The Big Bang was a great explosion generated billions of years ago. For what we know until now, with the Big Bang came along space and time, that didn’t exist before this phenomenon. To be more specific, the Big Bang happened 1.4 x 10^ 10 years ago. This apparently random explosion gave birth to our universe and galaxy. At the time, Earth wasn’t created yet and the galaxy was filled with asteroids and giant rocky planets. The second when the universe was born the temperature was crazily hot, around 5.5 billion degrees Celsius. After less than one second it had a small diameter of the earth-to-sun distance, which is 149.600.000 km. In the next second, it got bigger than 1 light-year, which is approximately 9.46 trillion kilometers. From here the size of the universe escalated very quickly to then get to a point where its growth was slowed down. Nowadays, 13.8 billion years later, the universe is still expanding at an approximate speed of 72 kilometers per second. Let’s try to get a clear idea of how fast the universe expanded at the beginning of his era and what molecules and particles were created.
10^-43 seconds after the big bang the grand unification epoch started. It was based on the four fundamental forces: strong nuclear force, electromagnetic force, weak nuclear force, and gravity. In this era, the fourth fundamental force, gravity, was separated from the other three, which remained united. The first particles and antiparticles were also being created. At 10^-34 – 10^10 seconds, there were the electroweak and quark epochs. These two consisted in the division of the strong nuclear force from the other two and the creation of multiple exotic particles in the first era and the creation of many quarks, electrons, and neutrons together with the division of all the fundamental forces, that assumed their present forms, in the second one.
In the quark epoch, the temperature in the universe started to decrease, going below 10 quadrillion degrees. Quarks and antiquarks destroyed each other upon contact, but some quark particles managed to survive and, in a process called baryogenesis, they created matter. 10^-10 seconds after the big bang the temperature decreased to one trillion degrees allowing quarks to fuse and create hardons. Electrons together with protons collided together to create neutrons. So, exactly one second after the big bang, protons and neutrons were already formed. Three minutes later the temperature had fallen down to one billion degrees letting out the atomic nuclei of hydrogen, helium, and lithium. After that, the development of the universe continued relatively slowly.
300 000 years later, after the period of formation of the first atoms, the dark age arrived. Without any stars to light the universe, everything immersed in darkness, giving birth to the era called in the same way. Meanwhile, radiation filled the universe with the so-called “Cosmic Microwave Background”. In that period the temperature kept falling getting below 475 degrees celsius. One million years after the beginning of everything atoms have been completely formed, stars and galaxies exist and the universe has finally become transparent to light. But it is still completely dark, as stars have not formed completely yet. One billion years after the big bang we could find the first stars already formed, due to the collapse of cosmic gas that created a nuclear fusion between hydrogen atoms. But they were still different from now. In fact, one of those was around one hundred times the mass of the sun. They were the so-called “Population III” also known as “metal-free” stars. That primitive universe is our present universe, filled with stars and galaxies, around 14 billion years after everything started.
Together with the physical development, the universe kept growing in size, especially in the first moments of its life. In fact, the smallest recordable size of the universe was 10^-35 seconds after the big bang, with a size of 17 centimeters. Approximately that of a football ball. But right after 10^-30 seconds of creation of the universe, it was already 168 meters big, approximately the length of the skyscraper Intesa Sanpaolo in Turin, Italy. In a few seconds, our universe had already passed the 150 000 000 kilometers mark, making it bigger than the distance between the earth and the sun. One second after it was already extending for more than one light-year (around ten trillion kilometers ). After that, the growth started to slow down. Right after the universe’s first year of age, it reached the size of the Milky Way, around 105 700 light-years. It continued expanding and it is still now in expansion with a current diameter of 93 billion light-years, filled with around 1000 billion galaxies and several quadrillions of several quintillions times bigger than it was one year from its creation.
In conclusion, in less than 14 billion years, our universe developed protons, electrons, and neutrons. Then atoms and matter, stars and galaxies appeared. And finally, planets, including ours, came to being. But, while all of this was happening, it went from around the size of our head, passing through different lengths, to the size of quadrillions of quintillions of our Milky ways, even though it “only” has 1000 billion galaxies and plenty of free space. In fact, more galaxies are expected to be created as it expands at a speed of 72 kilometers per second.