Unified Dark Matter in Scalar Field Cosmologies
ABSTRACT 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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ABSTRACT: We present an analysis of the cross-correlation between the CMB and the large-scale structure (LSS) of the Universe in Unified Dark Matter (UDM) scalar field cosmologies. We work out the predicted cross-correlation function in UDM models, which depends on the speed of sound of the unified component, and compare it with observations from six galaxy catalogues (NVSS, HEAO, 2MASS, and SDSS main galaxies, luminous red galaxies, and quasars). We sample the value of the speed of sound and perform a likelihood analysis, finding that the UDM model is as likely as the LambdaCDM, and is compatible with observations for a range of values of c_\infinity (the value of the sound speed at late times) on which structure formation depends. In particular, we obtain an upper bound of c_\infinity^2 \leq 0.009 at 95% confidence level, meaning that the LambdaCDM model, for which c_\infinity^2 = 0, is a good fit to the data, while the posterior probability distribution peaks at the value c_\infinity^2=10^(-4) . Finally, we study the time dependence of the deviation from LambdaCDM via a tomographic analysis using a mock redshift distribution and we find that the largest deviation is for low-redshift sources, suggesting that future low-z surveys will be best suited to constrain UDM models.Journal of Cosmology and Astroparticle Physics 02/2011; 1103(2011):039. · 6.04 Impact Factor
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ABSTRACT: We perform a detailed phase-space analysis of a class of k-essence cosmology. We find the critical points can be divided into three classes: points unstable but the model stable, both points and the model stable, points stable but the model unstable. Like the case of points unstable but the model stable, the case of points stable but the model unstable is not relevant from a cosmological point of view, though they can be late-time attractors for the universe. So in order to study the possible final state of the universe, it is important to investigate not only the stability of the critical points but also the stability of the model. The case of both points and the model stable can further be divided into two classes: points only presenting decelerating phases and points at which all decelerating, constant-speed, and accelerating phases can appear; the final state of the universe dependents on the potential.Classical and Quantum Gravity 06/2010; 28(6). · 3.56 Impact Factor
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ABSTRACT: In this work we analyze and review cosmological models in which the dynamics of a single scalar field accounts for a unified description of the Dark Matter and Dark Energy sectors, dubbed Unified Dark Matter (UDM) models. In this framework, we consider the general Lagrangian of k-essence, which allows to find solutions around which the scalar field describes the desired mixture of Dark Matter and Dark Energy. We also discuss static and spherically symmetric solutions of Einstein's equations for a scalar field with non-canonical kinetic term, in connection with galactic halo rotation curves.Advances in Astronomy 08/2010;