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The photon sphere of a black hole (BH) is the spherical region of radius 3/2 times the Schwarzschild radius where photons may circle in an (unstable) orbit. Light passing near the photon sphere may be deflected so much as to make a full circle around the BH before escaping. As the light ray come closer, it may make two, or more, circles around the BH before ...


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The answer is no, because these effects have been looked for and not found. The microlensing surveys of the 80s and 90s specifically set out to look for the lensing signatures of compact, massive objects, including black holes. Whilst some lensing events were seen, the numbers were low enough that it could be concluded that there was no significant dark ...


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Firstly, there is a requirement for the dark matter to be non-baryonic. There are manifold reasons for this. The most important are the comparison between the universal energy density implied by measurements of the cosmic microwave background (CMB) compared with the baryonic matter density in the universe implied by measurements of the primordial abundances ...


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The estimates for galaxy masses rely heavily on the mass-luminosity relationship, which is however quite uncertain for low mass stars. From the data in http://adsabs.harvard.edu/full/2004ASPC..318..159H one finds that masses for low mass stars deviate from the theoretical mass-luminosity curve by a factor 2 or more. Considering that about 50% of the stellar ...


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