"Big bang theory" redirects here. For the TV series, see The Big Bang Theory.
For other uses, see Big Bang (disambiguation).

The Big Bang model is that the universe began in an extremely dense and hot condition and has expanded. The theory suggests that redshift proves the universe is expanding.
The **Big Bang** is a physical theory about how the universe started expanding, and then made the stars and galaxies we see today. The Big Bang is the name that scientists use for the most common theory of the universe, from the very early stages to the present day.
The most commonly considered alternatives are called the Steady State theory and Plasma cosmology, according to both of which the universe has no beginning or end.

According to the theory the Big Bang began as a very hot, small, and dense superforce (the mix of the four fundamental forces), with no stars, atoms, form, or structure (called a "singularity"). Then about 13.8 billion years ago, space expanded very quickly (thus the name "Big Bang"). This started the formation of atoms, which eventually led to the formation of stars and galaxies. It was Georges who first noted (in 1927) that the universe is expanding. The universe is still expanding today, and getting colder as well.

As a whole, the universe is growing and the temperature is falling as time passes. Cosmology is the study of how the universe began and its development. Some scientists who study cosmology have agreed that the Big Bang theory matches what they have observed so far.

Naming
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Astronomer Fred Hoyle mockingly called the theory the "Big Bang" on his radio show. Scientists who did not agree with him decided to use it.

Evidence
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Scientists base the Big Bang theory on many different observations. The most important is the redshift of very far away galaxies. Cosmological redshift is the Doppler effect occurring in light. When an object moves away from Earth, its color rays look more similar to the color red than they actually are, because the movement stretches the wavelength of light given off by the object. Scientists use the word "red hot" to describe this stretched light wave because red is the longest wavelength on the visible spectrum. The more redshift there is, the faster the object is moving away. By measuring the redshift, scientists proved that the universe is expanding, and they can work out how fast the object is moving away from the Earth. With very exact observation and measurements, scientists believe that the universe was a singularity approximately 13.8 billion years ago. Because most things become colder as they expand, scientists assume that the universe was very small and very hot when it started. This does not take into account other possible (non-cosmological) causes of redshift.

Other observations that support the Big Bang theory are the amounts of chemical elements in the universe. Amounts of very light elements, such as hydrogen, helium, and lithium seem to agree with the theory of the Big Bang. The same is true, however, for Static State theory and plasma cosmology. Scientists also detected what they call the cosmic microwave background radiation. These electromagnetic waves are everywhere in the universe. This radiation is now very weak and cold, but is thought to have been very strong and very hot a long time ago.

Time
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It can be said that time had no meaning before the Big Bang. If the Big Bang was the beginning of time, then there was no universe before the Big Bang, since there could not be any "before" if there was no time. Other ideas state that the Big Bang was not the beginning of time 13.8 billion years ago. Instead, some believe that there was a completely different universe before the Big Bang, and it may have been very different from the one we know today.

Nonetheless, in November 2019, Jim Peebles, awarded the 2019 Nobel Prize in Physics for his theoretical discoveries in physical cosmology, noted in his award presentation that he does not support the Big Bang Theory due to the lack of concrete supporting evidence, and stated, "It's very unfortunate that one thinks of the beginning whereas in fact, we have no good theory of such a thing as the beginning."

Graphical timeline of the universe
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Nature timelineview • discuss • edit-13 —–-12 —–-11 —–-10 —–-9 —–-8 —–-7 —–-6 —–-5 —–-4 —–-3 —–-2 —–-1 —–0 —Dark AgesReionizationMatter-dominated  
eraAccelerated expansionWaterSingle-celled lifePhotosynthesisMulticellular  
lifeVertebrates←Earliest Universe←Earliest stars←Earliest galaxy←Quasar / black hole←Omega Centauri←Andromeda Galaxy←Milky Way spirals←NGC 188 star cluster←Alpha Centauri←Earth / Solar System←Earliest life←Earliest oxygen←Atmospheric oxygen←Sexual reproduction←Earliest fungi←Earliest animals / plants←Cambrian explosion←Earliest mammals←Earliest apes / humansL  
i  
f  
e(billion years ago)

Many things happened in the first picosecond of the universe's time:

More reading
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* Caleb, Weedon (2005). *Big Bang: the most important scientific discovery of all time and why you need to know about it*. Harper Perennial. ISBN 978-0-00-715252-0.

| * v
* t
* e
Cosmology |
| --- |
| Background | 
* Age of the universe
* Big Bang
* Chronology of the universe
* Universe
* Observable universe

 |
| History ofcosmological theories | 
* Discovery of cosmic microwave background
* History of the Big Bang theory
* Religious interpretations of the Big Bang
* Timeline of cosmological theories

 |
| Past universe | 
* Cosmic microwave background
* Cosmic neutrino background
* Gravitational wave background
* Inflation
* Nucleosynthesis

 |
| Present universe | 
* FLRW metric
* Friedmann equations
* Hubble's law
* Expansion of the universe
* Accelerating expansion
* Redshift

 |
| Future universe | 
* Future of an expanding universe
* Ultimate fate of the universe

 |
| Components | 
* Dark energy
* Dark fluid
* Dark matter
* Quintessence
* Lambda-CDM model

 |
| Structure formation | 
* Galaxy filament
* Galaxy formation
* Large quasar group
* Large-scale structure
* Reionization
* Shape of the universe
* Structure formation

 |
| Experiments | 
* 2dF
* 6dF
* BOOMERanG
* COBE
* Illustris project
* Observational cosmology
* Planck
* SDSS
* WMAP

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