Evidence of "modified gravity" in 150 galaxies strengthens dark matter alternative
Dark matter is currently the most widely accepted hypothesis for explaining some of the weirdness we see in the cosmos – but it’s not the only possibility. Now, a team of astronomers has discovered evidence in over 150 galaxies for a long-standing alternative model of “modified gravity.”
The idea of dark matter was conjured up in the 1930s in response to astronomical observations that flew in the face of Newton’s laws of gravity. Basically, the more mass an object has, the stronger its gravitational pull – but Swiss astrophysicist Fritz Zwicky noticed that galaxy clusters were rotating much faster than their gravity should allow, based on the mass of their visible matter. Zwicky came to the conclusion that these galaxy clusters contained much more mass than we could see, and he dubbed it dark matter.
The hypothesis has stood up to scrutiny in the decades since, with observations continuing to support the idea of dark matter. But one major piece of the puzzle is still missing – finding the stuff itself. Plenty of experiments have tried to detect particles of the elusive dark matter, or even create them, but so far none have been successful.
Perhaps that’s because it’s not really there after all, and instead it might be that our models of gravity and physics need some tweaking. This class of hypotheses is known as modified gravity, and now astronomers claim to have found evidence supporting one particular model, known as Modified Newtonian dynamics (MOND).
First proposed in 1982 by physicist Mordehai Milgrom, MOND suggests that at low accelerations, gravity’s effects are stronger than Newton’s laws describe. A side effect of this is that the motions of objects would depend not just on their own mass, but all other masses in their neighborhood. This phenomenon is known as the external field effect (EFE).
And now, researchers on the new study say they’ve observed the EFE in action in 153 different galaxies. The team was studying the rotation curve of the galaxies, which plots the orbital speed of stars and gas against their distance from the center of the galaxy.
The researchers discovered that galaxies in strong external fields slowed down much more frequently than galaxies in weaker external fields did. That’s a prediction made only by MOND, and the discovery surprised even the astronomers themselves.
“The external field effect on rotation curves is expected to be very tiny,” says Federico Lelli, co-author of the study. “We spent months checking various systematics. In the end, it became clear we had a real, solid detection.”
It’s an intriguing result, and it may lend some weight to the MOND hypothesis for further study. But it’s important to keep in mind that so far the bulk of the evidence still points towards dark matter, and it’ll take much more work to topple that hypothesis entirely.
The research was published in the Astrophysical Journal.
Source: Case Western Reserve University