Bertacca, Daniele (2008) Unified Dark Matter in Scalar Field Cosmologies. [Tesi di dottorato]
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In this thesis I have investigated the possibility that the
dynamics of a single scalar field can account for a unified description of the Dark Matter and Dark Energy sectors: Unified Dark Matter (UDM).
In particular considering the general Lagrangian of k-essence models, I study and classify them through variables
connected to the fluid equation of state parameter w_kappa. This allows to find solutions around which the scalar field describes a mixture of dark matter and cosmological constant-like dark energy (UDM) (Bertacca, Matarrese, Pietroni 2007).
Subsequently I also perform an analytical study of the Integrated Sachs-Wolfe (ISW) effect within the framework of Unified Dark Matter models based on a scalar field which aim at a unified description of dark energy and dark matter. Computing the temperature power spectrum of the Cosmic Microwave Background anisotropies I am able to isolate
those contributions that can potentially lead to strong deviations from the usual ISW effect occurring in a Lambda CDM Universe.
This helps to highlight the crucial role played by the sound speed in the unified dark matter models. This treatment is completely general in that all the results depend only on the speed of sound of the dark component and thus it can be applied to a variety of unified models, including those which are not described by a scalar field but relies on a single dark fluid (Bertacca and Bartolo 2007).
Finally I also investigated the static and spherically symmetric
solutions of Einstein's equations for a scalar field with non-canonical kinetic term (Bertacca, Bartolo, Matarrese 2007).
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