Once Before Time - Martin Bojowald [92]
Two hydrogen atoms can approach each other, initially not held back because of their electric neutrality. On the contrary, it is energetically favorable for two atoms to form a pair, a cohabitation allowing them to share the electrons more economically. In this way, a chemical bond is formed; the two atoms form a hydrogen molecule consisting of two nuclei and a shared orbital wave function of two electrons. The nuclei are kept at a safe distance, the molecular diameter, because they are both positively charged; this is the hydrogen molecule that one usually deals with in chemistry. On Earth, in the presence of atmospheric oxygen, even this molecule is difficult to keep stably in pure form, since hydrogen and oxygen can, again with energy gain, combine to form water in a heavy reaction. In the early universe, however, no oxygen had formed from the first protons and electrons of the big bang, and hydrogen is safe as the fuel of stars.
But how, then, do stars gain energy if there is no oxygen to burn their hydrogen? At first, after big bang nucleosynthesis and with further cooling of the expanding universe, wide hydrogen clouds formed. Like cosmic background radiation, which tells us about those times, and like all matter formed during nucleosynthesis, these clouds are initially nearly homogeneous. But since they are not perfectly homogeneous, more concentrated centers form where the mass density has a small surplus compared to the neighborhood. Even such a tiny mass surplus pulls in, by gravitational attraction, yet more hydrogen from the surroundings, further increasing the density. This is a slow process, but time is something the early universe has in abundance, so some regions can become very dense. As one can see, gravity shows not only those sadistic tendencies culminating in singularities, but also capitalistic ones: Regions already rich in mass become further enriched.
Hydrogen is compressed more and more so that, if gravity had its way, even the nuclei in a hydrogen molecule would approach each other. But they fight back, for they are equally charged and repel each other. An initial balance of forces arises, with electric repulsion counteracting gravitational attraction. This stability is the reason for the existence of planets such as Earth, realized in this case not for hydrogen but for the atoms in Earth’s core. Earth and other planets are stable to a high degree because they are not dense enough for gravity to overpower electrical forces between nuclei. There is, moreover, not much matter in their neighborhood, and so their masses cannot increase by further infall (disregarding small contributions such as meteorites). In the much less expanded early universe, this was entirely different: Around the density centers there was still plenty of hydrogen to reinforce the centers even more. Their density rose—so far that gravity pushed electric repulsion into a corner and made the hydrogen nuclei approach each other ever more closely.
That cannot go on for long. When two hydrogen atoms come too close, another reaction happens: Two nearby but separate protons require more energy than a single deuteron (a very close combination of a proton and a neutron; binding an electron, a deuteron forms an electrically neutral deuterium atom). These two particles can come much closer than two protons because the neutral neutron is not repelled electrically. Moreover, the deuteron combination of a proton and a neutron turns out to be stable, with the two particles attracted to each other and bound by the strong nuclear force, provided they