Cosmic Microwave Background Radiation is an electromagnetic remnant of the early Universe which contains very important information about creation, evolution, geometry, density of components and many other properties of the cosmos.
So this whole time we have been talking about how the theory of a Hot Big Bang is the most accepted theory in cosmology today, but it was not always that way. It finally started to gain popularity with the discovery and identification of the Cosmic Microwave Background. To begin, in the middle of the 20th century, George Gamow surmised that if matter had been created during decoupling, than the primordial radiation (i.e. The photons that decoupled from the matter) should be present with an almost perfect uniformity everywhere in the universe. He inaccurately predicted the temperature of this radiation to be about 50 K (-267°C), but his students later re-calculated the temperature to be 5 K. However, there was still no empirical proof of the existence of the radiation, so Gamow’s theory fell somewhat into obscurity.
Things started to heat up again in 1965 when Arno Penzias and Robert Wilson, radio astronomers working for Bell Laboratories, found a mysterious microwave signal causing background noise in their radio telescope. The strange part was that the signal seemed to come from everywhere. A group at Princeton was able to identify this radiation as the leftover remnants of the Big Bang, called the Cosmic Microwave Background, or CMB for short. The spectrum of the CMB fits that of a black body nearly perfectly, and so via the black body curve the temperature of the CMB has been determined to be about 2.7 K. Due to its near perfect uniformity, scientists conclude that this radiation originated in a time when the universe was much smaller, hotter, and denser. So as a result of the continual expansion of the universe, the light waves of this radiation have stretched out to longer wavelengths which today exist in the microwave region of the electromagnetic spectrum (which is why we call it the Cosmic “Microwave” Background).
There are a lot of reasons to look at the Cosmic Microwave Background (CMB). We hope we can find the answers to how and why the galaxies formed, and how the universe expanded. We are especially interested in what we call the geometry, or shape, of our universe. To start, we all know from Newtonian Mechanics that things that have mass exert a gravitational force which is proportional to their mass (i.e., The more mass something has, the more gravitational pull it exerts). For example, the Earth’s gravity is what keeps us from falling off it. Moreover, the galaxies in our local group are actually moving toward each other due to gravity. The density of matter in our group is high enough to overcome the force of the expansion of our universe. On a larger scale, our universe also has a specific density of matter, but that is a very hard quantity to determine because we do not know just how big the universe is. Based on the rate of expansion of our universe, however, we can figure out what the critical density of our universe is. The “critical density” refers to the minimum density of the universe that would be needed to have enough “gravitational pull” to overcome the expansion of the universe and cause it to re-collapse. We define a parameter omega as the (actual density of the universe) / (critical density). If omega is greater than 1, we say the universe has a positive curvature and call this scenario a closed universe because eventually gravity will overcome the expansion and cause the universe to re-collapse. On the other hand, if omega is less than 1, then the universe will expand forever; we call this an open universe with a negative curvature. The last scenario is when omega is equal to 1 because the force of gravity is exactly enough to counter the force of the expansion, and so the universe will continue to expand, but its rate of expansion will slow down asymptotically to 0; we call this a flat universe.
While the CMB is predicted to be very smooth, the lack of features cannot be perfect. At some level one expects to see irregularities, or anisotropies, in the temperature of the radiation.
These temperature fluctuations are the imprints of very small irregularities which through time have grown to become the galaxies and clusters of galaxies which we see today.
The cosmic microwave background is polarized at the level of a few microkelvin. There are two types of polarization, called E-modes and B-modes. This is in analogy to electrostatics, in which the electric field (E-field) has a vanishing curl and the magnetic field (B-field) has a vanishing divergence. The E-modes arise naturally from Thomson scattering in a heterogeneous plasma. The B-modes, which have not been measured and are thought to have an amplitude of at most 0.1 µK, are not produced from the plasma physics alone. They are a signal from cosmic inflation and are determined by the density of primordial gravitational waves. Detecting the B-modes will be extremely difficult, particularly as the degree of foreground contamination is unknown, and the weak gravitational lensing signal mixes the relatively strong E-mode signal with the B-mode signal.
Here is a useful review paper on the Cosmic Microwave Background radiation.