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Old December 17th 12, 11:27 PM posted to sci.astro
Steve Willner
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Default It seems that as Dark Energy increases, Dark Matter decreases as time goes on

In article ,
dlzc writes:
Having Dark Energy as "stuff" (as opposed to the cosmological
constant) allows for it to be non-uniformly distributed in space
and time, subject to observational support of course.


Sorry to be slow getting back to this.

Dark matter is just matter. Its exact nature is unknown except that
most of it is non-baryonic. (There's also baryonic dark matter, but
all the baryons dark and otherwise make up only 4% of the Universe in
today's standard cosmology.) In particular, the density of dark
matter varies in space and time, and the statistics of its
distribution can be calculated under any assumptions one likes.
(Typically these are that the non-baryonic dark matter interacts only
by gravitation and that its total amount is fixed, but other
assumptions could be put into the models. The calculations are not
perfect by any means, but they are probably OK for non-baryonic
matter at scales of whole galaxies and larger. Calculating what the
baryons do is extremely complicated because they interact non-
linearly to make stars, planets, and protoplasm among other things.)

Dark energy is conceived to be a property of space, independent of
the matter in it. A cosmological constant is one example. By
definition, the classical cosmological constant does not vary with
time, but it's easy to imagine "something like a cosmological
constant but potentially varying with time." That's what "dark
energy" means. It's perhaps not ideal terminology, but something was
needed to distinguish a parameter with potential time variation from
one without (cosmological constant). I think "time-variable
cosmological constant" would have been worse? (What is a variable
constant?!)

There is much more at
http://www.astro.ucla.edu/~wright/cosmo_constant.html

Measuring the time variation of dark energy is really hard. For now,
possible variation is usually represented as just a single parameter,
and all observations (so far as I know) are consistent with no time
variation, i.e., that dark energy is a cosmological constant. (This
is expressed as "w = -1;" see the link above.) Better supernova
measurements and baryon acoustic oscillations should pin this down
better in the next several years.

It could turn out that a yet more complex model with dark energy
varying in space is required to fit the data, but we are very far
from needing that right now.

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