Cosmic Microwave Background Radiation
Around 13.7 Billion years ago, Universe started with the big bang. The laws of physics break at this point, the singularity, and the current physics do not know what there was, what produced the bang or anything before that. But soon after the bang, as a matter of fact, until first 10-32 to 10-33 seconds, the universe expanded rapidly, by approximately a factor of 1026. The physicists do have a theory, the popular one developed by Alan Guth and his colleagues, about this inflationary period, and so we do know what happened at this time scale in the universe. As per their theory, the universe was set into bang due to a conjectured quantum field called the inflaton. But interestingly, the inflationary period came to an end and after this inflationary period, the universe continued to expand due to dark energy called the Hubble expansion.
When the inflation ended, the universe existed in a plasma state owing to the large temperatures. The temperature was so high, causing electrons had enough energy and it didn't bound with the protons to form atoms. Free electrons in the plasma, therefore, caused the Thomson scattering of photons and therefore the mean free path of photons remained quite low. In other words, photons were coupled with the matter. The universe, therefore, was quite opaque at this time.
In the background, as was said, the universe kept expanding. The expansion of the isolated system, adiabatic expansion, lead to its adiabatic cooling. After about 370,000 years, the universe cooled to reach a critical temperature in the history of the universe, that is, 3000K. At his temperature, electrons combined with the protons to form neutral atoms. This time is also called as the recombination epoch. Bound electrons absorb photons of only specific energies, therefore, the photons decoupled with the matter. This increased the mean free path of photons to the length of the universe. Over time, they have been travelling through space, which itself kept expanding. This caused the radiation waves to stretch, so much so, that they fall in the microwave region (therefore the name Cosmic Microwave Background or CMB). Penzias and Wilson, in 1965, while attempting to study the galaxy using their radio telescopes had found a uniform noise coming from all directions, irrespective of the time of year. Having tried all possible fixes, they connected to Dicke and Peebles, who knew that the noise was CMB.
Finer measurements later by COBE, WMP and Plank satellites found that the radiation is very homogenous, with minute anisotropies, peaking at 160.23 GHz. The spectrum actually fits very closely with the theoretical black body emission spectrum at 2.7K. The higher resolution data from subsequent missions found that the anisotropies are in the order 10-5K. Given the radiation is at thermal equilibrium with its surrounding this is what one may expect. The anisotropies match the predictions if small thermal variations, generated due to quantum fluctuations in small space, is made to expand to large volumes. These observations, therefore, strongly support the lambda-CDM model of the universe.
CMB fluctuations based on Plankdata. Red are hotter while blue is colder fluctuations. The mean temperature is 2.725K [6]
Plank data provided the power spectrum, which again provided an excellent fit with the theory as was discussed in the article about Baryonic Acoustic Oscillations. This also tells the importance of these fluctuations as these later led to the formation of large scale structures in the universe like clusters, galaxies etc.
However, there also exists some details which do not support the lambda CDM model and calls for further investigation, providing deeper insights into the universe like dark matter, dark energy etc. One such is the existence of asymmetry in the average temperatures across the two opposite spheres of the sky. There is found to be a slight excess of hot spots in the southern hemisphere and of cold spots in the northern part. Also, slight anomalies are observed when fitting standard models with the CMB both at small and large angular scales, although overall the fit was good. Another is the existence of a cold patch, much bigger than expected in the Southern hemisphere.
References:
[3] - PBS
[4] - Lectures
[5] - WSF
[8] - KnowTheCosmos
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