The Big Bang is something we all have heard of. We all know that it was the event that caused the formation of the universe, but there is a lot more to it than just that. That is what this article is about.

First off, lets make it clear what exactly the Big Bang is. You may think that it is an explosion in space, but that's not the case: matter does not appear out of nowhere and fill an empty universe, space itself expands and increases the distance between two objects. The Big Bang Theory describes how the universe expanded from an initial state of high density and temperature. and is the most widely accepted theory on the formation of the universe as it explains the evolution of the observable universe from very early on from its creation.

it explains a lot of observed phenomena: The abundance of light elements (like hydrogen and helium), the Cosmic microwave background radiation and large scale structure (like galaxy filaments) are some examples. critically, it agrees with the Hubble-Lemaître law, which is the observation that galaxies are moving away from Earth, and the further away they are, the faster they are receding. More about that at the end.

The Big Bang theory depends on two assumptions to work: The universality of physical laws and the cosmological principle. The universality of physical laws is one of the crucial principles of the theory of relativity. The cosmological principle states that the universe, on large scales, appears the same in all directions no matter what the location is. And we know these assumptions are most likely true: General Relativity has passed many difficult tests over the years, And since the universe appears the same in all directions from Earth, The cosmological principle may be correct , if the Copernican principle (which states that the earth does not have a special view of the universe) is true.

Another important consequence of the big bang theory are Horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not yet had time to reach us. This places a limit on the most distant objects that can be observed. Also, because space is expanding, and more distant objects are receding ever more quickly, light emitted by these objects will never reach us.

Using general relativity, we find that the beginning of the universe had infinite density and temperature, showing that general relativity is incomplete. The universe started growing in size exponentially only 1o^-47 seconds into its existence (which is 0.000000000000000000000000000000000000000000000001 seconds), dropping the temperature from almost infinite (10^32) by a factor of 100,000. Tiny irregularities in the quantum world now became huge voids. At one second in, all antimatter was annihilated, and atoms and elements as we know them only formed 379,000 years after the big bang. After this, the universe kept slowly cooling till stars, galaxies and galaxy clusters could form. (PS: it is thought that the early universe before inflation was so dense that pockets of space collapsed into black holes, Which grew rapidly to become the ones at the centres of modern galaxies.)

As it stands right now, the universe is getting bigger much faster then before, due to dark energy, a mysterious form of energy we know nothing of, and yet makes up 73% of the universe. And as the universe expands, there is less gravity in one area, meaning that the universe expands even faster. This will probably continue and lead to the universe's death. (Which is a whole other topic)

The Hubble-Lemaître law

In 1929, Edwin hubble found that the farther away distant galaxies were, the faster they were moving away from Earth. The spped of distant galaxies can be found be examining their redshift, or in other words, how much the light they emmit has been strecthed towards longer wavelengths. The only proper explanation for this is that the universe is expanding in all directions, and is expanding faster with time. it is considered the first peice of observational evidence for the expansion of the universe, and is one of the peices of evidence for the big bang theory. It is important to note that in 1927, Georges Lemaître independently theorized that recedding galaxies could be explained by an expanding universe, Hence the name of the law.

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