If the universe is expanding, how can galaxies collide?

The NASA/ESA Hubble Space Telescope has snapped the best ever image of the Antennae Galaxies. The galaxies — also known as NGC 4038 and NGC 4039 — are locked in a deadly embrace. Once normal, sedate spiral galaxies like the Milky Way, the pair have spent the past few hundred million years sparring with one another. This clash is so violent that stars have been ripped from their host galaxies to form a streaming arc between the two. Image credit: ESA/Hubble

The NASA/ESA Hubble Space Telescope has snapped the best ever image of the Antennae Galaxies. The galaxies — also known as NGC 4038 and NGC 4039 — are locked in a deadly embrace. Once normal, sedate spiral galaxies like the Milky Way, the pair have spent the past few hundred million years sparring with one another. This clash is so violent that stars have been ripped from their host galaxies to form a streaming arc between the two. Image credit: ESA/Hubble

This apparent contradiction comes from the way that scientists commonly explain the expansion of the universe; we say, “Imagine a balloon with a series of dots on the outside of it. Now inflate the balloon. All the dots move away from each other as the balloon, which is space, grows in size.”

Or, “Imagine you have a loaf of bread with raisins in the surface. As the dough rises, the raisins will spread further apart from each other.”

This is an accurate metaphor- the fabric of space is expanding as the universe ages. However, when we make these metaphors, we draw out our objects in space - the dots on the surface of the balloon, or our raisins in bread dough - in regular patterns. We put everything on a grid, so that the effects of the universe’s expansion are easier to spot. This is convenient because it means we’re only looking at one effect (the expansion), but it’s a big oversimplification of the universe.

Objects in the real universe aren’t laid out on a grid. The universe doesn’t do grids. Real galaxies are scattered randomly across the fabric of space, which means that sometimes you’re going to wind up with one or two or 50 galaxies pretty close to each other. Sometimes you’ll wind up with a galaxy with nothing around it at all.

When you have two enormously massive objects relatively close in the universe, another force takes over. Gravity. Gravity is an extremely powerful attractive force, and if two objects are near enough to each other to feel the gravitational pull of the other galaxy, it doesn’t matter that the universe is expanding; it’s not expanding fast enough to counteract the attractive force of gravity, and these two objects are going to fall towards each other. When they do, there’s a good chance they will eventually become a single, larger galaxy, and the process gives us magnificent images.

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