Observational cosmology with multi-wavelength surveys

Cosmology studies the large-scale properties and evolution of the Universe. As such, cosmology is arguably one of the most complete branches of physics in that it must be able to describe the large-scale distribution and motion of matter, governed primarily by the gravitational forces, but also the intricate interactions between subatomic particles that dictated the rules during and after the Big Bang, as well as the violent physical processes that take place in galaxies and clusters of galaxies. However, the most characteristic feature of cosmology that separates it from other branches of physics, is the impossibility to replicate experiments. We only have one set of data: the Universe, and we cannot repeat it. This originally put cosmology in an awkward position, where due to the lack of experimental data, progress was mainly driven by theoretical work based on fundamental premises. Astonishingly, as astronomical observations improved, many of these theoretical predictions were actually found to be valid, and the last couple of decades have seen cosmology grow into a fully fleshed science driven by experimental observations.

Since there is only one Universe to observe, the cosmologist's quest is to observe as much of it as possible: to map out the distribution of matter and energy in the entire observable Universe. The aim of this endeavour is not merely cartographic. Due to the finiteness of the speed of light, we see distant structures the way they were at the time the photons we observe were emitted. This way the cosmologist is also able to travel in time, and therefore the cosmologist's ideal map describes not only the current state of the Universe, but also its evolution since the moment of the Big Bang. So far we have only been able to collect separate pieces of this map, covering the early stages in the evolution of the Universe from measurements of the cosmic microwave background (CMB) emitted shortly after the Big Bang, as well as the late-time steps in this evolution, in terms of observations of the distribution of galaxies around us. However, in the next decade, large steps will be taken towards the completion of the cosmologist's ideal map: at least half of the observable sky will be jointly mapped by different experiments in a wide range of the electromagnetic spectrum, and these observations will cover far larger volumes than have been accessible so far.

However, the cosmological information is encoded into these datasets in the form of an absorbing puzzle: different experiments cover different ranges of radial and angular scales, as well as different energy regimes, and certain sections of the data end up being dominated by non-cosmological sources and instrumental effects. The beautiful cosmologist's map must therefore be carefully disentangled from the raw experimental data, lest it be inevitably contaminated. This project focuses on identifying the regions and combinations of these datasets that are valuable to reconstruct this map, and that contain the most relevant cosmological information, making use of state-of-the-art statistical and computational tools. As an example, one of the main objectives of this project is the detection of primordial gravitational waves, the ripples in space-time originated during the Big-Bang, which could teach us a lot about the physical conditions in the early Universe. These waves leave an imprint in the polarisation of the CMB with an amplitude significantly smaller than the emission of our own galaxy, and therefore the latter must be carefully removed from the data before the former can be studied.

With cosmology soon entering the era of "big data", as most other branches of science are currently doing, many of the algorithms and methods developed for this project will be useful for a wide range of disciplines, from atmospheric physics to the social sciences, and the computing needs of cosmological studies will also act as a driver for technological development.