Les Houches Summer School Lecture Notes

The aim of this school is to capitalise on the present and future experimental efforts to detect properties of the dark Universe, such as the nature of dark energy and dark matter, in conjunction with the rise of the physics of gravitational waves emitted by Black Holes and other dark objects. The implications and the link with deep theoretical questions such as the cosmological constant problem or the nature of dark matter will be developed.

Scientific Rationale/International Context

Precision cosmology has now been well established since the advent and the flurry of exciting scientific results provided by the Planck mission in the 2010s. The 2020s will see the furthering of this initial impulse with new results from multiple experiments probing the late and early time universe. For the late time universe, experiments such as DESI, EUCLID and LSST probe the structures through galaxy statistics. Complementarily, current (S4) and forthcoming (Litebird) experiments will provide new tests of the physics of the early Universe. Gravitational wave observations also provide a complementary approach and may help in understanding better fundamental questions related to both the early Universe, e.g. phase transitions, or the recent Universe, e.g. the distribution of dark matter. Hopefully, this will lead to challenging new results which will confirm or confront the present standard model of cosmology whose status may already be cracking at the seams with the existing tensions on the value of the Hubble rate, the possibility of dynamical dark energy and to a lesser extent the amount of structures in the Universe.

We organise a four-week school on the present and future of cosmology combining both its observational aspects and its theoretical underpinnings. The school is centred on three scientific themes. The first one concerns the physics of the late acceleration of the expansion of the Universe. Driven by forthcoming experimental results by DESI, EUCLID and LSST, the lectures focus on the resulting physics with thorny questions such as the link with high-energy physics, the status of the cosmological constant problem, the relevance of current theoretical models in understanding and challenges in modelling the data. Tools to analyse the experimental results are also provided with lectures on numerical modelling of large scale structures (e.g. N-body codes, CLASS) and data analysis (e.g. PBJ, BORG, ILI). A second theme covers the cosmology of the early Universe in its finest details as probed by current and future experiments. In particular, the search for non-Gaussianities or spectral distortions is detailed in advanced lectures on the physics of the Cosmic Microwave Background. We cover new probes such as 21 cm physics and the structure of the dark ages. Finally, with the absence of particle physics signals from dedicated searches for dark matter, it is particularly timely to present a panorama of alternatives with important consequences for cosmology. As a prime example, we aim to present the axion but other alternatives such as self-interacting dark matter will also be considered.

Overall, the school serves as a stepping-stone towards modern cosmology, bridging the gap between well-founded theoretical ideas and forthcoming observational results