cosmology was the desire to achieve critical density in a Friedmann universe. In a roundabout way, this addresses the need to have a more realistic big bang model by considering all the matter in the universe. However, we don’t live in a Friedmann universe (dark energy is not compatible with a Friedmann universe). Frankly, I never understood this bias among cosmologists. Even before the discovery of dark energy, the assumption of a Friedmann universe seemed like an unnecessary imposition. And even if we live in a Friedmann universe, why must its density be critical? The density of the universe ought to be a measured quantity, not an assumed boundary condition. The third line of evidence for dark matter is gravitational lensing. If a very massive object is nearly in the line of sight of a much more distant object, the strong gravity of the nearer object can distort spacetime so that the light of the more distant object is bent, resulting in an altered view of the more distant object. Since this bending is similar to the refraction of a lens, this phenomenon is called gravitational lensing. While there were several early publications suggesting the possibility of gravitational lensing, the phenomenon is most associated with Albert Einstein, who published a paper about it in 1936. These early treatments were primarily theoretical. The first practical discussion of gravitational lensing was the following year when Zwicky (1937a, 1937b), proposed that clusters of galaxies could act as gravitational lenses of more distant galaxies. Depending upon the geometry, gravitational lensing can take several forms. One form of gravitational lensing is two or more images of the same object. The first discovered gravitational lens was of this type (Walsh, Carswell, and Weymann 1979). The twin quasar SBS 0957+561 consists of two quasars separated by just six arcseconds and having the same redshift (z = 1.41) and nearly the same apparent magnitude. On images of SBS 0957+561, the giant elliptical galaxy Q0957+561 G1 with redshift z = 0.355 is asymmetrically located between the twins. Since Q0957+561 G1 has a much smaller redshift, it is presumed to be in the foreground of SBS 0957+561. The proximity of two quasars with similar spectra, identical redshifts, and nearly the same apparent magnitude suggested that they were two images of the same quasar. Confirmation came when identical variations in brightness of the two quasars separated in time by 417 days were discovered. This is interpreted as a delay due to different travel distances of light on two different paths caused by the galaxy Q0957+561 G1 not lying exactly along the line of site to the midway between quasar SBS 0957+561. The more common situation is gravitational lensing of a distant galaxy or galaxies by a nearer cluster of galaxies. One of the best examples of this is CL 0024+17 (aka ZwCl 0024+1652) (Anonymous, no date) (see Fig. 2). Most of the cluster members in this HST image of CL 0024+17 appear yellow. However, near the center of the cluster there are a series of blue concentric arcs that are gravitationally lensed images of more distant galaxies. Modeling the observed lensing allows determining the amount of mass required to produce the lensing, as well as the distribution of the mass (see Fig. 3). In every case of gravitational lensing caused by clusters of galaxies, the total inferred mass exceeds the lighted mass by a factor of 5-10. III. CREATIONISTS’ RESPONSES TO DARK MATTER The concordance from the three lines of evidence for dark matter on the amount of dark matter required to explain the observations is striking. Under most circumstances, such concordance constitutes a strong case, but astronomers were very reluctant to reach this concluFigure 2. A Hubble Space Telescope (HST) image of CL 0024+17. Photo credit: NASA/ESA/HST. Figure 3. The gravity map of CL 0024+17 determined from the amount of gravitational lensing superimposed on the image of Figure 2. Image credit: NASA/ESA/HST. FAULKNER Dark matter and dark energy 2023 ICC 4
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