Big Bang

Big Bang is the first observed event of Observable Universe, happened in approximately 13.8 billion years ago. It is also the first event of every single Universe, as it starts the formation of an Universe.

People thought the Big Bang caused the cosmic inflation, but that inflation actually happened in around 10-33 - 10-32 seconds after the Big Bang. Some scientists predict there was a gravitational singularity before this event, but we decided to use that scenario for our cosmology.

Planck epoch already happened immediately after it, lasting a single Planck time. No elementary particles and forces were already formed during that period. (t = 10-43 s) People theorized that an Universe was a single point with a size of a Planck Length, with Planck Temperature everywhere.

After that short period, ending the whole event of Big Bang and starting the Grand Unification epoch, when the four physical forces start working.

On Multiverses
In Multiverses, including ours, Big Bang is basically caused by colliding with two universes. This collision bents the areas nearby the borders beyond spacetime, transforming them to new gravitational singularities with infinite gravity at their centers. The heavy gravitational forces cause those singularities to inflate, which starts the Big Bang event on those points. You can see the full explanation at Universe Formation.

The expansion forces caused by a Big Bang pushed the nearby universes out, but the objects in those universes feel like it doesn't change at all.