# Independent age estimates

A variety of independent techniques for estimating the age of the Universe exist, all of which indicate that its age lies in the range of 10 to 20 billion years. Among the most prominent methods include the following:
The fact that these three very different dating methods yield the same age range for the Universe strongly supports the notion that all the phenomena in the Universe shares a common origin. This serves as strong, though indirect, supporting evidence for the theory of the Big Bang.

## The expansion of the Universe

Today's observed recession of galaxies pays witness to the sheer force of the Big Bang, and the enormous - almost inceivably large - reservoir of energy that it provided the Universe. During the 1920's, the astronomer Edwin Hubble was the first to realize the largescale expansion of the Univers by examining the "redshifted" spectra of a number of galaxies. (A "redshift" refers to a shift to longer wavelengths in the spectral lines of an object's spectrum, while a "blueshift" represents a shift to shorter wavelengths.)

To relate the recession velocity of a galaxy with its distance, Hubble proposed the relationship: v = H x R. The constant of proportionality, H, is known as "Hubbles' constant" and has units of km/sec/megaparsec. Modern astronomers disagree as to the precise value of H. The most commonly used estimate is around 50 km/sec/Mpc, but astronomical data indicates that the value can range up to 100 km/sec/Mpc.

Of immediate significance is the realization that Hubble's Law, and in particular Hubble's constant, cna be used to set limits on the age of the Universe. Inverting Hubble's costant, and canceling units of distance, actually yields a time estimate for the age of the Universe. For a value of H=50 km/sec/Mpc, we have an age of 10 billion years, while for H=100 km/sec/Mpc, the age is 20 billion years. Hence, we conclude that the age of the Universe, as derived from Hubble's Law, is somewhere between 10 and 20 billion years old.

## Age of Globular Clusters

Globular clusters are densely-packed, beehive-like conglomerations of stars which are situated at the outer edges of our Galaxy. They are by far the oldest existing objects in our Universe. The ages of globular clusters are detemined from both stellar evolution theory as well as dynamical considerations. Consisting of two different types, metal-rich and metal-poor, globular clusters range in age between 11 and 17 billion years. The highly metal-poor clusters, such as M15 and M92, are up to 17 billion years old, while the slightly more metal-rich clusters range in age between 11 and 12 billion years. (For measure, globular cluster stars are classified as population II stars, while the younger stars that populate the galactic disc known as population I stars). Assuming that the oldest of the globular clusters are some 17 billion years old, we conclude that the Universe is about 18 billion years old if we include another 1 billion years needed for the formation of our Galaxy.