The Cosmic Microwave Background or CMB was discovered in 1965 by two Bell Laboratories scientists.  They were building a radio receiver which picked up an unexpected microwave signal as extra noise.  Scientists Robert Dicke and Dave Wilkinson at nearby Princeton University heard about this unexplained radiation and realized that this signal must be the cosmic radiation background.  The CMB was actually predicted in 1948 by scientist George Gamow and in 1950 by Ralph Alpher and Robert Herman. The Big Bang Theory states that the early universe was hot and full of extremely high energy electromagnetic radiation.  Protons, electrons, and photons as well as other particles existed in a dense "fog" or "soup" bouncing off of one another. As the universe expanded, it cooled, and around 400,000 years after the Big Bang, the universe cooled enough that protons and electrons could combine to form hydrogen.  The photons were then able to travel free.  This period is known as the period of last scattering or recombination, since it was when the CMB last interacted with the other particles.  Now the formerly hot radiation is at a uniform temperature of 2.725 degree Kelvin.  Electromagnetic radiation of this temperature peaks in the microwave, making it invisible to our eyes, but relatively easy to detect with instruments on Earth. One of the most important aspects of the CMB is the near uniformity of the radiation.  Fluctuations in temperature (between cool and hot spots) have been measured to be only about .0002K in difference, as shown by the 1992 experiment COBE.  These fluctuations are tiny, but are believed to mirror the fluctuations of matter density in the early universe, which went on to create large scale structures, such as galaxies. WMAP has continued the mapping out of the cosmic background radiation, which holds much important information about the beginnings of the universe.

This image from the NASA/WMAP Science Team shows a detailed image of the Cosmic Microwave Background, giving us a glimpse of the early universe.  The colors indicate "warmer" red and "colder" blue spots, although the actual difference in temperature is about ± 200μK.    The white lines indicate the direction of polarization, which is explained briefly below.

So we know that that the CMB accounts for a hot early universe, but there are several other questions that the Big Bang Theory and the CMB do not explain. Why the Universe is so uniform at large scales, why the geometry of the universe is flat, and what caused the initial fluctuations in temperature and density great enough to create large scale structures are some of these questions. A theory developed by Alan Guth, Andrei Linde, Paul Steinhardt, and Andy Albrecht provides the answers to these problems by proposing that just after the universe began, it expanded exponentially in a fraction of a second. This rapid and quick expansion accounts for a flat universe, which is what recent missions have determined about the geometry of space.  It also explains why the universe is uniform on large scales since the expansion took place in such little time that matter was not able to scatter significantly. The rapid expansion was also able to increase tiny fluctuations in density and temperature to sizes suitable for creating large scale structures such as galaxies. 

Polarization of the CMB

The CMB is polarized due to Thompson Scattering of CMB photons by free electrons at the period of last scattering.  When a photon, or light wave "bumps" into a free electron, the electric field due to the photon acts as a force on the electron.  According to the equation the force exerted causes oscillatory motion for the electron and creates a dipole polarized in a certain direction.  Now in an isotropic radiation field, where radiation is uniformly spread out and is not dependent on direction this scattering effect would have no net polarization.  Since the CMB is polarized, there must exist a quadrupole moment in the initial radiation field.  This quadrupole moment is created by a temperature anisotropy or the intrinsic arrangement of "hot" and "cool" areas at the horizon of last scattering.  However since temperature (and density) are scalar quantities (not having any direction) the polarization will not have any curl component.   Another explanation for the quadrupole is the affect of a gravitational wave traveling through space.  This gravitational wave perturbs the metric, or you can think of it as stretching space, and this creates  quadrupole moment as well.   Gravity waves are tensor quantities and have both gradient and curl components which are referred to "E" and "B" modes respectively (analogous but not the same as the E and B field components of electromagnetic radiation).  Only gravity waves, and not temperature anisotropy, can cause the B modes.  The gravity waves created during inflation later interact with the CMB photons and give the CMB a tensor polarization.  This tensor polarization should be observable according to recent results from WMAP. Although the amplitude is not known, it is predicted to be in the range of 1 to 100 nK, which is about one percent of the total polarization of the CMB.  This is the signal that PAPPA is searching for, and for which new technology has been developed to aid in this extremely difficult task. 

For more information about the CMB and Polarization click on these sites

WMAP website -more information on the WMAP mission as well as background on the CMB

Wayne Hu's Tutorials on CMB -a series of tutorials on the CMB and polarization with material ranging from beginner to advanced

Ned Wright's Cosmology Tutorial