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Our observable universe is about 13,800,000,000 years old and that it came into being in the Big Bang out what? Out of something we can't describe.
That doesn't mean that there was nothing before the Big Bang.
And this applies not only to time, but to space itself.
The universe is supposed to have been concentrated in an infinitely small point at the moment of the Big Bang, and supposedly had a finite size, like less than one atom or whatever. It only applies to the visible part of the universe.
This doesn't mean at all that the universe wasn't infinite at the moment of the Big Bang. In fact, all the evidence suggests that the universe is infinite, which means that the universe was much denser, but still infinite at the very beginning[1].
Infinity...
Looking at a star, we are experiencing the gravitational pull of that object, even if that object is huge distances away from us. The peculiarity of gravity is that although it weakens through space, it never reaches zero.
The force of gravity would be enough to cause all massive objects in our universe to collapse sooner or later. We could never move far enough away from an object to not experience its gravitational effects at some level. Meanwhile, our universe is not collapsing.
In fact, the exact opposite is happening; we don't know why, but the universe is expanding, and it's doing so at an accelerating rate.
And since physicists have no idea what is causing the universe to expand, they usually call any hypothetical phenomenon that could accelerate the expansion of the universe as dark energy.
Rewinding time 13.8 billion years, back to the very beginning, we find a moment when, for reasons we don't fully understand, the universe's expansion suddenly accelerated. A section of the universe no bigger than a computer monitor expanded to the size of the observable universe, and beyond, in less time than it takes a quark to cross from one side of a proton to the other.
But then this expansion, for some unknown reason, slowed down. This has its own name - the cosmic inflation hypothesis. And we have no reason to say that cosmic inflation cannot happen again, for example, right now.
Since the universe is 13.8 billion years old since the Big Bang, then we should be able to see a maximum of 13.8 billion light years, but in fact we see further because of dark energy.
Today, the farthest distance we can see is a sphere with a radius of 46 billion light years. In the future, this sphere will grow due to the expansion of space, but only up to a certain limit. Approximately 33% more than the current one, up to 61 billion light years.
All objects outside our sphere will not be causally related to us and will not be. We will never be related to them.
More than 98% of the galaxies we could ever observe are already beyond our reach. As space continues to expand, the gravitational forces within galaxies will remain strong enough to hold them together, but gradually, every galaxy will be plunged into darkness. Gradually, stars will burn out, particles will decay, and black holes will evaporate via Hawking radiation. Eventually, the universe will be left as an empty space, with only dark energy driving the exponential expansion for eternity.
The stronger the gravity where the matter is more, the more dark energy where the less matter is. The density of dark energy remains constant with the expansion of universe.
As the expansion of the universe accelerates, there will be more and more empty space, which in turn will lead to an acceleration of the expansion process, which will lead to an increase in empty space, and so on.
In the end, the universe will be an empty space in which there is only dark energy, and the concept of time ceases to have meaning. Is this the end of the universe?
It is believed that physical processes at the microscopic level are either completely or mostly symmetrical in time.
Perhaps, entropy creates what we call time.
It is unlikely that as entropy goes low randomly, all the air in my room will organize, gather in one place and suffocate me, and then return to a disorganized state, with entropy going high, but it is possible.
Entropy is a measure of disorder or randomness in a system. It explains why things go from organized to disorganized over time.
That is, for some reason, the universe at the moment of the big bang was in an ordered state, low entropy. And the gradual movement from that very original hot ordered state to a cold disordered, high entropy state we call the movement of time.
The fact that we feel time moving in the same direction means that the universe has not yet reached the equilibrium state that it strives for according to entropy.
Having reached the state of total equilibrium, the universe will have maximum entropy, because from that moment on, the total entropy of the universe can no longer increase. The universe will be in state of total homogeneity.
In order for something to happen, energy must be transferred from one place to another.
But if the universe is infinite, with infinite time, everything will happen.
By putting a monkey at a desk with a typewriter and give it an infinite amount of time, it will write all the existing and all possible books an infinite number of times.
It is possible to reduce the disorder in a system, but it is extremely unlikely. But, if I were to sit in a room for an infinite amount of time, all the air molecules would eventually, by chance, gather in one corner, suffocating me. Given infinite time, it would happen with 100% probability.
With 100% probability...
