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Testing modified gravity theories with weak gravitational lensing

Spurio Mancini, Alessio

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‘Cosmic shear’ is the weak gravitational lensing effect generated by fluctuations of the gravitational tidal fields of the large-scale structure that induce correlations in the distortion of observed galaxy shapes. Being sensitive to spacetime geometry and the growth of cosmic structure, cosmic shear is one of the primary probes to test gravity with current and future surveys.

In this thesis we analyse the power of cosmic shear to constrain alternatives to the standard cosmological model that could explain cosmic acceleration. We focus in particular on a large class of alternatives to General Relativity, the Horndeski class, which includes the majority of universally coupled extensions to ΛCDM with one scalar degree of freedom in addition to the metric. Given a fixed background, the evolution of linear perturbations in Horndeski gravity is described by a set of four functions of time only.

First, we forecast the sensitivity to these functions that will be achieved by future cosmic shear surveys like Euclid. We produce our forecasts with two methods to analyse a cosmic shear survey: a tomographic approach, based on correlations of the lensing signal in different redshift bins, and a fully 3D spherical Fourier-Bessel decomposition of the shear field. We show how the latter produces tighter constraints on all cosmological parameters with a sensitivity gain of the order of 20% in particular on the ones that describe Horndeski gravity.

We then consider the possibility of using cross-correlations of cosmic shear with other probes to constrain Horndeski theories of gravity. We analyse a combination of cosmic shear, galaxy-galaxy lensing and galaxy clustering data from the Kilo Degree Survey and Galaxy And Mass Assembly survey and set constraints on the aforementioned Horndeski parameters. We also forecast the expected sensitivity to the same parameters that could be achieved with future cross-correlations of Stage IV cosmic shear, galaxy clustering and CMB experiments. While current constraints are not very tight, our implementation could be used in the future with data coming from Stage IV surveys, which we show to have great constraining power on these theories.

Finally, we present in detail the numerical techniques that we used to produce our 3D cosmic shear forecasts and compare our predictions with an alternative, independent method developed with the same purpose. We find excellent agreement between the two methods and use our simulated 3D cosmic shear covariance matrices within a new algorithm that we develop to generate 3D lensing random fields. We calculate the Minkowski Functionals associated to our random fields and use them to test our field-generation procedure, as well as to demonstrate the possibility of a new approach to cosmological inference leveraging the estimated Minkowski Functionals.

Item Type: Dissertation
Supervisor: Pettorino, Dr. Valeria
Place of Publication: Heidelberg
Date of thesis defense: 17 October 2018
Date Deposited: 19 Oct 2018 12:54
Date: 2018
Faculties / Institutes: The Faculty of Physics and Astronomy > Institute for Theoretical Physics
Subjects: 530 Physics
Uncontrolled Keywords: cosmology , gravitational lensing , dark energy , modified gravity
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