I want to learn about cosmology, maybe start with the book “An Introduction to Modern Cosmology” by Andrew Liddle and the lecture “The Early Universe” by MIT OpenCourseWare.

The standard Big Bang

It does not tell the cause of the Bang, but the aftermath of the Bang. It assumes all the matter already existed before the Bang.

Cosmic Inflation

A prequal to the Big Bang.

• gravity can be repulsive when pressure is negative. (Find out more about the relationship between gravity and pressure in the next part.)
  There is a patch of repulsive gravity material in the early universe, which is the cause of the Big Bang.
  The (mass/energy) density of the repulsive material is not lowered as it expands.The resolution is to introduce negative energy.

Complementarity: The relationship between gravity and pressure

Below is a high-level introduction to these ideas, followed by some of the key formulas in cosmology and general relativity that show why negative pressure can produce a repulsive gravitational effect.