Re: Doomsday-like argument in cosmology

From: Brent Meeker <>
Date: Sat, 17 Aug 2002 18:24:31 -0700

On 17-Aug-02, Wei Dai wrote:
> On Sat, Aug 17, 2002 at 04:55:59PM -0700, Brent Meeker
> wrote:
>> I think what the paper says is that when matter/energy
>> have thinned out enough so that we have essentially empty
>> space again, a de Sitter universe, a vacuum fluctuation
>> can start a new universe.

> You're not understanding the paper correctly. A de Sitter
> universe never thins out to essentially empty space. It
> thins out to a certain density and no further (I think
> because new vacuum energy is created as the universe
> expands.) So at any given moment there is always a minimal
> chance that the very sparse matter/energy can come
> together and recreate the present.

My understanding was from Weinberg, "Gravitation and
Cosmology". He takes the Einstein cosmology with
cosmological constant, sets k=alpha=0 and says, "In the de
Sitter model space is essentially empty and flat...there is
no matter".

Maybe by "vacuum energy" you mean the closed loop Feynman
diagrams of QFT. However, I think the bosonic closed loops
give a positive vacuum energy, while the fermionic loops
give a negative term - so the speculation is that they
cancel and that is why the (or a) universe can pop out of
the vacuum and not violate conservation of energy.

Brent Meeker
I am very interested in the Universe - I am specializing in
Universe and all that surrounds it.
      --- Peter Cook
Received on Sat Aug 17 2002 - 18:20:07 PDT

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