400,000 years after the initial Big Bang, when the era of exponential inflation
was over, things settled down to a slower and steadier rate of expansion.
As more space became available for the energy in it, the universe was
cooling things down. The early
universe was a nearly perfect “liquid plasma” saturated with energy, in
which quarks behaved as free particles. As
the cooling progressed, the only strongly interacting particles around, quarks,
organized themselves into composite mesons, protons, and neutrons.
By some process that remains obscure, there was a slight excess of
protons and electrons over their antimatter equivalents (antiprotons and
positrons). During the high-density
stages of the early universe, essentially all of the antimatter paired off with
its matter counterparts to annihilate, leaving behind the slight excess of
matter particles as “the only game in town”.
The cooling universe was a “soup” dominated by free electrons and
protons. In this environment, a
photon of light could travel only a short distance without being absorbed by
interacting with one of the free charged particles.
the negative electrons and positive protons tended to pair off, forming neutral
hydrogen atoms. In the process, the
dominance of free charged particles, which easily absorb photons, was being
replaced by light-transparent neutral atoms.
The “soup” of the universe was changing from murky black to crystal
photons that were present in that era had energies that were characteristic of
light emitted from an object (the universe) with a temperature of about 2,900 K.
(Here, K means “kelvin” and specifies the absolute temperature in
Celsius degrees above absolute zero.) As
long as the universe was murky black, they were caught in a “ping-pong
match” of repeated emission and re-absorption.
However, the growing transparency of the universe released them from this
trap, and they became free photons. Those
photons have been traveling through the universe ever since, and we detect them
today as the cosmic microwave background radiation (CMB).
Wilkinson Microwave Anisotropy Probe (WMAP) was launched into a high orbit on
June 30, 2001 from a Delta II 7425-10 rocket at
"Ellipsoidal Universe Can
Solve the CMB Quadrupole Problem", I. Campanelli, P. Cea, and L. Tedesco,
submitted to Physical Review Letters, September,2006, preprint astro-ph/0606266