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Definition: Big Bang from Dictionary of Energy

Earth Science. a term for the theory that the present universe originated about 15 billion years ago when a gigantic explosion (the “Big Bang”) released huge amounts of energy.


Summary Article: Big Bang Theory
from Encyclopedia of Time: Science, Philosophy, Theology, & Culture

Today, it has become widely accepted that our universe originated with the so-called big bang. This event marks the beginning of time and the further evolution of our universe. In the standard model of cosmology, this origin arises in a space-time singularity. The question what was “before” the big bang cannot be answered, because no physical description is available. More accurately, with reference to the singularity the concepts of time and space lose their meaning, as the scales involved (called Planck time and Planck length) are so incredibly small that spacetime can no longer be considered a continuum. Quantum gravitational effects come into play, and no consistent quantum gravitational theory has yet been developed. Physical description and understanding are therefore possible only after this period, which is called the Planck era. It is generally held that the big bang occurred about 15 billion years ago.

The idea of the big bang goes back to the Belgian mathematician Georges Edouard Lemaître (1894-1966). Many of its predictions are supported by astrophysical observations. The theory predicts that one fourth of all baryonic material should be made out of helium. Here, baryonic matter means mass consisting of “normal” nonexotic particles, like hydrogen, helium, and other nuclei, together with electrons. Observations of metal-poor objects in the universe indicate that this is indeed the case: Their chemical composition yields a helium amount of about 24%. By using this measured abundance of helium, it is possible to derive the different kinds of neutrinos (electrically neutral particles). It was theorized that three different kinds should exist, and this has since been confirmed by accelerator experiments.

Another important hint was the detection of the cosmic microwave background (CMB) by Arno Penzias and Robert Wilson in the 1960s. The astrophysicist George Gamow predicted this thermal blackbody radiation in 1946. Actual measurements by the satellites COBE and WMAP confirm this exact blackbody spectrum. Due to the expansion of the universe the temperature of the CMB has decreased from 3000 Kelvin (i.e., 2726.85 degrees Celsius) to a value of about 2.73 Kelvin (-270.42 degrees Celsius). Yet one more important point refers to the structures in the universe we observe today. They originate from small fluctuations in the density. These fluctuations should be imprinted on the CMB as fluctuations in temperature, which have been detected by COBE and WMAP. Altogether, the theory of the big bang is very promising.

Briefly after its birth, our universe underwent several different epochs. It is assumed that in the beginning, all natural forces were united into one elementary force.

The expansion of the universe led to a decrease in temperature and therefore shortly to the decoupling of the forces into the four basic forces: the gravitational force, the strong force, the weak force, and the electromagnetic force. Following the Planck era, the first one that split from the rest was gravitation. The remaining forces comprise the so-called GUT force. (GUT stands for grand unified theory.) The continued decrease in temperature then led to the separation of the others, starting with the strong force (which acts between the protons and neutrons in atomic nuclei), followed by the electroweak force, which divides into the electromagnetic force (describing the behavior of magnetic and electric fields) and the weak force (which acts between electrons and light particles). This happens after a time span of about only 10−12) seconds!

After the decoupling, the building blocks of the atoms, quarks, and electrons were mixed up together with the photons and other exotic particles within a hot soup. The ongoing expansion led to the gradual cooling of this soup and eventually to the condensation of the quarks into hadrons (protons and neutrons) with a slight excess over their antiparticles. The permanent decrease in temperature finally led to the building of atomic nuclei at T = 3,000 K. The primordial nucleosynthesis, as this event is called, describes the development of helium, deuterium, lithium, and beryllium. The other, heavier elements we measure today come from supernova explosions, which set free these elements from the interior of the stars, where they have been produced.

At a temperature of about T = 3,000 K, the first atoms formed. The universe cooled down enough so that was transparent for the photons, which means that interaction with other atoms was marginal. They decoupled and thereby resulted in the cosmic microwave radiation discussed earlier in this entry.

To summarize, the predictions of the big bang theory coincide well with subsequent observations. It has become an important foundation of the description of the universe.

Microwave Sky Map (©). This microwave sky map was made from I year of data taken by COBE (Cosmic Background Explorer) differential microwave radiometer (DMR) used in artist's concept depicting the crucial periods in the development of the universe after the big bang.

Source: Time & Life Pictures/Getty Images.

See also

Big Crunch Theory, Black Holes, Cosmogony, Cosmology, Cyclic, Gamow, George, Hawking, Stephen, Lemaître, Georges Édouard, Singularities, Universe, Age of, Universe, Evolving, Universe, Origin of, White Holes, Wormholes


  • Guth, A. H. ( (1997).). The inflationary universe: The quest for a new theory of cosmic origins. Cambridge, MA: Perseus.
  • Lidsey, J. E. ( (2000).). The bigger bang. Cambridge, UK: Cambridge University Press.
  • Singh, S. ( (2004).). Big bang: The origin of the universe. New York: Harper Perennial.
  • Smoot, G., & Davidson, K. ( (2007).). Wrinkles in time: Witness to the birth of the universe. New York: Harper Perennial.
  • Junk, Veronika
    Copyright © 2009 by SAGE Publications, Inc.

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