Origins of the Fundamental Interactions/Forces

[HeavyWater]

Are the four fundamental interactions (or "forces") e.g. gravity, weak, color, electromagnetic believed to be the result of "phase changes" as the universe expanded from the quark-gluon plasma?

 

[rootone]

Yes, this theory is believed by many, although it's not accepted as a certainty.
http://www.ctc.cam.ac.uk/outreach/origins/cosmic_structures_one
http://abyss.uoregon.edu/~js/ast123/lectures/lec18.html

 

[PeterDonis]

Are the four fundamental interactions (or "forces") e.g. gravity, weak, color, electromagnetic believed to be the result of "phase changes" as the universe expanded from the quark-gluon plasma?

This is basically what is believed, but the degree of confirmation is not the same for all the forces. (Also, if all four forces were in fact unified in the very early universe, then the state at that time would not have been a quark-gluon plasma; quarks and gluons would not have been distinct from any other particles at that time. In fact, this would not have been a good description any time before the Grand Unification phase transition; quarks and gluons only became distinct particles when the strong force and the electroweak force separated--see below.)

We are very confident that the weak and electromagnetic forces were unified before the electroweak phase transition, since we can probe energies at or somewhat above the energy of that phase transition experimentally (the LHC discovery of the Higgs particle was a result of this). The electroweak model is an integral part of the Standard Model of particle physics.

We are fairly confident that the strong (color) force was unified with the electroweak force before the Grand Unification phase transition (I think that's the usual name for it), because, although we can't do experiments at or near the estimated energy scale of that transition, extensions of the Standard Model that include it seem to do pretty well on predictions that we can test. But AFAIK it is not currently known exactly which extension of the Standard Model is the right version of grand unification.

The idea that gravity was unified with the other three forces at some point in the very early universe is, at this point, only a reasonable hypothesis. We can't probe energies anywhere near the scale of the phase transition that would have been involved, and we don't even have a good theory of quantum gravity at this point, so we don't have any way of making predictions based on the hypothesis that we can actually test. Many physicists like the idea because it obviously makes things simpler conceptually, but that's as much as we can say.

 

[HeavyWater]

Thanks Rootone and PeterDonis. Your answers were much better than I expected.