What Is Cosmic Microwave Background Radiation - Simplified
A Brief History of The CMB
Over fifty years ago, two astronomers made a groundbreaking discovery that revolutionized our comprehension of the universe. One day, two radio astronomers, Robert Wilson, and Arno Penzias were working on a radio receiver for Bell Telephones Laboratories, and when they tested their receiver, they noticed a form of white noise that was constantly being picked in every direction of the sky, all the time. After checking for technical issues, they couldn't find the reason why they were receiving this noise. Unaware to them, they had stumbled upon the Cosmic Microwave Background (CMB) radiation, the thermal radiation from the Big Bang, providing a snapshot of the infant universe and confirming key aspects of the Big Bang theory. Wilson and Penzias won the Nobel Prize in Physics in 1978 for their important discovery, making them key figures in the history of science.
Robert Wilson(left) and Arno Penzias(right) stand next to their radio receiver.
Image Credit: "Image from New Scientist, 'Father of Big Bang carries its hiss on his cellphone' article. Retrieved from New Scientist"
Before this discovery, a theoretical prediction of the CMB was made in 1948 by Ralph Alpher and Robert Herman, who, working with George Gamow, calculated that the early universe would have been extremely hot and dense. As the universe expanded, this heat would have been stretched to microwave wavelengths, resulting in background radiation that should still be detectable today. They estimated the temperature of this radiation to be around 5 K, which is quite close to the actual observed value of approximately 2.7 K.
How Did It Form
The universe was a young fiery baby at the dawn of space and time. After the Big Bang, every corner of space was filled with this fancy-named substance called a quark-gluon plasma. This plasma is made by elementary particles called quarks and gluons, which are the building blocks for protons and neutrons. These elementary particles are moving around so fast, that they can’t combine to form atomic structures. A couple of minutes after the Big Bang, the universe expanded and cooled down enough that quarks and gluons were able to come together to form atomic nuclei (protons and neutrons). But if we remember from chemistry class, we know that we are missing one crucial part to make a complete atom. Drum roll…… Electrons!
Now where are the electrons right now? Well, they are also zooming around really quickly along with atomic nuclei. This is where things get interesting. When a light particle, a photon get emitted through certain conditions, it can’t travel very far because it will get bounced back and forth or get absorbed by atomic nuclei or by electrons. So if we sent a camera back to that time, we wouldn’t see much as it would look very opaque like a foggy night. This is also the reason why we can’t see the very early stages of the universe, this foggy and opaque vieqw of the universe wouldn’t let us see what came before, putting a barrier on our ability to directly observe the moments immediately following the Big Bang. Fast forward to 380,000 years after the Big Bang, the universe was 84 million light-years in diameter. This was big enough for the universe’s temperature to cool down enough so that electrons could combine with the prefabricated atomic nuclei, hence the name of this epoch, Recombination. This was great news for photons since they now have more space and a lower probability of coming into contact with other particles. The photons that were emitted were in the form of visible light. But as the universe continued to expand, the light’s wavelength started to increase, causing it to lose its energy. The light has been redshifted.
If we look at the electromagnetic spectrum, the left side contains the longer wavelengths like radio waves or microwaves that have low energies and the right side contains shorter wavelengths and have higher energies. When a wave of light is redshifted, it moves left of the electromagnetic spectrum. So in the context of CMB, the first light that was emitted was in the form of visible light and was later red-shifted into the microwave due to the expansion of space, and that’s why we call it the Cosmic Microwave Background.
This Electromagnetic Spectrum shows that the CMB’s frequency is very low due to the expansion of the universe.
Image credit: Image from NDE Ed's page on 'Characteristics of Radiation.' Retrieved from NDE Ed
After the discovery of the CMB by Wilson and Penzias, science communities around the world set out on a quest to take a visual picture of the CMB. But how can you take a picture of something that is not in the visible spectrum? Well, the CMB is considered as a blackbody. A black body is an object that absorbs light, but doesn’t reflect it and emits a form of radiation back. This radiation is currently in the microwave spectrum. All you need are telescopes that are sensitive to microwaves. With this, you can scan the entire sky and map out the temperature variations and anisotropies in the CMB, providing a detailed image of the early universe's conditions.
After multiple attempts, this is the most accurate and detailed map of the CMB to date. It was created by the Planck Satellite by the European Space Agency after completing an entire sky survey.
Image credit: "Image from the European Space Agency (ESA) article 'Planck reveals an almost perfect Universe.' Retrieved from ESA”
What Robert Wilson and Arno Penzias discovered is the first light that could fly through the cosmos freely. It is like the leftover heat from a fireplace, waiting for someone to feel its presence. Their discovery provided the strongest form of evidence for the Big Bang theory and opened up new avenues for understanding the early universe's conditions and evolution.
