Eternity is a daunting concept, but modern cosmologists are not afraid to face it. Cosmology usually concentrates on the beginnings of the universe, but what might happen at the other end of time is just as intriguing—and by far the greater portion of the story. Adams and Laughlin, two leading astrophysicists (at the University of Michigan and the University of California, Berkeley, respectively) divide the life span of the universe into five acts, beginning with the Primordial Era, the time of the Big Bang and its immediate aftermath, when hydrogen and helium were first formed in an explosive birth. The Stelliferous Era is our present period, when stars fill the universe with visible light. The authors expect this to last another 100 trillion years. The universe doesn—t end with the fading of the visible stars, but enters a time dominated by lesser lights: brown dwarfs, white dwarfs, and other stellar remnants. This is the Degenerate Era, when the primary source of cosmic energy is proton decay, slow and feeble: a typical degenerate star might achieve the brightness of half a dozen ordinary light bulbs. An occasional stellar collision may light up the sky with a supernova. After all protons decay, the universe will enter its fourth act: the Black Hole Era. Black holes' enormous gravity protects them from losing mass and energy by ordinary processes, but they slowly dissipate through Hawking radiation and will become extinct after ten-to-the-hundredth-power years. This leaves only the most tenuous forms of matter and energy to fill out the Dark Era: electrons, neutrinos, and low-energy photons that interact only sporadically. The authors fill in this broad outline in fascinating detail, considering such questions as the long-term prospects for life and the possibility of recollapse to a singularity (a "Big Crunch") rather than a slow dying out of the fire. A thought-provoking treatment of the grandest of subjects, highly recommended to anyone interested in the world beyond tomorrow.
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