Study may solve the riddle of our looming crash with Andromeda

Simulation explains why Milky Way and Andromeda galaxies appear to violate the cosmic order.

- By Don Pinnock

Simulated movement of local galaxies.

(Ewoud Wempe and collaborators)

The two largest galaxies in our Local Group are heading straight towards each other, apparently disobeying the cosmic order. Barring unforeseen events, they will collide and eventually merge several billion years from now.

They're travelling at roughly 100 kilometres per second — cosmically, that's a crawl, but the reason is what's interesting.

The impending collision has been known for decades and sits uneasily alongside another longstanding observation: almost every other nearby galaxy appears to be moving away from the Local Group.

Galaxies act like floating markers, their movement revealing how gravity is shaping the local universe. Astronomers have measured Andromeda’s motion precisely, and the outcome is considered inevitable. What has puzzled scientists for decades is what isn’t happening around them.

Apart from Andromeda and the Milky Way, almost every other nearby galaxy is following the general expansion of the universe. Even more surprisingly, these galaxies do not appear to be strongly affected by the enormous combined gravity of the Milky Way and Andromeda.