The idea that the universe had a beginning is supported by a combination of theoretical and observational evidence. One of the key pieces of evidence is the observed expansion of the universe. In the 1920s, the astronomer Edwin Hubble discovered that galaxies are moving away from each other, and the farther away a galaxy is, the faster it is receding. This observation led to the development of the Big Bang theory, which suggests that the universe started from an extremely hot and dense state around 13.8 billion years ago and has been expanding ever since.
In addition to observational evidence, theoretical physics provides further support for the idea of a cosmic origin. The general theory of relativity, formulated by Albert Einstein, describes the gravitational interactions in the universe. When applied to the universe as a whole, it predicts that space, time, and matter were compressed to a singular point at the beginning of the universe—a singularity. However, the laws of physics as we currently understand them break down at this singularity, indicating the need for a more complete theory that can describe the conditions at the earliest moments of the universe.
Furthermore, the detection of cosmic microwave background radiation (CMB) provides strong evidence for the Big Bang. The CMB is a faint glow of radiation that permeates the universe and is considered the remnant of the hot, dense state that prevailed shortly after the Big Bang. Its discovery in 1965 by Arno Penzias and Robert Wilson was a significant confirmation of the Big Bang theory.
While these pieces of evidence strongly support the idea of a cosmic beginning, it's important to note that our understanding of the universe's origins is still an active area of scientific research. Scientists continue to investigate the early moments of the universe using particle physics, cosmology, and other fields in the pursuit of a more complete understanding of its birth and evolution.