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Re: [Phys-l] cosmology question



In my previous post I neglected to emphasize that for some of the potential scenarios I discussed for the universe, e.g. big bounces, crunches, etc, could correspond to unphysical values for the coefficients of the polynomial in the Friedmann equation. Specifically, in the last special case situation where I discussed all the conceivable potential behaviors for a spatially flat universe, I mentioned that:

.... OTTH, if [Omega]_lambda > 1 then the universe expands
forever after starting out from an initial state that was a big
bounce of finite non-zero scale factor 'size' from a previous
contraction.

This third range for [Omega]_lambda (> 1) is one such unphysical situation. Since the universe was taken as spatially flat this means that 1 = [Omega]_m + [Omega]_lambda. That means that this situation requires that [Omega]_m be negative. We know that ordinary matter can't have a negative mass density, and I have not heard of anyone seriously entertaining the possibility that the dark matter component could have a negative mass density, either. So it is reasonable to say this last parameter range would have to be considered as an unphysical one.

David Bowman