16 From Newton to Hubble
The globular clusters (GCs) are roughly spherically distributed stellar systems in
the spheroid of the Galaxy. During the majority of the life of a star, it converts hydrogen
into helium in its core. Thus the most interesting stars for the determination of푡Gare
those which have exhausted their supply of hydrogen, and which are located in old,
metal-poor GCs, and to which the distance can be reliably determined. A recent age
determination gives
푡GC= 14. 61 ± 0 .8Gyr.
This includes an estimated age for the Universe when the clusters formed.
Of particular interest is the detection of a spectral line of^238 U in the extremely
metal-poor star CS 31082-001, which is overabundant in heavy elements. Theoretical
nucleosynthesis models for the initial abundances predict that the ratios of neighbor-
ing stable and unstable elements should be similar in early stars as well as on Earth.
Thus one compares the abundances of the radioactive^232 Th and^238 U with the neigh-
boring stable elements Os and Ir (^235 U is now useless, because it has already decayed
away on the oldest stars). One result is
푡∗= 13. 5 ± 2 .9 Gyr. (1.24)Brightest Cluster Galaxies (BCGs). Another cosmochronometer is offered by the
study of elliptical galaxies in BCGs at very large distances. It has been found that
BCG colors only depend on their star-forming histories, and if one can trust stellar
population synthesis models, one has a cosmochronometer. From recent analyses of
BCGs the result is
푡BCG≳12 Gyr. (1.25)
Allowing 0.5–1.0 Gyr from the Big Bang until galaxies form stars and clusters, all
the above estimates agree reasonably with the value in Equation (1.21) (This correc-
tion was already included in the value from globular clusters.).
There are many more cosmochronometers making use of well-understood stellar
populations at various distances which we shall not refer to here, all yielding ages
near those quoted. It is of interest to note that in the past, when the dynamics of the
Universe was less well known, the calculated age휏Hwas smaller than the value in
Equation (1.21), and at the same time the age푡∗of the oldest stars was much higher
than the value in Equation (1.23). Thus this historical conflict between cosmological
and observational age estimates has now disappeared.
Later we will derive a general relativistic formula for푡 0 which depends on a few
measurable dynamical parameters determined in a combination of supernova analy-
ses, cosmic microwave background analyses and a set of other data.
1.6 Matter in the Universe
Since antiquity the objects in the sky were known by the visible light they emit, absorb
or reflect. Stars like the sun shine, planets and moons reflect sunlight, planets around
distant stars reveal themselves by obscuration, and intergalactic dust by dimming