Relativity Lite

Cosmology | 89 WHICH UNIVERSE DO WE LIVE IN? The debate for 75 years had been whether the universe would slow its expansion but con- tinue to expand forever or eventually stop and begin contracting into a Big Crunch. In 1998, two teams observing the brightness of distant Type Ia supernovae (stellar explosions of known intrinsic luminosity) independently determined that the expansion of the uni- verse is actually accelerating. * (The team leaders—Adam Riess, Brian Schmidt, and Saul Perlmutter—were awarded the 2011 Nobel Prize in Physics for their discovery.) See the right-hand side of figure 7. What is causing this acceleration? This substance, which has to make up for 71.4% of the energy of the universe to account for current WMAP observations of the light left over from the Big Bang, † has been dubbed Dark Energy, but no one really knows what it is. Another 24% of the energy of the universe is dark matter, whose composition is likewise unknown and interacts with conventional matter (4.6% of the energy of the universe) only gravitationally. Dark matter was first suggested by Fritz Zwicky as a way to explain the observed mo- tions of galaxies in the Coma cluster of galaxies: “[T]he average density in the Coma system would have to be at least 400 times greater than that derived on the basis of observations of luminous matter.” ‡ Vera C. Rubin and W. Kent Ford’s measurement of the flat rota- tional speed curves of regions in the Andromeda Galaxy in 1970, and subsequently many other galaxies, provided sufficient evidence that dark matter extended far beyond the op- tically bright edge of galaxies to make the assumption of its existence the norm within astronomy. § Figure 11 is a composite image: the Hubble space telescope was used to map the dark matter (colored in blue) using a technique known as gravitational lensing . Chandra X-ray Observatory’s data enabled the astronomers to accurately map the ordinary matter, mostly in the form of hot gas, which glows brightly in X-rays (shown in pink). It is significant that the blue lobes on either side, consisting of dark matter, have passed through the pink collision region with little to no interaction with the conventional matter. Indeed, there was little to no interaction with the oncoming dark matter from the other galaxy cluster other than the gravitational interaction that will eventually pull both ordinary matter and dark matter components into a rough ball in the central region. * Adam G. Riess et al., Astron. J. 116 , 1009 (1998), http://​iopscience​.iop​.org/​1538​-3881/​116/​3/​1009/​fulltext/; Astrophys. J. 517 , 565 (1999), http://​iopscience​.iop​.org/​0004​-637X/​517/​2/​565/​fulltext/ . † NASA/WMAP Science Team, “Universe Content—WMAP 9yr Pie Chart,” National Aeronautics and Space Adminis- tration, last modified April 8, 2014, https://​map​.gsfc​.nasa​.gov/​media/​121236/​index​.html. ‡ Fritz Zwicky, Helvetica Physica Acta 6 , 110 (1933), with an English translation by Heinz Andernach at https://​arxiv​.org/ ​ abs/​1711​.01693. See section 5. § Vera C. Rubin and W. Kent Ford Jr., Astrophys. J. 159 , 379 (1970).