Indian Institute of Technology, Kharagpur
Some questions are so surreal that it seems irrelevant at times to ask them. One of such question is why do galaxies exist in the way they do?
Think about this, there are lumps of matter scattered around in the vast emptiness of the space. There could be uniform matter everywhere and let me tell you the things would not be interesting in such a world if it exists. To go to the depth of this mystery we need to understand some fundamentals of the nature of the emptiness of the spacetime.
Zero-point energy and the Heisenberg Uncertainty Principle:
Let’s start with how will a particle behave if we try to minimize its energy, i.e. if we approach zero Kelvin. What would you expect? As all the energy is being sucked out so the particle should come to an absolute standstill with no movement at all. But if that would have been the case we would know the absolute position and momentum of the particle and thus we would be violating the famous Heisenberg Uncertainty Principle, which states that you cannot determine both the position and momentum of a particle precisely simultaneously. If you try to measure one to precision the other will go out of whack.
Thus we conclude even at Zero Kelvin the particle is not at rest and there is a threshold minimum known as Zero-point energy which is mandatory to exist even when there is nothing.
Now we switch gears from classical mechanics towards quantum mechanics and try to understand, How is a particle represented through quantum fields? In this theory, each fundamental particle of the standard model has its own field what is a field is a good question here to ask. So we can basically say, the field is something that has a value everywhere in space. For example, the gravitational field has a value defined at any point in the universe you point out. Similarly, there exists a field for electron for instance. And if the electron is present at a location can be expressed in field value at the point in space. The electron field has a specific value for that point in space. So the catch is Heisenberg’s Uncertainty Principle applies here too and it suggests that a quantum field can’t be completely quiet or rather completely flat similar to the zero-point energy. So there are always small fluctuations in any field which are so minuscule that they go unnoticed in our normal interactions. But when galaxies were being formed things weren’t that normal.
The Big Bang:
So it happened around 13.8 billion years ago? It’s no hard to wrap our small heads around but it is when time started and it is pointless to talk something before that because there was no time no laws of physics none of it before that. And thus the universe as we know it originated from a single point and is expanding since then and if you think rationally you would expect that the there was a smaller version of the current picture of the universe and we are just making it bigger and bigger at a constant rate But that’s not the case. The history of space-time is somewhat unique.
Image Source: phys.org
There are 4 phases here I know.
- At first, it started expanding more or less steadily
- Around 10-33 to 10-32 seconds after the big bang there was an exponential increase in the rate of expansion and spacetime blew up 1026 times its original size in around 10-36 seconds. This period is known as inflation. And it stopped as abruptly as it started.
- After inflation, the expansion continued but at a decreasing rate which is exactly what you would expect from something that has matter thus the gravitational attraction was slowing down the rate of expansion of the universe
- Only until 5-6 billion years ago that it started accelerating again and the cause is Dark energy Theory suggests that, at first even though the gravitational attraction was large enough to overpower dark energy, the distance kept on increasing so there came to a point where dark energy started to accelerate the expansion again and we currently are in this phase and so we say the universe is expanding at an increasing rate.
All this doesn’t explain why galaxies formed until you combine the 2 discussed theories together.
To summarize things up:
During the inflation, the space-time blew up in an unimaginable exponential way. Along with spacetime the small quantum fluctuations that otherwise remain unnoticed magnified to a scale that provided the seeds for the innumerable galaxies we observe today. Mere gas dust swirling around formed lumps and gravitational forces kept on increasing the size of these lumps until the nuclear fusion was not possible and we had our first stars in our universe now when these stars came together and formed large clusters galaxies were born many of which had a black hole or group of blackholes that the stars revolved around.
To wrap things up in a single sentence one can say the galaxies exist due to the Heisenberg Uncertainty Principle. However small things may seem to be they can always have an impact which can literally turn worlds around.