One Important Reply of mine (this old Willner's article from the year 2012) about Dark Matter:

tiistai 18. joulukuuta 2012 klo 0.27.20 UTC+2 Steve Willner kirjoitti:

In article <[email protected]>,
dlzc <[email protected]> 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.)

IMPORTANT NOTE:

Above definition of Dark Matter is WRONG. This other type of mass
(which is not correctly defined in the standard model of particle physics) exist only in contracting space areas like inside event horizon of Schwarzschild black hole for example. Interactions of this other type
of mass are gravitational interaction and
weak interaction (right neutrinos included, which are not same as sterile neutrinos).

Best Regards, Hannu Poropudas

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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Steve Willner Phone 617-495-7123 [email protected]
Cambridge, MA 02138 USA

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