🚀 go-pugleaf

scip1061@nuscc.nus.sg (Marc Paul Jozef) writes...
>If one however reads this `mutual gravitational attraction'
>in an intuitive `Newtonian' sense, then the assertion
>is false, of course: the supposed homogeneity of the
>universe implies namely precisely that the mutual attraction
>is zero (in one point, the net contribution of gravitation
>of other points is zero due to isotropy).

I turns out that if you treat the boundary conditions in a different way,
then the 'Newtonian sense' gives just the right answer. You have to
consider the limit of a finite universe as the ends go to infinity. Then
the expansion rate slows down just as GR predicts. In any case, if you don't
consider the boundary conditions, then ANY behavior is consistent with the
'Newtonian sense.' Infinity minus infinity can be anything you like.

>Therefore, in the Newtonian sense, one would expect the
>expansion-rate to be constant, regardless the amount of
>matter in the universe. Now Einstein's equations say
>differently, but it should be remembered that their
>global scale application is based on the somewhat thin
>assumption that the global matter-distribution can be
>idealized as that of a perfect fluid.

>If one would for instance idealize the stress-energy
>tensor as that of some discrete lattice (with matter-
>concentration in the lattice-points), then Einstein's
>equations would confirm the Newtonian expectation.

Not true. A lot of papers have been written on this subject, and it turns out
that unless density fluctuations are the same size as the characteristic radius
of the universe then the large scale behavior will be just like a homogeneous
distribution.

Dave Ring
dwr2560@zeus.tamu.edu