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By the term black hole we normally mean one of four spacetime geometries, the Schwarzschild, Reissner–Nordström, Kerr or Kerr-Newman metrics. The universe is (we believe) approximately described by the Friedmann–Lemaître–Robertson–Walker metric, and it is not a black hole. The Big Bang is not the same as the singularity at the centre of a black hole. For ...


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One can apply the underlying principle of relativity -- that all reference frames are valid and agree on the speed of light -- to expanding space, but one has to be careful. In particular, special relativity assumes reference frames are these global things that cover all of space and time. Picture a uniform grid clocks and rulers stretching as far as the ...


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Dark energy is not negative energy. It causes a repulsion because of its unusual equation of state, which causes it to behave as if it has a negative pressure. There is some discussion of this in the answers to Have negative pressures any physical meaning? and possibly also 'Negative pressure' counteracting gravity?. When general relativists talk ...


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Sometimes the word universe is just used colloquially and can just refer to everything on some side of a horizon (an event horizon, a causality horizon, etc.) But when used precisely, I'm sure different definitions are used in different fields. For instance, in mathematical general relativity, you assume that your universe is a connected four dimensional ...


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If the question is asking whether there is a definition that encapsulates our universe, then I believe the answer is No. This is because encapsulating a "space" into a formal system requires defining bounds. However, we don't know the bounds of our own universe--let alone what bounds might be possible for any universe. We can only describe what we can ...


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Of course not, you would have to also simulate the simulation, etc. ad infinitum. To address one of the OP's comments: no, this does not mean we can never have a theory of everything. A theory of everything is a theory that can describe every type of fundamental particle and interaction; there is nothing in this definition that says you have to simulate the ...


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The universe is not currently in (or even close to) thermodynamic equilibrium, so the kind of thermodynamics we teach in a first course (often called "equilibrium thermodynamics") is right out.


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What would we see just outside of it? Pure blackness or other expanding bubbles of multiverses? Outside our particle horizon we assume that everything is more or less the same like where we are, at least if the assumption of homogenity and isotropy holds. If we live in a multiverse there might also be other laws of nature beyond or horizon, but there is ...


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Is it legitimate to speak of distant red-shift galaxies as experiencing time more slowly in relation to our experience of time? No, it is not, since they are not moving relative to the hubble flow, which means that they are sitting on their comoving coordiantes and are therefore at rest relative to the CMB, just like we are (peculiar velocities ...


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Yes, but, the system is not in thermodynamic equilibrium, as said by @dmckee or @HritikNarayan, since the processes happen very fast, creating additional entropy; Yes, up to the experimental data of the moment, although we have theories that deal with the interaction among universes; No, because the reasons of my first answer (the thermodynamic processes ...


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I am assuming that you do want a simulation of the whole universe and not just a theory of everything. Your question should be decomposed into two questions. The first is really a mathematical question: Can a part (the simulator) simulate the whole ? Given a positive answer to the first question, the second is whether the mathematical structures thus ...


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This is a typical conundrum created in using classical thinking in combination with some relativistic concepts (e.g. photons experience no time) Infinite time dilation (i.e. no time elapsed for the photons) is immediately related to infinite length contraction (there is no distance to travel) and the only conclusion here should be: the frame of photons is ...


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This is a good question. In fact, as I'll explain, it leads right to some topics of current research. First, an important fact about entropy: if you look carefully at the proof Kittel gives, it should only be applicable to a situation in which you add degrees of freedom to a small system that is exchanging energy with some heat bath. However, if you ...


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I'm not sure I agree with either of your options... Don't think about the Universe as a big room with things in it, whose walls are moving outward. Rather it is like a ball of dough that is baking in an oven and the dough is expanding everywhere. The rate of expansion of a particular region of space is proportional to size of the region. So the space ...


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First, just a clarifying point: the fact that more of the universe becomes observable over time has to do with the finite speed of light, not the expansion of the universe (which is indeed happening at an accelerated rate). And of course it is best to consider the universe as a whole. Even though we can't see the dark side of the moon, we still know ...


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"Universe" can have several meanings. Some describe the visible universe (small u), others describe the whole Universe (capital U), whatever that might be. That we can described the "visible" universe, perhaps implies a visible and non-visible universe.


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even if the universe is infinite he could grow? Yes. Even if you have a universe containing ∞ m³ every m³ can still double. ∞×2 will still be ∞, but all distances will have grown nevertheless. Link 1, Link 2, Link 3


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Singularities exist in theoretical 'perfect' solutions to General Relativity, but when you look at actual natural Kerr-like objects spinning in a noise filled background of GR waves and other incoming radiation and matter, its likely that no physical real singularities exist. Brandon Carter, referring to spinning black holes (all real black holes spin): ...



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