chapter  2
20 Pages


We observe the Universe at many different wavelengths, visible light being most common. Observations are made mostly by ground-based telescopes. However, in order to eliminate atmospheric distortion, space-based telescopes, such as the Hubble Space Telescope, are also used. What we typically observe is the visible light emitted by different atoms and ions at the surface of stars. This gives us information about the temperature, relative density of different elements and their state of ionization at the stellar surface. We also observe the Universe at other wavelengths, such as radio, microwave, infrared, ultraviolet, x-rays, and gamma-rays. Excluding the radio, most of these observations are made using space-based telescopes, as these frequencies are strongly absorbed by the Earth’s atmosphere. Besides this, we also observe cosmic rays, which are high-energy particles and consist mainly of protons and atomic nuclei. The highest energy cosmic ray particles are observed to have energies on the order of 1020 eV. This energy is extremely large. In comparison, the maximum energy of protons that has been achieved in the laboratory is on the order of 1013 eV. This was achieved in the Large Hadron Collider (LHC), which consists of a beam pipe of circumference 27 Km, equipped with the most powerful magnets operating at liquid helium temperatures. How does a cosmic site generate energies that are 10 million times higher? So far we do not have a satisfactory answer to this question.