1.1.9. Models of a Dense Universe
In 1929, Hubble presented the observational evidence for an expanding universe. In 1934 Tolman made an analogy between the expanding universe and a thermodynamic state. Equating expansion with the thermodynamic concept of entropy led him to conclude that the expansion must cool any background which has a thermal spectrum. Later, in 1942, Chandrasekhar and Heinrich suggested that if the Universe in its early history achieved thermal equilibrium at conditions of temperature and density near 1010 K and 107 gm cm-3 then equilibrium abundances of lighter elements would have frozen out, in ratios that roughly agreed with observations. But this argument is only partially correct since the high matter density at the time would have required rapid expansion (to avoid early collapse of the Universe) and hence equilibrium calculations are not appropriate. Gamow (1942, 1946) used this as the basis for his set of arguments that physical processes in the early universe were dynamic in nature as they must have occurred in a rapidly expanding and cooling environment.
Gamow (1948) later reasoned that at sufficiently high temperatures, (kT in excess of the rest mass energy of a neutron), the energy density in the photon field must have greatly exceeded the energy density of the matter field. In such conditions, the radiation field can photo-dissociate any nuclei, meaning that the early universe had to consist of photons and free elementary particles. Without going into detail here, the ability for a neutron-proton capture to occur (thus creating deuterium and Helium) is strongly related to both the number density of neutrons and protons as well as the energy density in the photon field (which is fixed by the number density of the photons and the temperature of the Universe). The current cosmological abundance of deuterium (a difficult observation to make) and helium thus tells us much about these early physical conditions. Gamow used the estimated abundances of helium to predict that, at the current epoch, the photon field has been redshifted to millimeter wavelengths. Similar conclusions were also reached by Alpher and Hermann (1948).