The Big Bang Theory proposes that the observable universe began around 13.8 billion years ago as the primordial universe exploded from an initial extremely hot, dense point. According to the theory, the universe has been expanding ever since. Evidence for the theory includes the observation that other galaxies are moving away from our own, remnants of cosmic microwave background radiation dating to 380,000 years after the explosion, and abundances of light elements like hydrogen and helium matching predictions of nuclear fusion in the early universe. The theory was first proposed by a Belgian priest in the 1920s and gained support from later astronomical observations and discoveries.

1. Big Bang Theory

2. The most popular theory of our
universe's origin centers on a cosmic
cataclysm unmatched in all of history—
the big bang. This theory was born of
the observation that other galaxies are
moving away from our own at great
speed, in all directions, as if they had
all been propelled by an ancient
explosive force.

3. Before the big bang, scientists
believe, the entire vastness of the
observable universe, including all of
its matter and radiation, was
compressed into a hot, dense mass
just a few millimeters across. This
nearly incomprehensible state is
theorized to have existed for just a
fraction of the first second of time.

4. Big bang proponents suggest that
some 10 billion to 20 billion years ago,
a massive blast allowed all the
universe's known matter and energy—
even space and time themselves—to
spring from some ancient and
unknown type of energy.

5. The theory maintains that, in the instant—a
trillion-trillionth of a second—after the big bang, the
universe expanded with incomprehensible speed
from its pebble-size origin to astronomical scope.
Expansion has apparently continued, but much
more slowly, over the ensuing billions of years.

6. Origins of the Theory
A Belgian priest named Georges Lemaître
first suggested the big bang theory in the
1920s when he theorized that the universe
began from a single primordial atom. The
idea subsequently received major boosts by
Edwin Hubble's observations that galaxies
are speeding away from us in all directions,
and from the discovery of cosmic
microwave radiation by Arno Penzias and
Robert Wilson.

7. Big Bang Theory - Evidence
for the Theory

8. First of all, we are reasonably certain that the
universe had a beginning.
Second, galaxies appear to be moving away
from us at speeds proportional to their distance.
This is called "Hubble's Law," named after Edwin
Hubble (1889-1953) who discovered this
phenomenon in 1929. This observation supports
the expansion of the universe and suggests that
the universe was once compacted.

9. Third, if the universe was initially very, very hot as the
Big Bang suggests, we should be able to find some
remnant of this heat. In 1965, Radioastronomers Arno
Penzias and Robert Wilson discovered a 2.725
degree Kelvin (-454.765 degree Fahrenheit, -270.425
degree Celsius) Cosmic Microwave Background
radiation (CMB) which pervades the observable
universe. This is thought to be the remnant which
scientists were looking for. Penzias and Wilson
shared in the 1978 Nobel Prize for Physics for their
discovery.
Finally, the abundance of the "light elements"
Hydrogen and Helium found in the observable
universe are thought to support the Big Bang model of
origins.