The Universe is expanding, and individual, bound structures are all receding away from one another. How, then, are galaxies still colliding?
Here in our Universe, an astrophysical phenomenon continues to occur that seems paradoxical. The Universe is expanding, and the expansion itself is accelerating due to dark energy, causing distant objects to recede from one another at ever-increasing rates. When we look at galaxies, we see this directly: the farther away they are from us, the faster they recede from our perspective. Moreover, because of dark energy, if we watch any individual galaxy recede from us over time, we’ll find that it speeds up in its recession: exactly what we mean when we say that the Universe’s expansion is accelerating.
And yet, all across the Universe, both nearby and far away, galaxies — the very objects that should all be receding mutually away from one another — are seen interacting: merging, colliding, and cannibalizing one another. How are these two seemingly contradictory things both consistently true? How can the Universe be expanding, with galaxies receding away from one another, while galaxies are also finding each other, colliding and merging, simultaneously? That’s what Tom Peacock…
