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Cosmic Shear Analysis of the Kilo Degree Survey: Cosmological Constraints

Periodic Reporting for period 1 - COSMIC SHEAR (Cosmic Shear Analysis of the Kilo Degree Survey: Cosmological Constraints)

Reporting period: 2016-01-20 to 2018-01-19

'Dark matter' and 'dark energy' are arguably the most mysterious substances in our Universe. Using the technique of weak gravitational lensing, we are beginning to understand many of their properties, including their time evolution, their reaction under gravity, and their relation to observable galaxies.

In summary, light coming from distant galaxies is distorted in the presence of dark matter, in excellent analogy with the way a normal lens distorts the shape of background images. By measuring the coherence in these distortions as a function of time and space, we are able to construct maps of dark matter at different epochs. Analysing the images of millions of galaxies recorded for the Kilo Degree Survey by the European Southern Observatory, we observe that the clustering of dark matter evolves with time, and that it tends to live in haloes whose shapes, profiles and mass we can now measure.

By understanding these properties, we construct a clearer picture of what constitutes about 85% of the mass in our Universe.

Over the course of this action, I have combined the measurement of these shapes with the microwave observation from the Planck satellites and placed independent constraints on the abundance and clustering of dark matter in five time snapshots, which allowed me to further investigate its growth.

I have additionally constructed a massive state-of-the-art control sample of simulated galaxy catalogues and dark matter maps that allowed our international team to validate, test and calibrate our tools and results. This mock data has been used in a number of analyses thus far - more than 20 published papers- and consists in a key legacy that will take us through the next decade.
I carried out a key validation of our model of dark matter: I combined two maps of dark matter obtained by completely different techniques and by independent teams, and recovered a correlation over the overlapping area and epochs. This lead to two (first author) publications since January 2016.

Within the Kilo Degree Collaboration, I am leading the numerical modelling of the weak lensing observations, which is required for the interpretation of the measured cosmic distortions. This role places me at the centre of a group of scientists that intent to use my tools, control samples and mocks data. I am thereby involved in many projects, including the study of cosmic shear (Hildebrandt et al, 2017, MNRAS. 465, 1454), hot gas (Hojjati et al, 2017, MNRAS, 471, 1565) on cosmological scales, combined probe analysis (Harnois-Deraps et al, 2017, MNRAS, 471, 1619), test of models of gravity beyond Einstein General Relativity (Amon et al, 2017, arXiv:1711.10999) and the weak lensing investigation of extremely under-dense regions of the cosmos, also called cosmic voids (paper in preparation).

These studies provided tight constraints on many of the parameters from theoretical cosmological models.

I have also contributed to two independent studies (Sellentin et al, 2017, arXiv:1712.04923 plus one in preparation) that address issues of inaccurate inference in cosmic shear measurements, despite the data and modelling being correct.

All these studies paved the way for upcoming similar analyses with the next generation of galaxy surveys (e.g. LSST, Euclid). Many of the tools will be re-used and many of the analyses strategies will be adopted.

I am jointly supervising a PhD student, I supervised two summer projects and a Honours 1-term project taken by undergraduate students.
The mock data that I am developing is based on about 600,000 CA$ of calculation time on High Performance Super Computing Centers, and is truly unique in the field. It allows the weak lensing community to sharpen and validate its analysis tools, to investigate a number of statistical properties of the weak lensing measurements, to estimate the uncertainty on the data and to look for new features in the data, including systematic effects, unexpected correlations, biases, selection effects, etc.

This remarkable numerical product is what sets the Kilo Degree Survey data from the competing teams (the Dark Energy Survey, the Hyper Suprime Camera survey, mainly) and is a clear and well known advantage. The post processing of this mock data has been completed during the course of the Action (paper in prep.), and is expected to contribute to more than 20 peer-reviewed paper in the next two years. Notably, these mock data allowed our group to carry out unique analyses based on the combination of many cosmological probes (Joudaki et al, 2018, MNRAS, 474, 4894, van Uitert et al, 2017, arXiv:1706.05004) and with their public release, they will be central to more analyses outside of the Kilo Degree Survey Collaboration.
Mock weak lensing data: dark matter (color, to recover), halos (squares), the galaxies (yellow)