Why does Tipler say the age of the universe is 20 billion years old?
That was back in 1994, before the universe felt old enough to lie about its age (joking). The age of the universe was a much rougher guess until the results of the Wilkinson Microwave Anisotropy Probe (WMAP) were available (beginning in 2002). Scientists were using different figures depending what sources they used. There was even a time (a decade before Tipler’s use of 20) when some cosmologists went as low as 10 (with astronomers working separately to find the age of globular clusters to be as high as 11). With the WMAP data, the time elapsed since the emission of the Cosmological Microwave Background radiation is known relatively well (within 0.2 billion years). The CMB event is thought to be roughly 700,000 years after the Planck Time (which is what most people mean when they say the “beginning”). WMAP cannot explore that time period. Our present knowledge of Physics does not allow us to explore the time before the Planck Time (if there is such a time).
