Gravitational lensing in a clumpy universe

In the last century, cosmology has unveiled one of the greatest mysteries of fundamental physics. There exists an hypothetical form of matter that makes up approximately 83% of matter in our universe. Despite the overwhelming evidence of its existence, the nature of dark matter remains a mystery, and there are various theories about what it could be made of. One of the most promising, due to the first detection of a gravitational-wave signal reported by the LIGO/Virgo Collaboration in 2015, got back under spotlight, namely primordial black holes, an old dark matter candidate. The theoretical understanding of the clumpy structure of the matter distribution in our universe is therefore crucial for cosmology. One of the most efficient ways to learn about it is by using gravitational lensing, which is the bending of a wave due to the presence of a gravitational field and has the virtue of being directly sensitive to the presence of dark matter. The theoretical understanding of gravitational lensing in a clumpy universe is the motivation of this thesis, where we show that, despite their numerous differences, light and gravitational waves are both powerful probes to study and constrain the clumpiness of our universe.

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