Before the second world war, it was discovered by astronomers that, in any direction you look, the other galaxies are receding away from us (maybe our galaxy smells bad or something). Einstein’s General Relativity (GR) was still fairly new but, using some fancy razzmatazz mathematics, cosmologists were able to show that GR could explain this recession if the universe was expanding.
To cope with these observations, many competing theories popped up, but there were two main theories that most cosmologists chose between. One was called the Steady State Theory. This theory was that the universe looked pretty much the same forever. It would expand and galaxies would recede, but in this theory the act of expanding the universe caused matter to be created at regular intervals, which would then form new galaxies. And so the distribution of galaxies and matter would not actually change much; it would appear to always be the same, but expanding. The second theory that arose was that if you traced the paths of these galaxies backward in time, you’d find the universe gets hotter and denser further in the past and that it should all converge (kind of like how they trace the path of shrapnel backwards to find the location of an explosion in those CSI shows). This theory was later (jokingly) named the Big Bang Theory. At the time (this was now after WWII) there was mathematics to support both theories and, while they had very different predictions and implications, there was a distinct lack of observations that could show which was better. However, most cosmologists of the time favoured the Steady State theory, in part because the Big Bang theory meant there was a beginning to the universe and that idea seemed “repugnant” and overly religious. Supposedly, the supporters of the Big Bang criticized those reasons as biased and claimed their theory just followed observations with fewer assumptions.
This debate went on for almost two decades. Over that time, a few observations started popping up that hinted at the Big Bang model being more probable; but nothing that could definitively disprove the Steady State theory. However, one of the predictions of the Big Bang theory was that at some point in the past the universe would have been so hot and so dense that space itself wouldn’t be transparent. If one were to look out into space at that time, instead of seeing the familiar blackness of space with stars and whatnot dotting the sky, it would have instead seemed like every point in space was like looking at the surface of a bright orange star (your stove element gets hot enough that it glows red; the universe was hot enough to glow orange). The theory went on to say that at some point the universe quickly (quick meaning in only a dozen or so thousand years) turned see-through. At that point, any light that was produced when it was hot and opaque just shot out into space until it hit something (some of it never hit anything). As the universe continued expanding, it cooled and this light gradually shifted from orange to red and eventually into the range of microwave radiation (corresponding to a temperature that is 270.45°C below zero). Since this light originated from every point in the universe at about the same time, the Big Bang model said we should expect to see microwave radiation across the entire sky coming from a distance that would correspond to the time when the light was emitted (remember, it takes time for light to travel a distance. The farther out in space you look, the farther into the past you see). On the other hand, the Steady State model said since the universe always looks about the same, there wouldn’t have been a time when this “Cosmic Microwave Background Radiation” (CMB) could have been emitted, so we shouldn’t see anything of the sort. Well, in 1964, we discovered that anywhere you look in the sky, you’ll find microwave radiation just as the Big Bang theory said you would. As our lovable Dr. Stephen Hawking so poetically put it at the time, this was “the final nail in the coffin of the steady-state theory”. While there were a few scientists that still supported alternative theories, the Big Bang became the dominant accepted theory in cosmology.
But what exactly is the Big Bang? To answer this, let me first describe what I mean when I say the universe is expanding. Picture a partially inflated balloon. Now imagine two ants just chilling at two separate positions on the surface of the balloon. They aren’t moving; probably on vacation or something. This represents the universe as it is today and the ants are two giant space ants that aren’t going to move from their position. If we proceed to inflate the balloon, this is like the universe expanding. Notice that, while the ants don’t move, the distance between them increases. In fact, we see that every point on the balloon gets farther away from every other point and that more distant points move away faster, which is also true with the expansion of the universe. Then, if we deflate the balloon, this is like going back in the past and we see the distance between the ants shrinks, even though they still haven’t moved. This brings us to the definition of the Big Bang. If we were to continue deflating the balloon, there would come a time when the distance between any two separate points becomes zero. That doesn’t mean our ants are at the same point, it means there’s no distance between their locations. The Big Bang is defined as the moment in time when the distance between any two points in space was equal to zero. From this, we see that the Big Bang was not an event, like an explosion that happened at any one place, it was actually a moment of time, like “yesterday”. So if someone were to ask where the Big Bang happened, that’s like asking “Where did yesterday happen?”. Everywhere.
Great, so the Big Bang theory became the favourite theory and everything in life became wonderful forever, right? Nope. It was soon pointed out that there was one gaping hole in the Big Bang theory. When we look out at the CMB, we see that it is almost exactly the same in every direction, as if the temperature at every point in space had been able to equalize before the radiation was emitted. But hold on, nothing can travel faster than light and the light from one side of the sky is just now reaching us; at the halfway point between the two opposite sides of the sky, after travelling for about 13.8 billion years. So how could the two opposing sides of the sky ever have talked to each other in order to equalize their temperatures? In fact, when we did the math, we found that there was never a time when these regions of the universe could have been in communication, and yet we still see that they must have been in contact in order to equalize their temperatures (otherwise you’d expect at least some variation). This is the horizon problem of the old Big Bang model.
That is where inflation comes in. To solve the horizon problem (and a couple more mathy problems), the theory of inflation was proposed, which says that the universe expanded really slowly at first. This lasted an unimaginably short period of time, but since the distance between any two points was also unimaginably small, the speed of light was fast enough for everything to communicate and temperatures to equalize. Very soon, however, the expansion just took off like a bat out of hell. In the span of 10-30 seconds (or one billionth of one billionth of one billionth of a millisecond), the universe expanded by a factor of at least 1026 times. To put that in perspective, if you had two points that were separated by a distance of about the height of a person (around 6 feet) before inflation started, then, after it ended 10-30 seconds later, the distance between those same two points would be around 20 billion light years. Those points may have been able to talk to each other beforehand, but afterwards they clearly are too far apart to communicate at all.
The rapid expansion during inflation substantially cooled the universe, kind of like how expanding fluid is used to cool your refrigerator. However, after inflation, the expansion decelerated fast (really fast). So what happens when you’re in your car and you slam on the brakes? The brakes can get really hot. In a weirdly similar way, the expansion slowed down so fast after inflation that the universe heated back up (Pro tip: Don’t touch really hot brake pads or universes). The massive amount of energy in this reheating process also caused matter and radiation to first come into existence (there’s a much more complicated reason for this involving the decay of the field that drives inflation, but mostly it amounts to providing the energy to create everything). From there, the universe continued as the old big bang theory described and the horizon problem was solved.
In conclusion, I've checked and it turns out that after many years scientists have been able to repeatably verify that it is NOT butter, but I still can't believe it's not butter.