Cosmology science cases

Understanding the nature of the dark Universe and its accelerated expansion is one of the key questions in fundamental physics and cosmology. The large-scale structure (LSS) of the Universe is one of the most powerful tools because it encodes key fundamental information that will determine whether this acceleration is due to an unknown form of energy or requires a modification of General Relativity at large scales. Tracing the cosmic web, a network of sheets, filaments and nodes containing virialised dark matter halos (see Figure 3 left panel), with high accuracy on all scales is a pivotal goal of all ongoing and forthcoming cosmological surveys. Mapping the LSS in the redshift (z) range 1.5<z<4 will still be missing after the coming surveys are completed and thus WST yields a major advance in testing, for the first time early Dark Energy (DE) models and modified gravity at high redshift, as well as substantially tightening constraints on neutrino masses, primordial non-Gaussianities and subsequently the inflationary models.

The nature of the dark Universe and its expansion. 

The primary goal is to reconstruct the 3D map of matter in the Universe on large cosmological scales in the redshift range z~1-4 using multiple tracers (e.g., D4000, [OII]3727, UV absorption lines, Lya) for both quiescent and star-forming galaxies, while past and on-going spectroscopic campaigns (e.g., SDSS BOSS) were limited to z~0.7. For example, a 10m class WST can carry out an ambitious survey to map the large-scale structures (LSS) down to a limiting mag HAB<22 to observe 13,000 galaxies deg2 over 10,000 deg2, for a total number of galaxies of 1.3 x 108 much larger than Euclid, with the advantage of fibre spectroscopy and higher spectral resolution than Euclid for example (R~380). This will provide a map of the Universe with a dense sampling and high fidelity in the uncharted range of 1.5 <z <4 to accurately reconstruct the time evolution of the dark energy equation of state, w(z), which is key to discriminating among alternative gravity and dark energy models.

Left: The dark energy equation of state, w(z), for different models of early dark energy. WST will be able to constrain w(z) in the redshift range z~1-4 where the difference between the models is at its maximum. Right: a WST dedicated survey of the final 10-years LSST galaxy sample (cyan curve) would be able to return constraint on H(z) with fractional errors of the order of 10-3, at least an order of magnitude more precise than what will be achievable by any other facility. Adapted from Bull et al. (2021).