[Physltest] Re: [Phys-L] Re: Basic Facts About Dark Energy

[Brian A Powell <bapowell@buffalo.edu>]

On Mon, 10 Jan 2005, Jack Uretsky wrote:

> > 4.  It causes empty space to repel itself.
>
>         Nonsense.  We have no experience with "empty space".  "Dark
> energy" is just a catchy code phrase for the observation that the
> expansion of the Universe is apparently accelerating - according to the
> most recentt measurements.  The observation can be "explained" by invoking
> Einstein's "cosmological constant".  The difficulty, for many theorists,
> is that there is no a posteriori justification known for the observed
> value of the constant.

Well, I think empty in this context should be redefined as "devoid of
ordinary, everyday stuff" (I've always been very exact in my definitions
:-))

If, for instance, the current acceleration of the universe is due to the
nonzero vacuum energy of some scalar field (which could manifest itself in
Einstein's Eqs as a cosmological constant), and since such energy
gives rise to negative pressure, then the acceleration could be due to
empty space in the sense that there are no quanta needed.  What I find
tricky is the space repelling itself part.  A material whose stress-energy
takes the form of a cosmological constant, via EE, necessary gravitates
oppositely from ordinary matter- it repels itself( by itself here I mean
different regions of space with nonzero vacuum energy). Now, I suppose,
that since it is coupled to the background spacetime, that it drags this
with it as it repels itself.

So, it really comes down to what we mean when we casually say "empty
space."

Brian Powell
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