News and Views

CDM Introduction

The concept of Dark Matter was introduced because simulated Galaxy Rotation curves did not match observations. The observed curves are based on visible matter or ordinary matter. Ordinary matter are the chemical elements of the Periodic Table.
For a nice example see: Galactic rotation curves
What the sketch shows are two errors compared to observations:
  1. First the simulated curve is much lower at the right side compared to the observed curve.
  2. Secondly the observed curve all of a sudden stops at the right side. This implies that this the outer limit of the galaxy.
To solve both discrepancies the concept of Dark Matter was introduced which is nonbaryonic (invisible) matter and which resides in a halo surrounding the galaxy disc.
The question is why not using ordinary matter instead ?
The interesting point is that ordinary matter when you study that in small quantities is also invisible. Ordinary matter only becomes visible when you combine those small quantities into larger quantities, even much larger objects like our Earth and that the temperature increases to roughly 2500 Degrees C.

Comments about the article in Nature: Gone with the wind

Following is a discussion about this "NEWS & VIEWS" article in Nature Vol. 459 11 June 2009 by Marla Geha
In the last paragraph I explain my own opinion.

At the beginning of the document we read:

Neither the number nor the appearances of small dwarf galaxies conforms to the predictions of the otherwise highly successful cold dark matter CDM theory of galaxy formation.
This raises immediate the question: Exactly what are the predictions of the CDM theory?
Next we read:
Such predictions however, are usually derived from simulations that don't include stars.
I find this very strange (And I doubt if this is correct). How is it possible to make a simulation of a galaxy without including stars (ordinary matter) while you know that they exist. IMO any simulation without stars (only CDM) is wrong.
Next we read in the text:
This apparently glaring oversight is justified because visible matter plays a minor part in the much larger drama of galaxy formation.
Again I find this reasoning very strange. Figure 1 shows two galaxies. The right one is a real image of a galaxy. The question is what does each light blue dot represent ? IMO visible stars of ordinary matter. Not CDM.

Next we read:

The hidden actor is dark matter, whose effects are felt only through gravity.
But so does ordinary matter. In fact both can be described by Newton's Law.
Next we read:
On page 203 Governato etc demonstrate that visible matter also has a key role in modifying the properties of dwarf galaxies, thereby simultaneously solving two long-standing problems with CDM galaxy formation.
Again this raises the question: Why introducing CDM in the first place while you can do the same simulations using ordinary matter only, which is a much simpler physical concept allowing to model a very smooth evolution of galaxy creation.

A little further on we read:

Yet despite this uncertainty (CDM is a hypothesized particle) the prevailing model of galaxy formation based on CDM is tremendously successful, predicting a vast range of observational data.
IMO ordinary matter only based simulations will exactly do the same. They are simpler and require much less parameters.
Next we read:

Dwarf galaxies are small accumulations of stars with total masses one-tenth or less that of our Galaxy.
Exactly. That is why CDM only simulations are wrong. The issue is how do you physical go from nonbaryonic particles to the chemical elements of the periodic table in order to "create" stars. In a ordinary matter only Universe such a transformation is a natural and smooth process.
Next we read:
A fundamental prediction of CDM galaxy formation is that galaxies have central 'cusps' - the density of matter increases steeply towards a galaxy's center. etc. This is in sharp contrast to Observations. Dwarf galaxies are often seen to host large regions of nearly constant density in their centers.
In fact that means there is something wrong with the CDM model. IMO what observations reveal is that galaxy formation is a rather smooth process.
Next we read:
The standard CDM theory simply cannot explain such galaxies. Many solutions have been proposed etc - most dramatically abandoning CDM altogether in favour of an entirely new theory
IMO the last seems to me the most common sense thing to do and to try ordinary matter only simulations.
A little further on we read:
Testing this hypothesis requires simulating dark and luminous matter in a galaxy simultaneously - a challenge for even the most powerful supercomputers.
IMO what they should do is to simulate galaxy formation using both invisible and visible ordinary matter.
Next we read:
The authors models reveal that stars and gas have an active role in shaping a dwarf galaxy. In the normal course of star formation massive stars are produced that have very short lifetimes ending in spectacular supernova explosions.
All of this seems very logical in ordinary matter only simulations. In ordinary matter simulations supernova are not a must. Apparently in CDM simulations they are.
Next we read:
The dark matter must also react to gas removal in order to maintain dynamical equilibrium - by expanding outwards.
If I understand this correctly than what the supernova's do is to remove the dark matter from the inside to the outside. In fact you create a halo of dark matter not residing within the galaxy. IMO this is a physical very complex process which you can not prove if your simulations are correct because dark matter is invisible. Ordinary matter only simulations don't require this complexity.

