Cosmology science cases

Although it is often claimed we have entered an era of “precision cosmology,” the astro- physical community faces several fundamental puzzles. What causes the cosmic acceleration? Must we resurrect the “cosmological constant” with an energy density that cannot be physically understood? Does its value evolve with cosmic time? Or is dark energy simply an illusion caused by an incorrect theory of gravity on large scales? Moreover, although the standard cold dark matter model can successfully reproduce the observed growth of large scale structure from the epoch of recombination to the present day, anomalies on small scales
remain raising further questions.

The nature of dark energy and gravity in the early Universe

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 provides powerful tools for addressing this question because it encodes information that can reveal whether this acceleration is due to an unknown form of energy or requires a modification of General Relativity at large scales. Measurements of galaxy clustering constrain the nature of dark energy via the scale of baryon acoustic oscillations (BAO, pressure waves imprinted on the early universe) and gravity via the growth rate of structure (obtained from redshift space distortions induced by the peculiar velocities of galaxies)

While the current generation of redshift surveys exploiting telescopes with apertures up to 4 metres (e.g. DESI, Euclid, 4MOST) are probing the universe up to redshift z=2, a WST cosmology survey over a footprint of 18,000 deg2 will be able to constrain the scale of the BAO and the growth rate of structure with unprecedented precision at redshifts inaccessible to current facilities (2 < z < 5.5). Selecting targets galaxies through different techniques (e.g. Lyman-break galaxies, LBG), WST will be able to probe these high redshifts, when (in contrast to today) the Universe is dominated by matter, the expansion is decelerating, and structures grow with high efficiency. Such an ambitious survey will allow us to conduct sub-percent galaxy clustering measurements and consequently constrain fundamental cosmological constituents and forces in the early Universe with unprecedented sensitivity

WST (red curves) will allow to constraint the dark energy models to a level of precision unachievable from current or upcoming facilities (DESI, DESI-II) and open a completely unexplored parameter space at z>4. Image credit: William Doumerg (IFAE).

The cosmological model will ultimately also determine the properties of the most massive structures in the Universe: galaxy clusters. Galaxy clusters provide a privileged laboratory bridging cosmology and galaxy formation. As the latest virialised structures forming around the densest nodes of the cosmic web, they keep a memory of the primordial density field while hosting galaxies at the latest evolutionary stage. WST will be able to probe an unprecedented statistical sample of ~7000 clusters at redshifts 0.1<z<1.5 by fully exploiting its unique capabilities: exploring the inner regions with the IFS and the outskirts and connections to the cosmic web of large-scale structure with the MOS (Figure 11). The 3D galaxy density field on the periphery of the cluster can be mapped by MOS measurements of the positions and redshift of galaxies, from which their orbits and the velocity field can be reconstructed.  Simultaneously, the IFS can chart the position and 2D kinematics of galaxies. By synthesising the dynamics of the whole system and inferring the past orbits of cluster galaxies, the role of the environment in shaping galaxy evolution  can be assessed and the physical properties of galaxies in dense environments (metallicity, SFR, etc.) related to the types of environment (voids, sheets, filaments) they passed through on their journey into the cluster.

A schematic of the WST comprehensive view using MOS and IFU of the massive clusters in the Universe - measure of their mass (using lensing, X-ray and dynamics), how they grow with time and how they connect to the filaments and large-scale structures. Image credit: Johan Richard (CRAL).