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Originally Posted by renormalised
Right so far, but the question was would the mass of the cluster (+ its dark matter) have to be effectively greater in an expanding space in order to compensate for the change in the light path taken through the lens. Being that the change in the light path might not necessarily bend the light enough so the observer can see it....as compared to the same object in a static space. In an expanding space, you not only have to take into account the space expanding between the objects and the observer, you also have to take into account the space expanding at right angles to the light path and the objects (remember, space is expanding in all directions not just away in one direction from the observer). The path the light takes is actually a curved path through spacetime, not a straight line, so you also have to take into that into account.
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Agreed.
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In order to bend the light in to focus towards the observer, the object in an expanding space needs to compensate for that curved path and there's only one way that can happen. It has to compensate not only for a lengthening of the light path due to expansion but also a widening of the path and that becomes increasingly difficult the further the lensed galaxy is away from the central optical axis of the lens.
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I think I get what you're saying - the angle between the expected light path, (predictably curved due to expansion), and the actual, observed deflected angle, allows us to calculate the mass of the lens, right ?
The image we receive from a 'lensing' observation is merely distorted. The different types of distortion tell us different things about the intervening object's mass density/distribution. I'm not sure I get this 'focusing' concept. Are you saying that the expansion of spacetime causes problems in detecting the deflection angle ? The type of distortion we see depends on the relative positions of the 'distorted galaxy' and the 'lens galaxy', also, but I'm not sure I get the 'focus' thing ?
Cheers
PS: Just trying to understand ...
