We’ve long known that our Milky Way galaxy will collide with its neighbor Andromeda in about four billion years. But some European astronomers think this is a case of deja vu — that we have collided once before, long ago. This would explain puzzling structures in both our galaxies, and the odd existence of our tiny satellite galaxies like the Magellanic clouds.
But it would mean that dark matter does not exist. And our ideas about how gravity behaves on large scales, is wrong. It would change everything.
In 1930, the brilliant astronomer Fritz Zwicky noticed that the way groups of galaxies stay together despite their large individual speeds shows that they contain about six times more gravity than can be explained by all their stars, planets, black holes, and everything else. The way our galaxy spins supports this, too. Some unseen entity that has gravity must dominate the scene everywhere, and we call this dark matter. This is what must be gravitationally pulling us toward Andromeda at 70 mps so that the two of us can overcome the universe’s expansion.
But the need for dark matter assumes that gravity at great distances acts the same way as it does locally, which is what Newton believed, and Einstein supported in his modification of Newton’s model. Yet a few maverick astronomers have long wondered if it’s truly so.
Enter the modified gravity theory, called MOND, which says that gravity behaves differently on the largest scales. Instead of its power falling off rapidly with the inverse square of distance, the way it does in local neighborhoods, it weakens more and more gradually. If this is so, then we and our neighbor galaxy skimmed past each other in the distant past, but did not merge since the new gravity ideas would not have forced that to happen. It would have created the actual structures we presently see around us. It would solve a bunch of puzzles.
Moreover, if this is so, there is no need for dark matter to supply missing gravity, because no gravity is missing anywhere.
The Milky Way and Andromeda are still going to crash into each other in a few billion years. (Don’t worry about it: our individual stars and planets won’t collide because the spaces between everything is so enormous). But if supported by further studies, the new gravity model totally changes the age and dynamics of the whole universe. Our understanding of cosmic structure would have to be rewritten from scratch.