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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 the name of a scientific theory about how the universe started, and then made the groups of stars (called galaxies) we see today.

In the Big Bang theory, the universe began as very hot, small, and dense, with no stars, atoms, form, or structure (called a "singularity"). Then about 14 billion years ago,[1] space expanded very quickly (thus the name "Big Bang"), resulting in the formation of atoms, which eventually led to the creation of stars and galaxies. The universe is still expanding today, but getting colder as well.

As a whole, space is growing and the temperature is falling as time passes. Cosmology is the name given to how the universe began and how it has developed. Scientists that study cosmology agree 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. Since then, Fred Hoyle's reasons for not agreeing with the theory have been proven wrong.[2]

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, it looks reddish because the movement stretches the wavelength. The reddish color occurs because red is the lowest 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 can even work out how fast the object is moving. With precise observation and measurements, scientists believe that universe was a singularity approximately 13.8 billion years ago. Because most things become colder as they expand, the universe is assumed to have been very hot when it started.[3]

Other observations that support the Big Bang theory are the amounts of chemical elements in the universe. Amounts of 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. Even so, it is now very weak and cold, but a long time ago it was very strong and very hot.[1]

The Big Bang might also have been the beginning of time. If the Big Bang was the beginning of time, then there was no universe before the Big Bang, since there was no concept of "before" without 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 different universe before and it may have been very different from the one we know today.[3]

Graphical timeline of the universe

A great deal happened in the first second of the universe's life:

<timeline> ImageSize = width:720 height:1000 PlotArea = left:40 right:256 bottom:75 top:75

Colors =

 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

AlignBars = justify

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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

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 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,purple)  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."



More reading

  • Singh, Simon (2005). Big Bang: the most important scientific discovery of all time and why you need to know about it. Harper Perennial. ISBN 9780007152520