The Big Bang

Timeline of Universe

    The Big Bang, according to this theory, this is where it all started. Every star you see including ours, The Sun, our moon, other planets, and many more things originated from this phenomenon. Putting in simple terms, in the beginning, there was this singularity that had a very high temperature and very high density, exploded into the cosmos we know today. In 1927, Georges Lemaître identified that if the time was reversed, the nearby galaxies will converge into a single point. Later it was confirmed by Edwin Hubble and the event was named The Big Bang Theory.

    It is rather unclear that time existed before the big bang so it is convenient for us that the time originated from the moment of the big bang. The timelines can be divided into:

1. Singularity: Hubble's law tells us that galaxies move farther from the earth at a speed proportional to its distance to earth. Meaning that the farther a galaxy is from the earth the greater its speed is at which it moves away from earth. This can also be interpreted as that every galaxy moves away from the other galaxy. If we move back in time, we can see that everything originated from a singularity. This singularity was super hot and super dense. Around 13.7 billion years ago this singularity exploded and all its matter spread out in every direction. This explosion is called the Big Bang.
2. Inflation: The four fundamental forces, electromagnetic force, strong nuclear force, weak nuclear force, and gravitational force were unified for the first 10^-43 seconds (this unit is so small that light travels only 2.99*10^-35m, which is 3 times smaller than an atom, where the light travels around the globe 7 times in 1 second). The universe spaned 10^-35 meters and had a temperature of 10^32 degrees. After 10^-43 seconds, the gravitational force separated from the other forces. The temperature was too high to form any particles. At 10^-37 seconds, the expansion underwent inflation called the cosmic inflation in which the expansion rate increased exponentially that it was expanding faster than the speed of light. Because of this rapid expansion, the temperature decreased drastically. After the inflation stopped around 10^-32 seconds, the temperature was right about to cook some elementary particles. Still, the temperature was high to form any kind of complex particles. The particles and anti-particles due to their high relativistic speed were forming and annihilating. At some point in time, because of a reaction, the quantity of particles excesses over anti-particle. This resulted in the dominance of particle over anti-particle in the present universe.
3. Cooling: As the universe further expanded, the universe cooled to a degree that the gluons and quarks (both gluons and quarks are elementary particles which combine in different proportions to form protons, neutrons, and electrons which constitute the atom) combined to form protons and neutrons. The temperature was not high enough to form any particle-antiparticle pair. After a few minutes, the protons and neutrons combined to form deuterium and helium nuclei. The temperature was hot to form any stable atoms. After about 379,000 years, nuclei and electrons combined to form an atom, mostly hydrogen.
4. Structure Formation: After a long time, slightly dense regions attracted gravitationally to become denser and formed gas clouds which became stars and galaxies. Our galaxy, The Milky Way Galaxy started forming about 8.8 billion years ago. Our solar system started forming around 4.6 billion years ago.

    While there may be many pieces of evidence for this theory, or how many can be discovered in the future, we will never know the true origin of this universe. Maybe the universe goes into a cycle of inflation and deflation, maybe something else.