3. THE OLDEST OBJECTS IN OUR GALAXY

The universe cannot be younger than the oldest objects in it. Thus, estimates of the age of the oldest objects in our Galaxy are lower limits to the age of the universe (Table 2 and Fig. 2). A standard but simplified scenario for the origin of our Galaxy has a halo of globular clusters forming first, followed by the formation of the Galactic disk. The most recent measurements of the age of the oldest objects in the Galactic disk give tdisk = 8.7 ± 0.4 Ga (Table 2). The most recent measurements of the age of the oldest objects in the halo of our Galaxy give tGal = 12.2 ± 0.5 Ga (Table 2). The individual measurements are in good agreement with these averages. There are no large outliers. In contrast to the t0(h, m, ) estimates obtained above, all of these age estimates are direct in the sense that they have no dependence on a Big Bang model.

How old was the universe when our Galaxy formed? If we write this as tGal + t = t0, then what is the amount of time (t) between the formation of our Galaxy and the formation of the universe? If we had an estimate of t, then we would have an independent estimate of t0 to compare to t0 = 13.4 ± 1.6 Ga, obtained above. However, we have very poor constraints on t. The simple but plausible estimate t 1 Gy is often invoked, but estimates range from ~ 0.1 to ~ 5 Gy, and may be even larger (24, 25). This uncertainty in t undermines the ability of estimates of the age of the oldest objects in our Galaxy to tell us the age of the universe. Without t, we cannot infer t0 from tGal. The best estimate of t may come from the difference between the age reported here and the estimate of the age of our Galaxy (Table 2). Thus t = t0 - tGal = 13.4 - 12.2 = 1.2 ± 1.8 Gy.

The age measurements in Table 2 also indicate that there is a 3.5-Gy period between halo and disk formation (tGal - tdisk). If our Milky Way is typical, then this may be true of other spiral galaxies. With the best fit values obtained here for the three parameters, (h, m, ) = (0.72 ± 0.09, 0.23 ± 0.08, 0.65 ± 0.13), the ages tdisk and tGal can be converted into the redshifts at which the disk and halo formed: zdisk = 1.3+1.5-0.5 and zGal = 6.0+-4.3. Thus, a diskless epoch should be centered at a redshift between zdisk and zGal (1.3 zdiskless 6.0). We would expect fewer disks in the halolike progenitors of spiral galaxies in this redshift range. Studies of galaxy types in the Hubble Deep Field indicate that this may be the case (26).

The requirement that the universe be older than our Galaxy, t0 > tGal, is a consistency test of the Big Bang model. The best fit model obtained here passes this test. There is no age crisis. This is true even if the high values of h (~ 0.80) are correct. Only at h 0.85 is t0 tGal. This consistency provides further support for the Big Bang model, which the standard model (m = 1, = 0) is unable to match unless h < 0.55.