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The Big Bang model is that the universe begun in an extremely dense and hot condition and has expanded. The theory suggests, and measurements show, that the universe is still expanding today.[1]

The Big Bang is a scientific theory about how the universe started, 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,[2][3][4] from the very early stages to the present day.[5][6][7]

The universe 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,[1] 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 Lemaître who first noted (in 1927) that an expanding universe could be traced back in time to an originating single point. 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. Scientists who study cosmology have agreed that the Big Bang theory matches what they have observed so far.[1]

Fred Hoyle called the theory the "Big Bang" on his radio show. He did not believe the Big Bang was correct. Scientists who did not agree with him thought the name was funny and decided to use it.[8]

Scientists base the Big Bang theory on many different observations. The most important is the redshift of very far away galaxies. 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.[9]

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. Scientists also have found "cosmic microwaves background radiation". This radiation is known as radio waves, and they are everywhere in the universe. This radiation is now very weak and cold, but a long time ago it was very strong and very hot.[1]

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.[9]

Nonetheless, in November 2019, Jim Peebles, awarded the 2019 Nobel Prize in Physics for his theoretical discoveries in physical cosmology.[10] 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."[11]

Graphical timeline of the universe

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

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 id:period1  value:rgb(1,1,0.7) # light yellow
 id:period2  value:rgb(0.7,0.7,1) # light blue
 id:events   value:rgb(1,0.7,1) # light purple
 id:era1     Value:yellowgreen
 id:era2     value:lightorange
 id:time1    Value:coral
 id:time2    Value:lavender

DateFormat = yyyy Period = from:-430 till:155 TimeAxis = format:yyyy orientation:vertical # order:reverse does not work ScaleMajor = unit:year increment:10 start:-430 # second ScaleMinor = unit:year increment:2 start:-430

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TextData =

 text:Planck epoch
 text:Big Bang
 text:"0: Linear time"
 text:"Logarithmic time:"
 text:"10 · log10 second"
 text:The Stelliferous Era

LineData =

 at:123  frompos:358 tillpos:369   width:0.5  # Matter domination
 points:(369,878)(402,800)  width:0.5
 points:(402,800)(413,800)  width:0.5


 textcolor:black  fontsize:M
 bar:Clock  color:events  align:right  shift:(28,3)  mark:(line,teal)
 at:-120 text:"One picosecond"
 at:-90  text:"One nanosecond"
 at:-60  text:"One microsecond"
 at:-30  text:"One millisecond"
 at:0    text:"One second"
 at:36   text:"One hour"
 at:75   text:"One year"
 at:105  text:"One thousand years"  shift:(43,3)
 at:135  text:"One million years"
 bar:Era  mark:(line,white)  align:center  shift:(0,0)
 from:-430  till:135  color:era1  text:"The Primordial Era"
 bar:Periods  align:center  shift:(0,0)  mark:(line,white)
 from:-430 till:-360 color:period1  text:Grand unification epoch
 from:-360 till:-120 color:period2  text:Electroweak epoch
 from:-120 till:-60  color:period1  text:Quark epoch
 from:-60  till:0 shift:(-40,0) align:left  color:period2  text:Hadron epoch
 from: 0   till:23   color:period1  text:Lepton epoch
 from: 23   till:131  color:period2  text:Photon epoch
 from:131  till:155  color:period1  text:"Dark Ages"
 mark:(line, blue)  textcolor:black  fontsize:M
 bar:Events  color:events  align:left  shift:(30,-4)
 at:-430 shift:(30,-12) text:"Planck time, the smallest theoretically observable ~unit of time and the time before which science is ~unable to describe the universe. At this point, the ~force of gravity separated from the electronuclear ~force."
 at:-360           text:"Separation of the strong force from the ~electronuclear force."
 from:-360 till:-320 align:left shift:(-55,0) color:time1  text:"Inflationary epoch. The Universe expands exponentially"
 at:-320  shift:(30,4) text:"Reheating after inflation populates universe ~with quarks and anti-quarks."
 at:-120           text:"The weak force separates from the ~electromagnetic force resulting in the four ~separate forces we know today."
 at:-60            text:"Quarks become confined within hadrons."
 at:-30            mark:(line,white) text:"Formation of hydrogen nuclei."
 at:0              text:"Neutrinos cease to interact with other particles."
 at:23 shift:(30,-11)   text:"Lepton/anti-lepton pairs annihilate."
 from:23  till:31  align:center shift:(0,-4) color:time2  text:"Big Bang nucleosynthesis"
 at:31 shift:(30,2)  text:"3 to 20 minutes: Formation of helium nuclei"
 from:129 till:131  shift:(-114,-2) color:time1 mark:(line,white) text:"Recombination"
  1. at:123 shift:(30,-32) text:"70,000 years: Matter domination"
 at:123  mark:(line,white)  shift:(30,-83)  text:"70,000 years: Matter domination"
 at:131  text:"379,000 years: Hydrogen and helium nuclei ~capture electrons to form stable atoms. Photons ~are no longer able to interact strongly with atoms. ~Cosmic microwave background radiation streams ~freely."
 at:155  text:"100 million years: First star begins to shine."



  1. 1.0 1.1 1.2 1.3 NASA. "Universe 101:Big Bang theory". Retrieved 2010-01-28. 
  2. Overbye, Dennis (20 February 2017). Cosmos Controversy: The Universe Is Expanding, but How Fast?. Retrieved 21 February 2017. 
  3. Kurki-Suonio, Hannu (2018). Cosmology I. University of Helsinki. p. 9-10. Retrieved 30 January 2019. 
  4. Kornreich, Dave (27 June 2015). Can we find the place where the Big Bang happened? (Intermediate). Ask an Astronomer, Cornell University. Retrieved 20 October 2018. 
  5. Silk, Joseph (2009). Horizons of Cosmology. Templeton Press. p. 208. 
  6. Singh, Simon (2005). Big Bang: The Origin of the Universe. Harper Perennial. p. 560. 
  7. Wollack, Eddie J. (10 December 2010). "Cosmology: The Study of the Universe". Universe 101: Big Bang Theory. NASA. Archived from the original on 14 May 2011. Retrieved 2017-04-15. "The second section discusses the classic tests of the Big Bang theory that make it so compelling as the likely valid description of our universe." 
  8. Sullivan, Walter (August 22, 2001). "Fred Hoyle dies at 86; opposed 'Big Bang' but named it". New York Times. Retrieved 2010-01-28. 
  9. 9.0 9.1 Chris LaRocco and Blair Rothstein. "The Big Bang: it sure was BIG!". Retrieved 2010-01-28. 
  10. Hooper, Dan (12 October 2019). "A Well-Deserved Physics Nobel – Jim Peebles’ award honors modern cosmological theory at last". Scientific American. Retrieved 13 October 2019. 
  11. Couronne, Ivan (14 November 2019). "Top cosmologist's lonely battle against 'Big Bang' theory". Retrieved 15 November 2019. 

More reading

  • 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