A DYING UNIVERSE - THE LONG-TERM FATE AND EVOLUTION OF ASTROPHYSICAL OBJECTS

This paper outlines astrophysical issues related to the long-term fate of the universe. The authors consider the evolution of planets, stars , stellar populations, galaxies, and the universe itself over time sca les that greatly exceed the current age of the universe. Their discuss ion starts with new stellar evolution calculations which follow the fu ture evolution of the low-mass (M-type) stars that dominate the stella r mass function. They derive scaling relations that describe how the r ange of stellar masses and lifetimes depends on forthcoming increases in metallicity. They then proceed to determine the ultimate mass distr ibution of stellar remnants, i.e., the neutron stars, white dwarfs, an d brown dwarfs remaining at the end of stellar evolution; this aggrega te of remnants defines the ''final stellar mass function.'' At times e xceeding similar to 1-10 trillion years, the supply of interstellar ga s will be exhausted, yet star formation will continue at a highly atte nuated level via collisions between brown dwarfs. This process tails o ff as the galaxy gradually depletes its stars by ejecting the majority and driving a minority toward eventual accretion onto massive black h oles. As the galaxy disperses, stellar remnants provide a mechanism fo r converting the halo dark matter into radiative energy. Posited weakl y interacting massive particles are accreted by white dwarfs, where th ey subsequently annihilate with each other. Thermalization of the deca y products keeps the old white dwarfs much warmer than they would othe rwise be. After accounting for the destruction of the galaxy, the auth ors consider the fate of the expelled degenerate objects (planets, whi te dwarfs, and neutron stars) within the explicit assumption that prot on decay is a viable process. The evolution and eventual sublimation o f these objects is dictated by the decay of their constituent nucleons , and this evolutionary scenario is developed in some detail. After wh ite dwarfs and neutron stars have disappeared, galactic black holes sl owly lose their mass as they emit Hawking radiation. This review finis hes with an evaluation of cosmological issues that arise in connection with the long-term evolution of the universe. Special attention is de voted to the relation between future density fluctuations and the pros pects for continued large-scale expansion. The authors compute the evo lution of the background radiation fields of the universe. After sever al trillion years, the current cosmic microwave background will have r edshifted into insignificance; the dominant contribution to the radiat ion background will arise from other sources, including stars, dark-ma tter annihilation, proton decay, and black holes. Finally, the authors consider the dramatic possible effects of a nonzero vacuum energy den sity.