# Tag Info

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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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The cosmological constant has an interesting history behind it. Originally, when Einstein introduced his theory of general relativity in the early 20th century, the Einstein Field Equation, which was the equation for the gravitational field, described gravity as the effect of the curvature of space-time due to the presence of matter and energy. Perhaps you ...

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The cosmological constant is important for at least two reasons. Our universe is currently asymptotically evolving towards a universe where a constant energy density dominates the total energy density. The cosmological constant can be interpreted as exactly this. Therefore, analysis of the current state of our universe relies heavily on the concept of a ...

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This is all just terminology. 'Force' is a term from Classical mechanics really. 'Fundamental Force' is a term for any one of the set of four theories, gravity, and the three Standard Model interactions, Strong nuclear, weak nuclear and electromagnetic. The strong nuclear interactions (plural) for example could be said to be eight 'forces'. It's just that ...

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Unfortunately I can't add comments yet. How much knowledge theoretical physics do you know, before I make too many assumptions (or lack thereof!)? I don't think that this question can have an acceptable answer. This will depend greatly on what model for the Dark Energy (DE) you are using. There are dozens, and new ones continuily being created, and we do ...

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In an acceleratingly expanding universe, there will be an emission time of light from a distant galaxy after which we can never recieve newly emitted light. Old light will eternally be received, but even more dimmer and red shifted. See The Long–Term Future of Extragalactic Astronomy for more information.

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Because as far as we understand general relativity, it's not doing "the opposite of what gravity does." Gravity can be locally attractive or repulsive, depending on whether the stress-energy content satisfies or violates the strong energy condition. For ordinary matter, the stress-energy is dominated by the mass, the SEC holds, and its gravity is attractive. ...

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The term "Dark Energy" is just a name that we have given it while we try to determine what exactly it is and what a better name for it would then be. However, calling it an energy is appropriate. Our best model, the cosmological constant, says that dark energy has a constant energy density (that is a constant amount of energy per unit volume) in the ...

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The situation you are describing is very similar to (but not exactly like) a Schwarzschild-de Sitter universe. That is a spacetime that is flat, infinite in size, expanding with a cosmological constant, and contains a massive body (such as a black hole). The metric for such a spacetime is: ...

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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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Einstein once called the cosmological constant his biggest blunder, which he said when he was told by someone else the universe was expanding, and he then revised his theory, based on what little was known back then. He came up with it originally to explain a static, non-expanding universe, which was accepted until observations of redshift by Hubble. Energy ...

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No, it's not correct. There are many things outside the visible horizon that affect us. For example, the quantum vacuum, the void with nothing in it that causes dark energy to be sucked outward into it. Gravity doesn't end at the horizon, the gravity of the entire universe has an effect on us, slowing the expansion. Also, there are many things we can't ...

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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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There's a difference between curvature of spacetime and curvature of space. Extrapolating from what we can see around us and assuming the cosmological constant lives up to its name, spacetime will eventually approach curved de Sitter geometry, in contrast to flat Minkowski geometry or anti-de Sitter geometry of opposite curvature. This is something of an ...

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I think this is an example,universe is rotating about its own central axis.if this is the case,take a curve beaker and a flat plate with some water in them.First shake clockwise the curved beaker and then flat plate.In which,beaker or plate did the circular motion was seen about its center?I guess curved one.So,our universe is curved.

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It is only in the absence of dark energy that the correspondence between geometrical curvature and the ultimate fate of the universe is as straightforward as you describe. Measurements (primarily of the cosmic microwave background) indicate that our universe is flat or very nearly so, which should be interpreted geometrically (i.e. in terms of the sum of ...

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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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Energy cannot be created or destroyed, it only changes form. That's a pretty solid law to dismiss by saying that the total amount of dark energy is increasing. Where does it come from?

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My view is that dark energy is like hot air in a balloon. It's density does not stay constant, but decreases as the universe expands, like every other form of energy. Like other forms of energy, it would also be subject to the second law of thermodynamics and would cool down, slow down, and get sucked into black holes, converting it's pushing effect, into a ...

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