# Tag Info

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"I presumed the source of this energy was not coming from the conversion of other types of energy to dark energy, so it must violate conservation." This is where you go wrong. The positive dark energy is balanced by the negative energy in the gravitational field. As a volume of space expands more dark energy is created in the volume but this is balanced by ...

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It is not necessary to assume that the universe has walls in order for the matter content of the universe to have nonzero pressure. The standard assumption is that the matter/radiation content of the universe is infinite, but at a finite volume density. Also note that there could be some sort of "end of matter" at some radius beyond the cosmological ...

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There are the walls of the universe, at finite distance from here. But we cannot reach them because of length contraction: the closer we to the wall the more we contracted radially. It is the cosmic horizon. For eternal de Sitter (expanding) universe cosmic horizon is just de Sitter event horizon. Any particle approaching the horizon looses its speed due to ...

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This is a great question. I feel it necessary to point out the level of study and understanding that go behind asking this question. Well done! Here's the way I understand it. You analysis is flawless; in a radiation dominated universe, $a\propto\sqrt t$. That said, it is not correct to interpret this as the photons exerting some sort of pressure that ...

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I read this question but I didn't understand the physics equations used in the answer. Let me offer a simplified explanation. The origional Big Bang cosmology asserts that the universe is expanding, which if true means that the proper distance $D$ between any 2 points is increasing. Since you can think of this expansion as "space itself expanding," then ...

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The size of the universe is given by a scale factor, normally written as $a(t)$, that is a function of time and is calculated by solving Einstein's equation for an isotropic homogeneous universe. Once we've calculated $a(t)$ we can differentiate it wrt time to get $\dot{a}(t)$ and use this to calculate the recession velocities. The scale factor is a ...

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I always like simplistic but consistent facts to understand something a bit better. Let's neglect cosmological expansion first and rather focus on the immediate vicinity. A rocket A is fired and a rocket B is considered to be a point at rest with respect to A's initial position. The visible spectrum of light is sent form A to B and B can see A's spectrum ...

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There is a very nice paper exactly on this topic, where the expressions describing the fit curves are derived: http://arxiv.org/abs/hep-ph/9906447v1 If you have a look at the final expression (formula (6.5) on page 8), you will agree, that the relation between $\Omega_\Lambda$ and the luminosity is hard to describe by words. However, you can try to think ...

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It is common to hear in cosmology that the entropy of the universe is adiabatic (constant) at late times. In the standard model of cosmology, $\Lambda CDM$, the universe is dominated at late times by the cosmological constant that causes exponential expansion of the universe. During this epoch, the formation of new structure is limited by exponential decay, ...

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Yes and No Yes part If you think the whole Universe as a closed system , then the entropy must be constant. How come ??? According to Quantum mechanics , the evolution of a closed system is unitary. The unitarity forces us to believe that the whole universe can be thought of as a reversible computer and so a big fine quantum computer. Now since the whole ...

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Entropy is a natural tendency of the Universe to fall apart into disorder. In a reversible process, an increase in the Entropy of the system will be exactly equal to the Entropy decrease of the surroundings. Thus, the net change in the Entropy of the system and its surrounding will be zero. But in an irreversible process in an isolated system (for ...

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As far as I know, the expansion of the universe contributes into creating more and more microstates. This is almost equivalent to saying that entropy increases (because $S = k_B$ln$(\Omega)$). We cannot be sure that the law of entropy applies to the whole universe (There is debate if the universe is a closed system or not, if its infinite or finite, etc..) ...

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First, in the most common model of dark energy, $\Lambda$CDM, dark energy is a constant energy density, which means that the "energy" from dark energy does increase as the universe expands. Second, the law of conservation of Energy is only valid in a static universe. Because our universe is expanding, it is no longer the same at every moment of time and so ...

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We don't know how the relationship between gravity and dark energy changes over time as gravity decreases (from the rest of the universe), because one cancels out the other to a degree we don't know. It is not reasonable to assume that as the universe expands more strings of dark energy magically appear to keep the density constant. Einstein originally ...

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The dividing line meets at $(\Omega_m,\Omega_\Lambda)=(0,1)$. From the Friedmann equations, it follows that the scale factor $a(t)$ satisfies the relation $$\frac{\dot{a}^2}{H_0^2} = \Omega_m a^{-1} + (1 - \Omega_m - \Omega_\Lambda) + \Omega_\Lambda a^2.$$ The universe has no big bang singularity if the above expression is negative (or zero) for some ...

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Yes it can be, if the Universe attains equilibrium.

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The Milky Way is receding from the members of the Hydra-Centaurus Supercluster. The Hydra cluster has a red shift of 0.0548. The Centaurus cluster has a red shift of 0.0114. The Norma cluster has a red shift of 0.0157. The local group is and will continue moving away from the Hydra-Centaurus Supercluster.

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Space itself was once concentrated in an infinitesimally small point. During the Bang of the Big Bang all distances between points got bigger. If you try to measure the expansion of the universe from any point you will draw the conclusion that the expansion started from that point. It seems that the expansion happened everywhere, and nowhere at the same ...

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Remember, the Big Bang theory is just that- a theory. I predict astrophysicists will soon discover galaxies 15,20,25, billion years old. Then what?

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Other than noting that galaxies are moving away from us, we have no frame of reference in order to state how the universe is expanding. We would need to be out side the universe in order to get a description.

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As I understand it, the space is not actually moving, but expanding; Which means that objects in this space are "moving apart". Now, "moving apart" has not much to do with the physical term of moving. Essentially, one does not move space because that makes no sense; Not because it is impossible. (Or, more poetically: It is not even impossible to move ...

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The catch with dark energy is that it has a constant energy-density, despite the expansion of space$^1$. To paint a simple picture, as space expands, more dark energy is "created" so that the energy-density of dark energy remains unchanged. Thinking of dark energy as invisible/undetectable particles is perhaps not the most instructive way to think about ...

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Besides the comments, stars have formed well after the Big Bang. Depending on where they are in the universe, it will take a while for us to see them.

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In curved spacetime, you can no longer compare velocities at different points in the straight-forward manner we use in flat spacetime. Thus the claim that recession velocities should not be considerer 'real' (as in relative) velocities, but rather rates of expansion of space. If you want to get at the former, you need to parallel transport the source's ...

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No, EM radiation cannot be completely dissipated. It simply expands to fill the increasing size of the universe, so that each cubic meter contains less of its original free EM energy as the universe grows larger. That reduction is reflected by a decrease both in the frequency of the light within that volume and by an overall dimming of the light. There is ...

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