We should not act like we know that the universe began with the Big Bang... we'll see that there are strong suggestions that the Big Bang was perhaps… - Alan Guth

The gravitational repulsion created by this small patch of repulsive gravity material would be, then, the driving force of the Big Bang and it would cause the region to undergo exponential expansion... there is a certain doubling time, and if you wait the same amount of time it doubles again, and if you wait the same amount of time it doubles again... and it's because these doublings build up so dramatically, it doesn't take very much time to build the whole universe. In about 100 doublings this tiny patch of 10^-28 cm can become large enough, not to be the universe, but to be a small marble-sized region which will then ultimately become the observed universe, as it continues to coast outward after inflation ends.

It is said that there’s no such thing as a free lunch. But the universe is the ultimate free lunch.

A positive pressure produces an attractive gravitational field... Positive pressures are just sort of normal pressures and attractive gravity is normal gravity, so normal pressures produce normal gravity, but it is possible to have negative pressures, and negative pressures produce repulsive gravity, and that's the secret of what makes inflation possible.