The idea that the Big Bang represents the beginning of our universe is derived from a combination of observational evidence and theoretical models. Here are some of the key lines of evidence that support the Big Bang theory and the notion of a finite beginning:
Redshift of distant galaxies: In the early 20th century, astronomers such as Edwin Hubble observed that light from distant galaxies appeared to be shifted toward longer wavelengths, known as a redshift. This observation indicated that galaxies were moving away from us and from each other. The further away a galaxy was, the greater its redshift, suggesting an overall expansion of the universe. This became known as Hubble's Law.
Cosmic Microwave Background Radiation (CMB): In the 1960s, scientists discovered a faint, uniform background radiation pervading the entire universe. This radiation, called the Cosmic Microwave Background (CMB), is often referred to as the "afterglow" of the Big Bang. Its existence and characteristics, such as its nearly uniform temperature across the sky, provide strong support for the idea that the universe was once in a hot, dense state.
Abundance of light elements: The Big Bang theory successfully predicts the observed abundances of light elements, such as hydrogen and helium, in the universe. Shortly after the Big Bang, the conditions were suitable for the synthesis of these elements, and their predicted abundances match the observations.
Cosmic expansion and the age of the universe: The observed expansion of the universe, as indicated by the redshift of galaxies, can be extrapolated backward to a point where all matter and energy were concentrated in an incredibly hot and dense state. This point is often referred to as the initial singularity or the beginning of the universe. By estimating the rate of expansion and reversing it, scientists have inferred an age for the universe of approximately 13.8 billion years, which aligns with other lines of evidence.
It is worth noting that the Big Bang theory is not a complete description of the universe's history, particularly regarding what happened at or before the initial singularity. The theory does not address what may have caused the Big Bang or the nature of conditions prior to it. These remain open questions in cosmology and are areas of ongoing research and theoretical exploration.
Nonetheless, the combination of observational evidence, such as the redshift of galaxies and the CMB, along with successful predictions and consistency with other aspects of physics, has led to the widespread acceptance of the Big Bang theory and the idea that it represents the beginning of our observable universe.