“So the vacuum energy can result in negative pressure. And this is very interesting, because pressure is a form of energy. General relativity predicts that not only is mass gravitationally attractive, but positive pressure is also gravitationally attractive. And so, quite incredibly, negative pressure is predicted to result in gravitational repulsion! As Alan Guth says in The Inflationary Universe: "According to general relativity, a positive pressure creates an attractive gravitational field, as one might guess. A negative pressure, however, creates a repulsive gravitational field." So next time someone says that gravity is a force which is always attractive, you can correct them: negative pressure results in gravitational repulsion!”