Cosmology with a Primordial Scaling Field
ArXiv astro-ph/9711102 (1997)
Abstract:
A weakly coupled scalar field $\Phi$ with a simple exponential potential $V=M_P^4\exp(-\lambda\Phi/M_P)$ where $M_P$ is the reduced Planck mass, and $\lambda > 2$, has an attractor solution in a radiation or matter dominated universe in which it mimics the scaling of the dominant component, contributing a fixed fraction $\Omega_\phi$ (determined by $\lambda$) to the energy density. Such fields arise generically in particle physics theories involving compactified dimensions, with values of $\lambda$ which give a cosmologically relevant $\Omega_\phi$. For natural initial conditions on the scalar field in the early universe the attractor solution is established long before the epoch of structure formation, and in contrast to the solutions used in other scalar field cosmologies, it is one which does not involve an energy scale for the scalar field characteristic of late times . We study in some detail the evolution of matter and radiation perturbations in a standard inflation-motivated $\Omega=1$ dark-matter dominated cosmology with this extra field. Using a full Einstein-Boltzmann calculation we compare observable quantities with current data. We find that, for $\Omega_\phi\simeq 0.08-0.12$, these models are consistent with large angle cosmic microwave background anisotropies as detected by COBE, the linear mass variance as compiled from galaxy surveys, big bang nucleosynthesis, the abundance of rich clusters and constraints from the Lyman-$\alpha$ systems at high redshift. Given the simplicity of the model, its theoretical motivation and its success in matching observations, we argue that it should be taken on a par with other currently viable models of structure formation.Structure formation with a self-tuning scalar field
ArXiv astro-ph/9707286 (1997)
Abstract:
A scalar field with an exponential potential has the particular property that it is attracted into a solution in which its energy scales as the dominant component (radiation or matter) of the Universe, contributing a fixed fraction of the total energy density. We study the growth of perturbations in a CDM dominated $\Omega=1$ universe with this extra field, with an initial flat spectrum of adiabatic fluctuations. The observational constraints from structure formation are satisfied as well, or better, than in other models, with a contribution to the energy density from the scalar field $\Omega_\phi \sim 0.1$ which is small enough to be consistent with entry into the attractor prior to nucleosynthesis.Polarization-Temperature Correlation from a Primordial Magnetic Field
ArXiv astro-ph/9707115 (1997)
Abstract:
We propose a new method for constraining a primordial homogeneous magnetic field with the cosmic microwave background. Such a field will induce an observable parity odd cross correlation between the polarization anisotropy and the temperature anisotropy by Faraday rotation. We analyze the necessary experimental features to match, and improve, current constraints of such a field by measuring this correlation.Cumulants as non-Gaussian qualifiers
ArXiv astro-ph/9704261 (1997)
Abstract:
We discuss the requirements of good statistics for quantifying non-Gaussianity in the Cosmic Microwave Background. The importance of rotational invariance and statistical independence is stressed, but we show that these are sometimes incompatible. It is shown that the first of these requirements prefers a real space (or wavelet) formulation, whereas the latter favours quantities defined in Fourier space. Bearing this in mind we decide to be eclectic and define two new sets of statistics to quantify the level of non-Gaussianity. Both sets make use of the concept of cumulants of a distribution. However, one set is defined in real space, with reference to the wavelet transform, whereas the other is defined in Fourier space. We derive a series of properties concerning these statistics for a Gaussian random field and show how one can relate these quantities to the higher order moments of temperature maps. Although our frameworks lead to an infinite hierarchy of quantities we show how cosmic variance and experimental constraints give a natural truncation of this hierarchy. We then focus on the real space statistics and analyse the non-Gaussian signal generated by points sources obscured by large scale Gaussian fluctuations. We conclude by discussing the practical implementations of these techniques.The closet non-Gaussianity of anisotropic Gaussian fluctuations
ArXiv astro-ph/9704052 (1997)