What is cosmic microwave background and what is its connection with our expanding universe?

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It is a result of the red-shifting of light from distant objects, because of the expansion of the Universe. You can read about the red shift in the first part of this answer: Ilavenil Surya Thirumavalavan's answer to How scientists measure the age of light coming from stars?
After the Big Bang, there were various stages in which the Universe expanded and cooled. First the elementary particles formed (quarks, electrons, etc.) and then came the subatomic particles (protons and neutrons.) This was very chaotic, as particle-antiparticle pairs annihilated continuously. The light that was emitted would traverse a very short distance before it hit another particle, and would scatter.
At a certain stage in the very early Universe (377000 years later, but we’re talking about something that happened 13.6 billion years ago) the temperature of the Universe dropped enough for protons and electrons to form the first hydrogen atoms (shown as the Era of atoms in the above diagram). This meant light could travel through the Universe without constantly getting scattered, or, the Universe became transparent to light.
If the Big Bang Theory was true, the light that was emitted at that point should still be around. Also, it would be seen everywhere in the Universe, at the light was not emitted from one object or in one direction. When scientists calculated the amount of red shift, they saw that the light would now be so strongly shifted, that it would be in the microwave range. They termed this the Cosmic Microwave Background Radiation (CMBR) and it was observed first in 1964 by Penzias and Wilson. Further studies have conclusively proved that the CMBR exists, proving the Big Bang theory as well.
92 Views · View Upvoters · Answer requested by Karan Pratap Shaw
Akshay Khairnar
Akshay Khairnar, Blogger, Space Explorer, Technologist
I think this would be helpful,
The cosmic microwave background (CMB) is believed to be leftover radiation from the Big Bang, or the time when the universe created. As the theory goes, when the universe was born it suffered a rapid inflation and expansion. The CMB denotes the heat left over from the Big Bang.
You can't see the CMB with your naked eye, but it is everywhere in the universe. It is invisible to humans because it is so cold, just 2.725 degrees above absolute zero. This shows its radiation is most visible in the microwave part of the electromagnetic spectrum.
What’s the connection?
The CMB is useful to scientists because it helps us learn how the early universe was formed. It is at a constant temperature with only small instabilities visible with precise telescopes. By studying these fluctuations, cosmologists can learn about the origin of galaxies and large-scale structures of galaxies and they can measure the basic parameters of the Big Bang theory
I think you should check out this link, It will definitely give answers to your questions - What is Cosmic Microwave Background

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