I'm really confused on this topic. Pardon my French if my questions seem really stupid. Before Dark Energy was discovered in 1998 did scientists know that the cosmological constant calculated, I assume from vacuum energy, was so grossly tuned by 10 to the 120 powers as to exclude the existence of life or even a universe for that matter.? When scientists make this calculation which turns out to be too large how are they doing it? Is it from observations or from General Relativity ? Then Dark Energy is discovered and if we now change the cosmological constant to match what the expansion from Dark Energy is doing that it now means we have a user friendly universe and before we didn't? Since the discovery was that the universe was expanding faster than we thought one would think the badly tuned vacuum energy would have been 10 to the 120 powers off in the other direction. No? It could be I need a little introduction to the nuts and bolts of "vacuum energy". My degree is in Math so you may really have to stretch to help.
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$\begingroup$ Dark energy is assumed so General Relativity can account for the accelerating expansion of the Universe. It was the discovery of the accelerating expansion from careful measurements with the HST that required dark energy as a plug to General Relativity. $\endgroup$– Peter RCommented Jun 30, 2016 at 4:01
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$\begingroup$ Have you read the article on general relativity in wiki en.wikipedia.org/wiki/General_relativity#Einstein.27s_equations ? also this en.wikipedia.org/wiki/Cosmological_constant $\endgroup$– anna vCommented Jun 30, 2016 at 4:10
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$\begingroup$ We never had a way to calculate the cosmological constant and/or dark energy. What the mismatch between quantum field theoretical "predictions" and reality tells us is simply that quantum field theory is not the right tool to try this. That's not really surprising as QFT assumes a background metric (more precisely, it assumes a flat background metric to work well) and that's simply not useful for cosmological questions. $\endgroup$– CuriousOneCommented Jun 30, 2016 at 5:33
1 Answer
To start with, the cosmological constant, vacuum energy, and dark energy are mostly used interchangeably in cosmology. This is because the cosmological constant is observationally indistinguishable from vacuum energy by the Einstein field equations. The term "dark energy" was, as far as I am aware, introduced when the expansion of the universe was observed to accelerate. Why? Previously one had generally assumed that the cosmological constant was zero, but according to the standard model of cosmology only a positive cosmological constant can account for an accelerated expansion. As it now seemed the cosmological constant had physical relevance, its physical nature received a name. However, there have also been attempts with more dynamic dark energy solutions, whence dark energy must not always refer to a cosmological constant. It is important to realize that the "observational evidence" of dark energy is very much due to the model used to interpret our observations.
Contrary to the comments, there is room in general relativity for an accelerated expansion without the need of dark energy. In a spatially homogeneous solution where the fluid flow used to model matter content is orthogonal to the homogeneous spaces it remains true that we need dark energy, but if the fluid flow is tilted with respect to the homogeneous spaces there are other possible explanations. Since matter pressure is generally assumed zero for the current state of the universe (with good reason, I believe) the remaining possibility is a non-zero vorticity. Otherwise a pressure gradient may also produce an accelerated expansion.
Other possible explanations for the accelerated expansion include (see also this thread).
The treatment of the universe as spatially homogeneous may hide a gravitational effect from the inhomogenities we know exist. I believe that this requires that the inhomogenities are not perturbation effects on an otherwise homogeneous solution.
Or perhaps there are large-scale inhomogenities. As far as I understand things, this is strongly correlated to violation of the Copernican principle to match observation.
Perhaps general relativity, although having received strong observational confirmation on a local scale, turns out to be in larger error on a cosmological scale. Alternatively there could be some quantum effects at play.
Even within the standard model, the observational data may have been misinterpreted. For example, we could be wrong about the evolution of supernovae.
The grossly out of proportion calculation that you mention was indeed based on vacuum energy. It was made in the domain of QFT and therefore has nothing to do with the cosmological model at the time. It raises the issue that if dark energy is vacuum energy then why is it so small? Alternative theories may assume instead that the vacuum energy is non-gravitating, i.e. that it has no gravitational effects. One may also posit that this is more of an issue for particle physics, since it may be the calculation that is in error.
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$\begingroup$ It says avoid saying thanx? How am I supposed to thank someone for spending their time and answering my question. Piss on the rules. Thank you Erik, you have been most kind. $\endgroup$– user86411Commented Jul 1, 2016 at 16:44