Null cosmic strings and space-time physics

The report is devoted to the discussion of the one-dimensional topological defects of space-time: massless (zero) cosmic strings. Zero cosmic strings, if they exist in nature, generate a number of physical effects in the surrounding matter, such as mutual transformations of the trajectories of massive bodies and light rays, perturbations of the velocities of bodies, as well as additional anisotropy of the cosmic microwave background. In this report, I will discuss how such objects are described and consider their optical properties. Zero cosmic strings can carry important information about the content of space-time and physical processes in the early universe. An intriguing property of zero cosmic strings is that, like relic photons, they can encode the history of their propagation in the universe.