We take it for granted that our universe contains planets, stars and galaxies because those are the things we see
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but the only reason these big structures exist is because of the nature of nothingness - empty space.
But to understand why we have to go back to the beginning. The very beginning. The big bang. You know, I always thought that in the Big Bang the observable universe started as a point and then just expanded steadily more or less to the point that we're at today but that's not actually how it happened.
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There were four different phases in the universe's expansion. To start it was expanding steadily but then after just a tiny fraction of a second the expansion just blew up and the whole universe increased in size by like ten to the twenty six times in a very short period of time and that period is known as inflation and then just as abruptly as it began inflation stopped and after that the universe continued expanding but at a decreasing rate so the expansion of the universe was actually slowing down which is exactly what you expect if the universe is full of massive objects that gravitationally attract each other
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but then about five or six billion years ago the expansion of the universe started speeding up again and this is thought to be caused by dark energy, an energy tied to space.
So before that time there was enough matter, the matter density was high enough, that it was pulling everything back together slowing down the expansion but once the universe reached a critical size well then there was enough dark energy to start pushing things apart and that is the phase that we're still in the expansion of the universe is accelerating.
Now, this story doesn't really explain the formation of galaxies until you tie in the nature of nothingness now ordinarily we think of everything around us as made of particles of atoms and electrons, but our best theories of physics are actually field theories that say all these particles should be seen as just excitations in fields.
The word "field" always gets me because it makes me think of... well, fields. but a field is just something that has a value everywhere in space.
So every subatomic particle has its own field: an electron field, an up quark field, a down quark field, a neutrino field, and so on...
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And anywhere there's an excitation in this field that is some energy in the field well that is where we will observe particles.
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So out in completely empty space where the values for all of these fields are basically zero. But here's the thing it's impossible to make a field perfectly flat and 0. You cannot take a quantum field and make it completely quiet. It's the Heisenberg uncertainty principle. It says you can't take a particle and pinpoint it to exactly zero energy likewise you can't take a quantum field and make it exactly flat everywhere.
Now, this is important because ordinarily these fluctuations are really really tiny and they only affect subatomic processes but during that period of inflation the universe expanded in size so rapidly and so incredibly that those tiny fluctuations got blown up to the scale of the observable universe.
Now, without them we think the matter distribution in the universe would have been completely homogeneous completely uniform and that means the gravitational force on any object in the universe would have been the same in all directions which means nothing would ever have collapsed into the big structures that we see today
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but thanks to these fluctuations there were slightly denser and less dense regions and of course the denser regions had stronger gravitational fields so they pulled in the matter from around them and that clumped together the matter into huge gas clouds
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that would go on to be the galaxies
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that contain the stars and the planets
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and the things we know today, you can actually see the imprint of these quantum fluctuations in the leftover radiation from the Big Bang the cosmic microwave background radiation
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the quantum fluctuations in the fields were amplified to big classical fluctuations in the density of matter from place to place and those fluctuations in the density of matter show up in the universe today as temperature differences in the cosmic background radiation and ultimately in things like stars and planets and galaxies.
I think it is incredible that without these fluctuations in the vacuum these tiny insignificant things that we take for granted our universe might really contain nothing.
Thanks for reading