The Big Bang theory describes how the Universe began in a rapid expansion about 13.7 billion years ago and has evolved since that time. It is thought that all of space was created in this first moment.
Since the 1940s, when the modern form of the theory took shape, scientists have detected radiation from the early Universe with radio telescopes and satellites and named it cosmic microwave background radiation (CMB). The CMB, which is formed of microwaves and radio waves, is considered important evidence in support of the Big Bang because it matches theorists' predictions.
Image: A computer-generated image of the Big Bang
The Universe begins in a huge expansion.
Scientists probe the Universe's beginnings.
Cosmologists and other scientists have written a series of mathematical equations sometimes called the Standard Model of Cosmology (or the Lambda-CDM model) that describe the Universe today. Working backward in time, this model allows them to discover how the Universe started - the Big Bang.
Cosmologists try to explain the Universe's uniformity.
Cosmologists have written a series of mathematical equations sometimes referred to as the Standard Model of Cosmology that attempts to describe the Universe as it is today. Working the equations backwards in time has allowed scientists to predict how the Universe started - the Big Bang. However, they've encountered problems along the way. One issue is that the Universe's temperatures are uniform - something at odds with the expected vast temperature variations. This conundrum led particle physicist Alan Guth to develop the inflation theory.
Fred Hoyle attacks the Big Bang theory.
Astronomer Sir Fred Hoyle rejected the Big Bang theory. Instead, he proposed an alternative idea - the Steady State theory - which did away with the Big Bang theory's need for a start to the Universe.
Particle accelerators reveal creation.
Paul Dirac predicted the existence of antimatter in the early 1930s, but it wasn't until Carl Anderson detected the mysterious particles that the theory became widely accepted. Subsequently, particle accelerators gave clear evidence of matter and antimatter being created from energy, just as Einstein's theory of special relativity predicted.
The spacecraft gathers data on the cosmic microwave background radiation.
Three years after its 1989 launch, experts began to understand the data from the Cosmic Background Explorer (COBE), a satellite studying the cosmic microwave background radiation from the early Universe.
The Big Bang theory is the prevailing cosmological model for the early development of the universe. The key idea is that the universe is expanding. Consequently, the universe was denser and hotter in the past. Moreover, the Big Bang model suggests that at some moment all matter in the universe was contained in a single point, which is considered the beginning of the universe. Modern measurements place this moment at approximately 13.8 billion years ago, which is thus considered the age of the universe. After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, including protons, neutrons, and electrons. Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed. The majority of atoms produced by the Big Bang were hydrogen, along with helium and traces of lithium. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies, and the heavier elements were synthesized either within stars or during supernovae.
The Big Bang theory offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background, large scale structure, and Hubble's Law. As the distance between galaxies increases today, in the past galaxies were closer together. The known laws of nature can be used to calculate the characteristics of the universe in detail back in time to extreme densities and temperatures. While large particle accelerators can replicate such conditions, resulting in confirmation and refinement of the details of the Big Bang model, these accelerators can only probe so far into high energy regimes. Consequently, the state of the universe in the earliest instants of the Big Bang expansion is poorly understood and still an area of open investigation. The Big Bang theory does not provide any explanation for the initial conditions of the universe; rather, it describes and explains the general evolution of the universe going forward from that point on.
Belgian catholic priest and scientist Georges Lemaître proposed what became the Big Bang theory in 1927. Over time, scientists built on his initial idea of cosmic expansion, which, his theory went, could be traced back to the origin of the cosmos and which led to formation of the modern universe. The framework for the Big Bang model relies on Albert Einstein's theory of general relativity and on simplifying assumptions such as homogeneity and isotropy of space. The governing equations were formulated by Alexander Friedmann, and similar solutions were worked on by Willem de Sitter. In 1929, Edwin Hubble discovered that the distances to faraway galaxies were strongly correlated with their redshifts. Hubble's observation was taken to indicate that all distant galaxies and clusters have an apparent velocity directly away from our vantage point: that is, the farther away, the higher the apparent velocity, regardless of direction.Assuming that we are not at the center of a giant explosion, the only remaining interpretation is that all observable regions of the universe are receding from each other.
While the scientific community was once divided between supporters of two different expanding universe theories—the Big Bang and the Steady State theory, observational confirmation of the Big Bang scenario came with the discovery of the cosmic microwave background radiation in 1964, and later when its spectrum (i.e., the amount of radiation measured at each wavelength) was found to match that of thermal radiation from a black body. Since then, astrophysicists have incorporated observational and theoretical additions into the Big Bang model, and its parametrization as the Lambda-CDM model serves as the framework for current investigations of theoretical cosmology.