It was long thought that the universe was eternal with no beginning and no end. However a rather surprising discovery in the 20th century changed all that and confirmed that the universe did indeed have a beginning some 13.7 billion years ago. This was the beginning of all beginnings since nothing, not even time, existed before the universe came into being. This can be hard to imagine since we live in a world where time reigns supreme.
We humans have managed to figure out ways to get around most of the inconvenient laws of nature. Gravity can be overcome by the physics of rockets and planes, rust and rot can be overcome by the chemistry of protective paints, and infection can be overcome by the biology of antibiotics. Time however has always eluded human control to the point where we just accept it as untouchable. Yet even time did not exist before the universe. After all, time is simply the rate at which things move and change. Without a universe there is no motion, no matter, and therefore no items for time to act on. There was simply nothing.
And then it began. For reasons still unknown, out of an infinitesimally small point came... well everything. A rapid expansion occurred and a very hot, tiny universe emerged. The energy levels were so high that matter (the stuff that makes up everything we know and love) and antimatter were constantly being spontaneously created, only to meet and cancel each other out. This cancelling out is called annihilation and results in the matter and antimatter being converted back into pure energy. The conversion of mass into energy and vice-versa is something we know can happen thanks to one of the most famous equations of all time:
Einstein's equation states that the amount of energy E stored in a mass of matter or antimatter M is equal to that mass times the speed of light C squared. The speed of light is a huge number so even just a little mass can store a lot of energy. For example if a 60kg person were to have their mass converted into pure energy it would amount to 18 billion joules which is the equivalent of exploding over 4,300 kg of TNT!
Just don't tell airport security that you're basically a weapon of "mass" destruction
Matter and antimatter continued to pop into existence and take each other out of existence as the universe expanded and cooled, however it eventually reached a point where there was no longer enough energy to spontaneously create matter from energy. This was a crucial point because traditional wisdom says that all the remaining matter and antimatter should have cancelled each other out leaving nothing. However this didn't happen. Most of the matter and antimatter did cancel each other out but for some reason there was a very slight excess of matter over antimatter (something like 1 in 30 million). This little bit of extra matter is what survived to make up everything we know in the universe today.
Energy levels in the early universe were high enough to spontaneously produce matter-antimatter pairs
Matter and antimatter cancel out releasing the same amount of energy that went into creating them
Once all the antimatter was gone, the remaining matter was free to exist without being annihilated. As the universe continued to expand and cool, the basic building block of atoms were able to form: protons, neutrons, and electrons. We learned about how these particles formed atoms in The Not So Useless Neutron and Nuclear Reactions. You can refer back to these articles if you need to brush up for the sections that follow.
Everything up until this point has been highly theoretical, however once the energy levels in the universe dropped low enough to produce protons, neutrons, and electrons, they were also low enough for us to recreate in modern particle accelerators. Therefore everything from here on is pretty well known since it has been confirmed by lab experiments like those done in the Large Hadron Collider at CERN.
CMS Experiment at the Large Hadron Collider
At this point, we are now just under one second into the creation of the universe. Previously, energy levels were far too high for protons, neutrons, and electrons to form. If they did, they would quickly be torn apart. But after about the one second mark, energy levels were no longer high enough to rip these particles apart and protons, neutrons, and electrons began to appear all over the new universe.
Once these particles could form without being torn apart, the universe could be said to be full of hydrogen since the nucleus of a hydrogen atom is just a single proton, but this wasn't the case for long. The high energy levels of these newly created particles caused them to smash together with enough force to overcome their electrical repulsion and bond together in nuclear fusion reactions. These are the same reactions that happen at the core of the sun. Recall that it is only the number of protons that determines what the element is (ie. one proton is hydrogen, two protonsis helium). The number of neutrons determines which isotope of the element it is. For example helium has two common isotopes: helium-3 and helium-4. The number denotes how many particles in total there are in the nucleus so helium-4 for example has two protons (which is what makes it helium) and 2 neutrons which adds up to four particles in total making: helium-4.
Energy levels were high enough to fuse newly created protons and neutrons together
But eventually the universe expanded and cooled further to the point where there was no longer enough energy to produce these reactions either. When all was said and done, 75% of the universe was still made up of hydrogen-1 (single, uncombined protons) but about 25% had become helium-4 through reactions like the one above.
Although hydrogen and helium nucleii had now formed, it took a few hundred thousand years for things to cool off enough for electrons to bond with these nucleii. Nuclear bonds are very strong which is why nucleii were able to form so early in the history of the universe. Electrical bonds however are much weaker so the universe had to cool off considerably before electrons could stick to atoms without being immediately torn away.
At high energy levels, the attraction between positive protons and negative electrons is not strong enough to form bonds
At low energy levels, positive protons and negative electrons can bond without being torn apart
Before energy levels in the early universe were low enough to allow electrons to bond with nuclei, they simply bounced around together in a sort of particle soup. The early universe was also filled with photons (light) which were scattered as they moved through this particle soup. Since light could not pass through without being tossed about in all directions, the early universe would have looked like a thick fog. In the same way that fog scatters light in all directions, making it impossible to see through, the free electrons and nuclei in the early universe prevented light from travelling more than a short distance before being redirected.
Photons (light) could not easily pass through the early universe
Once the universe expanded and cooled to the point where electrons could bond with nuclei without being immediately torn away, photons suddenly found that there was very little to prevent them from travelling across the universe. At this point, a few hundred thousand years after the Big Bang, the universe finally became transparent.
Once electrons could bond with nuclei, light was able to pass through the universe with minimal interference
Right around this point you may be thinking that this all sounds well and good but how do we actually know any of this? After all nobody was around when the universe began so what proof do we have that any of this actually happened?
Scientific theories are made and broken based on their predictive power. Unless you can use a theory to predict something will happen then go into the lab and try it, there really isn't much value in that theory. Experiments are how we test whether our theories are able to predict how the real world will behave.
If you've been following closely, you'll be able to find two important predictions that the Big Bang theory makes. Each one of them can be tested:
1. The universe is expanding: If the universe was expanding initially, it should still be expanding today or there should be a good reason why the expansion has stopped.
2. A large number of photons were "freed" in the early universe: When energy levels were low enough for electrons to form bonds with nuclei, photons were suddenly able to pass freely through the universe. This light should still be visible today.
We will put each of these predictions to the test in the next article.