Transcript: Tracing the expansion rate back in time gives the age of the universe in a big bang model. Cosmological parameters, when they’re measured, can be used to predict the age of the universe. Based on the accurate measurements of the WMAP satellite, the age of the universe is between eleven and twelve billion years. Note that any universe where there is vacuum energy and a cosmological constant has an older age than a universe without such vacuum energy because the cosmological constant acts to accelerate the expansion and give an older age. The fundamental test of the big bang model is to look for the oldest objects where the age is measured using an astrophysical technique. In astronomy we use globular clusters, the oldest known stellar systems in the Milky Way galaxy. Their ages are measured by comparison of theories of stellar evolution with observations of the globular cluster. This technique is called isochrone fitting. These careful observations and the theories of stars that go with them show that the oldest globular clusters in the Milky Way galaxy have ages of about ten to eleven billion years allowing enough time for them to have formed along with the Milky Way. Thus, the big bang passes a critical test; it contains no objects older than the universe itself.