2.2. Possible Kinds of Cosmological Models

In terms of the parameters H, , t_glob and , five different combinations are consistent with extant data. The role of observations is to collapse these five combinations down to a single on that fits all the data. By convention, instead of using , cosmologists use another parameter which is just / _c. A spatially flat Universe has = 1. Although many combinations of these parameters are allowed, a cosmology in which H₀ is 100, = 1 and = 0 produces an expansion age of only 6.7 Gyrs and thus can be ruled out. As will be discussed later in this book, there is strong theoretical prejudice for = 1 which then necessarily pushes estimates of H₀ to lower values. Here then are the current cosmological options which are more or less consistent with observations:

Option 1: H₀ is 50, = 0, the ages of globular clusters are well understood with t_glob 16 ± 1 Gyrs, and 0.1 (the Universe is open) and most of the matter is baryonic. Arguments for this cosmology were first made in Sandage (1972) and again most recently in Sandage and Tammann (1997). If this cosmology is correct, its a nice self-consistent one in which the expansion age and the globular cluster ages agree. Of course, its difficult to account for inflation in this model.

Option 2: H₀ is 100, = 0, the ages of globular clusters are at their lowest possible limit, and the Universe must be very open. The age of the Universe in this cosmology is 10 Gyr. This option dates back to the work of Gerard De Vaucouleurs in the early 60s and was strongly revived in the early 80's by the work of Aaronson et al. (1980, 1985) Recent improvements in stellar evolutionary code, however, would now seem to preclude such young globular cluster ages and hence rule out this cosmology. In addition, other age indicators strongly suggest that the Galactic Disk is at least 10 Gyrs old. Like option 1, this option is also inconsistent with the inflationary scenario.

Option 3: H₀ is 75, is 0.1 - 0.3, the ages of globular clusters are marginally understood with t_glob in the range 12-15 Gyrs. This is the compromise option, in which overlapping error bars accommodate most observations and prejudices, although = 1 produces an age of only 9 Gyr with this value for H₀.

Option 4: H₀ is 50, the Universe is critical ( = 1) and the ages of globular clusters are marginally understood with t_glob being 12-13 Gyr. This option is preferred by inflationary cosmologies which predict a critical Universe. This cosmology was strongly favored in the 1980s to the point that observations which suggested H₀ 80 were essentially ignored by a large segment of the theoretical community since that would conflict with the = 1 constraint coupled with the ages of globular clusters.

Option 5: We understand the ages of globular clusters with current models putting them at 13-17 billion years. By the consensus of most of the last 10 years worth of data, H₀ lies in the range 70-90. Going where this data now leads then suggests a) the Universe is open or b) the inflationary constraint of flat space has to be satisfied by a combination of and . This option is a significant departure from the cosmological models of the 70's and 80's and as we will see later, has a fair bit of observational support.

Clearly the current situation is unsatisfying as a very large range of cosmological models remain consistent with the data. As we will see in Chapter 4, although the inflationary paradigm is an elegant one, there is very little observational support for it. At the same time, there seems to be real conflict between the ages of the oldest stars and the inferred expansion age based on measurements of H₀. We are thus left with either a positive value for or a low value for H₀ as the alternatives.