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## Hot answers tagged universe

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My essay Misconceptions about Virtual Particles explains in detail the difference between the popular science view of virtual particles popping in and out of existence and the physical science view of virtual particles as useful (but literally virtual) imagery for talking about complicated integrals for the computation of scattering amplitudes. From the ...

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That is the case. The conformal transformation of the metric $g_{ab}~\rightarrow~\Omega^2g_{ab}$ transforms the line element as $$ds^2~\rightarrow~ds^2~=~\Omega^2(du^2~-~dx^2~-~dy^2~-~dz^2).$$ Then for the case that $du~=~\Omega^{-1}dt$ we have the metric $$ds^2~=~dt^2~-~\Omega^2(dx^2~+~dy^2~+~dz^2),$$ that is the de Sitter spacetime metric for $\Omega^2~... 2 This is known as the "Heat Death" of the Universe which is based on the Second Law of Thermodynamics which says that Entropy will increase in an isolated system (and, we assume the Universe is isolated). So, as Entropy increases over time then ultimately every part of the Universe will be at the same temperature and same level of maximum Entropy such that ... 2 You need to learn to use mathematics to tackle such problems. If you try to do without math using only your intuition then you'll make many hidden assumptions that may not be valid. Physicists who understand some theory well enough can get away with using their intuition, but then that intuition is based on a rigorous mathematical understanding of the theory.... 2 So, there are several possible ways the universe could be baryon symmetric: A region of the universe where antimatter dominates. There is a problem with this theory, though - 30 years' worth of scientific research has calculated just how far away this type of region would have to be, and from these calculations it is considered very unlikely that any part ... 1 The idea of a "Zero-Energy Universe" is a theory held by a limited number of scientists. There are several stackexchange question that expand on the theory and may help you. Zero energy universe Total energy of the Universe 1 Infinity is a mathematical concept, as well as the concept of variables describing dimensions. Physics is about observations, either in the laboratory or of the cosmos, which are fitted with mathematical models. It started with the geocentric system, became the heliocentric system and then the realization that the galaxy is composed out of sun like stars, ... 1 The noboundary condition means there is no boundary that marks the end of space or time. With respect to time one might think of the lines of longitude on a globe as representing the time direction at different point in a spatial manifold modeled as the lines of latitude. As one looks further to the north, which is the big bang that eventually you look north ... 1 Sticking with the sphere analogy, first remember that in this analogy, the Universe is a shell, i.e. only the points on the surface of the sphere exist in the Universe, not points inside or outside. If the Universe has a spherical geometry, then the centre would be the centre of this sphere, which is not in the Universe anymore (which is why one would say ... 1 For the case$\Omega_M + \Omega_\Lambda = 1 = \Omega_\text{total}$, which is a good approximation for$t \gtrsim 10^8 \; \text{yr}$, an explicit formula for a(t) is $$a(t) = \left[ \frac{\Omega_{M,0}}{\Omega_{\Lambda,0}} \sinh^2 \left( \frac32 \sqrt{\Omega_{\Lambda,0}} \, H_0 \, t \right) \right]^{\frac13}$$ or, plugging in numbers from Pulsar's answer,$\$...

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