What does the term "universal expansion" refer to?

The term "universal expansion" specifically refers to the phenomenon of the increasing distance between celestial bodies over time, as described by the Big Bang theory and demonstrated through observations such as the redshift of distant galaxies. This expansion indicates that the universe itself is growing larger, with galaxies moving away from each other. This concept helps astronomers understand the dynamic nature of the cosmos and has been pivotal in developing theories about the origins and future of the universe.

In contrast, the other options focus on different aspects of astronomical phenomena. The decrease in cosmic radiation does not directly relate to the overall expansion of the universe, as cosmic radiation is more associated with the aftereffects of the Big Bang rather than the distances between galaxies. The formation of new star systems pertains to stellar evolution and the life cycle of matter in space, which is a separate process from universal expansion. Lastly, the stabilization of planetary orbits relates to gravitational interactions and does not reflect the expanding nature of the universe itself; these orbits can be influenced by various factors but are not indicative of universal expansion as a whole.