Re: sources of gravity

From: alistair (alistair_at_goforit64.fsnet.co.uk)
Date: 11/21/04 

Date: Sun, 21 Nov 2004 08:53:57 +0000 (UTC)

jdff1001@hotmail.com (jdff) wrote in message news:<3f96fbb1.0411190308.42bc2fa8@posting.google.com>...
> >
> > An energy density of 10^120 Joules per cubic metre (using m = E/c^2
> > this is 10^103 kg) corresponds to 10^52 kg (approximately the rest
> > mass of the universe as a whole) in a sphere of radius 10^-17
> > metres.So if space-time is curved very little, or not at all,at this
> > energy density,and given that the universe was extremely hot at the
> > outset,and so would probably have been able to expand with gravity so
> > weak,I conjecture that a classical calculation - not involving general
> > relativity - would show that our universe never got smaller than
> > 10^-17 metres, and that such a calculation is valid.
>
> Actually, this reasoning is incorrect, for the following 2 reasons
> (which are connected).
> Firstly, as is laid out in the FAQ, there is no universal definition
> of energy in GR, such that it is conserved in integral form. That is,
> locally energy is conserved in terms of flows, but integrated over the
> universe where the curvature is global, it is not.

I was suggesting that at very high energy densities gravity is so weak
that general relativity can be ignored.
>
> Secondly, when the Universe expands by scale factor L, the matter
> density falls as L^-3, but the radiation density is Doppler-shifted,
> so the radiation energy density scales as L^-4. The result is that in
> the early Universe, it was dominated by the radiation energy density.
> Which means that the total energy in the universe was much greater
> than it is now.

I agree with this.
>
> The second argument suggests that it would be a closer approximation
> taking the 3K microwave background, converting to an energy density,
> and then scaling by L^4 to see when it hits the Planck energy density.
> But I don't know whether detailed calculations would actually support
> that either. Because the microwave background tells you what happened
> when the radiation decoupled from matter as it de-ionised, and it is
> much further still to a radiation-dominated universe

The important point is that ,in principle,the energy density of the
universe when it was very small would,according to current inflationary
theory,have had to pass through a value of 10^120 Joules per cubic
metre,an energy density that might be associated with a very weak
gravitational field because the most successful and most experimentally
tested theory in physics- quantum mechanics- says that there should be
10^120 Joules per cubic metre and nobody can find a good explanation of
why this energy density is at odds with experiment. I am saying that we
should trust what quantum mechanics is telling us and find out why
vacuum energy does not gravitate.

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