How can galaxies collide in an ever-expanding universe?

I receive an email.

Most posts are press releases about astronomical discoveries – ok, forget that; most among them is spam, but science announcements are an easy second place. But I also get questions from readers about various aspects of the universe that they have difficulty understanding.

I like this! On the one hand, it shows that people are really curious about science, and we need to dig into that. On the other hand, it gives me the opportunity to explain counterintuitive concepts that probably bother other people too.

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A recurring question I get is about cosmology (an understandably confusing subject): if the universe is expanding, how can galaxies collide? Shouldn’t they move away from each other, no towards each other?

There are actually two reasons why galaxies can collide in an expanding cosmos. One is that expansion only dominates on very large scales, and the other is that expansion competes with gravity.

Okay, first thing to do: the universe is actually expanding. We have known this for over a century now and it is the basis of modern cosmology. This idea is called the Big Bang model – an unfortunate name because it suggests an expanding cosmos like an explosion, with galaxies moving away from each other through space like shrapnel.

But in fact space itself expandsand it’s different. It’s not that galaxies move through space; it is that the expansion of space carries them with it. This has many deeply strange implications. The first is that the further away a galaxy is from us, the faster it appears to be receding.

Imagine a tape measure made from an extremely flexible material. The two ends are of course one meter apart. In the center you can mark two points one centimeter apart.

Now grab each end of this fake meter stick and stretch them so the stick is now two meters long. (Ask an imaginary friend for help.) The two end points moved a meter away from each other, moving at a speed of, say, a meter per second. But those marks you made earlier that were an inch apart are now two centimeters apart because the whole meter has stretched. This means that these two points moved away from each other at a speed of only one centimeter per second, much slower than the end points. In other words, the further apart two points are in an expanding scale, the faster they move away from each other.

That’s the universe in a nutshell. We see more distant galaxies moving away from us faster and we can even measure this change in speed as a function of distance. Roughly speaking, for every megaparsec of distance (3.26 million light years, a convenient unit for astronomers but not for anyone else), space expands by about 70 additional kilometers per second. So a galaxy located, say, 10 megaparsecs away from us is receding at about 700 km/s.

It’s pretty fast. But a galaxy one megaparsec away is only moving away at 70 km/s. Even if it’s still fast – a quarter of a million km/hour! – it is possible that galaxies travel faster than this. through space, like a shrapnel.

The Andromeda Galaxy offers an excellent example. It is the closest large spiral to our Milky Way, and we both belong to a regional group of galaxies called the Local Group. 2.5 million light years away, Andromeda should be moving backward at a speed of about 50 km/sec, but in fact it is moving towards us at a speed of about 110 km/sec. Indeed, the two galaxies are close enough to each other that each is attracted by the other’s gravity – so strongly, in fact, that their mutual speed is far greater than the capacity of the universe to separate them. This is also why Andromeda and the Milky Way could one day collide and even merge, but not for perhaps eight billion years.

And that brings us to the second reason why galaxies can still collide in an expanding universe. We think of gravity as a force that brings things together. But according to Einstein’s general theory of relativitygravity is actually a curvature of space-time, like a dimple in a leaf. If an object passes next to an object with large mass, such as a planet or galaxy, this deformation causes a curvature of the object’s path.

If two objects have sufficient mass and are moving at relatively slow speeds, they can be gravitationally bound, meaning their speeds cannot overcome gravity, and they stay close to each other in what is called a closed orbit. This is how a moon orbits a planet or two galaxies, like the Milky Way and Andromeda.

This is where things get weird. According to relativity, if space expands, it can’t extend within this delimited region. The mutual gravity of the objects within this region holds them together; space extends around this volume but not inside it. This means that if two galaxies get close enough to each other, they can still collide. To explore this topic further (so to speak!), my colleague and fellow science writer Ethan Siegel also wrote about this.

It gets even stranger than that, because we now know that the expansion of the universe is not constant. In 1998, two teams of astronomers announced that the expansion is accelerating, more and more rapidly, because of a still mysterious entity called dark energy. This could mean, for some still theorized behaviors of dark energy, that even space within a bounded region can expand. This effect would be stronger on scales of greater distance, so, for example, the slowest galaxies near the edge of a galaxy cluster would be lost to expansion, torn from the cluster like the outermost leaves of a head of lettuce.

Given enough time and relentless cosmic acceleration, each linked structures would be torn apart, even those linked by forces other than gravity, such as molecules and even atoms themselves! Astronomers call this idea the Great Rip, for fairly obvious reasons, and it’s not a very reassuring fate. But we really don’t know what dark energy is or how it behaves over long periods of timethe great rupture is therefore only a possible scenario for an extremely distant future.

So don’t worry: whether it’s the Great Rip or a collision with Andromeda, the time scales at play are so immense that these events won’t happen for eons (if at all), so they don’t really affect your daily life – unless you’re an astronomer, in which case they TO DO. But we like to think about such things and share those thoughts with you. I hope they help you expand your mind.