In the 19th century, Edwin Hubble, a renowned astronomer, observed that the galaxies are moving away from each other, which confirmed that the universe is continuously expanding. For a better understanding – imagine you have a balloon on which you drew some circles. As you blow air into the balloon, the distance between the circles increases. Now, if the universe is the balloon and the circles are the galaxies, we can say that as the universe expands, the galaxies go further away from each other.
However, if you release air from the balloon, the circles get closer to each other. As you go back in time, the distances between the galaxies becomes smaller and smaller, and perhaps at some point, there was just one galaxy, and if you go back even further, the universe was smaller than the existing galaxies.
This observation gave rise to the infamous theory that we call The Big Bang Theory.
The Big Bang Theory Timeline
In the beginning of time, all the matter in the universe was concentrated into an infinitely small space of infinite temperature and pressure. From there, the tiny little universe started expanding rapidly. At more than 10 billion degree Celsius, the only particles that existed were called the gluons. Not many gluons could survive the heat, but the once that did, came together to form the elementary particles called quarks. These quarks destroyed each other, forming more gluons, which came together to form pairs of quarks.
This cycle continued till the universe expanded and cooled down enough, allowing the quarks to come together and form the most basic particles that most chemistry students know as protons and neutrons. At this point, the universe is one second old and it’s diameter is more than 100 billion kilometers!
Over the next few minutes, the universe kept expanding and its temperature kept decreasing. Around this point, the protons and neutrons decomposed to form the first atomic nuclei. After around 300,000 years, the universe cooled down further which allowed the nuclei to capture electrons and form the first atoms of hydrogen and helium.
Over the next millions and billions of years, hydrogen gases clumped together to form clouds of hydrogen, which in turn formed the galaxies, the stars, the planets and everything else in the observable universe.
The diameter of the observable universe is currently around 93 billion light years. The overall size of the universe could be at least 200 times larger than the observable size. Astronomy always reminds us how tiny we are compared to everything else.
There is one mystery unsolved by the Big Bang Theory. What had happened before the Big Bang? How did everything get concentrated into one single dot? The answer is – we don’t know. This is a very confusing timeline because now we are talking about a time before the beginning of time (Huh?!). And this leads us to ask, were there universes before ours? Were there multiple big bangs leading to the creation of a multiverse?