# Tag Info

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The Universe we see around us today including the space was once concentrated to a size smaller than an atom. This infinitely dense object may have had a past before that state but no information would make it from that past into our universe so we may as well say that time began at the big bang.

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Your argument is based upon the idea that all the matter in the universe was created at the moment of the Big Bang. If matter can be created from nothing at the moment of the Big Bang then it does seem reasonable that matter could disappear again, as you say. The trouble is that there is no experimental or theoretical support for the idea that matter was ...

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$C$ stands for charge. Charge symmetry implies that if the charge of all particles were to be inverted - particles become anti-particles and viceversa - the universe would look exactly the same. A universe made of matter would be indistinguishable from one made of antimatter. So if a given process allows for two matter particles (say, a proton and a ...

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I go with Time is the separation between distinct events that happen in the same place. which is very general and not quantitative at all, but covers the basics. Given three distinct events that happen at the same place we can determine which happened between the other from just the values of the three separations. And it agrees with the notion that ...

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The 7 fundamental quantities are hard to define. Time Displacement/position Mass Temperature Current Amount of substance (e.i. the mole) Luminous intensity More here: http://gravimotion.info/Physics_seven_basic_quantities.php Time is just one of them. If someone asked me, I would say something like "Time is how long something lasts" or "Time is the ...

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Here is how to see this answer: In the standard model of cosmology, which is given by the FLRW (Friedmann-Lemaitre-Robertson-Walker) solutions of Einstein's field equations, symmetries of isotropy and spatial homogeneity require that such universes be perfect fluid universes. As you know from thermodynamics, perfect fluids have their entropy conserved! So, ...

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The entropy of a black hole is proportional to its surface area. If the Universe follows the same rule, then as it expands entropy increases, but entropy per volume might be constant, or even decrease. For example, if life continues to increase in its ability to efficiently use Gibbs free energy from Sun photons, fossil fuels, and nuclear sources, it might ...

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Firstly, string theory is a mathematical hypothesis that is currently speculative. It is a possible candidate for a quantum theory of gravity - a unification of QM with Einstein's theory of gravity - General Relativity. The idea of superstring theory emerged from another theory called supergravity. Supergravity was an attempt at a supersymmetric theory of ...

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What Scott's saying Okay, so the basic assumption I can phrase as this: The universe is finite and has an end. During The End, cosmic inflation does not rip every particle apart into its own universe, and no cyclic model of cosmology predominates: The End of the universe is instead one big "soup" of particles in thermal equilibrium, all having some low ...

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Really, your confusion is rooted in the fact that the equation you give relating temperature and particle mass assumes that the thing's whose temperature you're predicting is a gas of particular matter that obeys Newtonian mechanics. Light, and really anything in the early universe, decidedly does not obey Newtonian mechanics. There are several ways one ...

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The way to understand this is as follows. Assume that the early-universe is radiation-dominated, and additionally assume that the early universe is of FLRW type with a single fluid that obeys a barotropic equation of state $p = w \mu$ where: $w = 1/3$ for radiation $w=0$ for dust $w=1$ for a stiff fluid, etc... Now, the Einstein field equations ...

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This is a very broad question but I'll try and answer it. I'll not include any mathematics in my answer. If you would like me to get into technical details then let me know and I'll edit my answer. The idea of multiverses has found resonance in multiple places in theoretical physics but I think Brian Greene's categorization of multiverses is perhaps the must ...

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Dear Carson: To understand why we see today the CMB radiation, after 13.7 billion years after the BB we do not need to know GR at all. Let's use a story to simply illustrate the problem, and get the solution. Suppose you awake in the middle of your street and you find yourself sorrounded by a terrible heavy and dense fog, with a yellow glow all over. So ...

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General Relativity is about describing (the dynamics of) a curved spacetime. So you need a collection of events, and a metric that tells you the interval between nearby events. That's it. You can a stress energy source term too. But anything else is either unphysical or a straight up bias brought into a theory for no reason. There are some easily ...

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Some people have very long explanations of why the universe can be created out of nothing "because the universe always does this". As a counter argument: There is no proof that the universe always does this. It is only theoretical. If you believe that the whole universe can be created out of nothing, it means you could create energy and matter out of ...

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These types of theories that physicists such as Krauss espouse of a "Universe Coming From Nothing" are quite flawed, as by no means are they talking about nothing! Further, the concepts of particles, mass, and energy are not even well-defined when talking about the universe in general. I wrote a paper on this (excuse the shameless self-promotion), it can be ...

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Can space be destroyed? I can only give an opinion here, not some authoritative answer. But my opinion is based on Big-Bang cosmology and general relativity, so hopefully it isn't total junk. We have evidence that the universe is expanding, and this evidence looks pretty good. When we project backwards by 13.8 billion years, we surmise that all the ...

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You're quite correct that measuring time from the Big Bang does separate spacetime into a time bit and a space bit, but this isn't arbitrary. When we want to describe the universe around us we need to choose some coordinate system that we can use to record physical quantities. Whatever coordinate system we choose will have one coordinate that behaves like ...

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