to dark matter offered by recent creationists. Tenev and Horstemeyer (2019) suggested that the evidence for dark matter could be explained by the inherent structure of space at galactic scales, with the curvature of space amplifying the gravity of ordinary matter in galaxies. While I may not agree with this proposal, I encourage this sort of original thinking. IV. DARK ENERGY In a finite universe governed by Newtonian mechanics, gravity will eventually collapse all matter to its center. For centuries, most scientists had thought the universe is eternal. But if the universe is eternal, then collapse should have happened long ago. The fact that this hasn’t happened ought to have convinced astronomers that the universe is not eternal. But rather than make this straightforward conclusion, astronomers instead postulated that the universe also is infinite. In an infinite universe, there is an equal amount of matter in all directions whose gravity pulling on matter in all locations so that there is no net motion of matter to bring about a collapse. Therefore, the universe was envisioned as being eternal, infinite, and with no net motion. This static universe prevailed for more than two centuries. Albert Einstein published his theory of general relativity (GR) in 1915. A year later, Einstein followed up this work by applying GR to the universe. However, one of the differences between Newtonian gravity and GR is that in a universe governed by GR, the universe will collapse under its own gravity, even if the universe is infinite. To preserve a static universe, Einstein included in his cosmology the cosmological constant, usually indicated by the Greek letter λ. The cosmological constant acts as a repulsion term that space has for itself. If the value of λ has the right value, then its outward force of repulsion balances the inward force of gravity, producing a universe with no net motion. In 1922, Alexander Friedmann showed that Einstein had failed to realize the general solution of GR applied to the universe. In the general case, the universe is either expanding or contracting. By insisting on a static universe, Einstein had settled on the intermediate case between the two extremes of the general case. Friedmann favored the expanding case, though it is not clear why. It may be that he thought expansion made more sense than contraction in a universe that has always been governed by naturalism. Or it may be that Friedmann was aware of Vesto Slipher’s work commencing in 1912 showing that what eventually were recognized as galaxies had large redshifts, consistent with an expanding universe. The little-known Carl Wilhelm Wirtz (1918) certainly understood the implication of Slipher’s work. Perhaps Friedmann had read Wirtz’s paper. At any rate, credit for discovery of the expansion of the universe generally goes to Edwin Hubble in 1929, though Georges Lemaître had published a similar thing two years earlier. The difference between the two was that Lemaître’s work was theoretical, based upon Friedmann’s cosmology, while Hubble’s approach was observational. Though Einstein originally opposed an expanding universe, he soon abandoned the static universe, calling his introduction of λ his greatest blunder. However, this assessment is a bit harsh. Einstein had thought the universe was static, so he enforced this boundary condition on the universe by including λ. Friedmann and Lemaître chose boundary values that permitted an expanding universe, with the preferred value of λ being zero. This cosmology was dominant for about 70 years, but in the late 1990s astronomers discovered the need to reintroduce something akin to the cosmological constant. What changed? Consider the effect of gravity on the expansion of the universe. The gravity of the matter of the universe tends to slow expansion, which is why Einstein introduced λ to counter this effect to preserve a static universe. If the universe is not static, then expansion will slow, with the amount of slowing in expansion related to the density of the universe. Density is an important parameter of the universe, so a good measure of density is important. Assuming a constant speed of light, distance amounts to a lookback time. If the expansion of the universe does not change, then galaxies will demonstrate a strictly linear relationship between their recession and distance. However, if expansion has slowed, then very distant galaxies will have greater recession than expected from a linear relationship observed in the local universe, which, assuming lookback time, has been subjected to slowing. Thus, slowing expansion would show up as an upturn in the Hubble relation at great distance. This expected behavior of the Hubble relation has been known for a long time, but limits of accurately measuring the distances of faraway galaxies made testing this impossible. Classical techniques, such as Cepheid variables, were limited to distances of tens of millions of light years, but this effect is not likely to show up except on the scale of several billion light years (for a survey of astronomical distance determination methods in the creation literature, see Faulkner [2013]). By the 1970s, astronomers realized that type Ia supernovae provided an opportunity to extend distance determination methods to billions of light years. This is because type Ia supernovae are very bright (far brighter than other standard candles) and are homogeneous in their maximum brightness. Therefore, if one observes a type Ia supernova at maximum brightness, then one knows its intrinsic brightness, and comparison to its observed brightness readily yields its distance. Type Ia supernovae are relatively easy to distinguish from other supernovae. The problem is that supernovae of all types are relatively rare, happening perhaps a few times per century in any given galaxy. Consequently, it may be centuries before a type Ia supernova may be seen in a galaxy that astronomers regularly monitor. By the 1990s, moderately large robotic telescopes were taking images of hundreds, if not thousands, of galaxies every clear night and comparing the images to reference images to search for supernovae. Subtracting a reference photo of a galaxy from a newly obtained photo in which there is no supernova yields a blank image. But if a supernova has occurred in a galaxy, then subtraction of the reference photo from a new photo will result in an obvious bright spot that computers can readily detect. Once a supernova was detected in a galaxy, alerts were sent to major observatories where astronomers could quickly turn very large telescopes to further study the erupting supernova. This process ended up producing many new supernova discoveries each year that would have been missed in the past. FAULKNER Dark matter and dark energy 2023 ICC 7
RkJQdWJsaXNoZXIy MTM4ODY=