Article

# A Separate Universe Approach to Quintessence Perturbations

Nuclear Physics B - Proceedings Supplements (Impact Factor: 0.88). 03/2005; DOI: 10.1016/j.nuclphysbps.2005.04.050

Source: arXiv

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**ABSTRACT:**The strongest evidence for dark energy comes presently from geometric tech-niques such as the supernova distance-redshift relation. By combining the mea-sured expansion history with the Friedmann equation one determines the energy density and its time evolution, hence the equation of state of dark energy. Be-cause these methods rely on the Friedmann equation which has not been inde-pendently tested it is desirable to find alternative methods that work for both general relativity and other theories of gravity. Assuming that sufficiently large patches of a perturbed Robertson-Walker spacetime evolve like separate Robertson-Walker universes, that shear stress is unimportant on large scales and that energy and momentum are locally con-served, we derive several relations between long-wavelength metric and matter perturbations. These relations include generalizations of the initial-value con-straints of general relativity. For a class of theories including general relativity we reduce the long-wavelength metric, density, and velocity potential perturba-tions to quadratures including curvature perturbations, entropy perturbations, and the effects of nonzero background curvature. When combined with the ex-pansion history measured geometrically, the long-wavelength solution provide a test that may distinguish modified gravity from other explanations of dark energy.The Astrophysical Journal 04/2006; · 6.73 Impact Factor -
##### Article: Evolution of perturbations in distinct classes of canonical scalar field models of dark energy

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**ABSTRACT:**Dark energy must cluster in order to be consistent with the equivalence principle. The background evolution can be effectively modelled by either a scalar field or by a barotropic fluid.The fluid model can be used to emulate perturbations in a scalar field model of dark energy, though this model breaks down at large scales. In this paper we study evolution of dark energy perturbations in canonical scalar field models: the classes of thawing and freezing models.The dark energy equation of state evolves differently in these classes.In freezing models, the equation of state deviates from that of a cosmological constant at early times.For thawing models, the dark energy equation of state remains near that of the cosmological constant at early times and begins to deviate from it only at late times.Since the dark energy equation of state evolves differently in these classes,the dark energy perturbations too evolve differently. In freezing models, since the equation of state deviates from that of a cosmological constant at early times, there is a significant difference in evolution of matter perturbations from those in the cosmological constant model.In comparison, matter perturbations in thawing models differ from the cosmological constant only at late times. This difference provides an additional handle to distinguish between these classes of models and this difference should manifest itself in the ISW effect. Comment: 11 pages, 6 figures, accepted for publication in Phys. Rev. DPhysical review D: Particles and fields 10/2009; - [Show abstract] [Hide abstract]

**ABSTRACT:**Dark energy perturbation affects the growth of matter perturbations even in scenarios with noninteracting dark energy. We investigate the Integrated Sachs Wolfe (ISW) effect in various canonical scalar field models with perturbed dark energy. We do this analysis for models belonging to the thawing and freezing classes. We show that between these classes there is no clear difference for the ISW effect. We show that on taking perturbations into account, the contribution due to different models is closer to each other and to the cosmological constant model as compared to the case of a smooth dark energy. Therefore considering dark energy to be homogeneous gives an overestimate in distinction between different models. However there are significant difference between contribution to the angular power spectrum due to different models.Physical review D: Particles and fields 03/2012; 86(4).

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