Читать книгу Origin and Evolution of the Universe - Группа авторов - Страница 13

Although the energy density of the microwave background dominated the Universe for the first 50,000 years after the Big Bang, it is even more significant during the first 3 minutes. One second after the Big Bang, the average energy of a photon was 3 million electron volts (MeV), which is a gamma ray. Gamma rays destroy any atomic nucleus; thus, 1 second after the Big Bang there were only protons (p or hydrogen nuclei), free neutrons (n), electrons (e), and positrons (e+), neutrinos (v_e− ν_μ and ν_τ), and their antiparticle counterparts, the antineutrinos. The three types of neutrinos correspond to the three “families” of elementary particles, but, except for the neutrinos, all the particles in the second and third families (such as muons and tauons), are so heavy and unstable that they decay during the first second after the Big Bang. Weak nuclear interactions such as

determined the ratio of neutrons to protons. The neutron is heavier than the proton; the neutron-to-proton ratio declines as the temperature falls. But eventually, at about 1 second after the Big Bang, the density of electrons and neutrinos falls to such a low level that reaction (9) is no longer effective. After this time, the neutron-to-proton ratio gradually falls because of the radioactive decay of the neutron,

which has a half-life of 615 seconds.

As neutrons decay, the Universe expands and grows colder. Eventually the temperature falls to the point where the simplest nucleus, the heavy hydrogen or deuterium nucleus (d, the nucleus having both one proton and one neutron), is stable. This occurs when the temperature is about 10⁹K, which occurs about 100 seconds after the Big Bang. At this point, the reaction

very quickly converts all neutrons into deuterium nuclei. Once deuterium is formed, it is quickly converted into helium through a network of interactions, with the net effect

Because almost all neutrons that survive until T is less than 10⁹K end up bound in helium nuclei, the helium abundance in the Universe provides a measurement of the time it takes for the Universe to cool to 10⁹K. If the Universe cools rapidly, there is a large helium abundance, but slow cooling gives low helium abundance because more of the neutrons decay. The standard Big Bang model, with three types of neutrinos, predicts a helium abundance that is correct to within the 1% margin of uncertainty of current observations. Reaction (12) requires collisions between two nuclei, and if the density of atomic nuclei is low, then a fraction of the deuterium will not react. Thus, the residual fraction of deuterium in the Universe is a sensitive measure of the density of atomic nuclei. Based on the abundance of deuterium and other light isotopes like ³He, the best estimate for the current density of nuclei of all sorts is equivalent to 1/4 hydrogen atoms per cubic meter (Copi, Schramm and Turner, 1995, Schramm, 1995). This is about 25 times less than the critical density. Because the density of the Universe must be close to the critical density to produce the observed clustering of galaxies, we find from the light element abundances that most of the mass of the Universe must be the mysterious dark matter.

In the 1940s, George Gamow and colleagues proposed that all the chemical elements were produced in the Big Bang. This proposal, described further in Virginia Trimble's Chapter 3, led to a prediction of a 5-K microwave background (Alpher and Herman, 1948), but this prediction was not followed up. The eventual discovery of the microwave background in 1964 was accidental. Why was this prediction ignored? The absence of stable nuclei with atomic weights of 5 and 8 means that the Big Bang produces only hydrogen and helium isotopes and a very small amount of lithium. When a model is supposed to produce all the elements from Z = 1 to 92, but actually only works for Z = 1, 2, and 3, its other predictions tend to be ignored. But in this case the predictions were right.