The universe began expanding 13.8 billion years ago according to which theory?

• A. Cosmic Inflation Theory
• B. Big Bang Theory
• C. Steady State Theory
• D. Oscillating Universe Theory

Answer: (B)

The key idea is that the universe has a finite age inferred from its expansion. Observations show galaxies are moving away from us, with speeds tied to their distance (Hubble’s law). If you trace that expansion back in time, you converge on a hot, dense origin—the core image of the Big Bang. The estimated age of about 13.8 billion years comes from combining the expansion rate with measurements of the cosmic microwave background—the faint afterglow from the early universe—and the predicted abundances of light elements from Big Bang nucleosynthesis. Inflation fits inside this framework as an extremely rapid early expansion that helps explain uniformity, but it doesn’t replace the origin story. The Steady State view, proposing no beginning and continual matter creation, conflicts with the observed cosmic microwave background and the finite age. The Oscillating Universe envisions cycles of expansion and contraction, which current data do not show. So the description that matches the observed expansion and this age is the Big Bang Theory.