Modern Research Project

How sure are we that there exists dark matter?

In many fields of modern astronomical research the dark matter problem plays an important role in finding the answers to some fundamental questions. Particularly for those who study the evolution and dynamics of galaxies and clusters of galaxies, this is of great importance. Dark matter has great consequences for cosmology as well, whether it is in understanding the first moments after the Big Bang, or the large-scale structure of the universe, or indeed, how our universe will continue to evolve.
Given the import and the mysterious nature of dark matter, we decided to take an in depth look at the observations and theories behind it. The fact that apparently as much as 90% of the universe should consist of this mysterious dark matter - which cannot be observed directly - brought us to our main question: How sure are we that there exists dark matter?
After some initial reading we decided to examine the assumptions that go into a few of the most important sources of evidence for the existence of significant amounts of dark matter, and to take a look at an alternative explanation of the dark matter problem. They can be catergorized as follows: 1. The evidence from rotational curves for the existence of dark matter in galaxies; 2. The evidence from gravitational lensing for the existence of dark matter in clusters of galaxies; 3. The evidence from the value of for the existence of dark matter; 4. An alternative explanation provided by the MOND theory.
A brief summary of the aims and results of each of these is listed below:

1. How strong is the evidence from rotational curves for the existence of dark matter in galaxies?
Contributions by Olof and Douwe

We can conclude that the errors in the assumptions of the formulae for the determination of the rotation velocity aren't big enough to cancel dark matter out. From the General Rotation Formula there is the largest error but this is too small to decrease the amount of dark matter a lot (about 1.1 times smaller). Determination from redshift is even more accurate unless there would be a systematical error that hasn't been detected. Douwe states that the visible matter decreases with almost the same factor (only a little bit smaller because of remaining gas) which makes our attempts to decrease the amount of dark matter futile. Out of this we have to conclude that dark matter does exist. Maybe the figures aren't precise, but there is a lot.

2. How strong is the evidence for dark matter following from gravitational lenses
Contribution by Steven

We estimate the amount of dark matter in Abell 2218 from literature values of the lensing mass of the cluster, its luminosity and intracluster gas mass, and an estimate for the M/L of its member galaxies. The estimate shows that 94%±6% of the total mass consists of dark matter. This amount cannot be brought down to lower than 60%. A rough estimate of the intergallactic dark matter shows that 10%-90% of matter in the member galaxies consists of dark matter. No wrong assumptions or systmatic errors are found in the lensing analysis that can explain the huge cluster mass discrepancy. Only a theory like MOND would seem to be a viable alternative to dark matter.

3. How strong is the evidence for the existence of dark matter following from the value of ?
Contribution by Stefan

According to direct measurements has a value of about 0.4, but according to estimations of based on detectable mass this value should be about 0.1. Since both give values, which are, considering the assumptions that went into them, probably too high, they can't be made agree on the value of . Since the amount of dark matter is proportional to the difference between them, this shows, that about 65% to 85% of the mass in the universe is dark matter.

4. Could MOND be a viable alternative to dark matter?
Contribution by Peter

I found that although MOND is a viable option that nicely removes the need for dark matter there is no physical evidence whatsoever of the existence of MOND.And there is for Dark Matter.So I don't think it could replace the dark matter problem but it does show it doesn't have to be dark matter like most people believe.

Our conclusions lead us to believe that unless there is a MOND-like explanation that applies both to galaxies and clusters, to rotational curves and gravitational lensing, there are indeed significant amounts of dark matter in the universe (at least 60% of the total mass). The evidence for that is quite convincing, even though for galaxies there is still room for argument about the actual amounts involved.

• Dark matter, cosmology and large-scaled Structure of the Universe - Jonathan Dursi
• What is Dark Matter
• An article without a title
• Another one without a title
• Dark Matter - Allen E. Martin
• Dark Matter in the Universe: Missing Mass Continues to Baffle - Wesley Colley
• http://zebu.uoregon.edu/1996/ph123/fate.html