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
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.
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 above image has a typical dipole appearance because our Galaxy is moving in a particular direction. The result is
one side of the sky will appear redshifted and the other side of the sky will appear blueshifted. In this case, redshifting
means the photons are longer in wavelength = cooler (so backwards from their name, they look blue in the above
diagram). Removing the Galaxy's motion produces the following map:
This map is dominated by the far-infrared emission from gas in our own Galaxy. This gas is predominately in the plane
of our Galaxy's disk, thus the dark red strip around the equator. The gas emission can be removed, with some
assumptions about the distribution of matter in our Galaxy, to reveal the following map:
This CMB image is a picture of the last scattering epoch, i.e. it is an image of the moment when matter and photons
decoupled, literally an image of the recombination wall. This is the last barrier to our observations about the early
Universe, where the early epochs behind this barrier are not visible to us.
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.
Fluctuations and the Origin of Galaxies :
The density fluctuations at recombination, as measured in the CMB, are too large and too low in amplitude to form
galaxy sized clumps. Instead, they are the seeds for galaxy cluster-sized clouds that will then later break up into galaxies.
However, in order to form cluster-sized lumps, they must grow in amplitude (and therefore mass) by gravitational
instability, where the self-gravity of the fluctuation overcomes the gas pressure.
The CMB fluctuations are a link between Big Bang and the large scale structure of galaxies in the Universe, their
distribution in terms of clusters of galaxies and filaments of galaxies that we observe around the Milky Way today.
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