Eugene Ulrich said:
How certain can we be that the four fundamental forces as identified in modern cosmology, owe their existence to broken symmetry?
I think that the running idea at present is that it seems to be a reasonable conclusion that the standard model of particle physics (gravity not included, as PeterDonis stated) derives from a broken symmetry. It's not firmly-established by evidence, but it's basically the only way to make the standard model into a theory that makes any sense.
The standard model, as it stands, requires 19 different constants as input values (Wikipedia lists them
here). There's nothing in the theory which sets these values. But they all seem to be constant numbers that are the same everywhere in the observable universe.
Usually when you've got a bunch of different parameters, it means that there's something more fundamental going on that reduces the parameters. In the most general sense these could be:
1) There are some physics which we don't currently understand which relates these parameters to one another. E.g., if parameter A takes value 2, then parameter B must take value 3. The standard model has these parameters independent, but maybe a more fundamental theory will tell us that we have fewer parameters to work with. In the ideal case, all 19 parameters simply had to take the specific values they take because of the way the more fundamental theory behaves.
2) There are some dynamics in the early universe which tend to explore a bunch of different values for these parameters. The parameters are "sticky" in that entire regions end up with the same values.
But in either event, going down either path requires discussions of symmetry breaking. These 19 parameters represent a lack of symmetry in the standard model. Any more fundamental theory which has fewer free parameters would, by definition, have more symmetry. Symmetry that is broken within the observable universe, and broken in such a way that these 19 parameters keep the same values throughout the observable universe.
The first option taken above can involve theories which explore different values of the 19 parameters in various ways, but the universe tends to evolve towards a specific state, e.g. the lowest-energy state. This direction of investigation is the least controversial of the two approaches, but so far has proven largely fruitless.
The second option tends more towards theories which tend to produce a lot of different observable universes with different values for the 19 parameters. These often lead to discussions of the anthropic principle. This approach is far more controversial, but may have already produced at least some limited successes. But it's really difficult to do it correctly, and, as I said, very controversial.
Either way, symmetry breaking is almost certainly a feature of our universe. The questions are all about the nature of the symmetry (what specific symmetry was broken), and how it was broken.
