Cosmic microwave background – The light from the past
Have you ever wondered about our existence? Why earth is at a precise distance from the sun? if it was a little farther or closer, the earth would have been a completely different place. Digging deeper into the mysteries of this vast universe will eventually take us to some supernatural answers. For a long time, philosophers and scientists thought that our universe was a cube surrounded by a medium, that they called ‘ether’. After a long time due to a lot of research and observations, scientists came across a reasonable explanation of our universe. Different cosmologists came up with different models and explanations of how everything started. The most acceptable theory among them was the big bang theory, according to which the universe started from a big explosion followed by an exponential expansion called inflation. One can find many observational evidence for big bang theory like the increasing distance between galaxies, cosmological redshift, and so on. One of the important and wonderful discoveries was the detection of Cosmic microwave background (CMB). This article is about CMB and how it helped us to increase our understanding of our universe.
History of CMB Discovery
In 1948, two famous scientists, George Gamow and Ralph Alpher predicted the existence of cosmic microwave background. They estimated the temperature of this radiation to be 28 Kelvin. But this idea did not get attention from the public until, in 1965, Arno Penzias and Robert Woodrow Wilson detected this microwave radiation using a Dicke radiometer which was made for measuring the CMB radiation in 1964. Penzias and Wilson were doing experiments on radio astronomy and they surprisingly came across some background noise which they thought because of a pigeon. But an interesting fact was the radiation was isotropic. They scared away the pigeons and checked again. Surprisingly, that noise was still there. They published their findings and they confirmed the existence of CMB. In 1978 both of them received the Nobel prize in Physics for this extraordinary finding (Griswold, 2016).
What is Cosmic microwave background (CMB)?
CMB is the leftover electromagnetic radiation from the big bang. This radiation is invisible to the naked eye and for optical telescopes. Only radio telescopes can detect this faint signal which is coming from 13.8 billion light-years away from the earth (which is considered to be the maximum distance, a human can ever look at and consider as the age of the observable universe).
According to the big bang theory, the universe started from a single point with infinite density and heat. Due to this, the universe became unstable and exploded, and started expanding exponentially (inflation). At this time, the universe was so hot and only consisted of hot white plasma (Charged particles). There were no neutral atoms or even nuclei. After 3,80,000 years after the big bang, the universe slowly cooled down and the charged particles combined (recombination), which lead to the formation of the first neutral atom (Griswold, 2016). For the first time, the universe became transparent. The atoms were in an excited state due to the high energy and they released energy to be stable. This energy was photons (Photon decoupling) which we can now see as cosmic microwave background radiation. Due to the expansion of the universe, these photons became redshifted and we detect them as microwave radiations.
The concept of CMB was controversial at first because the cosmologists believed in the steady-state model at that time. They argued that this is the scattered light from distant galaxies. But, they couldn’t prove their theory through observations.
Temperature fluctuations or CMB temperature anisotropy
If we observe the cosmic microwave sky, we can see that the temperature of the CMB is not the same in all directions as it supposes to be according to the theoretical calculations. These small fluctuations in the temperature with an average temperature of 2.725Kelvin across the sky are the microwave background temperature anisotropies. Scientists say that this is due to two reasons – one is an intrinsic temperature variation at the last scattering surface (the sphere formed by the intersection of our past light con and the hypersurface corresponding to the time of photon decoupling) and another one is the temperature variation due to the redshift of the photons that they undergo during their journey towards us.
The observed temperature anisotropy of the CMB is composed of two contributions,
\begin{equation}
\Big(\frac{\delta T}{T}\Big)_{obs}=\big(\frac{\delta T}{T}\big)_{intr}+ \Big(\frac{\delta T}{T}\Big)_{jour},
\end{equation}
where the first term 
Models of universe and CMB
Being CMB as the most prominent and reliable data, any proposed model regarding the universe should agree with the initial data with the CMB. So far, the most popular model is the Inflationary cosmology which explains the initial radiation (CMB) to a larger extent. But, any popular model can be questioned for its certainty and precision whereby other promising models such as Conformal Cyclic Cosmology, Quantum Loop cosmology, Steady-state model, etc are also in the game.
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Great explanation, very helpful