This story is Part II of a two-part essay on the “Hubble tension.” Read Part I.

“Some of my colleagues told me the other day that we are reaching the ‘end of physics,’ and that there is nothing new to learn,” says Miguel Aragon-Calvo, a cosmologist at the Public University of Mexico City (UNAM), Mexico. “History tells us that whenever we think that, we’re wrong and things are about to change.” That paradigm shift could be triggered by the puzzle of the Hubble tension–a mystery regarding why different experiments measure incompatible values for the rate of the expansion of the universe (see Part 1). Many astronomers think the discrepancy will only be explained by rethinking the standard model of cosmology, perhaps with the inclusion of a new type of “early dark energy.” But others disagree, arguing that our theoretical understanding of the universe is correct–and one or more of our experimental techniques is flawed.

Standard Candles

Before declaring that the standard model of cosmology is dead, physicists must first rule out any silly mistakes with their experiments. For much of the past decade, most astronomers have had their money on there being an overlooked problem with the local measurements, made by various groups monitoring stars and galaxies. That’s because gauging how far away these objects lie requires constructing a complicated “distance ladder” from multiple overlapping observations, any one of which could be flawed. 

To form the basis of the ladder, astronomers triangulate the distance to certain nearby stars, called Cepheids, from the shift in their position on the sky, as the Earth moves round the sun. The Cepheids handily burn with a well understood intrinsic luminosity for a portion of their lives, so they can then serve as a ‘standard candles’: if astronomers spot one in a nearby galaxy, they can estimate its distance, from how bright or dim it appears, providing the first rung of the ladder. 

To build the second rung, astronomers search for galaxies with both Cepheids and certain supernovae that also have a well-understood luminosity. The third rung of the ladder then involves searching for more such supernovae at ever greater distances. Combined with observations of how much the light from these objects has been redshifted, astronomers can calculate the Hubble constant H₀ and the expansion rate of the universe.

But a slight error in any one rung of that ladder would be compounded–and the Cepheid rungs have fallen under suspicion, says Batten. It may be that the amount of carbon and oxygen contained in the Cepheids affects their luminosities in a more complicated way than thought, throwing off their calibration. To get round this possibility, some astronomers are using different standard candles. Batten, his Swinburne colleague Jeremy Mould and others, have been observing certain red giant stars, with well-understood luminosities, with the Hubble Space Telescope. But Batten and Mould’s preliminary analyses suggest that these red giants give a similar value for H₀ to the Cepheids. The tension persists.

Astronomers using the Dark Energy Spectroscopic Instrument (DESI) in Arizona took an even bolder leap, using entire galaxies–whose shapes can be related to their brightness and distance–in the nearby Coma cluster, as standard candles, in place of Cepheids. In August 2024, the DESI team reported that to calibrate their distance ladder and pin down the Hubble constant, they needed an accurate measure of the distance to the cluster. Luckily enough, Scolnic, Riess and colleagues were already monitoring the Coma cluster for supernovae. “When that DESI paper came out, we just dropped everything and I didn’t sleep for a solid month, I was just measuring this supernova,” says Scolnic. In September, Scolnic, Reiss and colleagues reported their result. Yet again, it matched the higher value for H₀, vindicating the Cepheids. “The Coma cluster analysis produces very strong evidence for a Hubble tension,” says Riess.

Gravitational Sirens

If the Cepheids are not the culprit, then maybe astronomers are misunderstanding supernovae, hazards Mould. “Supernovae are exploding stars that have an enormous amount of complicated astrophysics and yet we assume they are all the same,” he says. If this is problem, new and improved observations underway with the JWST, could help. “We get better telescopes, we get better computers, and the science moves forward,” says Mould. “There’s a reason to be optimistic.”

Astronomers also hope that new detections of gravitational waves–the ripples in spacetime created when black holes or binary stars merge–could bypass the distance ladder entirely. “Gravitational waves can act a standard ‘sirens’ for measuring the Hubble constant in a different way,” says Eleanora Di Valentino, a cosmologist at the University of Sheffield, UK, and former member of the Planck team. A study from 2017 that examined the gravitational waves and traditional light signals released simultaneously from a binary-star merger came up with a value for H₀ of 70 km/s/Mpc. It had a relatively large margin of error, but future measurements should bring those errors down.

But to Di Valentino, it seems unlikely that these new observations will change the picture that’s been painted over the past decade. “At this point, it is clear, at least to me, that it is not a question of one single measurement from one team, or one kind of calibration, or what kind of object we are looking at,” she says. “All of the local measurements are giving a similar measure of the Hubble constant.”

Something Strange in the Neighborhood

While Di Valentino and many others are convinced, not everyone agrees that the Hubble tension is a real problem. Okay, different local measurements give a similar high value for the Hubble constant, but there is another possible–and rather humdrum–explanation for why they are all too large, says cosmologist Aragon-Calvo. Maybe they are skewed because we live in a strange neighborhood. For instance, a supercluster of galaxies nearby would pull at objects, speeding their motion and giving the illusion this is due to the expansion of the universe. “For instance, last year, Nobel Laureate Jim Peebles published observations of huge structures–there is a mega-structure much larger that what we would expect to see just at our doorstep,” says Aragon-Calvo.

Ironically, Scolnic and colleagues were in the midst of investigating precisely this possibility, by looking at the effects of the Coma cluster and others, when the DESI paper came out and diverted their attention. Yet, Scolnic is skeptical that it’s the answer. He notes that the DESI team analyzed galaxies from different directions of the universe compared to earlier experiments, and still came up with the same answer. “They’re not seeing anything funky about our place in the universe,” Scolnic says.

Together, these results have warmed more cosmologists towards the possibility that the standard model that they have come to love needs refining or replacing. And there is no lack of alternatives, says Di Valentino, who along with Riess and other colleagues considered some 300 theoretical proposals for resolving the Hubble tension in a mammoth review in 2021–and found most to be wanting. Now that total reaches 500, she says, and “many are fantastical.” 

Physicists can easily tweak a parameter here, throw in a new exotic particle or force there, or come up with other ways to fudge the numbers and fix the Hubble tension. The trouble is these revised models then disagree with other astrophysical observations that already do match perfectly with the standard model.

Evolving Dark Energy

There is one suite of modifications though that intrigues Mould, Riess and Scolnic, however. These target dark energy, which is usually thought to be a constant, pushing space outwards with the same strength throughout the cosmos, ever since the Big Bang. But the Coma analysis hints that the baby  universe got an extra push, from some extra “early dark energy,” for around 300,000 years–just enough to make the CMB’s calculated value of H₀ a tiny bit larger. “It’s the perfect explanation that just solves the tension,” Scolnic adds. “So I am watching that right now.”

This idea also had a boost in 2021 when new CMB measurements from the Atacama Cosmology Telescope seemed to fit better with models containing a supercharged dark energy compared with the standard model, although later results from that experiment and the South Pole Telescope are more ambiguous. Taken with other observations, the Coma analysis “may be providing hints about evolving dark energy,” says Riess.

If nothing else, the recent results remind cosmologists to be humble and recognize that the cosmos can always surprise us. “We got it in our heads that the universe is pretty boring,” says Scolnic. “But I feel like now we’re a bit more open to the excitingness of the universe and all bets are off.”

This story is Part II of a two-part essay on the “Hubble tension.” Read Part I.

Zeeya Merali is a London-based science journalist and author of the popular physics book, A Big Bang in a Little Room.

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