Final Report Summary - STRINGCOSMOS (String Cosmology and Observational Signatures)
The ERC Starting Grant "StringCosmOS" has allowed the establishment of the Theoretical Cosmology group at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute). Over the course of the grant, the group has pursued research in several complementary topics, starting with the elaboration of early universe models that are in agreement with current cosmological observations while predicting interesting signatures for future observations. These so-called ekpyrotic models assume the existence of a contracting phase prior to the current expanding phase, and two coupled scalar fields. While these models manage to explain the standard cosmological puzzles, just as inflation does, they are distinguished by predicting small but particular non-Gaussian corrections in the distribution of temperature fluctuations observed in the cosmic microwave background. Thus, with future observations we may hope to find or rule out indications for the existence of such a contracting phase in our past.
We have also made attempts to understand/overcome the big bang singularity by studying models of classically non-singular bounces, i.e. models where a previous contracting phase smoothly bounces into the current expanding phase. Such transitions are of obvious relevance for the cosmological models just described. A particular achievement is the cosmological super-bounce, which is the first description of a cosmological bounce in supergravity. This model also allows one to unambiguously propagate cosmological perturbations across the bounce, showing how information from a previous phase gets transferred to the current expanding phase of the universe. Importantly, we found that the long-wavelength perturbations of observational interest are essentially unaffected by such non-singular bounces, and hence one may reliably follow cosmological perturbations from their generation during the contracting phase into the current expanding phase of the universe.
The super-bounce was based on a general formalism for treating higher-derivative kinetic terms for scalar fields in supergravity, which was developed over the course of the project. This formalism has many applications, in particular also to inflationary models in string theory and supergravity. In this context we discovered an interesting effect, namely that in supergravitational inflationary models the potential can be affected by the kinetic terms, i.e. the dynamics and the potential of the theory affect one another.
More conceptual issues that we have studied include the issue of how out of initial quantum fluctuations classical density perturbations can be generated, and gravitational tunneling by which a region of the universe can make a quantum transition into a physically distinct phase.
Finally, we have developed the no-boundary proposal further, which is a theory of initial conditions for the universe proposed by J. Hartle and S. Hawking. In this context we discovered new solutions, which have a very high probability and describe the emergence of classically contracting universes out of an initial quantum state. The existence of these solutions promises to have considerable significance for the early history of our universe. We also established the interesting result that within gravitational models coupled to a scalar field, only inflation and ekpyrosis manage to explain the emergence of a classical spacetime out of an initial quantum state. This result reveals an unexpected link between models that can dynamically explain the flatness of the universe and the very classicality of space and time. Inflation and ekpyrosis are thus even more special than we thought!
We have also made attempts to understand/overcome the big bang singularity by studying models of classically non-singular bounces, i.e. models where a previous contracting phase smoothly bounces into the current expanding phase. Such transitions are of obvious relevance for the cosmological models just described. A particular achievement is the cosmological super-bounce, which is the first description of a cosmological bounce in supergravity. This model also allows one to unambiguously propagate cosmological perturbations across the bounce, showing how information from a previous phase gets transferred to the current expanding phase of the universe. Importantly, we found that the long-wavelength perturbations of observational interest are essentially unaffected by such non-singular bounces, and hence one may reliably follow cosmological perturbations from their generation during the contracting phase into the current expanding phase of the universe.
The super-bounce was based on a general formalism for treating higher-derivative kinetic terms for scalar fields in supergravity, which was developed over the course of the project. This formalism has many applications, in particular also to inflationary models in string theory and supergravity. In this context we discovered an interesting effect, namely that in supergravitational inflationary models the potential can be affected by the kinetic terms, i.e. the dynamics and the potential of the theory affect one another.
More conceptual issues that we have studied include the issue of how out of initial quantum fluctuations classical density perturbations can be generated, and gravitational tunneling by which a region of the universe can make a quantum transition into a physically distinct phase.
Finally, we have developed the no-boundary proposal further, which is a theory of initial conditions for the universe proposed by J. Hartle and S. Hawking. In this context we discovered new solutions, which have a very high probability and describe the emergence of classically contracting universes out of an initial quantum state. The existence of these solutions promises to have considerable significance for the early history of our universe. We also established the interesting result that within gravitational models coupled to a scalar field, only inflation and ekpyrosis manage to explain the emergence of a classical spacetime out of an initial quantum state. This result reveals an unexpected link between models that can dynamically explain the flatness of the universe and the very classicality of space and time. Inflation and ekpyrosis are thus even more special than we thought!