In an expanding universe, why do galaxies collide?

Galaxies collide all the time.  How is this possible
when the universe is expanding?
(credit: NASA, STScI)

Here's another great question from a Space Fan:

"If the universe is expanding in all directions how is it that galaxies are able to collide?  Like dots on an expanding balloon I would think space between all the galaxies would be ever increasing."

The universe is expanding, observations clearly bear that out - that is, when we look at all the distant galaxies everywhere, we see them all flying away from each other.

The distant galaxies, that is.  For the ones close to each other, and the ones in our local group, including the Andromeda Galaxy, it's a different story.  The galaxies near one another are not necessarily flying away.  In fact, many are not.

The expanding universe, it turns out, is an overall effect, not a local one.  In fact, it may even be true that the universe is expanding at different rates at different times in different places.  There are even some theories suggesting that inflation is still happening in remote regions of the cosmos.

But let me reign this in a bit.  Let's get back to what we KNOW is happening:  the universe is accelerating as it expands.  So, why do we still have galaxy collisions?

When astronomers first started investigating the redshifts of distant galaxies, many expected to find that the universe is slowing in its expansion.  Due to the effects of gravity slowly overcoming the forces from the Big Bang and cosmic inflation that flung everything outward to begin with, the most distant galaxies would appear to be slowing down.

They didn't find that,  instead, the cosmos is speeding up and getting faster with each passing day.  Something is pushing the universe apart, spacetime itself appears to have a repulsive quality, one that is opposite gravity.

This repulsive quality is far from understood, but many are calling whatever-it-is-that-pushes-out, Dark Energy.

This force is not very strong on small scales, at least not yet.  What do I mean by small scales?  Things that are closer together than, say, a group of galaxies is too small for Dark Energy to dominate.

The reason we don't see the atoms in our bodies, the kitchen table, our house, or even the Earth blowing apart is that other - much stronger than Dark Energy - forces are at work holding things together.

On scales like the Earth, gravity overcomes the repulsive force and keeps the planet whole.  On the molecular level, atomic forces overwhelm Dark Energy and hold the atoms together.

Even scales as large as individual galaxies and clusters of galaxies are too small, Dark Energy is too weak to have an effect.  The gravitational pull of the stars within a galaxy or the pull of one galaxy on another is stronger than Dark Energy's effects.

All galaxies in the universe swarm in a seething cauldron of activity.  Some are swirling around each other: going both towards and away in a huge cosmic dance, choreographed by gravity.

This is what's happening in our local group:  as you may know, the Milky Way and the Andromeda Galaxy are on course for a head-on collision in a few billions years time.  This is possible because the force that fuels the universal expansion isn't strong enough locally to overcome the relatively nearby effects of gravity on the two galaxies.

On REALLY large scales however, like the entire universe or large parts of it, Dark Energy rules.  When we look at the entire cosmos, the repulsive push of enormous quantities of Dark Energy spread everywhere begins to be seen.

The really interesting question about Dark Energy (at least to me) is, what if Dark Energy changes with time? What if it gets stronger as the universe gets bigger?  What would that be like?

What if, as the universe slowly evaporates and the galaxies become lonely, isolated islands, Dark Energy gets stronger?  What happens then?

In such a case, if Dark Energy is getting stronger, then it's possible we will reach a point when the stars in a galaxy DO start growing apart.  The local strength of Dark Energy could get strong enough to push the stars apart, breaking up the galaxy.

What's more, if it continues to get stronger, Dark Energy could overcome the gravity holding the planets around the Sun, the molecules in our bodies, the atoms in our kitchen tables, and possibly even break up the atoms that make up all matter.

Including us.

This brings spontaneous human combustion to a whole new level.

Keep Looking Up!