General relativity and dark matter

[cymric]

Recently, astronomers have reported the observation of what they think of as dark matter in two colliding galaxies. They used observations from two sources: the Chandra X-ray observatory for the distribution of visible matter, and a gravitational lense for the distribution of other---dark---matter. Lucky observations, brilliant deduction.

What I am curious about is how people incorporate dark matter into general relativity. The material is hypothesized to be detectable through it gravitational effect only, but in order to make meaningful calculations (such as the location and size of the dark matter blobs), you need to have some idea of the magnitude, right? In addition, the lensing effect is a consequence of curved spacetime, so how was the strength of that lense established? Am I correct in assuming that you don't need to know what actually causes the curvature in order to use its effect? (Otherwise you'd have to take into account the possibility that the lense itself is composed of dark matter which you don't know much about, etcetera.)

Please bear in mind that I have no background in general relativity beyond a very qualitative description of what it is and does---most engineers don't really need general relativity on a daily basis. If my questions seem odd or obvious, then you understand why .

 

[George Jones]

According to general relativity, matter (and energy), including dark matter, causes the curvature of spacetime, so the strength of the lense is a measure of how much matter is where.

Measurements indicate that, in this case, unseen matter, i.e., dark matter, is largely resposible for the curvature that causes the lensing effect.

 

[Dr_Strangelove]

Oh the logic of it all !

Here's a simple logic test given a Mr. P and his answers:

"If A is true then B will be observed."

B is observed.
1) What is A?
Choose one: ___True ___False ____Indeterminate?
Mr. P answers True.
He is wrong, the correct answer being Indeterminate. Although A cause B, it may not be the only cause.

"If A and C are true then B should be observed."
B is observed.
2) What is A:
Choose one: ___True ___False ____Indeterminate?
Mr. P's answer is True.
He is wrong again, the correct answer being indeterminate.

3) What is C?
Choose one: ___True ___False ____Indeterminate?
Mr. P's answer is True.
He is wrong again, the correct answer being Indeterminate.

"If D is true then B will be observed"
B is observed in a situation where the effects of D are minimized.
4) What is D?
Choose one: ___True ___False ____Indeterminate ____ A and C are both True.
Mr. P chooses the absurd answer "A and C are both True".

Of course Mr. P flunks the test with a score of Zero.
Nevertheless, the science news headlines all read "Mr. P proves C exist".


Now let's meet the contestants:

Mr. P is one of many well respected physicists.
A is General Relativity
B is gravitational lensing
C is dark matter
D is interstellar dust

And F is Mr. P's grade in simple logic.

Oh, by the way, if you're wondering why A wasn't mentioned in the headlines it is because it was developed by a Mr. E who nobody questions.