Random Thoughts on the Universe

Wednesday, September 09, 2009

The Dark Matter Conspiracy?

In the last few years, there has been an increase in the number of articles, books, and blogs which claim there is a conspiracy to favor the particle dark matter model over alternatives, such as modified gravity. While it is useless to argue about conspiracies, it is worth mentioning some of the reasons why dark matter is more popular currently and the evidence for it. Of course that doesn't mean that it will be what nature has created and used, but any alternative will have to first overcome a lot of data.

  1. Galaxy dynamics: This was where dark matter was first discussed, and the most common target for alternative theories. When the mass of galaxies is measured from the number of stars and such, and the speed of the stars moving in the galaxy, it is found that a large amount of matter is missing. Other experiments can rule out planets, dust, or any other form of matter we are aware of. It is weak evidence of dark matter though, because better models in general relativity can reduce the mass needed, while modified theories of gravity can remove the dark matter entirely.
  2. CMB/BBN: These results were only really important in the last decade or so. Various experiments have measured microwave radiation that was created just after the Big Bang, and based on the fluxuations in temperature they were able to 'weigh the Universe'. We know how much matter is in any part of the Universe on average. The same data proves that most of this matter cannot be atomic in nature - which rules out all forms of stable massive matter that we know about. Other experiments measured the abundance of light elements created in the Big Bang, and they also prove that there simply isn't enough of the kind of matter we understand to form the Universe we observe. These results are very difficult to generate with anything but dark matter.
  3. Gravitational Lensing: Only a few years ago came the gravitational lensing data from observations of the Bullet Cluster. According the theory of relativity, and massive object bends light, and as such very massive objects will create a lens in space. When astronomers studied the Bullet Cluster in deep space, they observed most of the mass was in dark regions away from the cluster itself. Since the cluster was formed by a collision of two older massive clusters, this is the result that would be expected in dark matter models since the dark matter halos pass through each other while the ordinary matter sticks together. This is very strong evidence for dark matter, although there are one or two modified theories of gravity that can be tuned to produce the same effect.
  4. Simplicity & History: This is perhaps the strongest argument for dark matter. Dark matter can be modeled as a single particle and still explain all of the data. There is nothing simpler than that. The best modified gravity models still have to add several new fields or particles or create complicated mechanisms - and even then they need to be specifically designed to explain a single data set. It is possible that someone will invent one that is as simple as dark matter is, but as yet such models do not exist. The second part is the trend of modern physics experiments - in the last 95 years since general relativity was developed there have been no observations of modifications to gravity. As far as the experiments can tell GR is still the only theory of gravity. However in that same time frame hundreds of new particles have been discovered, studied, classified, explained, and broken up again. The entire Standard Model of particle physics was developed in the last forty years, and it still isn't complete, while gravity has so far never been observed to deviate from GR. Of course that isn't a valid reason for rejecting modified gravity, but it does add weight to the arguments for dark matter.
This article is not meant as any form of criticism of alternate models, because they are interesting theories to study and discuss. It is simply a review of how much data and theory must be overcome before the dark matter models can be ruled out, and how difficult it will be for a modified gravity model to usurp the status of dark matter.

But as with all science, it is important to always question the prevailing theories, and challenge conventional wisdom...

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