Once Before Time - Martin Bojowald [86]
One of the big puzzles of cosmology and particle physics is why protons and neutrons were able to live until the present day in large numbers, instead of having been completely annihilated in encounters with antimatter.8 One can be fairly certain that all visible matter is indeed matter and not antimatter: Even between galaxies there is hydrogen gas that, when annihilated at an antigalaxy, would emit clearly visible radiation identifiable by its characteristic energy. To solve the puzzle, one cannot simply assume that matter and antimatter are separated from each other, their Armageddon prevented by the peacekeeping force of endless space. A disparity in favor of matter must either have existed from the very beginning, or have come about in the course of time.
But this view, too, brings only troubles. An initial disparity could not be explained but at most be postulated. On the other hand, it is not easy to envision a disparity building up under the known physical laws. (Possible scenarios were first discussed in 1967 by Andrei Sakharov.) There are symmetries between particles and their antiparticles, strongly correlating their production and interaction rates. While these symmetries are not perfectly achieved, the degree of violation, known from accelerator experiments, is very small. No convincing mechanism has been found yet which would naturally lead to a sufficiently large disparity in favor of matter.
Quantum gravity cannot offer much either, and so we have to accept this fact for now. The universe, then, contained a sea of elementary particles that by and large left one another alone and, except for electrons, remained undisturbed by antimatter: There were electrons and positrons, protons, neutrons, and the ubiquitous neutrinos as well as highly energetic photons. With its expansion, the universe had became so cool by the end of the big bang phase that most of the freely moving particles had lost much of their energy and were at last allowed to settle down by forming stable nuclei. This transition is called big bang nucleosynthesis.
Compared to the multitude of chemical elements that can now be found on earth, big bang nucleosynthesis produced only a humble handful—merely a heavy isotope of hydrogen called deuterium (with not only a proton but also a neutron in its nucleus), helium, and a few other light elements such as lithium; but the exact ratios of these elements as they emerged back then are important for the course the universe and its contents subsequently took. The largest fraction came from hydrogen—75 percent—followed by helium with almost all of the remaining 25 percent. Deuterium contributed only about one nucleus out of a hundred thousand others, and lithium just a few nuclei out of ten billion others. The nuclei of all the heavier elements together were created with a mass fraction of just one percent. Nucleosynthesis in particular of the heavy elements continued later on in stars, where it produced the whole of planetary matter; this thread will be taken up again in the section on the first stars in our next chapter. But for the correct element ratios as they can be seen now, the distribution provided by big bang nucleosynthesis was already decisive.
The processes dominating big bang nucleosynthesis are so sensitive that the smallest variations in the ratios of elementary particles and in electromagnetic radiation can lead to strong deviations from observations. This is another opportunity for theories of gravity to prove themselves, because the quantities of particles are determined by the dilution behavior of the expanding universe, and vice versa: The total quantity of different particles influences the expansion rate as a result of their gravitational attraction. From the ratios of elements shortly after the big bang one can