Two fundamental parameters, the Hubble expansion rate (Hubble constant) and the age of the universe are key to our present understanding of how our universe formed and how it evolves. The Hubble Space Telescope is making progress in measuring the expansion rate of the universe. However, the age of the universe is still in debate. Recent measurements of the Hubble constant and the ages of globular clusters yield very puzzling result: the age of the oldest globular clusters in the Milky Way galaxy appears even older than the universe in which the cluster is located. This "age problem" has motivated the resurrection of the infamous cosmological constant, first suggested by Albert Einstein in 1918, as the remedy of this conundrum. More recently, we found that the UV upturn of giant elliptical galaxies is best understood if these galaxies are some 3-4 billion years older than the oldest globular clusters in the Milky Way galaxy. If true, the age of the universe is increased by 3-4 billion years, and this would be considered as strong evidence for the cosmological constant from time scale test.

 

Recently, we have developed a new age dating method of galaxies based on the UV spectral energy distribution of early-type galaxies (Lee 1994; Park & Lee 1997; Yi et al. 1999). In this model, the origin of the observed far-UV radiation from these systems is mostly due to hot horizontal-branch (HB) stars and their post-HB progeny produced by the metal-poor tail of the wide metallicity distribution in galaxies. The systematic variation of the UV upturn depends on the contribution from metal-poor hot HB stars and their progeny, which in turn depends on the ages of old stellar populations in galaxies. Our result implies a prolonged epoch of galaxy formation, in the sense that more massive galaxies in denser environment formed first. The available space UV data suggest that the age difference between the giant elliptical galaxies and the spiral bulges of the Local Group is about 3 billion years. If our work is correct, it will confound cosmologists trying to estimate the age of the universe. Logically, the universe must be older than its galaxies. But as many cosmologists believe the universe is less than 15 billion years old - younger than the Milky Way seems to be. If the giant ellipticals are billions of years older than the Milky Way, the disparity is greater. We are currently participating in the development of NASA's new UV space telescope program (GALEX) as an official partner. The new UV telescope will be used, among others, to test our models on the age spread among galaxies. The development of UV telescope itself will be carried out in the form of international collaboration with Caltech and NASA. In order to achieve both scientific and technical goals of our project, we divide our team into three subgroups: (1) space telescope group, who will participate in the development of new UV telescope, (2) space observation group, who will perform observations, data reduction, and analysis using the space telescope, and (3) theory group, by which the theoretical models of the observed UV spectra will be constructed.

The proposed UV space telescope (GALEX) will obtain the data for estimating more accurate ages of galaxies, and therefore the age of the universe. In addition, possibility of other explanations such as metallicity effect will be examined from the analysis of look-back time observations. These new informations will provide more strong constraints on the age of the universe and eventually the nature of the cosmological constant.