Something is missing in our universe. We see signs of it in the way that galaxies rotate, which suggests that there's extra mass out there beyond what we can observe. Scientists have a name for it—dark matter—and while we've never detected a single particle, its presence would fill in certain key missing pieces of the puzzle. Dark matter is thought to make up a quarter of the universe. (Less than 5 percent is normal matter.)
Scientists have been searching for a dark matter particle for decades, trying to prove its existence, but these searches keep coming up empty. What if there's no such thing as dark matter? What if the effects we're seeing are instead a property of gravity that we don't fully understand?
Watch Erik Verlinde deliver a live public lecture on the "new view" on gravity from the Perimeter Institute for Theoretical Physics Wednesday at 7 PM E.T.:
This is a minority view, to be sure—most scientists agree there's a dark matter particle out there, and that our increasingly sophisticated detectors will find it.
A few others are challenging this. One of them is Erik Verlinde, a theoretical physicist and string theorist based at the University of Amsterdam. "Dark matter became a tool for understanding the universe, even though there's no evidence for it," Verlinde told me in a Skype call in July. "I think now people might start thinking, maybe we went too quickly in making these conclusions. Maybe there is something else going on with gravity."
Physicists say there are four fundamental forces in nature, including gravity. Verlinde, who published a high-profile paper on his ideas in Nov. 2016, argues that gravity is an "emergent" phenomenon: Just like how temperature grows from the movements of particles, "gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime," a university news release explains. Dark matter is an illusion created by "an interplay between ordinary matter and dark energy," he told Quanta magazine last year. According to that way of thinking, we don't need the particle.
As for particle physicists who haven't given up on the idea of a dark matter particle, "I don't think they have thought about gravity in a much deeper way," he told me. "Their background is particle physics and they think everything is a particle."
Others disagree. "I think it really is a minority view that there's a problem," Michael Hudson, a professor of physics and astronomy at the University of Waterloo, told me. He argues that the case for dark matter has "gotten stronger" through observations of the cosmic microwave background (leftover radiation from the Big Bang). "Particle dark matter is essential to obtain the correct predictions for the temperature fluctuations in the cosmic microwave background," theoretical physicist Sabine Hossenfelder wrote in Forbes earlier this year.
Another case for dark matter, many scientists point out, is the Bullet Cluster, in which dark matter seems to have been pulled apart from normal matter by the collision of two galaxies.
Even so, Verlinde isn't the only one who's unconvinced there's a dark matter particle out there. Stacy McGaugh, a professor of astronomy at Case Western Reserve University, is a leading advocate of modified Newtonian dynamics, or MOND, a theory that modifies Newton's laws to account for certain properties of galaxies. "It's a terrible name because nature doesn't modify itself," he told me in a July phone call. "It's just what nature is doing."
McGaugh published a paper last November, not long after Verlinde's, looking at 153 spiral galaxies. It found that the stars' speeds (and inward accelerations) could be explained by the distribution of stars and gas. It "just screams MOND," he told Science magazine.
McGaugh, whose twitter handle is @DudeDarkmatter, sees the dark matter particle as a way to talk about the "discrepancies" in how we understand the universe, he told me.
"This is one of the things that disturbs me," McGaugh said in an interview. "[Dark matter] has taken on overtones of religion." he said there's some "self-selection" happening in the field of dark matter particle searches, where "people who stay are believers."
On Wednesday at 7 PM Eastern time, Verlinde will be describing his ideas about gravity at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario. You can watch it yourself through the webcast above. Meanwhile, dark matter searches continue.
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