Grand Unification - 5.4x10 -44 to 10 -36  Seconds
Gravity having gone its separate way, the Strong Nuclear Force, The Weak Nuclear Force, and Electromagnetism, remain unified in a single electronuclear force.  The temperature is falling from its value of around 1032K at the start of this period.  Assuming the process of grand unification is correct, at the end of this period the strong nuclear force separates from the other forces after 10-36 of a second. The temperature falls below 1027K, the Grand Unification temperature, so no more X and Y bosons are created.  If the theory is correct, and X and Y bosons did exist, they couple quarks to leptons. 

The problem is that this would permit violation of the conservation of baryon number (B) and the conservation of lepton number (L), but would conserve (B minus L).  This implies that protons would be unstable and would decay over a long enough period of time; for example, if one of its quarks changed into an electron.  A proton, with B = 1 & L = 0, decays into a pion, with B = 0 & L = 0, and a positron, with B = 0 & L = -1.  Thus B minus L starts with a value of (1 - 0) = 1, and ends with a value of ((0 + 0) - (0 - 1)) = 1.  Despite numerous experiments, no evidence of proton decay has been found. 

The X and Y bosons decay into various combinations of up/down quarks/anti-quarks, electrons, electron antineutrinos and positrons.  The violation of the conservation of baryon number may result in a small excess of matter over antimatter, providing one possible explanation for Baryogenisis (see section "A Particle Soup").  Once all this is over, it signals the start of the Electroweak epoch where the weak nuclear force and electromagnetism remain joined as the electroweak force, while the Strong Nuclear Force goes its own way. 

Astronomy & Cosmology -

The Early Universe

(or "What Banged in the Big Bang?")