Re: Hartle, Gravity: An Introduction to Einstein's General Relativity 1st Edition, 2002

[Brent Meeker]

On 2/26/2025 12:22 AM, Alan Grayson wrote:


On Tuesday, February 25, 2025 at 10:07:41 PM UTC-7 Brent Meeker wrote:



    On 2/25/2025 7:59 PM, Alan Grayson wrote:


    On Tuesday, February 25, 2025 at 6:40:35 PM UTC-7 Brent Meeker wrote:



        On 2/25/2025 3:48 PM, Alan Grayson wrote:


        On Tuesday, February 25, 2025 at 12:46:46 PM UTC-7 Brent
        Meeker wrote:

            I think all cosmologist, like Hartle, recognize that
            /the *observable* universe/ was much smaller in the
            past.  Which is perfectly compatible with /the universe/
            be spacially flat and infinite.

            Brent


        I fully anticipated that response. But why would the
        observable universe behave radically different from the
        entire principle, particularly in light of the Cosmological
        Principle? AG
        It's not radically different.  It's different in exactly the
        way that finite subsets of infinite sets behave.

        Brent


    But if the observable universe contracts to zero volume, the
    entire universe has a singularity, which is inherently
    contradictory. So, the model is, to say the least, inconsistent. AG
    It's not contradictory or inconsistent, it's unphysical, i.e. it
    can't be physically realized; which just means the theory of
    general relativity doesn't work there.  This is not a surprise
    since GR is not a quantum theory and if you're concerned with a
    subatomic scale region you'll probably need a quantum theory.

    Brent


My conjecture is that there's a fifth force, repulsive in Nature, that 
prevents the mass of a high mass collapsing star to reach zero volume. AG

But the reason GR predicts a singularity is that all forces contribute 
to mass-energy density and the higher the density the higher the 
curvature (that's Einstein's equations).  So adding a repulsive force 
just makes the singularity more certain; still with the caveat that this 
is a classical equation.

Brent

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