The universe is pulling itself apart faster than expected, and no one knows why. This research tackles the biggest open question in cosmology: what is driving the accelerated expansion of the cosmos? The leading suspects are a mysterious "dark energy" or a breakdown of Einstein's general relativity on vast scales. Tensions between measurements of the early and late universe suggest the standard model of cosmology may be wrong. The researcher will map how matter clumped together across cosmic history, using data from two major surveys just coming online—the ESA Euclid space mission and the DESI project, the largest spectroscopic galaxy survey ever built. By combining galaxy clustering, gravitational lensing, and quasar distributions, the analysis will achieve percent-level precision over an unprecedented range of cosmic time. If successful, this work could confirm or rule out broad classes of alternative cosmological models, potentially rewriting the fundamental physics that governs the universe. This is fundamental science with no immediate practical application—but similar investigations into the nature of space and time have historically underpinned technologies from GPS to particle accelerators. A deeper understanding of dark energy could, decades from now, reshape how we think about energy, gravity, and the fabric of reality itself.
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The accelerated expansion of the Universe in recent cosmic times is one of the most important unexplained observations in modern physics. It has profound implications in that the cosmos could be permeated by a new, exotic matter component known as dark energy, or that one of the most successful theories of physics, Einstein's general relativity, breaks down on cosmological scales. Emerging tensions between measurements in the late and early Universe are tantalising signs of a potential paradigm change away from the current cosmological standard model. The most promising route to shedding light on this key open problem is to explore the cosmic large-scale structure with multiple probes extracted from large galaxy surveys. I will map the growth of fluctuations in the matter distribution over an unprecedentedly wide range of cosmic history at per-cent level precision, enabling decisive conclusions on cosmic tensions and broad classes of alternative cosmological models. To this end, I will exploit two of the leading galaxy surveys of the decade, which are about to start delivering data: the ESA Euclid space mission and the DESI project, the largest spectroscopic galaxy survey ever undertaken. I will employ combined measurements of the clustering of galaxies, the gravitational lensing effect on galaxies, as well as the spatial distribution of quasars and of imprints of large-scale structure in their spectra. The joint analysis in overlapping cosmic volumes ensures a maximum of robust constraints on cosmology and on the astrophysical processes that link galaxies to the underlying dark matter distribution. The fidelity of results is advanced through dedicated calibration measurements pushed far beyond the state of the art, and by abandoning the conventional data analysis in favour of a novel forward-modelling approach via fast, highly multiplexed simulations and machine learning-assisted inference techniques.
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