ABSTRACT
2.1 Overview
In Chapter 1 we were able to parameterize the important equations by the observational parameter H = R(t)/R(t), which specifies the rate of change of the scale factor R (t). This rate of change today is known as the Hubble constant (H0) which is a measure of the present-day expansion rate of the Universe. Since the Universe has mass, the expansion rate is slowing down due to gravitational attraction, so the value we measure for H today is different (smaller) than its value in the past. The importance of determining an accurate value for H0 with respect to our cosmological model is threefold:
It specifies the expansion age of the Universe and therefore provides a consistency check with other, independent ways of dating the Universe. As we will see, current values of H0 indicate an expansion age less than the ages of the oldest stars, which is clearly physically impossible.
It specifies the value for the critical density of the Universe which, when compared to the observed value (a difficult parameter to measure), determines whether the Universe is open (expands forever) or closed (expands to a maximum radius then contracts)
It sets the overall scale of the Universe.
