Basic Techniques and their Limitations

Practically, all information on the distant universe is based on the analysis of the electromagnetic radiation reaching us from high-redshift objects. In principle, relativistic particles, which are traveling at almost the same speed, neutrinos and gravitational waves can also convey information on distant galaxies. However, relativistic particles are normally charged. As a result, they interact with magnetic fields, with cosmic matter, and with the radiation field. These interactions tend to change their propagation direction, which makes it difficult to determine the origin of the observed relativistic particles. Neutrinos and gravitational waves propagate like light rays. But the low sensitivity of the corresponding detectors makes it impossible to observe high-redshift sources of neutrinos and gravitational waves.