In my former career as a geologist, I was used to contemplating vast stretches of time. The basic unit we used was a million years; the fossil communities I was studying lived about 500 mybp (million years before present). If my students boggled, as they sometimes did, at the thought of the ice ages taking place hundreds of thousands of years ago, I would smugly say, "Oh, that's nothing - just yesterday!" Little did I know, that for truly overwhelming timescapes, you need to turn to astronomy. In their new book "The Five Ages of the Universe" (1999, Simon & Schuster), Fred Adams and Greg Laughlin consider the longest timescales imaginable - not the past only, but the far future of the universe. Many years ago, physicist Philip Morrison narrated what I think is the finest short science video ever made - "Powers of Ten". This starts at a familiar human scale, and zooms out by a factor of ten every ten seconds until reaching the size of clusters of galaxies; then reverses the process and zooms in to a proton in a carbon atom; in effect creating a logarithmic scale model of the universe. Adams and Laughlin apply the same logarithmic concept to time instead of distance. They speak of "cosmological decade n" when the universe is 10^n years old. For example, we are now living in the tenth cosmological decade, since about 10^10 years have passed since the Big Bang.
The five eras of the universe, then, are:
The Primordial Era (-15 < n < 5) From the Big Bang to 10,000 years later - inflation, the excess of matter over antimatter, primordial nucleosysthesis, the horizon and flatness problems, and the recombination of electrons with nuclei making the universe transparent - the cosmic background radiation.
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The Stelliferous Era (6 < n < 14) We are living in the middle of this era, with matter in the universe organized into galaxy clusters, galaxies and stars. Most energy is released from fusion within stars, which provides the opportunity for earth-like life to form. Towards the end of this era, all the gas in galaxies will have been used up, and the heavy and middle-weight stars will have burned out in one way or another. Only miserly red dwarf stars will remain.
The Degenerate Era (15 < n < 39) The universe is cold and dark, with no light from ordinary stars left. Matter is locked up in brown dwarfs, white dwarfs, neutron stars, and black holes. These objects continue to interact gravitationally, with some being ejected
from the dead galaxies, and others colliding. Very occasionally, two brown dwarfs might collide to create a new low-mass star. Dark matter is swept up into white dwarfs, providing a continuing energy source. At the end of the Degenerate Era, the protons and neutrons themselves decay, and the white dwarfs and brown dwarfs made form them slowly dissipate into radiation.
The Black Hole Era (40 < n < 100) Unaffected by proton decay, only black holes are now left. Even they, however, are not eternal; they evaporate through an even vastly longer quantum mechanical process known as Hawking radiation. A stellar black hole might take 10^65 years to evaporate; a galaxy sized black hole perhaps 10^100. When the largest are gone, the black hole era is over.
The Dark Era (n > 101) No condensed matter is left. The universe consists of extremely long wavelength photons, electrons, positrons, and neutrinos. Nothing much happens.
The book consists mainly of a detailed but non-technical look at the various processes which mediate these transitions. It also considers the possibilities for different kinds of life to form long after life based on liquid water is obsolete. As the temperature of an environment becomes lower, the processes of life and the "rate of experience" (my phrase) of an organism slows down, but the time available to evolution stretches out as well.
All this assumes that the universe is open or at least flat, which seems quite likely at present. It also assumes a lot about modern physics which is still highly conjectural. This book does remind us once again that our universe is surpassingly strange, and that its strangeness is distributed through time as well as through space.
