An independent method for determining the age of the universe

Abstract

The oldest white dwarf stars in the solar neighborhood are still observable, and their luminosity distribution can be used to determine directly the age of the Galactic disk and, somewhat less directly, the age of the universe. The observed number of white dwarfs in a given volume of space increases monotonically with decreasing luminosity, as expected from cooling rate considerations (hotter objects cool more quickly). However, their number drops abruptly at a luminosity of log (L/ L0 ) ""' -4.5, due to the finite age of our Galaxy. Comparing this sudden drop in the observed luminosity distribution with the best theoretical evolutionary white dwarf models available to us, and allowing for a mean pre-white-dwarf lifetime of 0.3 Gyr, we derive an age for the Galactic disk of 9.3 ± 2.0 Gyr. To obtain the age of the universe, we must add the time between the big bang and the first appearance of stars in the Galactic disk. Rather than assume a particular cosmological model, we prefer to choose a value (and stated error) that can include ali of the currently reasonable models describing this early era. On this basis we estimate the age of the universe to be 10.3 ± 2.2 Gyr. This new technique provides a way of determining tlie age of tlie Galactic disk that is largely independent of ali previous methods. Further, its current uncertainties can be materially reduced by direct measurements of the rate at which variable white dwarf stars cool tlirough the three known instability strips in the H-R diagram.