Comments about "Dark matter" in Wikipedia

This document contains comments about the document Dark matter in Wikipedia
This particular version is called "Rev2". There exist also a previous version which is called "Rev1". In the last paragraph I explain my own opinion.



In this section we read:
Astrophysicists hypothesized dark matter due to discrepancies between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the "luminous matter" they contain: stars, gas, and dust.
There is nothing wrong with this sentence. The major problem is the word dark matter because it reflects a human capabity. A better name would be: missing matter.
Next we read:
by Fritz Zwicky in 1933 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters.
Fritz Zwickt uses the correct thermenology.
Subsequently, many other observations have indicated the presence of dark matter in the universe, including the rotational speeds of galaxies by Vera Rubin etc and more recently the pattern of anisotropies in the cosmic microwave background.
Here I have a problem. You have to make a clear distinction with missing matter in individual galaxies, clusters of galaxies and the universe (in total) because the amounts of missing matter and specific its build up (baryonic versus nonbaryonic) can be different. Next we read:
According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle
The major problem of this sentence is that you can only go this route if you clearly know:
  1. That the missing matter can not be solved by assuming baryonic matter. That means invisible dust and gases.
  2. That it is important to make a disctinction between solar system, galaxies, clusters and the universe

1. History

The first sentence in this sections reads:
In 1933, Swiss astrophysicist Fritz Zwicky, who studied galactic clusters while working at the California Institute of Technology, made a similar inference. Zwicky applied the virial theorem to the Coma galaxy cluster and obtained evidence of unseen mass that he called dunkle Materie 'dark matter'.
To apply the virial theorem to a galaxy cluster in order to estimate the mass of each galaxy is extremely tricky.
The most reasonable explanation for the unseen mass is ordinary baryonic matter.

1.1 Cosmic microwave background radiation (CMB)

2 Observational evidence

2.1 Galaxy rotation curves

2.2 Velocity dispersions of galaxies

2.3 Galaxy clusters and gravitational lensing

2.4 Cosmic microwave background

2.5 Sky surveys and baryon acoustic oscillations

2.6 Redshift-space distortions

2.7 Type Ia supernova distance measurements

2.8 Lyman-alpha forest

2.9 Structure formation

3. Composition

3.1 Baryonic and nonbaryonic dark matter

This paragraph starts with the sentence:
There are three separate lines of evidence that the majority of dark matter is not made of baryons, ordinary matter including protons, electrons and atoms:
In short most of dark matter is nonbaryonic.
Also here you should make a difference between galaxies and the universe as a whole.
Next we read:
The theory of Big Bang nucleosynthesis, which very accurately predicts the observed abundance of the chemical elements, predicts that baryonic matter accounts for around 4–5 percent of critical density of the Universe.
We have the following issues:
  • First of all why using the word predict? (twice)
    Predict means that you describe what happens in the future based on past and present observations using a law or model. In the case Big Bang nucleosynthesis this is not the case ?
    My interpretation is that we measure (partly the amounts of Hydrogen and Helium). The Big Bang nucleosynthesis describes the processes (steps) which led to these results.
  • Secondly why use the words critical density?
    The text in the introduction reads:
    According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the universe contains 4.9% ordinary matter,
    When you compare both sentences somewhere something is wrong.

However there is more. When you follow the link Big Bang nucleosynthesis you can read that in order to understand nucleosynthesis the concept of dark matter is more or less introduced ad hoc. (See "History")
Next is written:
Large astronomical searches for gravitational microlensing, have shown that only a small fraction of the dark matter in the Milky Way can be hiding in dark compact objects; the excluded range covers objects above half the Earth's mass up to 30 solar masses, excluding nearly all the plausible candidates.
This sentence is not clear.
The issue is the total mass of all jupiter sized objects in the disc of our galaxy. This is baryonic matter. It is easy possible that this mass can explain flat galaxy rotation curves.

4. Detection (of WIMPS or Axions)

4.1 Direct detection

4.2 Indirect detection

5 Synthesis

6 Alternative theories

6.1 Mass in extra dimensions

6.2 Topological defects

6.3 Modified gravity

6.4 Spacetime fractality

7 In popular culture

8. See also

Following is a list with "Comments in Wikipedia" about related subjects


To divide matter or mass into two categories as visible versus invisible is highly subjective and is linked to human observations. Chemical processes, in particular the evolution of the Universe, should be described independent of any human perspective. That is why the term dark matter should not be used. Infact a much better name is missing matter.
There is nothing wrong to divide matter into baryonic versus non baryonic matter, because there is clear chemical difference between the two. The major problem is to call all invisible matter non-baryonic, while almost most missing matter could be baryonic. This depents about the size of the object and the external temperature of the object. Probably most missing matter are small objects or gas clouds.

The planets around the sun at larger distances are all invisible. All the planets are baryonic which shows a discrepancy.

If you want to give a comment you can use the following form Comment form
Created: Rev1 8 August 2013
Updated: Rev2 3 November 2016

Go Back to Wikipedia Comments in Wikipedia documents
Back to my home page Contents of This Document