In essence, the Cosmic Microwave Background (hereinafter referred to as CMF) is light, but we cannot see it with the naked eye. The type of light we can see is called visible light, but there are other types of light. 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.

Originally, CMF was very high-energy X-ray light. Over time, it lost energy and became ordinary microwave radiation.

In fact, CMF is light from the beginning of the universe. At this point in time, the universe was very hot and dense and filled with particles called electrons and protons. These particles have an electric charge, and when light reaches one of the particles, the electric charge sends it in another direction. This prevented the light from traveling too far.

CMF image from Planck telescope (colors show temperature of different points of CMF) / ESA photo

cooling of the universe

Over time, the universe expanded and cooled. Eventually, when the universe cooled enough, electrons and protons began to bond together to form hydrogen atoms. These atoms have no electrical charge, so they do not affect light like electrons and protons. Light can pass through them and pass through the universe as if it were completely empty.

The universe was cooling at the same rate everywhere, so this process was happening everywhere at the same time. Suddenly, light can travel far and fast from all over the universe at the same time. This light is still traveling today, and it is what has now reached Earth in the form of the Cosmic Microwave Background.

CMF light has always been in the universe, but it couldn’t spread far until the first atoms were formed. In fact, we know that it began to spread 380,000 years after the Big Bang. This seems like a long time between the Big Bang and the beginning of the CMF expansion, but it was when the universe was very young, as the universe is almost 14 billion years old.

The CMF “tells” us a lot of important information about what the universe was like a long time ago. According to the Big Bang theory, the early universe was very hot and filled with radiation. As the universe expanded and cooled, this radiation was eventually released. It even has the temperature predicted by the Big Bang theory, and therefore we can say that CMF is the proof of the Big Bang theory’s accuracy.

an accidental discovery

The cosmic microwave background was discovered by accident. Two American scientists, Robert Wilson and Arno Penzias, used a microwave telescope and consistently saw the same extra signal no matter where the antenna was pointed. They speculated that the extra signal might be due to a malfunction in their telescope, or even pigeon droppings on their antennae.

15 meter Holmdel horn antenna used by NASA to detect KMF / Photo

They later realized that they were the first humans to discover CMF, which the Big Bang theory predicted existed. They received the Nobel Prize for their invention.

Since then, humans have sent many telescopes into space to get better images of the CMF. A look at the oldest light in the universe can help us understand how everything we see today came to be.