In recent years, CITA has been a center of activity for studies of the early universe. Kofman and collaborators Frolov and Felder have been studying the implications of string theory and the braneworld scenario for the expansion of the universe in its early stages, and the generation of scalar and tensor perturbations. Beloborodov and Frolov has also conducted research in classical general relativity.
Together with the summer PhD student Ting Ting Lu, Levin is analysing the performance of a proposed pulsar-timing array in detecting of the gravitational-wave background.
Antony Lewis studied vector modes and their observational signature in the CMB. There is a regular primordial mode with a potentially interesting observational signature in the CMB polarization, for which he computed the theoretical predictions. Vector modes are also sourced by magnetic fields, and he computed numerically the effect of primoridal inhomogeneous magnetic fields on the CMB power spectra, identifying some new effects and problems with previous results.
Marco Peloso's activity in the past year was focused on braneworld models in extra dimensions, and on modifications of standard 4d gravity. Together with J. Martin, G.N. Felder, A.V. Frolov, and Lev Kofman (CITA) he studied the dynamics of braneworlds in the early universe. In most cases, the evolution is too complicated to be solved analytically. A numerical program was specifically designed for this study, and it was then applied to a variety of configurations. The results obtained were in some cases very different from the ones assumed by several authors on the basis of analytical approximations. Two other works in the context of extra dimensions were done in collaborations with L. Kofman and J. Martin (CITA), and with C.R. Contaldi and L. Kofman (CITA). In the first of these works, he studied analytically the system of scalar perturbations when the extra dimension is stabilized by a bulk scalar field. This calculation is necessary to compute how these modes are coupled to brane fields, which can then be used to set limits/predictions from accelerators experiments. In the second work, he studied how the stabilization of the extra space is affected by the expansion of the standard spatial coordinates; stabilization in the early universe is typically more difficult than at present. A different topic was investigated in collaboration with L. Sorbo (UC Davis). This work discusses the phenomenology of ghost condensation, which is a class of model recently introduced as an infrared modification of gravity. The motion of the sources was shown to have a drastic effect on the phenomenology of these models, invalidating the main signatures previously considered.
Dmitry Podolsky considered the structure of cosmological singularity in string theory inspired Veneziano's cosmological model of Pre-Bing-Bang. The universe is shown to acquire a domain structure near singularity, with an anysotropic expansion regime of its own being realized in each domain.
Working with Alexei Starobinsky of Landau Institute of Theoretical Physics, Moscow, Dmitry Podolsky investigated generic properties of expansion regime near the singularity in the class of cosmological models with scalar field potential unbounded from below and constructed general asymptotic solution of Einstein equations. Being different from standard BKL asympotics, this solution turns out to be of quasi-isotropic type. There is a "phase transition" between BKL and quasi-isotropic solutions driven by the strength of scalar field potential. The point of transition corresponds exactly to the potential which appears in supergravitation type theories.
Podolsky constructed the kinetic equation describing the process of thermalization after preheating in the early universe. While being selfconsistently derived from Keldysh diagrammatic technique, this equation contains additional terms with respect to weak turbulence kinetic equation. Their presence is related to the fluctuations of fields under mass shell.
Kofman and Podolsky discussed dynamics of moduli fields during inflation. As shown, due to the presence of effective potential for moduli their both background and stochastic components are generated during inflation. It is shown that moduli give essential contribution into CMB spectrum due to the generation of modulated cosmological fluctuations from preheating.
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