And a completely empty universe only at first glance seems empty because even where there are no particles, there are still quantum fields that never behave absolutely quietly. Quantum fields do not maintain a constant value and their value at any point in space always fluctuates. If you are ready to wait long enough, a very long time, from time to time, truly strange things happen due to unpredictable fluctuations capable of shifting some part of the system to a state of lower entropy, lower homogeneity.
Therefore, even if we accept that the universe had a beginning, and that it is not infinite in space, nevertheless, in an expanding universe, time and space becomes so much that extremely unlikely events can and will happen in it with 100% probability. It's unlikely that a piano will spontaneously assemble itself in empty space but if you wait for a period of time equal to a trillion, trillion times the age of the universe, you'll see one appear right before your eyes[2].
With infinite space and time, any unlikely event will happen an infinite number of times.
The same events will continue to happen over and over again throughout eternity.
The origin of our universe and everything that happens in it can be seen as the result of random fluctuations in an ever-expanding space containing only dark energy. At some point in space, quantum processes cause a jump to a very low-entropy state and a universe is created, which then gradually returns to its normal high-entropy state, becoming homogeneous again.
In the same way, when all the air in a room is organized and collected in one place, it gradually spreads evenly throughout the room again, and everything repeats itself over and over again.
Of course, not every specific big bang will repeat the history of that universe exactly the same as the history of our universe. But having infinite time, sooner or later our universe will repeat itself with any accuracy, and we will be here again.
Having reached the state of total equilibrium, the universe will have maximum entropy, because from that moment on, the total entropy of the universe can no longer increase. The universe will be in state of total homogeneity.
In order for something to happen, energy must be transferred from one place to another.
But if the universe is infinite, with infinite time, everything will happen.
By putting a monkey at a desk with a typewriter and give it an infinite amount of time, it will write all the existing and all possible books an infinite number of times.
It is possible to reduce the disorder in a system, but it is extremely unlikely. But, if I were to sit in a room for an infinite amount of time, all the air molecules would eventually, by chance, gather in one corner, suffocating me. Given infinite time, it would happen with 100% probability.
With 100% probability...
And a completely empty universe only at first glance seems empty because even where there are no particles, there are still quantum fields that never behave absolutely quietly. Quantum fields do not maintain a constant value and their value at any point in space always fluctuates. If you are ready to wait long enough, a very long time, from time to time, truly strange things happen due to unpredictable fluctuations capable of shifting some part of the system to a state of lower entropy, lower homogeneity.
Therefore, even if we accept that the universe had a beginning, and that it is not infinite in space, nevertheless, in an expanding universe, time and space becomes so much that extremely unlikely events can and will happen in it with 100% probability. It's unlikely that a piano will spontaneously assemble itself in empty space but if you wait for a period of time equal to a trillion, trillion times the age of the universe, you'll see one appear right before your eyes[2].
With infinite space and time, any unlikely event will happen an infinite number of times.
The same events will continue to happen over and over again throughout eternity.
The origin of our universe and everything that happens in it can be seen as the result of random fluctuations in an ever-expanding space containing only dark energy. At some point in space, quantum processes cause a jump to a very low-entropy state and a universe is created, which then gradually returns to its normal high-entropy state, becoming homogeneous again.
In the same way, when all the air in a room is organized and collected in one place, it gradually spreads evenly throughout the room again, and everything repeats itself over and over again.
Of course, not every specific big bang will repeat the history of that universe exactly the same as the history of our universe. But having infinite time, sooner or later our universe will repeat itself with any accuracy, and we will be here again.
If the entire universe can emerge from a vacuum due to quantum fluctuations, then it is much more likely that a single galaxy can do this because it is less complex, although it is more likely that a single solar system will emerge, no, one planet, or maybe just this day, but it is more likely that a single brain will emerge that contains all the memories and imagines that it lives in a complex world. Because it is much more likely that a single brain will emerge than the entire universe in which galaxies will emerge, then stars, then planets, then life will evolve into you.
It's speculative and for entertaining purposes.
- Linde, A. D. (1982). "A new inflationary universe scenario: A possible solution of the horizon, flatness, homogeneity, isotropy and primordial monopole problems." Physics Letters B, 108(6), 389-393.
- Anthony Aguirre, Sean M. Carrol, & Matthew C. Johnson. (2011). "Out of equilibrium: understanding cosmological evolution to lower-entropy states" (Report No. CALT 68-2845)
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. Will read later 
