what is cosmic microwave background radiation?

What is Cosmic Microwave Background Radiation? Did it happen after the Big Bang? —Sreehari, age nine, Kerala State, India

The Cosmic Microwave Background (CMB for short) is light: the oldest and farthest light we can see in the entire universe. It comes from shortly after the Big Bang – which is considered the beginning of the universe.

However, it is not made up of light that you or I can see with the naked eye. The type of light we can see is called visible light, but there are other types of light as well. Microwaves are a type of light, just like the X-rays we use to check for broken bones and the radio waves we use to listen to music in the car.

In the beginning, the CMB was very energetic X-ray light. Over time, it has lost energy and turned into lower energy microwaves.

Oval image of red, green and blue points of light
An image of the CMB from the Planck telescope. The colors indicate the temperature of different parts of the CMB.
ESA and the Planck collaboration, CC BY-SA

The CMB is the light from the beginning of the universe. At that time, the universe was very hot and dense, and full of particles called electrons and protons. These particles have an electrical charge, and when light reached one of the particles, the electrical charge would send the light in a different direction. This prevented the light from traveling far.

Cooling down

But over time, the universe expanded and cooled. Finally, when the universe cooled down enough, the electrons and protons began to bond together and form hydrogen atoms. These atoms have no electrical charge, so they don’t affect light in the same way that electrons and protons do on their own. Light could pass through it and on through the universe as if it were completely empty.

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The universe cooled at the same rate everywhere, so this process happened at the same time everywhere. Suddenly, light could travel very far and fast from all over the universe at the same time. This light still travels today, and it is what is now reaching us on Earth as the CMB.

The CMB light was always in the universe, but could not travel far until the first atoms were formed. We even know that it was released 380,000 years after the Big Bang. This sounds like a long time between the Big Bang and the CMB release, but since the universe is nearly 14 billion years old, this happened when the universe was very young.

The CMB tells us a lot of important information about what the universe looked like long ago. According to the big bang theory, the early universe was very hot and full of radiation. As the universe expanded and cooled, this radiation would eventually be released. This is exactly what we now see as the CMB. It even has the temperature predicted by the big bang theory, which is why we can say that the CMB is proof that the big bang theory is correct.

An accidental discovery

The CMB was accidentally discovered. Two scientists in the US, Robert Wilson and Arno Penzias, used a microwave telescope and saw the same extra signal wherever they pointed the antenna. They thought the extra signal could be caused by a malfunction in their telescope – or even pigeon droppings on their antenna.

Black and white image of aerial equipment with two men on it
The 15-meter Holmdel horn antenna at Bell Telephone Laboratories in Holmdel, New Jersey was used by radio astronomers Robert Wilson and Arno Penzias to discover the CMB.
NASA via Wikimedia Commons

Eventually, they realized they were the first humans to ever discover the CMB, which the big bang theory predicted would exist. They won the Nobel Prize for their discovery.

Since then we have sent many telescopes into space to get better and better images of the CMB. If we look at the oldest light in the universe, we can understand how everything we see today came to be.