"Jonathan Thornburg" schreef in bericht
...
From: "Jonathan Thornburg [remove -animal to reply]"

Subject: formation of dwarf galaxies in CDM cosmology
Newsgroups: sci.astro.research

Robert L. Oldershaw wrote:
In the Jan. 14th issue of Nature is a paper that claims to resolve a
serious problem that has plagued the CDM cosmology for a long time.

For the benefit of others, the paper in question is this one:

F. Governato, C. Brook, L. Mayer, A. Brooks, G. Rhee, J. Wadsley,
P. Jonsson, B. Willman, G. Stinson, T. Quinn & P. Madau
"Bulgeless dwarf galaxies and dark matter cores
from supernova-driven outflows"
Abstract:
For almost two decades the properties of `dwarf' galaxies have
challenged the cold dark matter (CDM) model of galaxy formation1.
Most observed dwarf galaxies consist of a rotating stellar disk2
embedded in a massive dark-matter halo with a near-constant-density
core3. Models based on the dominance of CDM, however, invariably
form galaxies with dense spheroidal stellar bulges and steep central
dark-matter profiles4, 5, 6, because low-angular-momentum baryons
and dark matter sink to the centres of galaxies through accretion
and repeated mergers7. Processes that decrease the central density
of CDM halos8 have been identified, but have not yet reconciled
theory with observations of present-day dwarfs. This failure is
potentially catastrophic for the CDM model, possibly requiring a
different dark-matter particle candidate9. Here we report
hydrodynamical
simulations (in a framework10 assuming the presence of CDM and a
cosmological constant) in which the inhomogeneous interstellar
medium is resolved. Strong outflows from supernovae remove
low-angular-momentum gas, which inhibits the formation of bulges
and decreases the dark-matter density to less than half of what it
would otherwise be within the central kiloparsec. The analogues of
dwarf galaxies-bulgeless and with shallow central dark-matter
profiles-arise naturally in these simulations.
Nature volume 463, number 7278, pages 203-206 (14 Jan 2010)
http://www.nature.com/nature/journal...ture08640.html
doi:10.1038/nature08640
preprint (open-access!) at http://arxiv.org/abs/0911.2237

Nature also has an "Editor's summary" and a "News and Views" article
about this work,
Marla Geha
"Galaxy formation: Gone with the wind?"
Nature volume 463, number 7278, pages 167-168 (14 Jan 2010)
http://www.nature.com/nature/journal...l/463167a.html
doi:10.1038/463167a


What is CDM and why do we need this ?
Accordingly to the Book "The Big Bang" by Joseph Silk
At page 182 we read:
"The dark matter responds to gravity, and initial density fluctuations
grow in contrast, just as they do with ordinary matter"
That means the behaviour dark matter can be described by Newton's
Law, but it is different from ordinary matter i.e. dark matter is not
described by the chemical elements of the periodic table,
which are the building blocks of the Sun and the Earth.
If you combine those chemical elements in small quantities you get
small pieces (grains) of ordinary matter, which are all dark and cold.
Those pieces only become visible if they grow in size and become as
large as the size of the Earth and if the temperature increases
to more than 2000 degrees C at the outside! (There are exceptions)
What important is that those small pieces (objects) of cool ordinary
matter are impossible to detect.

This raises for me the question:
Why introducing CDM which consists of nonbaryonic particles.
See: http://en.wikipedia.org/wiki/Dark_matter
The answer is partly because the observed rotation curves
of galaxies don't match the calculated rotation curves based
on the observed(visible) values of ordinary matter.
One specific discrepancy is on the right side where the rotation
curve assumes high speeds of something while outside that region
(away from the bulge) no (visible) mass can be detected.
The solution is to invent the concept of (invisible) dark matter
(in a halo) which behaves mathematical the same as ordinary matter
but physical not. There exists no inter exchange.
I 'am astonished by this reasoning. Why not introducing ordinary
matter in small quantities (inside the disc) ? This is a much simpler
concept which allows for a free mixing (colliding) environment.

At page 167 at the above mentioned document we can read:
"Yet despite this uncertainty (hypothesized particle) the prevailing
model of galaxy formation based on CDM is tremendously
successful, predicting a vast range of observational data"
What I would like to know which data CDM predicts correct and
why a model only based on ordinary matter would fail based on
the same circumstances.
I think to describe the evolution in galaxy formation only using
ordinary matter is much simpler.
CDM models require supernova in order to move dark matter
outwards. Ordinary matter models are less strict.
The whole issue is how fast small objects form larger objects
which form stars at the same time converting invisible matter
into visible matter. This whole process depends about the initial
density of the original grain clouds.

Page 167 shows one image of a real galaxy (right) and one
simulation image of a galaxy. The right image shows ordinary
visible matter (stars). However this is only a part of the total
ordinary mass of that galaxy. What is not shown is a type of
small objects in the plane of the disc (like the Kuiper Belt)
and a type of Oord cloud within and surrounding the galaxy

Page 205 shows two galaxy rotation curves: The real one and
a simulation using CDM.
IMO simulating the same galaxy rotation using only ordinary
matter is not more difficult. In fact the total galaxy rotation curve
is much larger and stretches far into the Kuiper Belt.

Nicolaas Vroom
http://users.pandora.be/nicvroom