September 13, 2005

September 13

Robert H. Brandenberger tries to Look Beyond Inflationary Cosmology. Quoting him freely: Scalar field-driven inflationary cosmology suffers from serious conceptual problems many of which relate to ultraviolet issues it is likely that the same new fundamental physics required to address the ultraviolet problems of the Standard Model.

Since the paper is a conference contribution he lists the conceptual problems of conventional scalar field-driven inflation. The first problem, the amplitude problem, relates to the amplitude of the spectrum of cosmological perturbations. A more serious problem is the trans-Planckian problem: all scales inside of the Hubble radius today started out with a physical wavelength smaller than the Planck scale at the beginning of inflation. A third problem is the singularity problem. Recently, the singularity theorems have been generalized to apply to Einstein gravity coupled to scalar field matter where a past singularity at some point in space is unavoidable. The Achilles heel of scalar field-driven inflationary cosmology is the cosmological constant problem.

Then he goes over to suggest solving these problems using string theory.

An immediate problem which arises when trying to connect string theory with cosmology is the dimensionality problem. The moduli fields must be stabilized. Resolving the singularity problem is another of the main challenges. One must also solve the flatness problem. Finally, there must a mechanism to produce a nearly scale-invariant spectrum of nearly adiabatic cosmological perturbations.

The way to attack these problems is to focus on symmetries and degrees of freedom which are new to string theory (compared to point particle theories). The symmetry he makes use of is T-duality, and the new degrees of freedom are string winding modes.

Postulating that T-duality extends to non-perturbative string theory leads to the need of adding D-branes to the list of fundamental objects in string theory. With this addition, T-duality is expected to be a symmetry of non-perturbative string theory.

My comments: The role of UV problems in going beyond current theories has been known for long. In this paper the known issues of cosmology are traded for new questions in string/brane cosmology. This is how progress is usually made, provided the new problems can be solved. We remain to wait with interest.