FACTBOX: What is the Big Bang?

(Reuters) - The European Organization for Nuclear Research, known as CERN, on Wednesday began an experiment to recreate conditions surrounding the Big Bang, which scientists believe gave birth to the universe.

Its Large Hadron Collider (LHC) will seek to collide two beams of particles at nearly the speed of light. Scientists will circulate a beam in one direction around the accelerator, then the other, later sending beams both ways to cause collisions.

Following are some facts about the Big Bang and CERN's particle-smashing experiment:

* RECREATING THE BIG BANG:

-- The final tests involved pumping a single bunch of energy particles from the project's accelerator into the 27-km (17-mile) beam pipe of the collider and steering them counter- clockwise around it for about 3 km (2 miles).

-- The collider aims to simulate conditions milliseconds after the "Big Bang" which created the universe around 13.7 billion years ago.

-- The collisions, in which both particle clusters will be traveling at the speed of light, will be monitored on computers at CERN and laboratories around the world by scientists looking for, among other things, a particle that made life possible.

-- The elusive particle, which has been dubbed the "Higgs Boson" after Scottish physicist Peter Higgs who first postulated nearly 50 years ago that it must exist, is thought to be the mysterious factor that holds matter together.

* WHAT IS THE BIG BANG?

-- Recreating a "Big Bang," which most scientists believe is the only explanation of an expanding universe, ought to show how stars and planets came together out of the primeval chaos that followed, the CERN team believes.

-- Its essential feature is the emergence of the universe from a tiny speck about the size of a coin but in a state of extremely high temperature and density.

-- The name "Big Bang" was coined in 1949 by British scientist Fred Hoyle to disparage a then emerging theory about origins that countered his own "steady state" view: that the universe had always existed and was evolving but not expanding.

-- According to the Big-Bang model, the universe expanded rapidly from a highly compressed primordial state, which resulted in a significant decrease in density and temperature. Soon afterward, the dominance of matter over antimatter (as observed today) may have been established by processes that also predict proton decay. During this stage many types of elementary particles may have been present. After a few seconds, the universe cooled enough to allow the formation of certain nuclei.

-- The theory predicts that definite amounts of hydrogen, helium, and lithium were produced. Their abundances agree with what is observed today. About 1,000,000 years later the universe was sufficiently cool for atoms to form.

* WHAT IS CERN:

-- CERN, the European Organization for Nuclear Research, is one of the world's largest and most respected centers for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works.

-- Founded in 1954, the CERN Laboratory sits astride the Franco-Swiss border near Geneva. It was one of Europe's first joint ventures and now has 20 Member States, plus 6 actively participant observers including the United States and Russia.

Sources: Reuters/Britannica.com/CERN