What is the role of negative gravity in the Big Bang theory?

[TR345]

As i understand, the current idea is that before the big bang, the four forces were combined to make one force and gravity breaking apart triggered inflation and that gravity was for a period after the big bang negative and at some point became posative. What is this part of the theory based on? How does the idea of negative gravity clash with general reletivity? Is this idea just speculation about the big bang theory, or is it part of the theory?

 

[hellfire]

The negative gravity that had driven inflation can be explained with the properties of scalar fields, a very special kind of matter that has not, however, been experimentally discovered yet. In a state in which the kinetic energy is negligible against the potential energy, scalar fields extert a negative pressure. In general relativity a negative pressure has the property of acting as repulsive gravity accelerating the expansion of space.

It is usually assumed that inflation was driven by a scalar field called inflaton. However, the standard model of cosmology tells us neither about the origins of the inflaton nor about how it got to that state of high potential energy. It is scope of quantum cosmology to describe the adequate conditions for the start of inflation.

Quantum cosmology is the branch of cosmology that studies the physics near the big-bang singularity. The theory of general relativity breaks down when describing singularities like the big-bang, and can only describe to the evolution of the universe at a time before the Planck time. Before the Planck time the quantum effects of the gravitational field become important due to the high energies and small distances involved. In order to formulate quantum cosmological models a theory of quantum gravity is necessary that unifies general relativity with quantum mechanics.

Different proposals for theories from quantum gravity exist (string theory and loop quantum gravity as the most promising proposals) that lead to different models of quantum cosmology. Most of these models decribe a universe in a quantum gravity phase that ends in, or contains, an inflationary phase, and explain how the inflaton got to the state of high potential energy. On the other hand, other models eliminate the need for an inflaton. This is the case of some models of loop quantum cosmology, in which the negative gravity is a property of the gravitational field at very high energies. Or also some string models, in which the scalar field that drives inflation is only an effective description at low energies of the behaviour of strings.

If you are interested in some details you may take a look to some of the last entries of https://www.physicsforums.com/blogs/viewblog.php?userid=3727 , where I have described the different approaches to quantum cosmology.