology seems to fit nearly everything else about the universe remarkably well.
Freedman and her colleagues may have finally found a way to reconcile the two measurements. Traditionally, the first two rungs of the distance ladder are variable-brightness stars called Cepheids and type 1a supernovae. Freedman and her team re-analysed all of the data from the Hubble Space Telescope on Cepheids, and they used the James Webb Space Telescope (JWST) to observe more of these objects. They also observed two other types of stars, called carbon stars and tip of the red giant branch stars, to fortify the distance ladder.
They found a Hubble constant of 69 km/sec/Mpc, which close enough to be in agreement with the CMB method. These more precise measurements may have resolved the Hubble tension at long last. “If it turns out that there is a tension, it is a big conundrum because it means there is something fundamental that we don’t understand,” says Rocky Kolb at the University of Chicago. “[If the tension were to be resolved] it would be a relief to many, including me.”
Not all cosmologists are certain that the tension is fixed or even on the verge of being fixed. While these results are promising, they have not been fully collated and published yet, nor have the data been made public. Some other measurements with JWST have yielded higher Hubble constants. Even Freedman agrees that it’s not time to break out the champagne just yet.
“My philosophy is that we need different ways of measuring it,” she says. “Ultimately, I would like to see more gravitational-wave sirens”, she says, referring to the use of ripples in space-time to measure cosmic distances. “That’s a completely different type of physics, and if that agrees it’ll tell me it’s done. But these data are pointing the way.” These new observations have given us light at the end of the Hubble tension tunnel, a prospect that I find equal parts exciting and disappointing. After all, what cosmic paradox will I have to obsess over once this one is gone?
