Which phenomenon explains the expansion of the universe?

The phenomenon that explains the expansion of the universe is the Big Bang. This theory posits that the universe began as a singular, incredibly hot and dense point approximately 13.8 billion years ago and has been expanding ever since. The expansion refers to the increase in distance between galaxies over time, which is a direct consequence of the universe's initial explosive expansion.

Evidence supporting the Big Bang includes the observed redshift of galaxies, which indicates that they are moving away from us, and the Cosmic Microwave Background Radiation, which is the afterglow of that initial explosion, now cooled and spread throughout the universe. While the Cosmic Microwave Background provides substantial evidence for the Big Bang theory and the universe’s expansion, it doesn't explain the expansion itself.

In contrast, the Big Crunch refers to a theoretical scenario where the universe's expansion could eventually reverse and lead to a collapse, while the Doppler Effect explains how the observed frequency of waves changes due to the relative motion of source and observer, which can describe how we observe the redshift of galaxies but does not explain the root cause of the universe's expansion. Thus, the Big Bang is the foundational event that marks the beginning of the universe's expansion.