Alan Guth Quote

What it takes to produce a gravitational repulsion is a negative pressure. According to general relativity, it turns out... both pressures and energy densities can produce gravitational fields, unlike Newtonian physics, where it's only mass densities that produce gravitational fields.

Lecture 1: Inflationary Cosmology: Is Our Universe Part of a Multiverse? Part I. - The Early Universe (2012)