The overly simplified explanation I'm giving myself right now is dark energy causes the opposite of what gravity does, that's why the universe is expanding. Now where gravity is a force, why dark energy is "energy"? Why it's not called "Dark Force" instead? I think I must be missing something here.
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$\begingroup$ What's in a name? Would a rose by any other name not stimulate the particulate receptors in your nasal cavity that triggers an olfactory response in the the brain just as sweet? $\endgroup$– JimApr 9, 2014 at 19:11
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2$\begingroup$ Remark: in general relativity, gravity is not a force. In fact, dark energy doesn't do the opposite of what gravity does, it does the opposite of what regular energy does (and mass, which is just some form energy). $\endgroup$– WouterApr 9, 2014 at 19:39
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$\begingroup$ Hate to digress, but Jim: Shakespeare ref? $\endgroup$– Doryan MillerApr 9, 2014 at 19:53
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2 Answers
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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. But this needn't be the case in general.
Since gravity depends on more than the (mass-)energy content, in cosmological models, the universe accelerates or decelerates proportionally to $\rho+3p$, where $\rho$ is the energy density and $p$ is the pressure. The factor of $3$ for the pressure comes from the fact that there are three spatial dimensions, but only one temporal dimension. In particular, the cosmological constant corresponds to the case of a perfect fluid with $p = -\rho$ (which can be generalized to other "equations of state"), and so a positive "dark energy" density has a repulsive effect, not because it's "the opposite of what gravity does", but rather because its negative pressure gravitates in addition to its energy.
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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 universe. Think about this, if you take an expanding box that represents the universe and put into it some energy (a hunk of matter or some radiation or whatnot), then at the initial time its energy density is the energy in the box divided by the volume. At a later time, the energy density is bound to be lower because the volume grew but you didn't put more energy into it. Dark energy has a constant energy density; it is an "energy" that adds more of itself to the universe the bigger it gets.
But I digress, it is not called a dark "force" because it acts more like an energy. However, we could have called it anything we wanted. Some people argue that a more appropriate name for it would be "Dark Pressure" because it is a constant energy density, which is the same as pressure. It is described as a negative pressure. But since, in cosmology, we like to put everything into units of mass (energy), we like to call everything an energy.
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$\begingroup$ Now I'm even more confused by this sentence- "it is an "energy" that adds more of itself to the universe the bigger it gets". Doesn't that violate law of conservation of energy? How would the total amount of energy increase in the whole Universe? $\endgroup$– MehdiApr 9, 2014 at 19:33
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3$\begingroup$ @MHK: Thinking a bit naively, if the box increases its volume by $\Delta V$ and it still has the same energy density $\rho$, then energy content increased by $\rho\Delta V$. But the pressure remained constant, so the work done is $p\Delta V$, where $p$ is the pressure. If $\rho = -p$, as it is for a cosmological constant, energy increase is balanced by negative work. I say "naively" partly because "the total amount of energy in the Universe" is (usually) not a well-defined quantity in GTR in the first place. $\endgroup$ Apr 9, 2014 at 20:09
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1$\begingroup$ @MHK additionally, there is no law of conservation of energy in a universe that is different at different times (a time-dependent expansion). The stress-energy is conserved, but not the total energy $\endgroup$– JimApr 10, 2014 at 3:49