CMB

Cosmic Microwave Background Radiation

In 1965, Penzias and Wilson of Bell Laboratories made the startling discovery of an all pervasive background radiation from the Universe. Since then several further missions, COBE and WMAP, have probed deeper into the properties and meaning of the CMB. Indeed, WMAP has pushed the current picture of our Universe leading to the startling result that around 68.3 % of the Universe is composed of Dark Energy and that although the rest of the Universe is composed primarily of matter, more than 80 % of the matter is of the form of Dark Matter and is composed of, as yet,an unknown particle. Only 4.9 % of the Universe is composed of normal matter.
The CMB has properties:

The radiation comes from everywhere and has very low temperature, T ~ 2.725 Kelvin or -455 Farenheit (or -270 C) !!!, based on the fact that its spectrum is well-described by a blackbody spectrum.
The radiation is isotropic, i.e., it is very close to the same temperature all across the sky -- temperature differences of < 0.004 % on angular scales of 7 degrees (excluding a well-known 0.12 % variation known as the dipole anisotropy and finer, lower amplitude temperature variations). In the next sections, we consider the low amplitude temperature fluctuations in detail.

The dipole anisotropy arises due to the motion of the Milky Way galaxy through the Universe. The Local Group of galaxies which contains the Milky Way is moving toward Hydra with a speed of around 500 kilometers per second.

Structure Formation
The timeline to the left shows when the various structures in the Universe are thought to have formed.

The low-amplitude, small-scale fluctuations seen in the CMB indicate the sizes of the fluctuations which eventually led to the observed structure of the Universe at the time the CMB formed, roughly 380,000 years after the Big Bang. The existence of the flucutations is thus good but may turn out to be rather vexing at the same time.

We know that on certain scales the Universe is inhomogeneous, there are observed structures in the Universe like galaxies, clusters of galaxies, clusters of clusters of galaxies, voids, ... .
These structures represent fluctuations in the material content of the Universe. For example, the average density of the Universe is less than 1 hydrogen atom per 100,000 cubic centimeters. The average density in our galaxy is something like 1 hydrogen atom per cubic centimeter. That is, galaxies are something like > 100,000 times denser than the average chunk of the Universe.

Presumably such structures were formed by the mutual gravitational attraction of the particles which make up the galaxies. A similar argument can be made for clusters of galaxies, clusters of clusters of galaxies, voids, ...
The upshot is that we can figure out roughly how long the formation process should take, and how large fluctuations need to be around the Epoch of Recombination when the CMB was formed.
The current size of structures in the Universe are not consistent with the measured smoothness of the CMB if normal matter dominates the Universe. However, if Dark Matter is included then things can be made right.

At early times, there are no large fluctuations. As time goes by, fluctuations driven by inflation start to grow. If there is a lot of Dark Matter compared to normal matter, then these fluctuations may be large because clumping of dark matter is not inhibited by the intense energy of the radiation; also, because it does not strongly affect light, the radiation of the Universe is not strongly affected by Dark Matter! As the fluctuations (both dark matter and normal matter) grow, structure will be produced without strongly affecting the light, that is, the CMB when it forms at 380,000 years! The stucture of the current Universe strongly constrains the balance between Dark Matter and normal matter in the Universe.

The left panel shows the CMB and a blow-up of a small portion of the CMB. You can see much fine structure in the temperature of the CMB. It is fairly random looking. Note--although the plots make the temperature fluctuations look large, they are actually quite small less that 0.0001 Kelvin compared to 2.73 Kelvin for the average CMB. The middle and right panels show how cosmologists make sense of patterns in the fluctuations. For example, consider the right hand panel. The figure in the upper left hand corner shows the full WMAP image of the CMB. In the picture to its right, notice that the small-scale (rapid changes are smoothed over) leaving only a more gradual changing component. The smoothing, for example, can be performed by running a filter over the image and averaging the CMB with the filter,

This can reveal structure; structure in the CMB is apparent when one does this and then compares the two images. The other six images do the same exercise except that each smooths out different fluctuation to reveal how the CMB looks on different scales. The right hand panel shows a neat way to look at the CMB. The central panel is another way to represent the picture in the right panel. In this case, the center panel shows which smoothed pictures contribute the most and least to the overall picture of the CMB.

The largest structure seen in the CMB map, the first big bump between 1 and 2 degrees is a result of the Baryon Acoustic Oscillation (BAO). The angular size of this feature reflects the size of the horizon of the Universe at the time of formation of the CMB, that is, at the time of Recombination. This makes sense as the horizon tells the size of the region over which different parts of the Universe may have communicated over the current lifetime of the Universe. This peak, when the math is done, tells us the curvature of the Universe. That is, this first BAO peak tells us that the Universe is flat.

The following peaks that occur at smaller angular sizes, give us more detailed peeks at the Universe. The next peak tells us that dark energy is around 68 % of the Universe.

The next step in the analysis of the CMB is to take all of the proposed models for the evolution of the Universe; open, closed, flat spacetimes, universes filled with dark matter, normal matter, dark energy or combinations of the above, inflationary universes, universes without inflation, steady-state universes, and any other models one can conjure up. The test then becomes to see which models can reproduce the snapshot of the Universe at 380,000 years, the CMB, and to then compare these results with results based upon other tests, the Hubble-type analyses, and studies of the distribution of galaxies.

WMAP RESULTS
Remarkably, an inflationary model with Ω = 1 and the breakdown shown at right is the best-fit model for the Universe.
Furthermore, at the far right are presented the best-fit models for CMB from WMAP and the best-fit universe from the Type Ia SNe observations. Both groups (independently) arrive at nearly the same model for the Universe!!

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