REVIEW Age Estimates of Globular Clusters in the Milky Way: Constraints on Cosmology Lawrence M. Krauss 1 * and Brian Chaboyer 2 * Recent observations of stellar globular clusters in the Milky Way Galaxy, combined with revised ranges of parameters in stellar evolution codes and new estimates of the earliest epoch of globular cluster formation, result in a 95% confidence level lower limit on the age of the Universe of 11.2 billion years. This age is inconsistent with the expansion age for a flat Universe for the currently allowed range of the Hubble constant, unless the cosmic equation of state is dominated by a component that violates the strong energy condition. This means that the three fundamental observ- ables in cosmology—the age of the Universe, the distance-redshift relation, and the geometry of the Universe—now independently support the case for a dark energy– dominated Universe. Hubble’s first measurement of the expansion of the Universe in 1929 also resulted in an embarrassing contradiction: Working back- ward, on the basis of the expansion rate he measured, and assuming that the expansion has been decelerating since the Big Bang —as one would expect given the attractive nature of gravity—allowed one to put an upper limit on the age of the Universe since the Big Bang of 1.5 billion years ago (Ga). Even in 1929 this age was grossly inconsistent with well- accepted lower limits on the age of Earth. Although this contradiction evaporated as further measurements of the expansion rate of the Universe yielded a value that was up to an order of magnitude less than Hubble’s esti- mate, much of the subsequent history of 20th- century cosmology has involved a continued tension between the so-called Hubble age— derived on the basis of the Hubble expan- sion—and the age of individual objects with- in our own galaxy. Of special interest in this regard are per- haps the oldest objects in our galaxy, called globular clusters. Compact groups of 100,000 to 1 million stars with dynamical collapse times of less than 1 million years, many of these objects are thought to have coalesced out of the primordial gas cloud that only later collapsed, dissipating its energy and settling into the disk of our Milky Way Galaxy (see Fig. 1). Those globular clusters that still pop- ulate the halo of our galaxy are thus among the oldest visible objects within it, a fact confirmed by measuring the abundance of heavy elements such as iron in stars within such clusters. This abundance can be less than one-hundredth of that measured in the Sun, which suggests that the gas from which these objects coalesced had not previously experienced significant star formation and evolution. Thus, an accurate determination of the age of the oldest clusters can yield one of the most stringent lower limits on the age of our galaxy, and thus the Universe. Globular cluster age estimates in the 1980s fell in the range of 16 to 20 Ga (1–3), producing a new apparent incompatibility with the Hubble age, then estimated to be 10 to 15 Ga on the basis of an estimated lower limit on the Hubble constant H 0 of 50 to 75 km s -1 Mpc -1 . This provided one of the earliest motivations for reintroducing a cos- mological constant into astrophysics. Such a term in Einstein’s equations results in an increased Hubble age because it allows for a cosmic acceleration, implying a slower ex- 1 Departments of Physics and Astronomy, Case West- ern Reserve University, 10900 Euclid Avenue, Cleve- land, OH 44106, USA. 2 Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Labora- tory, Hanover, NH 03755, USA. *To whom correspondence should be addressed. E-mail: lmk9@po.cwru.edu, chaboyer@heather. dartmouth.edu Fig. 1. Theoldestglobularclusters(compactcollectionsofstarsshownasbrightdotsinthefigure) are thought to have coalesced early on from small-scale density fluctuations in the primordial gas cloud,whichitselflatercoherentlycollapsed,dissipatingitsenergyandsettlingintothediskofour MilkyWayGalaxy.Asaresult,theseobjectspopulatearoughlysphericalhaloinourgalaxytoday. This sequence of events is shown schematically in four stages, from upper left to lower right. G LOBULAR C LUSTERS www.sciencemag.org SCIENCE VOL 299 3 JANUARY 2003 65 S PECIAL S ECTION on April 3, 2011 www.sciencemag.org Downloaded from