Ask Ethan: If Gravity Attracts, How Can The ‘Dipole Repeller’ Push The Milky Way?
“What are the mechanics behind a dipole repeller? How can an area of space void of matter repulse galaxies to any meaningful extent (or at all?)?”
There’s been a longstanding puzzle in astrophysics that’s finally coming to a head. For nearly a century, we’ve known that our Universe is expanding, and that the distance to a galaxy determines its average apparent recessional speed from us. But on top of that is an additional motion – a peculiar velocity – caused by the local gravitational field of the Universe. When we look at the motion of our own galaxy, we see it’s moving about twice as fast in one direction as the attractive masses would allow. But underdense regions, where there’s less mass and gravity than average, can serve as an effective repeller, failing to attract other matter just as much as overdense regions can attract it. Thanks to a newly mapped and accounted for cosmic void, we might finally understand how our galaxy is moving through the Universe.