Towards a nonsingular bouncing cosmology
Abstract
We present a nonsingular bouncing cosmology using single scalar field matter with nontrivial potential and nonstandard kinetic term. The potential sources a dynamical attractor solution with Ekpyrotic contraction which washes out small amplitude anisotropies. At high energy densities the field evolves into a ghost condensate, leading to a nonsingular bounce. Following the bounce there is a smooth transition to standard expanding radiation and matter dominated phases. Using linear cosmological perturbation theory we track each Fourier mode of the curvature fluctuation throughout the entire cosmic evolution. Using standard matching conditions for nonsingular bouncing cosmologies we verify that the spectral index does not change during the bounce. We show there is a controlled period of exponential growth of the fluctuation amplitude for the perturbations (but not for gravitational waves) around the bounce point which does not invalidate the perturbative treatment. This growth induces a natural suppression mechanism for the tensor to scalar ratio of fluctuations. Moreover, we study the generation of the primordial power spectrum of curvature fluctuations for various types of initial conditions. For the pure vacuum initial condition, on scales which exit the Hubble radius in the phase of Ekpyrotic contraction, the spectrum is deeply blue. For thermal particle initial condition, one possibility for generating a scaleinvariant spectrum makes use of a special value of the background equation of state during the contracting Ekpyrotic phase. If the Ekpyrotic phase is preceded by a period of matterdominated contraction, the primordial power spectrum is nearly scaleinvariant on large scales (scales which exit the Hubble radius in the matterdominated phase) but acquires a large blue tilt on small scales. Thus, our model provides a realization of the "matter bounce" scenario which is free of the anisotropy problem.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 August 2012
 DOI:
 10.1088/14757516/2012/08/020
 arXiv:
 arXiv:1206.2382
 Bibcode:
 2012JCAP...08..020C
 Keywords:

 High Energy Physics  Theory;
 Astrophysics  Cosmology and Extragalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology
 EPrint:
 21 pages, 11 figures, relevant references added