of the Universe
As we shall discuss further in connection with the big bang, there is strong evidence that the Universe has not always existed, but instead came into being a finite amount of time ago. There are several measures of the age of the Universe. Let us discuss two: (1) the age of globular clusters and (2) the inverse of the Hubble constant.
where v is the velocity of recession, H is the Hubble constant, and d is the distance. Thus, from this equation, we have that 1/H = d/v. but d/v is distance divided by velocity, which is time (e.g., if I travel 180 miles at 60 miles/hour, the time required is t = d/v = 180/60 = 3 hours).
Thus, the Hubble time T is just the inverse of the Hubble Constant:
Taking a value of H = 20 km/s/Mly (where Mly means mega-light years),
where all the factors are necessary to convert the time units to years and I've rounded some numbers to simplify the display.
The physical interpretation of the Hubble time is that it gives the time for the Universe to run backwards to the Big Bang if the expansion rate (the Hubble "constant") were constant. Thus, it is a measure of the age of the Universe. The Hubble "constant" actually isn't constant, so the Hubble time is really only a rough estimate of the age of the Universe.
Reasonable assumptions for the value of the Hubble constant and the geometry of the Universe typically yield ages of 10-20 billion years for the age of the Universe. For example, H near 50 km/s/Mpc gives a larger value for the age of the Universe (around 16 thousand million years), while a larger value of 80 km/s/Mpc gives a lower value for the age (around 10 thousand million years). Therefore, we shall take this information, and additional information from other methods to estimate the age of the Universe that we have not discussed, to indicate an age of approximately 15 billion years for the Universe.