Dark Matter

[Stochasty]

Of course, rotation curves are independent of lensing results such as the bullet cluster; however, it's interesting to note that we also use a Newtonian approximation to compute mass estimates from lensing results, so non-linear effects might be just as important here as they are in rotation curves.

I just saw a very interesting paper posted to the arXiv which discusses use of numerical integration to compute weak lensing for galaxy clusters versus the typically (Newtonian) thin lens approximation; the paper is posted here. They find potentially significant deviations from the thin lens approximation for extended objects (such as galaxy clusters), with deviations growing larger the more diffuse the object is.

To wit: "We conclude that in translating the weak gravity to different astrophysical phenomenon one has to take care of global relativistic features. We know from general relativity that, in contrast to the Newtonian dynamics, it is not possible to have an overdensity structure within an otherwise underdensity background except by the mediation of a void which compensates the overdensity [10]. Now, the familiar technology of thin lensing using a NFW density profile dismiss this void which could easily be included in a modified density profile. Adding a void and using a modified NFW density profile correct this deficiency and one obtain a result almost identical to the exact general relativistic model. The deviation of the normal NFW density profile from the exact relativistic model depends on the compactness of the structure. The higher the compactness the less is the difference to the exact model. We should therefore see for such effects in the astrophysical observations of galaxy clusters of different compactness. This effect may influence our knowledge of the cluster masses and also the so-called concentration mass relation of the galaxy clusters [31]."