Question: If not gravity, how else could distant galaxies be held together?

Benjamin Bose answered on 11 Mar 2016:

I’m sure there are a hundred and one possible answers to that question, but they’d all need a serious complicating of what we think we know about the universe. Simplicity is usually the guide scientists use to figuring how things work 🙂

Basically for any two things to be held together, from everything we see around us, requires a force of some kind :

If I throw a ball in the air, it comes back because of gravity.

The particles of your computer are held together because of electromagnetism.

Those particles are held together because of forces.

So we say this is true for galaxies too – they group together because of some force.

Now gravity has the longest range from all the forces we know – it works exactly how we predict it to within our solar system – pretty big but not as big as groups of entire galaxies! We make the big ASSUMPTION that this same gravity works within the Milky Way galaxy and even beyond – on groups of many many galaxies – but this may not be true and there is good reason to do so.

The gravity we think works everywhere has a few issues – it needs a special type of energy to work well at very very big distances, a type of energy which works in the opposite way of normal energy, i.e. it pushes everything apart!

The idea behind modified gravity is saying FORGET the ASSUMPTION gravity works everywhere – say there is some other force, or some change in gravity, that becomes outside the solar system. By changing the force itself we wouldn’t need to add in strange energies.

In summary, if not gravity, then some other type of force – very similar to gravity perhaps – but not exactly what it is here in the solar system – weaker perhaps, different sources perhaps (perhaps mass doesn’t cause it!), anything the imagination can conjure really (and which helps solve the problem gravity has at big big distances!)…..

….or perhaps they’re just all held together by strings 😀