The Five Ages of the Universe

The Primordial Era is defined as "−50 < n < 5". In this era, the Big Bang, the subsequent inflation, and Big Bang nucleosynthesis are thought to have taken place. Toward the end of this age, the recombination of electrons with nuclei made the universe transparent for the first time. The authors discuss the horizon and flatness problems.

The Stelliferous Era, is defined as, "6 < n < 14". This is the current era, in which matter is arranged in the form of stars, galaxies, and galaxy clusters, and most energy is produced in stars. Stars will be the most dominant objects of the universe in this era. Massive stars use up their fuel very rapidly, in as little as a few million years. Eventually, the only luminous stars remaining will be white dwarf stars. By the end of this era, bright stars as we know them will be gone, their nuclear fuel exhausted, and only white dwarfs, brown dwarfs, neutron stars and black holes will remain. In this section, Olbers' paradox is discussed.

The Degenerate Era is defined as "15 < n < 39". This is the era of brown dwarfs, white dwarfs, neutron stars and black holes. White dwarfs will assimilate dark matter and continue with a nominal energy output. As this era continues, the authors hypothesize that protons will begin to decay (violating the conservation of baryon number given by the Standard Model). If proton decay takes place, the sole survivors will be black holes. If so, life becomes nearly impossible as planets decay.

The Black Hole Era is defined as "40 < n < 100". In this era, according to the book, organized matter will remain only in the form of black holes. Black holes themselves slowly "evaporate" away the matter contained in them, by the quantum mechanical process of Hawking radiation. By the end of this era, only extremely low-energy photons, electrons, positrons, and neutrinos will remain.

The Dark Era is defined as "n > 101". By this era, with only very diffuse matter remaining, activity in the universe will have tailed off dramatically, with very low energy levels and very large time scales. Electrons and positrons drifting through space will encounter one another and occasionally form positronium atoms. These structures are unstable, however, their constituent particles must eventually annihilate. Other low-level annihilation events will also take place, albeit very slowly. Essentially, the universe will eventually turn into a void.