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As noted dark energy is a catch all term to explain accelerated expansion. In most general terms dark energy must be of a form such: ($\rho_{dark \: energy} + \rho_{all \: other \: matter}) + 3(P_{dark \: energy} + P_{all \: other \: matter}) <0$ Where $\rho$ is energy density and $P$ is pressure (they have the same units). As all other terms are ...

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Short and sweet: A cosmological constant is the special case of dark energy with constant density; Dark Energy may also be non-constant, we do not know if this is actually the case.

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When I read your post : so that the universe has a great big ball of virtual particles surrounding it and pulling it outwards slightly I worry that you might think the universe is a giant 3D ball of matter sitting inside an infinite 3d space. That would indicate that we have to be far from an edge (since we don't see an edge), which harks way back to ...

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General relativity was written to describe all behaviour of spacetime. And expanding universe is one small subset of the whole general class of spactimes in the phase space of general relativity. So yes, relativity does apply to an expanding universe. At least general relativity does.

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