What led to the electroweak and strong forces splitting?

Is the reason for the split believed to be spontaneous symmetry breaking? If so, did SSB occur because the Universe was cooling rapidly from extremely high temperatures?

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what makes you think they were "together" in the first place? –  Timtam Jul 30 '13 at 16:16
@ Timtam: Sorry, I know it is a theory and has not been proven –  curiousGeorge119 Jul 30 '13 at 16:18
How else would spontaneous symmetry breaking happen other than by cooling from high temperatures? –  BebopButUnsteady Jul 30 '13 at 16:38
@BebopButUnsteady: Thanks, I appreciate your comment! That is helpful, since I am not a physicist –  curiousGeorge119 Jul 30 '13 at 16:42
@Timtam ever heard about GUT models, that could have been relevant in the early universe or that could still be relevant at high enough energies? –  Dilaton Jul 30 '13 at 18:59

There is no fully worked out theory of how the strong and electroweak forces split, because there is no fully worked out unified theory to describe them. Various attempts have been made, e.g. Georgi and Glashow's SU(5) theory and Georgi (and others?) SO(10) theory, however none have proved fully satisfactory.

The idea that the electroweak and strong forces unify is an attractive one, and is supported by the convergence of the coupling constants if (and only if) supersymmetry intervenes in the right way. However grand unification remains unproven.

If the electroweak and strong forces do unify then it seems likely it would be broken by spontaneous symmetry breaking just as at the electroweak transition. This would happen when the average particle energies fell below around $10^{16}$ GeV.

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Thank you for this information, John Rennie! Does this mean SSB is the reason electromagnetic and weak force split, but not the reason the electroweak and strong force split? –  curiousGeorge119 Jul 30 '13 at 18:08
SSB is definitely the reason for electroweak symmetry breaking and a probable mechanism for electroweakstrong symmetry breaking as well. The details of the breaking mechanism are different in the two cases, but the idea is broadly the same. However take this with a large pinch of salt as neither SU(5) nor SO(10) give an adequate description of the real world. –  John Rennie Jul 30 '13 at 18:28
Interesting! Thank you very much! –  curiousGeorge119 Jul 30 '13 at 18:41
You should also mention that GUT symmetry breaking specifically requires new Higgs fields, to produce the SSB. The Higgs we know about breaks electroweak symmetry, to break further symmetries requires further Higgses that we don't know about. (Or it requires something, e.g. a condensate of new fermions, to be the causal agent, the point is that SSB doesn't just happen by itself.) –  Mitchell Porter Jul 30 '13 at 23:35
@MitchellPorter: Thank you very much for commenting, I have not read that!! –  curiousGeorge119 Aug 1 '13 at 15:40