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 Quote by Royce What caused inflation in the big bang theory? I've read a few things that seem pure speculation designed to give a cause for something thought to necessarily have happened such as an antigravity force that faded at the end of the inflationary period. It seems too contrived to me.
Basically inflation is antigravity in the sense that there is a gravitational repulsion. There are several ways for such a repulsion to occur. One is a non-zero cosmological constant. The other is vacuum energy and the third is as explained above, i.e. negative presssure. To see how pressure can act as a source in GR consider the weak field limit for a relativistic fluid which mass density $$\rho$$ and pressure p. Then

$$\nabla^2\Phi = 4\pi\G(\rho + 3p/c^2)$$

The necessary (but not sufficient) condition for a period of inflation is

$$\rho_{active} = \rho + 3p/c^2 < 0$$

which produces gravitational replusion since the right hand side of the first equation acts like a negative mass density. The term $$\rho_{active}$$ is the active gravitational mass density.

A similar thing happens with a positive cosmological constant.

Note: Regarding the inflaton - As I understand it, it is the vacuum energy of the scalar field which drives inflation and not the scalar field itself.

To see this from the man himself, i.e. Alan Guth, the creator of the infation theory, see

http://ocw.mit.edu/OcwWeb/Physics/8-...otes/index.htm

Go down to Guth's lecture and click on the material to the right which you'd like to see.

He also discussed the possibility of creating a universe in the laboratory. [:-)]