The three major theories of the origin of the universe are:
- The Big Bang Theory
This is the most widely accepted theory, proposing that the universe began about 13.8 billion years ago from an extremely hot, dense singularity that rapidly expanded. This expansion continues today, forming galaxies, stars, and planets. Evidence supporting this theory includes the cosmic microwave background radiation, the observed redshift of distant galaxies indicating expansion, and the abundance of light elements
- The Steady State Theory
This theory suggests that the universe has no beginning or end in time and is always expanding, but new matter is continuously created to fill the gaps left by the expansion. Thus, the overall density of the universe remains constant over time. It posits a universe that looks essentially the same at all times and places
- The Pulsating (Oscillating) Theory
According to this theory, the universe undergoes endless cycles of expansion and contraction (big bangs followed by big crunches). After expanding for a time, the universe eventually stops expanding and contracts back to a dense state, which then triggers another big bang, repeating the cycle indefinitely
Other notable theories include the Eternal Inflation Theory , which extends the Big Bang by suggesting continuous creation of multiple universes (multiverse), and various alternative hypotheses involving black holes or symmetry breaking in particle physics, but the above three are traditionally considered the major foundational theories explaining the universe's origin
