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 universe from the earliest known periods through its subsequent large-scale evolution. The model accounts for the fact that the universe expanded from a very high density and high temperature state, and offers a comprehensive explanation for a broad range of phenomena, including the abundance of light elements, the cosmic microwave background, large scale structure and Hubble's Law. If the known laws of physics are extrapolated beyond where they are valid, there is a singularity. 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, and later simple atoms. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies.
Since Georges Lemaître first noted, in 1927, that an expanding universe might be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion. While the scientific community was once divided between supporters of two different expanding universe theories, the Big Bang and the Steady State theory, accumulated empirical evidence provides strong support for the former. In 1929, from analysis of galactic redshifts, Edwin Hubble concluded that galaxies are drifting apart, important observational evidence consistent with the hypothesis of an expanding universe. In 1965, the cosmic microwave background radiation was discovered, which was crucial evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the universe before it was discovered. More recently, measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy's existence. The known physical laws of nature can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature.