If the FLRW metric equations are assumed to be valid all the way back to the beginning of the universe, they can be followed back in time, to a point where the equations suggest all distances between objects in the universe were zero or infinitesimally small. (This does not necessarily mean that the universe was physically small at the Big Bang, although that is one of the possibilities.) This provides a model of the universe which matches all current physical observations extremely closely. This initial period of the universe's chronology is called the "Big Bang". The Standard Model of cosmology attempts to explain how the universe physically developed once that moment happened.
