Introduction physical eschatology
As we take a longer-term view of our future, a host of astrophysical processes are waiting to unfold as the Earth, the Sun, the Galaxy, and the Universe grow increasingly older. The basic astronomical parameters that describe our universe have now been measured with compelling precision. Recent observations of the cosmic microwave background radiation show that the spatial geometry of our universe is flat (Spergel et al, 2003). Independent measurements of the red-shift versus distance relation using Type la supernovae indicate that the universe is accelerating and apparently contains a substantial component of dark vacuum energy (Garnavich et al., 1998; Perlmutter et al., 1999; Riess et al., 1998).l This newly consolidated cosmological model represents an important milestone in our understanding of the cosmos. With the cosmological parameters relatively well known, the future evolution of our universe can now be predicted with some degree of confidence (Adams and Laughlin, 1997). Our best astronomical data imply that our universe will expand forever or at least live long enough for a diverse collection of astronomical events to play themselves out.
Other chapters in this book have discussed some sources of cosmic intervention that can affect life on our planet, including asteroid and comet impacts (Chapter 11, this volume) and nearby supernova explosions with their accompanying gamma-rays (Chapter 12, this volume). In the longer-term future, the chances of these types of catastrophic events will increase. In addition, taking an even longer-term view, we find that even more fantastic events could happen in our cosmological future. This chapter outlines some °f the astrophysical events that can affect life, on our planet and perhaps Dark energy' is a common term unifying different models for the ubiquitous form of energy permeating the entire universe (about 70% of the total energy budget of the physical universe) and causing accelerated expansion of space time. The most famous of these models is Einstein's smologicoi constant, but there are others, going under the names of quintessence, phantom energy, and so on. They are all characterized by negative pressure, in sharp contrast to all other forms of energy we see around us. elsewhere, over extremely long time scales, including those that vastly exceed the current age of the universe.
These projections are based on our current understanding of astronomy and the laws of physics, which offer a firm and developing framework for understanding the future of the physical universe (this topic is sometimes called Physical Eschatology - see the review of Cirkovic, 2003). Notice that as we delve deeper into the future, the uncertainties of our projections must necessarily grow. Notice also that this discussion is based on the assumption that the laws of physics are both known and unchanging; as new physics is discovered, or if the physical constants are found to be time dependent, this projection into the future must be revised accordingly.
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