“A controversial approach to gravity that challenges Albert Einstein and suggests dark matter doesn’t exist has passed its first test.
The vast majority of physicists agree that gravity acts according to rules laid down in Isaac Newton’s law of gravitation and Einstein’s theory of general relativity. Yet observations of the universe show that the motion of the galaxies can’t be explained by the gravitational pull of all the ordinary matter out there – hence the belief in unseen, dark matter that exerts its own pull.
Now, a team of astronomers studying the distribution of matter in more than 30,000 galaxies say their observations can be explained by an alternative theory that does away with dark matter. If this “modified gravity” is correct, it would up-end hundreds of years of fundamental physics.
To their surprise, they discovered the observed lensing could just as readily be accounted for by a new model of gravity, without invoking dark matter.
Erik Verlinde, a theoretical physicist at the University of Amsterdam in the Netherlands, has been developing a competing model of gravity that borrows heavily from quantum mechanics, relativity, information theory and string theory. It also builds on controversial models of so-called modified gravity, such as the Modified Newtonian Dynamics (MOND) theory of Mordehai Milgrom.
Verlinde’s calculations fit the new study’s observations without resorting to free parameters – essentially values that can be tweaked at will to make theory and observation match. By contrast, says Brouwer, conventional dark matter models need four free parameters to be adjusted to explain the data.
“The dark matter model actually fits slightly better with the data than Verlinde’s prediction,” says Brouwer. “But then if you mathematically factor in the fact that Verlinde’s prediction doesn’t have any free parameters, whereas the dark matter prediction does, then you find Verlinde’s model is actually performing slightly better.”
Brouwer’s study takes advantage of catalogues of distant galaxies released in 2011 and 2015 and looks at regions close to the visible disc of each galaxy. These regions are where gravitational lensing should be bending light from distant galaxies beyond.
Using statistical algorithms that consider the shape and color of the background galaxies, the researchers inferred a lensing profile for the foreground galaxy. It’s a bit like projecting an image onto a warped and uneven sheet of glass and then, knowing what the original image looks like, figuring out the optical properties of the glass sheet from what we see on the far side.
From the gravitational distortions inferred for each foreground galaxy, the researchers devised a lensing profile based on Verlinde’s gravitational model, and another based on a conventional dark matter approach.
So if Verlinde’s is the better match, what’s the problem? Gravitational heresy. Verlinde’s gravity is stronger and dies off more slowly with distance compared with the models of Newton and Einstein.
To most physicists and astronomers today, that’s an issue, to put it mildly. Newton’s and Einstein’s theories of gravity have been so rigorously and comprehensively validated experimentally that it borders on sacrilege to suggest gravity could be something other than what they describe. String theorist Lubos Motl savaged Verlinde’s ideas in a recent blog post: “I wouldn’t okay this wrong piece of work as an undergraduate term paper.”
Milgrom, however, supports the work. He also points out that according to his own 2013 analysis of gravitational lensing data in galaxies, MOND produces similarly impressive results as Verlinde’s gravitational model does in Brouwer’s study.
“My equations work differently than Milgrom’s, and in the case of [galaxy] clusters this can be quite important,” Verlinde says. But in the case of Brouwer’s work, “They put in the formula I get,” he says, “and I have to admit it’s the same formula that Milgrom would have got, and… they just put it on the data. It looks like a fit.””