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Proposing Institution

Argelander-Institut fuer Astronomie, Bonn
Project Manager

Dr. Tommaso Giannantonio
Auf dem Hügel 71
53121 Bonn
Abstract
Observational evidence from Type Ia supernovae indicates the acceleration of the expansion of the Universe. On the other hand, there is no indication for spatial curvature from standard rulers such as the cosmic microwave background (CMB) anisotropies. This may be solved by postulating the existence of dark energy, or some other mechanism such as a modification of general relativity on the largest scales.So far, the simplest possible LCDM model, obtained adding a small cosmological constant to general relativity (GR), is in good agreement with all the observations, but it lacks elementary physics foundations, and it is therefore interesting to explore other possibilities.As a first check, it is possible to test at the background level the expansion history of the Universe. However, it turns out that most modified theories of gravity include some extra degree of freedom which can be tuned in order to obtain the correct expansion behaviour. This means that additional observables are required.GR, or any alternative theory of gravity, is based on laws relating geometry and energy. At perturbation level, these laws can be written as equations between the perturbative fields for density \delta and velocity \theta, and the gravitational potentials (\Phi and \Psi in the Newtonian gauge): in GR, these relationships consist of the Poisson, Euler, continuity, and anisotropy equations.An optimal way to study gravity would then ideally be to reconstruct the potentials in the past light-cone and, combining them with the observed density and velocity fields, to test for deviations from the GR relationships [Song et al 2008]. Unfortunately, the potentials are not useful astronomical observables; nevertheless they can be traced rather easily by some quantities which we can measure, like the galaxy or cluster counts, the peculiar velocities, weak lensing, and the integrated Sachs-Wolfe (ISW) effect [Jain & Bertschinger 2007]. This is why we decided to use a combination of data from weak lensing, the ISW effect, peculiar velocities, and galaxy counts, to break the degeneracies which remain at background level between GR and modified gravities.Furthermore, since there is no preferred model for modified gravity at the moment, we decided to perform a completely general Principal Component Analysis (PCA) following the ideas of Zhao et al 2009, to isolate and constrain deviations from GR as deviations from the usual Poisson and anisotropy equations.We plan to run Monte Carlo Markov chains over the modified gravity parameters using a modified version of the Cosmomc code, and using the aforementioned combination of data.
Impressum, Conny Wendler