Strong lensing and the Hubble constant
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USING GRAVITATIONAL LENSING TO MEASURE DARK MATTER When it comes to more distant examples ± say, two clusters of galaxies lining up the situation is different. In fact, as we described in Chapter 3, both Einstein and Eddington considered that the odds were so remote that while it was a nice idea, it had virtually no practical value (though in 1936 Einstein did publish the idea of gravitational lensing). Zwicky, on the other hand, reasoned that gravitational lensing would be visible if a galaxy were along the same line of sight as a more distant object such as a quasar. Because quasars are so bright and are generally seen at much greater distances than ordinary galaxies, they constitute bright background light-sources. As mentioned in Chapter 3, Zwicky extended this idea by proposing that not only was galactic gravitational lensing possible, but that the gravitational lensing of a distant quasar by a much closer galaxy or galaxy cluster would be an ideal way of studying the amount and distribution of dark matter in the foreground cluster. As it turned out, Zwicky was right, though it was fully sixty years after Eddington's famous eclipse experiment that the first gravitational lens was detected. Moreover, gravitational lensing has turned out to be an ideal way of studying the masses of clusters of galaxies. Since galaxy clusters are among the most massive aggregates of matter seen in the Universe that are collected into relatively small regions of space ± that is, they are small in comparison with the distances between us and background light-sources ± they exactly meet the criteria for gravitational lensing mentioned earlier. As for background lightsources, the Universe is very rich in galaxies ± particularly distant ones. If you can imagine a cone enveloping the cluster, with ourselves at the apex, beyond the cluster the cone envelopes a huge volume of space, increasing the chances of encountering a galaxy, or galaxies, which can be lensed. As we look deeper and deeper into space we see ever-increasing numbers of galaxies, providing a ready supply of background light-sources.
STRONG LENSING AND THE HUBBLE CONSTANT When looking in the direction of rich galaxy clusters, astronomers often see what is referred to as `strong lensing', which produces multiple images of a background object. One or more of those images is likely to be magnified and so will appear brighter than normal, though some of the multiple images may be fainter. Strong lensing offers a potential solution to an important problem in cosmology: the determination of the much-disputed Hubble constant (described in Chapter 2). Pinning down the Hubble constant has been a major challenge for astronomers for the last hundred years. Remember that Hubble showed that the further the distance of a galaxy, the faster its recession. This is because every galaxy is receding from every other galaxy. If we see two galaxies, one beyond the other, we will see the nearer galaxy moving away from us. The more distant galaxy will appear to be receding twice as fast as the nearer one because it is