Here's the promised brief introduction into the age of the universe.

Until the 20th century, astronomers believed that the universe was eternal, unbounded and unchanging. It had no beginning and no end. This world view was shaken in 1929 when Edwin Hubble measured the spectra of distant galaxies.

Hubble observed that the spectra are shifted to the red. The darker the galaxy was, i.e. the more distant, the more its spectrum was shifted.

Hubble (wrongly) assumed that the red shift was caused by a light doppler effect due to a movement of galaxies. Some sort of central explosion must have caused all galaxies to move away from each other. The more distant a galaxy, the higher its speed. This seemed to him a simple way to calculate the distance of galaxies from their red shift... if he only knew the relation between red shift and distance!

Unfortunately, for getting that relation he needed to know first the distances of some galaxies independently of their red shift. However, in the 1930s astronomy could only directly measure the distance of close stars, up to several thousand light years, with the parallax method. There were some tricks and approximations available for star distances up to several hundred thousand light years, but no precise method to measure distances greater than a million light years.

However, the red shift has another consequence: When all galaxies are moving away from each other, there must have been a time when they all were together at the same position. The universe had a beginning. As Hubble assumed some kind of explosion, the beginning was dubbed "Big Bang".

In the 1950s with the development of the hydrogen bomb, astronomy began to understand in detail the nuclear mechanism of a sun, which is nothing else but a hydrogen bomb under gravity pressure. During its life span - our sun is 5 billion years old - a star characteristically changes its brightness and temperature depending on its initial mass. Now astronomy had a quite reliable method to calculate the distance of stars from their directly observed temperature-brightness-relation.

They found that the farthest galaxies known at that time were several billion light years away. From their speed, one could calculate that they all were close together at a point about 10 billion years ago. This was the first rough approximation of the age of the universe.

More precise distance measurements and comparisions with supernova records led to the discovery that the red shift was not caused by a doppler effect, as Hubble assumed, but by an expansion of space itself. Thus the galaxies are not moving, it's the space between them that is permanently widening. This leads to a slightly different distance-redshift relation than a Doppler effect (which is still wrongly mentioned in some school books as the cause of the red shift).

In 1950..1960, quantum electrodynamics was largely understood and allowed to calculate a model of the Big Bang. The model predicted that 400,000 years after the the Big Bang, radiation escaped from the initial dense plasma. This radiation would cool off with a predictable rate, and should still be observable today. If we only could measure it's today's temperature, we'd know precisely the date of the Big Bang and the age of the universe.

This happened 1964. Two US physicists discovered the cosmic background radiation. This was considered the final proof of the Big Bang. The background radiation has a temperature of 2.7 Kelvin, which puts the age of the universe at about 10..20 billion years.

Later, the Hubble space telescope and other advanced telecopes allowed the measurement of far galaxy distances with higher precision, especially from the analysis of supernovae. This led to the discovery that some galaxies were older than the 10 billion years assumed so far for the age of the universe. This puzzle was solved by the discovery that the universe is expanding with increasing velocity. Until then, it was believed that the expansion velocity was decreasing. The new data put the age of the universe at 13.8 (+/- 0.4) billion years.

This is the formula used today for the Distance / Red Shift relation in an accelerated expanding universe:



where D = proper distance, z = red shift, c = light speed (ca. 300,000 km/s), H0 = Hubble constant (ca. 22 km/s per light year), Omega0 = proportion of Dark and Baryonic matter in the universe (ca. 0.27) and OmegaLambda = kosmologic constant, i.e. proportion of Dark Energy in the universe (ca. 0.73).

In the 1990s the age of the universe was again calculated with a complete different method, measuring uranium isotopes. Uranium came into existence through the nuclear process in the first stars. This put the age of the universe at 14.5 (+/- 1.1) billion years.

Finally, also in the 1990s the COBE and WMAP satellites did a high precision measurement of the background radiation, resulting in 13.7 (+/- 0.2) billion years for the age of the universe.

So we have three different methods for calculating the age of the universe, all producing the same result and thus giving creationists a hard time. By the end of the 20th century, the Big Bang model and the age of the universe was accepted by all main religions (unlike evolution theory that is still not fully accepted by Islam). In 2007 the PLANCK space probe will again measure the background radiation, and is expected to determine the age of the universe with an accuracy of about +/- 50 million years.

The above stuff is from my website. If you find something unclear or wrong, please ask - I'll check and go further into details.