arXiv subjects

R. Durrer

Publications and source records attributed to R. Durrer.

40 records · Page 3Linked to original sources

arXiv1997

Cosmic Microwave Background Anisotropies from Scaling Seeds: Fit to Observational Data

We compute cosmic microwave background angular power spectra for scaling seed models of structure formation. A generic parameterization of the energy momentum tensor of the seeds is employed. We concentrate on two regions of parameter space inspired by global topological defects: O(4) texture models and the large-N limit of O(N) models. We use $χ^{2}$ fitting to compare these models to recent flat-band power measurements of the cosmic microwave background. Only scalar perturbations are considered.

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arXiv1995

Large Scale Structure Formation with Global Topological Defects. A new Formalism and its implementation by numerical simulations

We investigate cosmological structure formation seeded by topological defects which may form during a phase transition in the early universe. First we derive a partially new, local and gauge invariant system of perturbation equations to treat microwave background and dark matter fluctuations induced by topological defects or any other type of seeds. We then show that this system is well suited for numerical analysis of structure formation by applying it to seeds induced by fluctuations of a global scalar field. Our numerical results are complementary to previous investigations since we use substantially different methods. The resulting microwave background fluctuations are compatible with older simulations. We also obtain a scale invariant spectrum of fluctuations with about the same amplitude. However, our dark matter results yield a smaller bias parameter compatible with $b\sim 2$ on a scale of $20 Mpc$ in contrast to previous work which yielded to large bias factors. Our conclusions are thus more positive. According to the aspects analyzed in this work, global topological defect induced fluctuations yield viable scenarios of structure formation and do better than standard CDM on large scales.

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arXiv1994

A new Contribution to Cosmological Perturbations of some Inflationary Models

We show that there are inflationary models for which perturbations in the energy momentum tensor, which are of second order in the scalar field, cannot be neglected. We first specify the conditions under which the usual first order perturbations are absent. We then analyze classically, the growth and decay of our new type of perturbations for one mode of fluctuations $\deϕ_k$ in the scalar field. We generalize this analysis, considering the contribution from the whole spectrum of $\deϕ$ to a given wavelength of geometrical perturbations. Finally, we discuss the evolution of the perturbations during the subsequent radiation dominated era and discuss the resulting spectrum of density fluctuations. In the case of a massless scalar field we find a spectral index $n=4$. For massive scalar fields we obtain $n=0$ but the resulting amplitude of fluctuations for inflation around a GUT scale are by far too high. Hence, 'conventional' inflationary models must not be influenced by this new type of perturbations, in order to lead to acceptable matter and CMB fluctuations.

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arXiv1993

Microwave Anisotropies from Texture Seeded Structure Formation

The cosmic microwave anisotropies in a scenario of large scale structure formation with cold dark matter and texture are discussed and compared with recent observational results of the COBE satellite. A couple of important statistical parameters are determined. The fluctuations are slightly non gaussian. The quadrupole anisotropy is $1.5\pm 1.2\times 10^{-5}$ and the fluctuations on a angular scale of 10 degrees are $ (3.8\pm 2.6)\times 10^{-5}$. The COBE are within about one standard deviation of the typical texture + CDM model discussed in this paper. Furthermore, we calculate fluctuations on intermediate scales (about 2 degrees) with the result $\De T/T(θ\sim 2^o) = 3.9\pm 0.8)\times 10^{-5}$. Collapsing textures are modeled by spherically symmetric field configurations. This leads to uncertainties of about a factor of~2.

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