Non-baryonic cold dark matter ("CDM"), the observational evidence

[Skwinty]

I apologise for this shameless bump, but I want to hear more on this topic.

 

[sol invictus]

I apologise for this shameless bump, but I want to hear more on this topic.

In previous threads I've tried to lay out my (philosophical, if you want) view of this sort of issue.

Scientific theories need to pass several tests to be worth considering.

• They must be specific and quantitative, so that they are capable of making precise predictions.
  
• They must be internally self-consistent.
  
• They must be consistent with all existing experimental data.

Given two such theories, how can we decide between them? I think the best answer is to use a combination of goodness of fit and number of parameters. Usually theories contain some continuous parameters (the density and temperature of dark matter, for example). The more parameters you add, the easier it is to fit the data (add another type of dark matter with a different temperature and different density and find the best fit to data for those four parameters - it will always be at least as good, and usually better, than the two parameter model).

Theories with lots of parameters are less predictive, less elegant, and should be strongly disfavored. So generally one wants to strongly disfavor theories with lots of parameters, and mildly favor theories that fit the data well. There are mathematical formulas for this, although they're a bit ad hoc.

On the topic of this thread - PC passes none of the three tests above and therefore cannot be compared to a scientific theory like CDM. Lambda-CDM on the other hand is extremely successful by this kind of measure, much more so than any alternative I'm aware of.