Big Bang
Definition
The hot, dense beginning of our universe 13.8 billion years ago — space itself expanding from an extremely compact state.
Technical definition
The cosmological model describing the universe's expansion from a hot dense state ~13.8 billion years ago. Evidence: Hubble expansion, cosmic microwave background (2.725 K relic radiation), and light-element abundances matching Big Bang nucleosynthesis predictions.
Example
The cosmic microwave background — faint microwaves from every direction — is the cooled afterglow of the Big Bang, now stretched to 2.7 K.
Common confusion
The Big Bang was not an explosion IN space — it was the expansion OF space. There is no ‘center’ it blew out from, and ‘before’ the Big Bang may be a meaningless question in standard cosmology.