Along with the particles of light (photons) neutrinos are the most abundant particles in the Universe. Unlike for photons, the Universe is transparent for neutrinos. Neutrinos are almost not absorbed by matter, nor deflected by magnetic fields since they are electrically neutral. A tremendous flow of neutrinos arrive at the Earth along straight lines pointing back to their sources. They carry undistorted information about the phenomena, objects, and events where they have been produced, even if the events took place far away in the most distant cosmic corners. They may promote our understanding of the early stage of the Universe’s evolution, about dark matter and dark energy, about processes the generation of chemical elements, the evolution of stars, the inner structure and composition of the Sun and the Earth, and the properties of the neutrino itself. Large neutrino telescopes are placed deep in transparent natural media in various geographical areas of the Earth — at present in Lake Baikal, at at the South Pole and in the Mediterranean Sea.
The method of deep underwater neutrino detection was first proposed by M.A. Markov in 1960. It consists in recording Cherenkov radiation from secondary muons and/or high-energy showers produced by the interaction of neutrinos with matter in transparent natural media.
Baikal-GVD will study the most violent processes in the Universe, which accelerate charged particles to highest energies, far beyond the reach of laboratory experiments on Earth. These processes must be accompanied by the emission of neutrinos. The large detection volume, combined with very good angular and energy resolution and the moderate light background in fresh water of Lake Baikal allows for an efficient study of the diffuse neutrino flux and of neutrinos from individual astrophysical objects, be they steady or transient. Multi-messenger methods will be used to relate our findings to those of classical astronomers and with X-ray or gamma-ray observations. A high-energy diffuse astrophysical neutrino flux has been observed recently by IceCube, using track-like and cascade-like events. GVD-I will have a detection volume for cascades of about 0.4 km3, which is approximately the same as the fiducial volume of IceCube for this detection mode. That guarantees the detection of astrophysical neutrinos during the GVD’s first years of operation. We will scrutinize the IceCube result and study in detail the energy spectrum, the global anisotropy and the neutrino flavor composition of the diffuse neutrino flux. The high angular resolution of GVD for track-like or cascade-like events (~0.25° for muon tracks and ~2° for cascades, respectively) provides a high capability for identifying point-like cosmic-ray accelerators. The closest (with respect to a terrestrial observer) astrophysical objects that are currently assumed to be capable for emitting high-intensity neutrino fluxes are located mainly in the vicinity of the Galactic center and in the Galactic plane. Supernova remnants, pulsars, the neighborhood of the black hole Sgr A* at the Galactic center, binary systems comprising a black hole or a neutron star, and clusters of molecular clouds are the most promising Galactic candidates for neutrino emission. Extragalactic objects — like Active Galactic Nuclei (AGN), Gamma-Ray Bursts (GRB), starburst galaxies and galaxy clusters — are another class of neutrino sources to be targeted by Baikal-GVD. Baikal-GVD will substantially contribute to multi-messenger astronomy studies. Multi-messenger astronomy is the combination of observations with cosmic rays, neutrinos, photons of all wavelengths and even gravitational waves. It represents a powerful tool to provide a multi-faceted picture of the physical processes driving the non-thermal Universe. The alert system of GVD will allow for a fast, on-line reconstruction of neutrino events recorded by GVD and – if predefined conditions are satisfied – for the formation of an alert message to the other communities. Combined analyses of cosmic high-energy neutrinos with spatially or temporarily coinciding gamma rays (or spatially coinciding ultra-high energy cosmic rays) may lead to a higher significance of the combined results.