Answer the following question given this paragraph: 

H0 = 20km/s ⋅ Mly. • Explanations of the large-scale characteristics of the universe are intimately tied to particle physics. • The dominance of matter over antimatter and the smoothness of the CMBR are two characteristics that are tied to particle physics. • The epochs of the universe are known back to very shortly after the Big Bang, based on known laws of physics. • The earliest epochs are tied to the unification of forces, with the electroweak epoch being partially understood, the GUT epoch being speculative, and the TOE epoch being highly speculative since it involves an unknown single superforce. • The transition from GUT to electroweak is called spontaneous symmetry breaking. It released energy that caused the inflationary scenario, which in turn explains the smoothness of the CMBR.


Q: The epochs of the universe are known back to very shortly after what 'explosive' event?


A:
big bang