When did Higgs particles first appear in the early universe? For example, did they first appear during the electroweak epoch, the quark epoch, or...?
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The Higgs boson gains mass by "eating" the Goldstone mode corresponding to spontaneous electroweak symmetry breaking, so it appeared at the end of the electroweak epoch. Before the electroweak epoch, all particles were completely massless and travelled exactly at the speed of light.
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$\begingroup$ Now you've lost me. I thought Goldstone bosons are massless. The Higgs boson is massive as we all know. Right? $\endgroup$ Commented Sep 19, 2016 at 6:12
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$\begingroup$ @flippiefanus You're absolutely right. Sorry about that, I've corrected my answer. See the last line of page 3 of home.fnal.gov/~lykken/Physics_Today.pdf $\endgroup$– tparkerCommented Sep 19, 2016 at 6:17
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2$\begingroup$ Actually it is still not quite right. The Higgs scalar field (not the "Higgs boson") contains 4 degrees of freedom. After electroweak symmetry breaking three of these become the massless Goldstone bosons which are eaten by the gauge fields to give masses to the W and Z bosons. The remaining degree of freedom of the Higgs field becomes the massive Higgs boson. $\endgroup$ Commented Sep 19, 2016 at 6:24
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$\begingroup$ @flippiefanus I'm not an expert on the matter, but it sounds like you are - you should answer the OP's question yourself! :-) $\endgroup$– tparkerCommented Sep 19, 2016 at 6:28
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$\begingroup$ When it comes to what happened during the early universe, I'm not an expert either. :-) $\endgroup$ Commented Sep 19, 2016 at 7:21