MadSci Network: Physics |
General Relativity (GR) does permit hypothetical universes with no matter or radiation density. Indeed, one of the first proposed solutions to the GR field equations was the de Sitter Universe, which has space but no matter/radiation, and "expands" due to the existence of a cosmological constant. In the future, as our Universe expands and the matter and other forms of energy are diluted, so it would become more and more like a de Sitter Universe. The modern viewpoint tends to regard a cosmological constant, if it exists (as seems likely from observations), as probably being due to an energy density of the vacuum, and in that sense there is not zero energy-density. Another early cosmological model, perhaps closer to what you are imagining, was the Milne model, which postulates no matter, radiation or cosmological constant. It therefore describes a universe with no gravitational forces at all, and exists in the Minkowski space-time of Special Relativity. Of course, although this model is not forbidden, our observations of gravitating matter/energy means the Milne model is not a viable description of our Universe. The requirement of the ultimate unification of GR and quantum mechanics is rather independent of these considerations, and can be thought about in a number of different ways. In particular, it would seem necessary to find some quantum theory of gravity if we are to understand the singularities which afflict current theory when applied to Black Holes or the Big Bang. For example, if we extrapolate our conventional theory back to the Planck time (about 10 to the power -44 of a second) then quantum mechanics predicts the spontaneous creation of virtual particles which would themselves be black holes. Exactly how nature brings about this unification is yet to be found, but theoretical problems of this sort suggest it must occur somehow!
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