Dear Steve Willner:

On Monday, December 17, 2012 3:27:20 PM UTC-7, Steve Willner wrote:
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.

Based on what is this conclusion reached?

(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.)

"Standard" was defined based on (now) significant errors in calibrating to a spiral galaxy. The center is swept clear. Intensity is a function temperature, the center is hot, and the disk is not. Vast pools of ionized normal matter, and even individual stars we find outside our disk.

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.)

There lay the glory in which we are immersed.

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?!)

Let's see:
- Hubble constant
- VSL comsologies
.... it is a choice of word, with foundations in history. Should not be considered to be a pronouncement.

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.

Or even seeing it. Thank you.

David A. Smith