- #1
kurros
- 453
- 15
So I recently had a conversation with a mathematician friend of mine who studies Einstein's equations, and he asked me this: Why do physicists call it "dark energy"? It isn't like the dark matter problem, where there is almost certainly some massive "stuff" out there gravitationally influencing the motions of stars and galaxies and galaxy clusters and so on. We already have a perfectly good model for "dark energy", he says: GR. It just works already, why all the fuss? The cosmological constant is more or less just a constant of integration that appears naturally. It is just another free parameter, like the speed of light, or Newton's constant. Why not just fix it to what it needs to be and call it a day? Why all the talk of a dark energy "problem"?
For the purposes of this question I am supposing or assuming that the "cosmological constant problem" is actually a different issue. That problem asks why Lambda has the value it does, not why does it exist. Naive QFT calculations give a clearly incorrect answer for a vacuum energy contribution, so just forget about those, QFT or quantum gravity people will fix that one day. We are here asking why not just use the cosmological constant as-is? Why even associate it with QFT vacuum energy? Why all the searching for stuff to add into the energy-momentum tensor to emulate a cosmological constant, or modifications of GR? Of course it would be nice if there are some new quintenssence fields or some modified gravity, or even if it IS the QFT vacuum, or something fun, but don't we already have the solution in front of us?
Just curious how others here would have responded to my friend :). I guess this is a question about what physical motivation there is to suppose that the cosmological constant is something more than just another constant in Einstein's equations.
For the purposes of this question I am supposing or assuming that the "cosmological constant problem" is actually a different issue. That problem asks why Lambda has the value it does, not why does it exist. Naive QFT calculations give a clearly incorrect answer for a vacuum energy contribution, so just forget about those, QFT or quantum gravity people will fix that one day. We are here asking why not just use the cosmological constant as-is? Why even associate it with QFT vacuum energy? Why all the searching for stuff to add into the energy-momentum tensor to emulate a cosmological constant, or modifications of GR? Of course it would be nice if there are some new quintenssence fields or some modified gravity, or even if it IS the QFT vacuum, or something fun, but don't we already have the solution in front of us?
Just curious how others here would have responded to my friend :). I guess this is a question about what physical motivation there is to suppose that the cosmological constant is something more than just another constant in Einstein's equations.