Inflationary theory versus the ekpyrotic/cyclic scenario.
Abstract
I will discuss the development of inflationary theory and its present status, as well as some recent attempts to suggest an alternative to inflation. In particular, I will argue that the ekpyrotic scenario in its original form does not solve any of the major cosmological problems. Meanwhile, the cyclic scenario is not an alternative to inflation but rather a complicated version of inflationary theory. This scenario does not solve the flatness and entropy problems, and it suffers from the singularity problem. We describe many other problems that need to be resolved in order to realize a cyclic regime in this scenario, produce density perturbations of a desirable magnitude, and preserve them after the singularity. We propose several modifications of this scenario and conclude that the best way to improve it is to add a usual stage of inflation after the singularity and use that inflationary stage to generate perturbations in the standard way. This modification significantly simplifies the cyclic scenario, eliminates all of its numerous problems, and makes it equivalent to the usual chaotic inflation scenario.
 Publication:

The Future of Theoretical Physics and Cosmology
 Pub Date:
 November 2003
 arXiv:
 arXiv:hepth/0205259
 Bibcode:
 2003ftpc.book..801L
 Keywords:

 Theoretical Physics;
 High Energy Physics  Theory;
 Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology;
 Physics  Popular Physics
 EPrint:
 22 pages, 8 figs, a talk at Stephen Hawking's 60th birthday conference, Cambridge University, Jan. 2002