The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to this theory, the Universe began from an extremely hot and dense state about 13.8 billion years ago and has been expanding ever since. The term "Big Bang" was originally coined by British astronomer Fred Hoyle during a BBC radio broadcast in 1949, somewhat derisively, as he himself supported a competing theory known as the steady state model. The theory was formulated based on observations indicating that galaxies are moving away from each other at speeds proportional to their distance, a discovery made by Edwin Hubble in 1929, which suggested that the universe was expanding.
The evidence for the Big Bang comes from several critical observations. The first is the expansion of the Universe, which can be observed through the redshift of galaxies, indicating they are moving away from us. Another cornerstone is the cosmic microwave background radiation, which is the afterglow of the initial explosion, discovered accidentally by Arno Penzias and Robert Wilson in 1965. This radiation provides a snapshot of the infant Universe, about 380,000 years after the Big Bang, when atoms first formed and the universe became transparent to radiation. Further support comes from the relative abundances of light elements such as hydrogen, helium, and lithium, predicted by Big Bang nucleosynthesis and observed throughout the universe.
An important aspect of the Big Bang theory is the concept of inflation, a period of extremely rapid expansion that occurred a fraction of a second after the Big Bang. Proposed by physicist Alan Guth in 1981, inflation helps explain why the universe appears to be flat and homogeneous in all directions, solving the horizon and flatness problems in cosmology. During inflation, tiny quantum fluctuations were stretched to cosmic scale, forming the seeds for large-scale structure formation in the universe. These initial fluctuations led to the distribution of galaxies and cosmic voids observed today.
Despite its widespread acceptance, the Big Bang theory is not without its challenges and unanswered questions, such as the mystery of dark matter and dark energy, which together make up about 95% of the total content of the universe. Dark matter is inferred from its gravitational effects on visible matter and the structure of the universe, while dark energy is hypothesized to be driving the accelerated expansion of the universe observed in distant supernovae. These components remain elusive, and their exact nature is one of the foremost pursuits in physics and astronomy today. Furthermore, the theory does not provide insight into what preceded the Big Bang or why it occurred, leaving space for theories like the multiverse and ideas related to quantum gravity to speculate on these profound questions.