If the Universe is expanding, how can galaxies collide?

We may think that galaxies will never be able to find themselves in our fleeing cosmos, but in fact gravity can sometimes overcome even the expansion of the Universe itself.

A recurring question that astronomer is asked Phil Plait concerns cosmology, an understandably perplexing topic: if the universe is expanding, how can galaxies collide? Shouldn’t they be moving away from each other instead of getting closer?

In fact, there are two reasons why galaxies might collide in an expanding cosmos, explains Plait in an article in .

One of these reasons is that expansion only dominates on very large scales; and the other is that the expansion competes with gravity.

First of all: the universe is in fact expanding, despite this expansion being possible. We have known this for over a century, it was recently, and it is the based on modern cosmology.

This idea is called Big Bang modelo que, diz Plait, It’s an unfortunate namebecause it evokes the image of a cosmos expanding like an explosion, with galaxies flying away from each other through space like shrapnel.

But in reality It is space itself that is expandingand this is different. It’s not that galaxies are moving through space; is the expansion of space that drags them with it.

This has many profoundly strange implications. One of them is that the further away a galaxy is from us, the faster it seems to be moving away.

Imagine a one meter ruler made of an extremely flexible material. The two ends are, of course, one meter apart. In the center we can mark two points that are one centimeter apart.

Now let’s grab each end of this imaginary ruler and stretch it so that the ruler stay with two meters in length. We can ask an imaginary friend for help, jokes Plait.

So, both ends of the ruler moved one meter awaymoving, say, to one meter per second. But those marks we made before, which were an inch away, are now two centimeters awaybecause the entire ruler has stretched.

This means that these two points moved away from each other at a speed of just one centimeter per secondmuch slower than the extremities. In other words, the further away they are two points on an expanding scale, the faster they move away from each other.

This is the Universe, explained simply. We see more distant galaxies moving away from us more quickly, and we can even measure this variation 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 is moving expandir-se at uns 70 km/s additional. Therefore, a galaxy that is, say, 10 megaparsecs from us, is moving away at about 700 km/s.

It’s quite fast. But a galaxy to a megaparsec away is just move away at 70 km/s.

The galaxy of Andromeda offers an excellent example. It is the closest largest spiral to our Milky Way, and both belong to a regional cluster of galaxies called Local Group. 2.5 million light years from us, Andromeda it should be moving away at about 50 km/sbut it is actually heading towards us at approximately 110 km/s.

This happens because both galaxies are close enough for each is pulled by the gravity of the othera — pulled so hard, in fact, that their mutual speed is much greater than the ability of the universe to separate them.

This is also why it even merges, although not for perhaps another eight billion years. And this brings us to the second reason why galaxies can still collide in an expanding universe.

Normally, we think about gravity as a force that holds things together. But according to Einstein’s theory of general relativity, gravity is actually a curvature of spacetimelike a depression in a sheet. If an object passes close to something with a lot of mass, like a planet or a galaxy, this deformation causes the object’s trajectory to bendse curve.

If two objects have enough mass and if they are moving at relatively low speeds, may be gravitationally boundwhich means their speeds cannot overcome gravity, and they remain close together in a call closed orbit. A moon orbiting a planet is like this — or two galaxies, like the Milky Way and Andromeda.

This is where things get weird. According to relativity, if space is expanding, it cannot expand within this bound region. The mutual gravity of objects within that region keep us together; space expands around that volume, but not inside it. This, in turn, means that if two galaxies get close enough, they could still collide.

That said, there is no reason to worry: if Andromeda ever collides with our Milky Way, it will be billions of years from now. We won’t be here anymore. And in fact, even .