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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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The problem is analogous to the physics of gases. Do we need to describe the chaotic motion of every molecule before we can determine overall properties? No.


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this may or may not be a misunderstanding, but there is no centre of the universe. Imagine the universe as being the surface of an expanding balloon, with all the galaxies and stars on the surface, being stretched away from each other. Just as there is no centre, for example, of the Earth's surface, there is no centre of the universe. If by 'centre' you mean ...


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This is a good idea... Dark matter by definition doesn't interact electromagnetically (i.e. it has no charge). Therefore its cross section $\sigma$ for absorbing radiation and being pushed away from an accreting object is $0$, at least to first order. You could look at higher-order effects, like its neutrino-absorption cross section, to calculate some ...


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This is a difficult question for many reasons. One reason is likely because most of the introductory thermodynamics textbook problems that we are familiar with from childhood do not involve gravity. To illustrate this difficulty with gravity consider, for example, this snippet from an article in the New York Times Review of Books by physicist/mathematician ...


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It's an example of adiabatic expansion. If you have a container full of gas and you expand the container, the gas cools. Entropy is preserved. Adiabatic processes preserve entropy. Any decrease in entropy due to lowered energy, and correspondingly fewer possible velocities for the particles, is offset by an increase in entropy due to the expanding volume, ...


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Let me show you that there is no contradiction by pointing out e.g. that for ordinary expansion periods (that is away from first order phase transitions, decouplings...) the total entropy is actually constant in time while the universe is getting bigger and cooler. Or, going back in time, the universe is getting hotter while S is kept constant. How is this ...


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Yes you are correct Sophia. This is a major component of the standard model itself, which concludes the opposite (that space is expanding in an infinite cosmos), but the only reason a conclusion like that comes about, is because what we observe is indistinguishable from being at the centre of a finite universe that races away from precisely us, uniformly in ...



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