• International Year of Astronomy - The Big Bang
The CMB temperature fluctuations from the WMAP data seen over the full sky. The average temperature is 2.725 K (cooled since it was emitted) and the colors represent the tiny temperature fluctuations—red is warmer and blue is colder by about 0.0002K. NASA

International Year of Astronomy - The Big Bang

<Edwin Hubble IYA Home Annie Jump Cannon> 

The Big Bang

The Big Bang is currently the most accepted cosmological
model of the origin and subsequent development of the Universe, and is
supported by a series of strong scientific evidence and independently collected

The Big Bang theory postulates that about 13.7 billion years
ago, at time zero, the Universe began as a point with infinite density and
temperature known as a singularity. The Universe then went through a burst
phase called inflation, where it expanded exponentially and cooled rapidly.
Elementary mass particles were formed but were moving at relativistic speeds
(close to the speed of light) and photons (or light particles) were continually
emitted and absorbed.

A few minutes into the expansion, the Universe was cool
enough that neutrons and protons could combine to form helium, deuterium,
lithium, and beryllium nuclei in a process called nucleosynthesis, though most
protons remained uncombined as hydrogen nuclei. At this point in time, the
Universe was still opaque to light because photons continued to interact with
electrons and were constantly being scattered. After about 379,000 years, the
temperature was low enough (about 3,000 K, where 1 K equals -272.15 °C) that
electrons could join nuclei and form neutral atoms in what is called
recombination, and light decoupled from matter and continued through space
mostly free of interactions. The light from this time, known as the epoch of
last scattering, is what now is detected as the Cosmic Microwave Background
Radiation (CMBR).

Over a long period of time, slightly denser regions of space
attracted nearby matter and grew even denser. After about 400 million years,
these regions had attracted enough matter to produce the first stars. Galaxies
and the other astronomical structures were formed eventually. Heavy elements,
such as those present in terrestrial planets, are created during the life cycle
of stars.   

The Big Bang theory assumes that the Universe has the same
characteristics at any point within it, appears the same in all directions, and
the same laws of physics apply everywhere. 
The so-called three supporting pillars of the Big Bang model are: the
observed expansion of the Universe or Hubble Law, the abundance ratios of light elements
versus hydrogen as predicted by nucleosynthesis, and the existence and
observable characteristics of the CMBR.

<Edwin Hubble IYA Home Annie Jump Cannon>