Ask Ethan: Why will the Milky Way and Andromeda collide?

Of all the galaxies in the Universe that lie beyond the Milky Way, none looms larger than our “big sister” in the Local Group: Andromeda. Andromeda has more stars, more mass, and a larger physical extent than the Milky Way in all three dimensions. It spans a larger angular extent in our sky than six full Moons all lined up next to one another, and despite its location some 2.5 million light-years away from us, it’s actually moving in our direction, setting up a collision that should happen 4 billion years in our cosmic future. Another 3 billion years later, the greatest galactic merger in our Local Group’s history will be complete, leaving just one behemoth of a galaxy at its core: Milkdromeda. But why is this happening? After all, not only is the Universe expanding, but the expansion of the Universe is accelerating, too! How could these two seemingly paradoxical points both be true: the expanding Universe is accelerating, but Andromeda is heading towards us and is destined for a collision-and-merger with us? That’s what Robert Asselta wants to know, writing in to inquire: <a href="https://medium.com/starts-with-a-bang/ask-ethan-why-will-the-milky-way-and-andromeda-collide-eda48c9f35dc">Learn More</a>