One of the foremost cosmological discoveries was the detection of the cosmic background radiation. The discovery of an expanding Universe by Hubble was critical to our understanding of the origin of the Universe, known as the Big Bang. However, a dynamic Universe can also be explained by the steady state theory.

The steady state theory avoids the idea of Creation by assuming that the Universe has been expanding forever. Since this would mean that the density of the Universe would get smaller and smaller with each passing year (and surveys of galaxies out to distant volumes shows this is not the case), the steady-state theory requires that new matter be produced to keep the density constant.

The creation of new matter would voilate the conservation of matter princple, but the amount needed would only be one atom per cubic meter per 100 years to match the expansion rate given by Hubble's constant.

The discovery of the cosmic microwave background (CMB) confirmed the explosive nature to the origin of our Universe. For every matter particle in the Universe there are 10 billion more photons. This is the baryon number that reflects the asymmetry between matter and anti-matter in the early Universe. Looking around the Universe its obvious that there is a great deal of matter. By the same token, there are even many, many more photons from the initial annihilation of matter and anti-matter.

Most of the photons that you see with your naked eye at night come from the centers of stars. Photons created by nuclear fusion at the cores of stars then scatter their way out from a star's center to its surface, to shine in the night sky. But these photons only make up a very small fraction of the total number of photons in the Universe. Most photons in the Universe are cosmic background radiation, invisible to the eye.

Cosmic background photons have their origin at the matter/anti-matter annihilation era and, thus, were formed as gamma-rays. But, since then, they have found themselves scattering off particles during the radiation era. At recombination, these cosmic background photons escaped from the interaction with matter to travel freely through the Universe.

As the Universe continued to expanded over the last 15 billion years, these cosmic background photons also `expanded', meaning their wavelengths increased. The original gamma-ray energies of cosmic background photons has since cooled to microwave wavelengths. Thus, this microwave radiation that we see today is an `echo' of the Big Bang.

The discovery of the cosmic microwave background (CMB) in the early 1960's was powerful confirmation of the Big Bang theory. Since the time of recombination, cosmic background photons have been free to travel uninhibited by interactions with matter. Thus, we expect their distribution of energy to be a perfect blackbody curve. A blackbody is the curve expected from a thermal distribution of photons, in this case from the thermalization era before recombination.

Today, based on space-based observations because the microwave region of the spectrum is blocked by the Earth's atmosphere, we have an accurate map of the CMB's energy curve. The peak of the curve represents the mean temperature of the CMB, 2.7 degrees about absolute zero, the temperature the Universe has dropped to 15 billion years after the Big Bang.

Where are the CMB photons at the moment? The answer is `all around you'. CMB photons fill the Universe, and this lecture hall, but their energies are so weak after 15 billion years that they are difficult to detect without very sensitive microwave antennas.

Ionization:

The last stage in matter production is when the Universe cools sufficiently for electrons to combine with the proton/neutron nuclei and form atoms. Constant impacts by photons knock electrons off of atoms which is called ionization. Lower temperatures mean photons with less energy and fewer collisions. Thus, atoms become stable at about 15 minutes after the Big Bang.

These atoms are now free to bond together to form simple compounds, molecules, etc. And these are the building blocks for galaxies and stars.

Radiation/Matter Dominance :

Even after the annihilation of anti-matter and the formation of protons, neutrons and electrons, the Universe is still a violent and extremely active environment. The photons created by the matter/anti-matter annihilation epoch exist in vast numbers and have energies at the x-ray level.

Radiation, in the form of photons, and matter, in the form of protons, neutrons and electron, can interact by the process of scattering. Photons bounce off of elementary particles, much like billiard balls. The energy of the photons is transfered to the matter particles. The distance a photon can travel before hitting a matter particle is called the mean free path.

Since matter and photons were in constant contact, their temperatures were the same, a process called thermalization. Note also that the matter can not clump together by gravity. The impacts by photons keep the matter particles apart and smoothly distributed.

The density and the temperature for the Universe continues to drop as it expands. At some point about 15 minutes after the Big Bang, the temperature has dropped to the point where ionization no longer takes places. Neutral atoms can form, atomic nuclei surround by electron clouds. The number of free particles drops by a large fraction (all the protons, neutrons and electron form atoms). And suddenly the photons are free to travel without collisions, this is called decoupling.

The Universe becomes transparent at this point. Before this epoch, a photon couldn't travel more that a few inches before a collision. So an observers line-of-sight was only a few inches and the Universe was opaque, matter and radiation were coupled. This is the transition from the radiation era to the matter era.

CMB Fluctuations :

The CMB is highly isotropy, uniform to better than 1 part in 100,000. Any deviations from uniformity are measuring the fluctuations that grew by gravitational instability into galaxies and clusters of galaxies.

Images of the CMB are a full sky image, meaning that it looks like a map of the Earth unfolded from a globe. In this case, the globe is the celestial sphere and we are looking at a flat map of the sphere.

Maps of the CMB have to go through three stages of analysis to reveal the fluctuations associated with the early Universe. The raw image of the sky looks like the following, where red is hotter and blue is cooler:

The clumpness of the CMB image is due to fluctuations in temperature of the CMB photons. Changes in temperature are due to changes in density of the gas at the moment of recombination (higher densities equal higher temperatures). Since these photons are coming to us from the last scattering epoch, they represent fluctuations in density at that time.

The origin of these fluctuations are primordial quantum fluctuations from the very earliest moments of are echo'ed in the CMB at recombination. Currently, we believe that these quantum fluctuations grew to greater than galaxy-size during the inflation epoch, and are the source of structure in the Universe.