A fight over the very nature of the universe has turned ugly on social media and in the popular science press, complete with accusations of “cheating” and ad hominem attacks on Twitter. Most of the universe is hiding, and some scientists disagree over where it has gone.
It’s quite literally a story as old as time. Wherever you look in the cosmos, things don’t seem to add up. Our human observations of the universe’s structure—as far back as we can observe—suggest that there’s around five times more mass than we see in the galaxies, stars, dust, planets, brown dwarfs, and black holes that telescopes have observed directly. We call this mystery mass, or the mystery as a whole, “dark matter.”
Several thousand physicists researching these dark matter-related mysteries will tell you that dark matter is a particle, the way that electrons and protons are particles, that only appears to interact with other known particles via the gravitational pull of its mass. But there are a few dozen physicists who instead think that a set of ideas called “modified gravity” might one day explain these mysteries. Modified gravity would do away with the need for dark matter via a tweak to the laws of gravity.
The story dates back to the 1980s, as physicists were looking for ways to explain the cosmic mysteries they observed. Dark matter as a particle has been the prevailing paradigm since then for good reasons: It exactly predicts temperature fluctuations in the most distant light we can see, called the cosmic microwave background. It also explains the universe’s large-scale web structure, as well as the strange warping of space near clusters of galaxies that appears to be caused by nothing. Proponents of modified gravity haven’t yet figured out how to explain the universe’s web structure, but they think their ideas better explain the confusing way that galaxies rotate.
As dark matter continues to go undetected, some of the modified gravity supporters have gotten more vocal, leading to the most recent spat.
This past spring, scientists gathered at a series of conferences at the University of California, Santa Barbara’s Kavli Institute for Theoretical Physics to debate the nature of dark matter. Then, in June, the most sensitive dark matter particle-hunting experiment, called XENON, announced it had once again failed to find a dark matter particle. A story titled “Is Dark Matter Real?” followed in the August issue of Scientific American, penned by vocal modified gravity proponents—physicist Sabine Hossenfelder from the Frankfurt Institute for Advanced Studies in Germany and astronomer Stacy McGaugh from Case Western Reserve University in Ohio.
In that story, the scientists put forth evidence in support of modified gravity. They explained that the outer regions of galaxies rotate too quickly, compared to what the usual laws of physics would predict. The speed of rotation suggests there’s a lot more mass than what we actually observe. Updating gravity’s laws so that they change in the outer regions of galaxies would explain this anomalous behavior more elegantly and more easily than a dark matter particle alone. It also explains the “Tully–Fisher relation,” a relationship between a galaxy’s brightness and how fast it spins. They proposed that modified gravity could explain some dark matter mysteries as well as particles could, and asked to be taken more seriously. “Perhaps modified gravity is wrong, but perhaps the scientific community is not putting in a good faith effort to know for sure,” they wrote near they story’s end.
Ethan Siegel, astrophysicist-blogger behind Forbes’ Starts With a Bang! blog, responded with a post titled “There’s A Debate Raging Over Whether Dark Matter Is Real, But One Side Is Cheating.” He wrote that, in order to win favor from the public, Hossenfelder and McGaugh were setting up a false narrative by treating the fight as an even one, even though the support for a dark matter particle far outweighs the opposing side. He reminded readers that modified gravity currently fails to recreate the behavior of pretty much everything larger than a galaxy. He sums up his post:
“It’s only if you ignore all of modern cosmology that the modified gravity alternative looks viable. Selectively ignoring the robust evidence that contradicts you may win you a debate in the eyes of the general public. But in the scientific realm, the evidence has already decided the matter, and 5/6ths of it is dark.”
Naturally, McGaugh and Hossenfelder were not happy to be called cheaters. McGaugh argued that he’s published far more scientific papers than Siegel has. “Ethan is a blogger,” he tweeted. Siegel told Gizmodo that he’d not seen McGaugh’s tweets. The discussion has since died down, but the sentiment hasn’t—Siegel repeated to Gizmodo that he still thought that modified gravity theorists were “cheaters,” and continued: “What’s not a responsible thing to do is say, ‘let’s throw away all of cosmology, and now tell the story about how it’s wrong—we just have to throw away general relativity, replace it with a theory we don’t have, and then we’ll have a new theory of gravity and solve the problem, not with dark matter but with modified gravity.’”
Meanwhile, McGaugh and Hossenfelder reiterated that they felt like they and their cohort of a few dozen were being ignored by the thousands-of-physicists-strong dark matter community.
