We live in a Universe really big, and really old. 13.7 billion years old, to be exact. And it has evolved a lot through all this time, eventually becoming one which could support stars, galaxies, and life itself.
The Universe began with a bang, really – the Big Bang. About 13.7 billion years old, there was no Universe. It is not as easy to comprehend – it is not that space and time were empty or zero, they didn’t even exist. For some reason, the Universe came into existence out of seemingly nowhere, and expanded rapidly.
It is a very simple explanation at best. A lot was happening as the Universe grew older, right from the first millionth of a microsecond.
After 10−43 seconds, gravitation separated from the other three fundamental forces, giving the Standard Model of Physics. The separation of gravitation is believed to coincide with cosmic inflation. Inflation was an event when the Universe expanded much faster than light itself. Yes, the speed of light is the speed limit while traveling through spacetime. But space itself can expand much faster than that.
The inflation is important, as it explains the Cosmic Microwave Background, and also created irregularities in division of energy. Without this, the Universe would always be homogeneous, and gravitational effects would just cancel out everywhere. At this point, the Universe is hot. So hot that all energy is present in the form of high frequency gamma rays, getting redshifted by the expansion of space.
At around 10−32 seconds, the strong force also separates, leaving the electroweak force. It is still too hot for any matter to exist yet. At this point, the Universe is still the size of a grapefruit. And already, so much has happened.
At smaller times, we observe pair productions of quarks and anti – quarks. This takes place at around 10−10 seconds, and the Universe is now full of quark – gluon plasma. The electroweak force also splits into the electromagnetic and weak force. Higgs mechanism comes into play, and the quarks obtain mass by interacting with the Higgs Field.
The leptons and hadrons are obtained about one second after the big bang. It is no longer hot enough for the quarks to exist independently, and QCD makes them pair up into hadrons. However, it is still hot, so the electrons are flying with very high energies and not bound to atoms.
Around 1013 seconds later, electrons are slowed down enough to finally bond with positively charged protons, thus making the very first atoms. The radiation from the big bang has cooled down significantly, and is now in the visible range. Since atoms have formed, the Universe now is cool enough to be transparent, and the light can travel throughout the cosmos. Let there be light!
For 1013 seconds, the Universe was essentially a soup of protons, neutrons and electrons. The Universe, in this time, has expanded to about 42 million light years across, from the size of a football when the electrons and protons first formed.
About 380,000 thousand years ago, we had hydrogen and helium. However, gravity takes time. It would take a lot of time for the gases to get into dense enough state to form stars. Gravity slowly brings the clouds of gases and dust together into a clump, before it finally gets hot enough to perform nuclear fusion. 150 million years ago, for the first time, clouds of gases got so compressed to fuse hydrogen into helium. The first ever stars were born.
From all that time, the Universe has expanded a lot, being 92 billion years across (and that is just the observable part) Stars have taken birth and died out, forming new stars, white dwarfs and black holes. The cosmic microwave background is now really low energy radio waves. From temperatures of trillions of Kelvin, the current Universe now has an average temperature of a meager 2.7 Kelvin.
Indeed, the most crucial part of the expansion of the Universe was within the first second of its life. Had inflation not occurred, or if the strong force was a bit stronger or gravity a bit weaker, the Universe we know would never exist.