Bulgeless dwarf galaxies and dark matter cores from supernova-driven outlaws

Figure 3 at page 205 shows the rotation curve of a simulated dwarf compared to that measured for a real galaxy.
What amazes me that in fact only a very small part of the galaxy rotation curve is shown. The curve should extend on the right hand side.
Below figure 3 is written:
The underlying dark matter density is proportional to r^-0.6 in the central kiloparsec, consistent with observational estimates and shallower than a dark matter only simulation that would predict a steeper profile proportional to r^-1.3
But dark matter is not visible ? or am I missing something ?. Anyway what is the purpose of a dark matter only simulation which does not exist. Comparing your result with such a condition is also misleading. In fact you should compare it with a ordinary matter only simulation.


One of the important documents to study is the following:
The cosmological simulation code GADGET-2 by Volker Springer. This document explains the software being used in order simulate galaxies i.e. N Body simulations. Gadget 2 stands for: "Galaxies with dark matter and gas interact"
This document shows that (astro) physics is very important in order to study galaxy evolution.
The following text in the Introduction section demonstrates this:
Given that many astrophysical phenomena involve a complex interplay of physical processes on a wide range of scales, it seems clear that the importance of simulation methods will continue to grow.
The software is also used to include supernovae (See page 1109):
Their model also accounts for energetic feedback by galactic winds, and includes a basic treatment of metal enrichment. More detailed metal enrichment models that allow for separate treatments of Type II and Type I supernovae while also properly accounting for the lifetimes of different stellar populations have been independently implemented by etc.
For more information about this read the following document: Simulating the effect of AGN feedback on the metal enrichment of galaxy clusters by L Tornatore e.a.

The question if there exists dark matter in ellipital galaxies is discussed in the following document:Lost & found dark matter in elliptical galaxies

For a very recent document discussing darkmatter in the Local Group including the Milky Way and its satellites and the Andromeda galaxy by means of simulations, read this: Local Group galaxies in a LambdaCDM Universe by Li, Yang-Shyang


At page 203 of the article we read:
To study the formation of dwarf galaxies in a Lambda CDM cosmology etc.
For more information about Lamdba CDM see: Lambda-CDM
At page 203 right column we read:
Showing that the properties of dwarf galaxies can be accurately predicted by the CDM scenario would end the 'small scale crisis' and further constrain the properties of the dark-matter particle candidate.
What the simulations show is that supernova are important to simulate CDM dwarf galaxy formation. However they are no prove that the CDM scenario is correct. Supernova are in general important because they are the source of all the different heavier chemical elements in the new born stars. In fact their purpose is more of a mixer than a remover

Galaxy formation starts as matter cools and collapses gravitational. This should be a relative slow process. As soon as when this process starts there is no way back. It is easy to assume that supernova's serve as some type of separator i.e. separating dark matter (which is gravitational bound) from the inside of a dwarf galaxy to the outside into a halo, but physical a very difficult to describe in detail how this happens in time. A supernova is an explosion and the birth place of invisible ordinary matter. It is also very difficult to describe and simulate individual Supernova's in 3D. Of course you can devellop equations which mimic all of this but that does not mean that it is physical correct.

IMO the most logical step is to delete the concept of CDM all together and skip to ordinary matter only scenarios, there by completely bypassing the problem of transforming baryonic matter into stars.

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Created: 25 January 2010

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