Scientists agree the universe is expanding. Scientists disagree how fast the universe is expanding.

The argument over the Hubble Constant–the scientific law defining that expansion rate–is heating up. But this isn’t just some nerdy squabbling in the ballroom of whichever hotel is hosting the latest astrophysical symposium.

No, our entire conception of how the universe works is at stake.

On one side of the fight are theorists such as Nikita Blinov, a physicist at the Fermi National Accelerator Laboratory near Chicago. The theorists are willing to toss out decades, even centuries, of settled physics in order to explain why different observations of galaxies and stars give us different numbers for the Hubble Constant.

“There’s a well-established standard cosmological model that does a good job of describing most observations, but runs into a problem with this Hubble tension,” Blinov told The Daily Beast. “People then come up with different modifications of the standard model, test these against data and see whether the tension persists or not.”

Objecting to these efforts are some traditional astronomers, such as University of Chicago cosmologist Barry Madore, who urge caution and restraint. Maybe our observations of the universe’s expansion disagree because the observations themselves are imprecise, Madore told The Daily Beast.

In that case, there’s no justification for ripping up the old models and writing new ones to replace them. “Whether you want to go find a solution to this problem depends on whether you’re a theorist who thinks they have bright ideas about cosmology,” the study of everything, Madore said.

The problem, for Madore and like-minded scientists, is that inventing a whole new kind of physics isn’t just unnecessary—it’s actually destructive. In throwing out old scientific laws that actually still work, they claim, you risk setting back entire fields of study and wasting a lot of scientists’ precious time.

You can’t take exception to curiosity, Madore conceded. “But you do have to be rational scientists about where you put your effort.”

To Blinov, challenging old assumptions is rational. “By their nature proposals to explain an anomaly like the [Hubble Crisis] are speculative, and therefore, high-risk, high-reward,” Blinov said. “Some people enjoy working on such problems, others don’t.”

This current crisis has been a decade in the making. Strictly mathematical calculations of the universe’s expansion rate, dating back to the 1920s, turned up a Hubble Constant value of 68 km/s/Mpc. That’s a galaxy that’s one megaparsec from Earth—a measurement of distance in space that’s roughly 3.3 million kilometers—moving outward from the universe’s center at a rate of 68 kilometers per second.

Disagreements among scientists began to flare up in 2014 as astronomers developed new methods of measuring the movement of stars. Calculating the path of red giant stars near Earth gives us a Hubble Constant of 70 km/s/Mpc. Meanwhile, tracking very faraway Cepheid stars (which dime and brighten periodically) leads to a Hubble Constant of 74 km/s/Mpc.

Maybe just one of those figures—or some future fourth figure—is correct. “Seldom do things agree on the first pass,” Madore explained.

Or maybe all the calculations are accurate. Maybe the universe expanded fast during its early history then suddenly slowed down. An abrupt downshift that would produce different Hubble Constants depending on what you’re looking at: a star that’s really far away–and thus older at the moment we glimpse it–or some star that’s a lot closer and thus younger when we see it.

If a slowdown is what happened, cosmologists figure it would need to have taken place between 50 and 150 million years ago in order to explain the assortment of conflicting Hubble Constants we’ve calculated in recent years.

If you assume the universe slowed down, you should try to explain why. There’s nothing in settled physics to explain it, so you might have to posit theoretical particles doing weird, theoretical things on a vast scale.

The theorists doing this work are the newcomers to the Hubble Constant party. They aren’t afraid to challenge long-held assumptions. “This is what they like to do,” Adam Riess, a Johns Hopkins astrophysicist, told The Daily Beast.

Madore and Blinov agree it’s entirely possible that new instruments that are now coming on-line might help settle the Hubble Crisis through old-fashioned astronomy—i.e. staring at stars through telescopes. NASA’s new James Webb Space Telescope alone could lead to new observations and new data that might render moot any attempt to smash today’s physics and replace it with something brand new.

University of Chicago astronomer Wendy Freedman, who has looked hard at red giant stars and is in line to use the JWST for further observations, said there’s no need for “new physics” to explain all those different Hubble Constants, because a better application of you might call “old physics” can do the trick without requiring a top-to-bottom rethink of how the universe works.

The explanation for all those different constants is probably pretty simple, Freedman told The Daily Beast. There’s a good chance there are errors in our observations of faraway stars, and thus inaccuracies in our calculations of those stars’ movements, she explained.

Courtesy University of Chicago

Addressing those errors should be the priority. Only after we’ve made a good faith effort to get better data on star-movement should we even consider, say, writing new laws of physics in order to accommodate an apparent slowdown in the expansion of the universe. “Eliminating systematic errors is essential before we can definitively say the cause of the tension is new physics,” Freedman said.