“I think that people aren’t aware of how the size of this community affects their judgment,” Hossenfelder told Gizmodo. “Yes, particle dark matter does better with the cosmic microwave background, but it doesn’t explain why modified gravity works so well in galaxies. I feel really queasy about people who want to wipe it off the table like it’s not there.”
This isn’t a new discussion—it’s gone on since 1981, when Israeli physicist Mordehai Milgrom put forth his theory of Modified Newtonian dynamics, or MOND. Further observations of more distant realms of the universe have, as of today, tipped the scales far in favor of particle dark matter.
The MOND theory’s continued existence and evolution into modified gravity more generally stems from increasingly sensitive experiments not finding any dark matter particles. And perhaps more importantly, astronomers like McGaugh have noticed that modified gravity theories can explain the way galaxies rotate with fewer human assumptions than dark matter does. When they ask why their theories seem to work, McGaugh and Hossenfelder feel like they keep hearing answers about the cosmic microwave background rather than about the galaxies. McGaugh told Gizmodo: “If [the standard dark matter paradigm] is correct, then why does modified gravity ever get anything right?”
In Gizmodo’s conversations with 13 physicists studying dark matter, a pretty clear picture emerged: Dark matter as an undiscovered population of particles that influence the universe through gravity is the prevailing paradigm for a reason, and will continue as such until a theory comes along with the same predictive power for the universe’s grandest features. Within particle dark matter, there are a wealth of different ideas for what the particles might look like that could explain different features of the universe, and physicists are working on various experiments to try and find them.
“Everywhere the dark matter theories make predictions, they get the right answers,” Scott Dodelson, a Carnegie Mellon physics professor, told Gizmodo. But he offered a caveat: “They can’t make predictions as well on small scales,” such as the scales of galaxies. Galaxies, of course, are smaller than the entire universe, but in some ways they require more detail to understand.
The motion of individual galaxies is incredibly difficult to model, after all. Dodelson said that dark matter particles can seem clunkier than modified gravity at trying to explain galaxies, where the interaction between regular matter particles plays an important role in the way that they move. So far, simulations of galaxy formation just haven’t been strong enough to help physicists answer whether dark matter particles or some other idea best explains why they rotate too quickly at their edges.
“Galaxies form over a long period of time, and until recently, simulations of galaxy formation couldn’t simulate galactic disks,” Tracy Slatyer, a professor of physics from MIT, told Gizmodo. Perhaps better simulations will weaken—or strengthen—the case for modified gravity theories in galaxies.
But nearly all of the physicists we spoke to noted that it’s important to keep an open mind toward other theories with solid mathematical and theoretical foundations, in case dark matter continues to go undiscovered. Dodelson said that “both sides are wrong” and that it’s an “act of hubris” to think that a simple model of dark matter, dark energy, and cosmic inflation could explain the universe’s largest scales. For now, though, he said that “any theory worth its salt should get things right,” or explain the bulk of the dark matter anomalies and not just a specific piece. “MOND does not and dark matter does.”
Kathryn Zurek, a physicist at Lawrence Berkeley National Lab in California, compared MOND to trying to reconstruct the story of a crime using only two out of 10 pieces of available evidence, when all 10 pieces taken together seem to support a different story. But she said she’s not opposed to physicists trying to see how they can modify gravity more generally to change the dark matter paradigm away from particle dark matter theories.
The debate is most certainly a lopsided one, where one side has more evidence and more supporters, while the other has used popular media in order to get their voices heard. But even so, most of the other physicists we spoke to felt it was important to listen to the arguments for modified gravity.
Simona Murgia, physicist at the University of California, Irvine, helped plan the dark matter conference where physicists discussed many of these ideas, including modified gravity. She and the other organizers had several conversations about how much speaking time to give to the modified gravity researchers. Outside of Twitter spats, she didn’t see it as a fight at all, but rather a community of researchers with lots of different ideas trying to answer a fundamental question about the universe.
Murgia felt it was important to give modified gravity researchers a seat at the table. “We should be open and listening because these [modified gravity] people are pretty good scientists and I can’t personally rule out what they’re saying,” she said. “But what they’re saying isn’t yet the full story, and I think they’d probably agree.”
Particle dark matter experiments such as LUX-Zeplin, XENON 1T, and the Axion Dark Matter eXperiment will continue their hunt for various possible dark matter particles. It may be several decades before the search is complete. But the longer scientists go without detecting a dark matter particle, the more pressure they’ll face from folks with other ideas, and the more heated this debate is likely to become.
Update: A contextual clarification regarding observable mass of the universe has been added.