We propose a conceptual design study of an innovative 10-m class wide-field spectroscopic survey telescope (WST) with simultaneous operation of a large field-of-view (5 sq. degree) and high multiplex (20,000) multi-object spectrograph facility with both medium and high resolution modes (MOS), and a giant panoramic integral field spectrograph (IFS). The ambitious WST top-level requirements place it far ahead of existing and planned facilities. In just 5 years of operation, the WST MOS will target 250 million galaxies and 25 million stars at medium resolution + 2 million stars at high resolution, and 4 billion spectra with the WST IFS. Through spectroscopic characterization, WST will potentiate the understanding of the data from many other huge imaging surveys that merely detect and classify sources. WST will achieve transformative results in most areas of astrophysics: e.g. the nature and expansion of the dark Universe, the formation of first stars and galaxies and their role in the cosmic reionisation, the study of the dark and baryonic material in the cosmic web, the baryon cycle in galaxies, the formation history of the Milky Way and dwarf galaxies in the Local Group, characterization of exoplanet hosts, and the characterization of transient phenomena. WST telescope and instruments will be designed as an integrated system and mostly use existing technology, aiming to minimize its carbon footprint and impact on local environment. Our ambition is to make WST the next large ESO project after the 40m ELT. ESO is the major inter-governmental organization for European astronomy with a unique suite of large telescopes in Chile. We will perform the study together with Australian colleagues, who have an ongoing strategic partnership with ESO and plan to apply soon for full membership. We have formed a large consortium of very experienced institutes plus ESO, representing 9 European countries and Australia. The team brings 60 years of in-kind staff effort to the study.

We propose to perform the first measurement of Lyman-alpha (Lya) emission from the average intergalactic medium at redshifts between 3 and 6.5, a crucial experiment in observational cosmology that so far was impossible because of a lack of sensitivity. We will combine two new and unique datasets obtained by us with the integral-field spectrograph MUSE, constructed under our leadership, with the Intensity Mapping (IM) formalism (adapted to our specific needs) to maximise the sensitivity of the measurement. The observational data comprise the revolutionary "MUSE eXtremely Deep Field" (MXDF) with 155 hours of exposure in a single pointing, augmented by several additional MUSE deep fields of 25-30 hours exposure each. Building on our recent successful work with MUSE we expect to reach an unprecedented surface brightness limit (1sigma for a single Lya emission line) of 3. We underline that our project differs conceptually in several points from most other ongoing or planned IM experiments: (i) We operate directly on 3D spectroscopic datacubes with full area coverage. (ii) While the field of view of MUSE is small, the total volume sampled is still substantial by virtue of the large redshift range covered. (iii) Most importantly, we explicitly aim at removing the dominant part of the signal that can be related to individual galaxies, isolating only the weak glow from the extremely tenuous matter far away from galaxies.