2.2. Why the Universe is Just So Old
Why is the universe not much older than it is? In the anthropic view, part of the reason must be the decrease in new star formation, which both globally and within the Galaxy has decreased by almost an order of magnitude during the 4.5 billion years since the solar system formed (Fukugita et al. 1996, Lilly 1998, Madau 1999). Galaxies have converted the bulk of their original gas to stars or ejected it altogether, and the larger reservoir of intergalactic gas is now too hot to cool and collapse to replenish it (Fukugita et al 1999, Cen and Ostriker 1999).
The decrease in star formation rate also means that the heavy element production rate is decreasing, and therefore the mean age of radioactive elements (especially those produced by Type II supernovae, whose rate is closely tied to current star formation) is increasing. The new planets which are forming now and in the future are less radioactively alive than Earth was when it formed. Since abundant live radioactive nuclei in the Earth's core (especially uranium 238, thorium 232, and potassium 40, with half-lives of 4.46, 14 and 1.28 Gy respectively) are needed to power vulcanism, continental drift, seafloor spreading, mountain uplift, and the convective dynamo which creates the Earth's magnetic field, new planets even in the rare instances where they do manage to form will in the future not have these important attributes of the Earth. Life is also sensitive to other features of the detailed composition inside the Earth: the correct iron abundance is needed to provide sufficiently conductive core flows to give a strong magnetic field. Without its protection, the solar wind would erode the atmosphere as it appears to have done on Mars since the magnetic dynamo ceased there (Acuña et al. 1999, Connerney et al. 1999). The coupling of bioevolution with astrophysics thus defines a fairly sharp window of habitability in cosmic time as well as space (Ward and Brownlee 1999): New stars and new habitable planets are becoming increasingly rare.