As Freedman fine-tunes her observations of distant stars using better and better telescopes, she’s getting more-refined data on stars’ movements. She’s optimistic that, with a little more effort and a turn or two with the JWST, she and other old-school astronomers are going to pin down a new calculation of the Hubble Constant that erases all the recent contradictions.

“I have come to the conclusion that we do not require fundamental new physics,” she said.

Darker Ideas

But the theorists aren’t waiting around for the JWST or some other new telescope to end the crisis. They’re busy proposing profound new theories that could explain why the universe might expand fast for a few billion years, then suddenly slow down.

One leading theory that has fallen from the spotlight recently involves dark matter, the invisible stuff that we can’t observe at all but that scientists think makes up as much as a third of all matter in the universe. Maybe a mysterious kind of “warm” dark matter abruptly produced a whole bunch of particles with strong gravity that, starting around 100 million years ago, dragged on stars and planets and asteroids and slowed down everything.

This “decaying warm dark matter” theory gained popularity among the theorists over the last couple years before losing some of its luster in recent months—thanks in part to work that Blinov led in 2020.

“Warm” dark matter is distinct from slow-moving “cold” dark matter and faster “hot” dark matter. As huge masses of warm dark matter decay over eons, they break down into neutrino particles, which are so small and nearly massless that they pass right through other matter. There are so many of them, however, that together they can exert a strong gravitational pull on everything else.

A lot of extra neutrinos popping up across the universe “could impact the expansion history of the universe in a way that helps to ameliorate the long-standing tension” over the Hubble Constant, Blinov and his coauthors Dan Hooper, a Fermi cosmologist, and University of Chicago grad student Celeste Keith wrote in their article, eventually published in 2020 in the Journal of Cosmology and Astroparticle Physics.

Ultimately, Blinov and his team concluded that, while interesting, decaying warm dark matter didn’t entirely make sense as a resolution to the Hubble Crisis.

An Iranian team followed up this year and agreed with Blinov and his coauthors that decaying warm dark matter, if it really exists, probably wouldn’t function as a parking brake on the whole universe. “Decaying dark matter seems not a promising candidate to address the cosmological tensions,” Zahra Davari and Nima Khosravi, both physicists at Shahid Beheshti Universityin Tehran, wrote in their preprint paper, which appeared online in March though has not yet been peer-reviewed.

To be clear, we don’t actually know for sure that warm dark matter even exists. But assuming our mixed bag of different Hubble Constants are all accurate, then something had to cause a universe-wide slowdown, Blinov explained. If it’s not decaying warm dark matter, it could be some other weird theoretical thing. “I don’t have a favorite explanation,” Blinov said.

A Call for Caution

The problem is, if we’re counting on an entirely hypothetical form of matter to resolve the Hubble Crisis, then we’ve got to rethink our basic understanding of the structure of, well, everything. It’s this grasping in the theoretical dark, running roughshod over settled scientific law, that alarms scientists like Madore.

In Madore’s view, the theorists should relax, quit pitching far-out ideas, and wait for new telescopes to give us a clearer view of those faraway Cepheid stars—and thus new and potentially more reliable estimates of the Hubble Constant. “Doesn’t it make a little more sense to say something is wrong with our understanding of the Cepheids rather than saying, ‘New physics!’?” Madore asked.

Freedman’s turn with the JWST this summer could help pin down the disagreeing constants. “Our proposal is to use the infrared capabilities of JWST to measure the Hubble Constant,” she told The Daily Beast. “We are using three different methods to do so, all within the same galaxies so that we can compare the results and thereby get a robust estimate of the uncertainties.”

Maybe with time and better instruments, all those other measurements of the constant—the ones that currently disagree—will start to converge. In that case, patience, not decaying warm dark matter or some other strange notion, might resolve the Hubble Crisis without forcing us to toss all our existing physics textbooks in the trash.

That’s the conservative position, scientifically speaking. The liberal one, so to speak, is that it never hurts to explore the fringes of cosmology. “These theoretical ideas are the way theorists approach these research problems and it feels kind of harsh to tell them they are wasting their time or that we shouldn’t listen to their ideas,” Riess said.

A sort of compromise is possible, of course. Both camps could just… go about their business. The cosmologists could keep proposing wacky new particles that may or may not exist but, if they do exist, might make sense of all those competing Hubble Constants.

Meanwhile, the old-school astronomers could take their turn at one space telescope or another, steadily plugging away at traditional measurements of the movements of stars, hoping to eventually arrive at a single figure for the Hubble Constant that satisfies everyone.

In reality, that’s what’s happening. It’s just that less adventurous scientists such as Madore think a lot of theorists are wasting their own time, their students’ time and research grants that should be going toward studies that don’t depend on imaginary particles to solve a problem that might not be a problem much longer.

“It’s both understandable and unfortunate that so much brain power has been put to answering this question, when the question itself hasn’t been honestly addressed as to whether it’s the right question,” Madore said.