The Cosmic Evolution
Age: 10^-43 - 10^-32 second. Initially, the four known forces are unified as one single force and no matter exists. Owed to a tiny puff, gravity splits off 10^-43 second and at 10^-35 second the strong force departs from the unified electro-weak force. At this stage the universe undergoes an exponential expansion - from 10^-25 times the size of an atom to the size of a grapefruit. Now the universe appears like an extremely hot ball of fire. (The figure 10^-43 is short for 10 millionth trillionth trillionth trillionth. If you feel uneasy about such a small fraction of a second - just remember that the space-time continuum itself expands and "measures" expands accordingly. More about this later).
Age: 10^-32 - 1/100 second. This era is characterized by the formation and the interactions of elementary particles swirling around with a speed at or close to the speed of light. At 10^-10 second the electromagnetic and the weak forces split. Now all the known species of elementary particles have come into existence - quarks, electrons, muons, tauons, neutrinos, photons, gluons and W-, Z- and Higgs bosons. A variety of interactions known from high-energy particle physics laboratories take place but the kinetic energy is too high for any pair to get a lasting hold of each other.
Age: 1/100 - 100 seconds. As the universe continues its expansion, more and more kinetic energy is turned into matter. At 1/100 second the fireball is cool enough for nucleosynthesis to set in - i.e. quarks combine to form protons and neutrons. At 100 seconds protons and neutrons start to fuse and form helium nuclei. The formation of elements now has begun. However, the heat is still too extensive for nuclei and electrons to combine and form atoms.
Age: 100 seconds - 150,000 years. The formation of helium and a few other light nuclei continues until 1000 seconds when the temperature of the fireball is too low for the nucleosynthesis to continue. For the next many years, the universe is a hot and dense plasma which almost entirely consists of photons, neutrinos, unbound electrons, unbound protons and helium nuclei. All quarks are bound and confined in the protons and neutrons. The scene is totally dominated by electromagnetic radiation - i.e. massless photons moving at the speed of light. Neutral atoms cannot gain a stronghold because the energetic photons continuously collide with the electrons and thereby strip these off the nuclei.
Age: 150,000 - 15 million years. As the universe expands, the density of photons diminishes. Simultaneously, the photons become less energetic. At 150,000 years the universe reaches a temperature of 4000 degrees and photon-electron scatterings loose their importance so that electrons and nuclei can combine and form atoms. Photons are now free to move large distances - i.e. the universe become transparent to radiation. As the impact of radiation diminishes and the amount of matter raises, gravity starts to grow in importance.
Age: 15 million years - present (8 - 20 billion years). The cosmic evolution is now dominated by matter clumping - stars, galaxies and galaxy-clusters are formed. The first galaxies are formed somewhere between 15 million and 1 billion years after the Tiny Puff set it all off. At the preceding stages only the light elements, hydrogen and helium, were formed. The heavier elements form in the interior of stars. Stars come and go and when a star reaches its end, its outer layers are blown out in space as interstellar dust. Once upon a time, sufficient amounts of stellar dust are gathered somewhere in a galaxy named Milkyway, eventually leading to the formation of the solar system. Some leftover dust, lying on the surface of the planet Earth, begin to interact with photons from the star Sun ... and humanity comes into existence.
After this brief history of space-time we are now ready for the theoretical foundations of the cosmological model prevailing in the rather cool glimpse called the second milennium.
