All three pieces of evidence play important roles in supporting the Big Bang theory, but if we were to assess their relative significance, the cosmic background radiation (CMB) is often considered the most compelling evidence. Here's why:
The expanding universe: The observation of an expanding universe, first made by Edwin Hubble in the 1920s, is a crucial piece of evidence for the Big Bang theory. The discovery that galaxies are moving away from each other and that the universe is expanding suggests that at some point in the past, everything was densely packed together in a hot and dense state. This supports the idea of an initial singularity and an explosive event from which the universe originated.
The abundance of light elements: The abundance of light elements, such as hydrogen and helium, is another important piece of evidence. The Big Bang theory successfully predicts the observed ratios of these elements in the universe. During the early stages of the Big Bang, the universe was extremely hot and dense. As it expanded and cooled, the conditions were just right for the formation of light elements through nucleosynthesis. The measured abundances of these elements match the predictions of the Big Bang theory, providing strong support for its validity.
The Cosmic background radiation: The detection of the cosmic microwave background radiation, which is a faint, uniform radiation pervading the entire universe, is considered one of the most compelling pieces of evidence for the Big Bang theory. The CMB was discovered accidentally in 1965 by Arno Penzias and Robert Wilson, and its existence had been predicted by the Big Bang theory. This radiation is essentially the "afterglow" of the hot, dense early universe. It is isotropic, meaning it is the same in all directions, and exhibits a nearly perfect black body spectrum. The CMB's characteristics match the predictions of the Big Bang theory remarkably well, providing strong confirmation of the theory.
While all three pieces of evidence are important, the discovery and precise measurements of the cosmic background radiation have been instrumental in confirming the predictions of the Big Bang theory and ruling out alternative explanations. The CMB's isotropy, its black body spectrum, and its temperature fluctuations as revealed by satellite missions like the Cosmic Microwave Background Explorer (COBE) and the Planck satellite have significantly bolstered the credibility of the Big Bang theory as the leading explanation for the origin and evolution of the universe.