The End of the Age Problem, and the Case for a Cosmological Constant Revisited

The lower limit on the age of the universe derived from globular cluster dating techniques, which previously strongly motivated a non-zero cosmological constant, has now been dramatically reduced, allowing consistency for a flat matter dominated universe with a Hubble Constant, H0 ≤ 66kms−1Mpc−1. The case for an open universe versus a flat universe with non-zero cosmological constant is reanalyzed in this context, incorporating not only the new age data, but also updates on baryon abundance constraints, and large scale structure arguments. For the first time, the allowed parameter space for the density of non-relativistic matter appears larger for an open universe than for a flat universe with cosmological constant, while a flat universe with zero cosmological constant remains strongly disfavored. Several other preliminary observations suggest a non-zero cosmological constant, but a definitive determination awaits refined measurements of q0, and small scale anisotropies of the Cosmic Microwave background. I argue that fundamental theoretical arguments favor a non-zero cosmological constant over an open universe. However, if either case is confirmed, the challenges posed for fundamental particle physics will be great. The cosmological model perhaps most strongly favored by the data over the past few years has involved a proposal which is heretical from an elementary particle physics perspective. In order to reconcile a flat universe—favored by both inflationary models and by the longstanding flatness problem in cosmology—with the apparent age of globular clusters, and the fact that many estimates for the clustered mass density on large scales suggest that insufficient non-relativistic matter exists to achieve a flat universe, the idea that the cosmological constant is non-zero has been invoked[1]. Most recently the inclusion of additional arguments associated with the baryon density of the Universe and large scale structure have further strengthened the case for a cosmological constant [2, 3]. The problem with this from a fundamental perspective is that a cosmological constant— -associated in modern parlance with a non-zero vacuum energy density in the universe—on a scale which would be cosmologically relevant and yet still allowed today would take a value which is roughly 125 orders of magnitude smaller than the naive value one might expect based on considerations of quantum mechanics and gravity (see for example [4]). This apparent discrepancy would involve the most extreme fine tuning problem known in physics, and for this reason many particle physicists would prefer any mechanism which would drive the cosmological constant to be exactly zero today. The possibility that cosmology might force physicists to have to directly confront this longstanding issue in fundamental physics is exciting, but at the same time its potential significance warrants a careful and continued examination of the data which motivates the confrontation. In this regard, the recent reanalysis of globular cluster age estimates based in part on new parallax measurements obtained from the Hipparcos satellite is very significant. Globular Cluster ages are obtained by fitting the observed color-magnitude diagram to the predicted distribution for a system of stars of different masses which all form at the same time and which are then evolved on a computer to a certain age. A number of different fitting techniques are employed, with different uncertainties. Because theoretical models predict the instrinsic stellar luminosity, while measurements yield an apparent magnitude, in order to compare theory and prediction, the distance to the globular cluster must be known. Using an extensive Monte Carlo analysis approach begun several years ago[5] it was demonstrated