I read that before the beginning of the quark epoch at t = 10–12 sec., particles don't have any mass. Does that mean that all particles before t = 10–12 sec travel at c? In other words, can a particle without mass travel at less than c? The Wikipedia article on massless particles says, "The behavior of massless particles is understood by virtue of special relativity. For example, these particles must always move at the speed of light." I would be happy to adjust the Wikipedia article if this statement is inaccurate. If such particles do always move at c, and all particles at this epoch are massless, how can it be said that space exists during this epoch? What non-c "observer" measures spatial separation between objects (or temporal duration for that matter)? It seems that mass/sub-c travel is required for the concept of "reference frame" to have any ontology or meaning. Any help with this conundrum would be appreciated -- thank you.