Left over from the Big Bang, the cosmic microwave background gives astronomers an insight into the entire history of our Universe. Ezzy Pearson explains
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Light is the window through which astronomers view the cosmos. It fills the Universe. Once light has been emitted, it travels on and on, moving constantly at the speed of light. It only stops when it hits something, be it a gas particle, a planet or even a black hole.
Because of light’s eternal nature, we see distant galaxies not as they are now, but as they were when their light was emitted. If it has taken the light from an object a billion years to reach us from a galaxy, then we see that object as it was a billion years ago. Look far enough away, and you could view all the way back to when the first stars began to shine.
Look back even further and it’s possible to see light from the very earliest days of our Universe’s existence, which we see now as the cosmic microwave background (CMB).
“The simple way to explain the CMB is as the very first light ever emitted in our Universe,” says Erminia Calabrese from Cardiff University. “It’s been travelling from that moment all the way to today.”
In the beginning…
The Big Bang theory states that in the very first moment of the Universe, all the energy and matter of the cosmos was at an infinitely dense point. In the initial fractions of a second of existence, this rapidly expanded, cooling as it went. When the Universe was a second old, it was cool enough for that energy to take on the form of matter particles – such as protons, neutrons and electrons. It also formed light particles known as photons, setting the entire Universe aglow long before the first stars began to shine. Within the first few minutes, protons and neutrons joined together to form positively charged nuclei, but the negatively charged electrons were so hot they could evade being caught. They floated free and so were able to easily absorb and re-emit the light photons that filled the cosmos. For over 380,000 years, the Universe was filled with an impenetrable fog that prevented light from travelling more than a few fractions of a nanometre.
Then came a time known as Recombination. The Universe had cooled enough that electrons could no longer escape the electromagneti
