ABSTRACT

Radiation, radioactivity and nuclear energy are intrinsic properties of matter. We have already seen how these aspects are investigated in the laboratory (Chapters 2 to 4). This chapter shows the facilities that the universe itself provides for such studies. Basic information on the present state of the universe is given, and an overview of data is provided with respect to an event which began some 15,000 million years ago with a "Big Bang”.

Modern concepts ofthe nature, origin and history of the universe are based essentially on two findings: first, the galaxies are receding in a manner that suggests flight from a central point with steadily increasing velocities; second the universe literally bathes in a sea of microwave radiation. Recession of the galaxies implies an explosive beginning and the temperature of the fossil radiation (2.7 degrees kelvin), after thousands of millions of years of cooling, shows that a superhot fireball must have exploded during the “Big Bang”.

A detailed theory of the course of events in the early universe is widely accepted as the “standard model”. It offers valuable insight into both the evolution of the universe and the nature of matter, especially with regard to its elementary constituents and basic forces.

Nucleosynthesis in the early universe produced the nuclei of deuterium and helium. Once the stars were formed, violent nuclear processes in the stellar cores initiated the syntheses of nuclei heavier than hydrogen and helium, and these processes still operate. The explosions of supernovas are the sites of syntheses of nuclides of the heaviest chemical elements.

Supernova explosions also contribute to the formation of cosmic rays, the radiation that fills interstellar space and bathes all celestial bodies. It is a mixture of charged particles and electromagnetic radiations of various energies. Protons are the predominant constituents, and the particle composition reflects that of the elemental abundance in the universe. The energies and fluxes of cosmic rays vary enormously.

Studies of antimatter in the laboratory are still at a primary stage. Potential military aspects already seem frightening.

Invisible dark matter dominates the material content of the universe. Its true nature is still unknown and it represents a challenge for cosmologists and particle physicists.