Once Before Time - Martin Bojowald [155]
5. OBSERVATIONAL COSMOLOGY
1. The observable universe at that time had just about one-billionth of its current volume.
2. Penzias and Wilson had not developed their microwave antenna with the aim of cosmological measurements in mind; the discovery was connected to cosmology only in retrospect. Other researchers had independently planned similar experiments but were scooped by Penzias and Wilson. Among these researchers was David Wilkinson, a name to be met again in the context of the satellite WMAP, which currently provides the most precise data about cosmic background radiation.
3. At any given time, the combination of all observations can show us only a finite part of the universe. What if the part we have seen so far is just a tidy special region in a much more irregular universe? This possibility can never be ruled out; cosmological predictions of the extremely long-term kind can escape this conundrum only with extra assumptions or the postulation of general principles.
4. For some time, Reines and Cowan had even played with the thought of constructing the detector near an exploding nuclear bomb.
5. One could just as well call the present acceleration “inflation” instead of dark energy. But since these are two apparently distinct phenomena, they should be separated conceptually. And because accelerated expansion in the early universe was historically investigated first, it had already claimed the name “inflation.”
6. For many researchers in cosmology, inflation obtained from quantum gravity shows strong appeal. In loop quantum cosmology, I published the first humble (and, in hindsight, probably wrong) possibility in 2003, meant as an initial indication not to be taken too seriously. Much later, I learned that this article was the main stimulus for Parampreet Singh, who now is among the active researchers in loop quantum cosmology, to enter the field.
7. Why unequal amounts of matter and antimatter currently exist is an open problem. These two ingredients must either have been separated from each other, or have emerged in different amounts owing to some asymmetry in their production. Neither of these possibilities can currently be supported completely by known physical processes, as we will briefly discuss later in this chapter.
8. What we call matter and what antimatter is, of course, only convention. Matter is what we normally see, and it has a dual relationship with antimatter produced at high energies. Since we do not know the cause for the dominance of matter, there is no feature distinguishing it physically; it is distinguished only by the apparently random fact that it dominates our cosmos.
9. Employing this mechanism for tests of quantum gravity was first proposed by Giovanni Amelino-Camelia, John Ellis, Nikolaos Mavromatos, Dimitri Nanopoulos, and Subir Sarkar in 1997. Some of them developed specific models and estimates for time delays in certain nonstandard (so-called noncritical) versions of string theory; the standard, critical string theory is not expected to give rise to detectable time delays because it retains the classical structure of space-time to a high degree. In loop quantum gravity, such models were provided by Rodolfo Gambini and Jorge Pullin in 1998 for photons, and by Jorge Alfaro, Hugo Morales-Técotl, and Luis Urrutia in 1999 for neutrinos. With recent detections of high-energy gamma particles from very distant bursts, observations are now approaching sensitivities