Let’s say you make a cup of tea (or coffee or hot chocolate), and then you leave it out on the kitchen counter because you got distracted by something (i.e. your dog or your children). What will happen to the hot drink? We all know that it will cool down over time. If given enough time, it will reach thermal equilibrium with its surroundings… which means it will reach room temperature.
Well, that’s kind of what’s been happening to the Universe for the last 13.82 billion years, and now the Universe is a very chilly 2.7K (-270C or -455F), which is only a few degrees warmer than absolute zero. Just like hot iron glows different colors when heated up, the Universe also “glows” based on its temperature.
But instead of visible light, the Universe “glows” uniformly in the microwave region of light (specifically about 160GHz). No matter which direction you look, you’ll find this microwave light corresponding to this temperature.
This was actually a very bizarre discovery. Currently, a lot of satellite communication is done using microwave light (contrary to popular belief, most long-wave radio cannot pass through Earth’s atmosphere, so satellites very far from Earth communicate with microwave instead). Back in the 1960s and 1970s, in the era of early space exploration, there was a push to try to communicate using microwave light. While working on their radio / microwave light detectors, Arno Penzias and Robert Wilson (researchers at Bell Labs) kept seeing this very annoying static noise, no matter how much they tuned their detectors. They pointed them every which way. They ruled out that it came from nearby New York City and they even cleaned out the dung droppings inside their dish from bats… And yet the static continued. Almost giving up, thinking their equipment was just faulty, they contacted Robert Dicke, a physicist at Princeton, hoping he would look into their electronics. Dicke, however, suggested that the pair had actually detected the “cooling of the universe” or the “cosmic background radiation” left over from the Big Bang. This was critical evidence in supporting the Big Bang model. Penzias & Wilson were awarded the Nobel Prize in Physics in 1978 for their accidental discovery.
It’s kind of like this. In the early Universe, matter and energy were basically one thing. It was extremely hot right after the Big Bang. So hot, that not even protons and neutrons were formed yet… that required some cooling down. As matter and energy spread out in all directions, the particles began to cool down & form into the atomic particles we all know and love (protons, neutrons, others). Cooling further, they formed the earliest of elements (hydrogen and helium). Once the hot gas had spread far enough apart, gravity started to take over in some of the denser regions. A few more millions of years and the first stars are formed. Groups of stars form galaxies. All the while, everything is still mostly moving apart (continued expansion of the Universe).
So the Universe is kind of like when you forget about your tea.
Using this mostly uniform cosmic microwave background & the corresponding 2.7K temperature, you can work your way backwards and figure out that the Universe has been cooling down for 13.82 billion years and had once been an extremely hot, condensed ball.
13.82 billion year old tea.