|The age of the Universe|
|If the universe has been
expanding for some while then at some time in the past, all the matter
it contains must have been concentrated together in a state of high density.
For a galaxy moving with a constant velocity v the time taken to
travel distance D is
D=vt therefore t=D/v
but from the Hubble Law we
substituting in for v gives us
Using the last equation we get (approximately) t=13000 million years. From this, we can see that the reciprocal of the Hubble constant tells us the age of the universe and that its value changes with time as the universe expands. This is why cosmologists are so interested in obtaining a reliable estimate of it.
|If all the matter in the universe was at some time concentrated at a single point then about 13000 million years ago some kind of gigantic explosion must have occurred to cause the universe to start expanding. Cosmologists call this event the Big Bang.|
for the Big Bang
|As well as the Hubble law
there is another important piece of evidence that points to a hot Big Bang
as being the origin of the universe. In 1964 Arno Penzias and Robert Wilson,
two scientists at Bell Telephone Laboratories in New Jersey, were carrying
out experiments using a microwave antenna for satellite communications.
As they pointed their antenna towards the sky, their receiver registered
a faint 'hiss' coming from all directions that would not go away. The hiss
was highly isotropic, constant with time and could be detected at any time
of the day or year. What Wilson and Penzias had discovered was a relic
from the Big Bang - the thermal radiation from the Big Bang itself!
Cosmologists have theorised that shortly after the Big Bang occurred, the universe was filled with very high energy blackbody radiation consisting of gamma rays of very short wavelength. As the universe expanded it cooled and the wavelength of this radiation red-shifted down to the microwave region of the electromagnetic spectrum with a blackbody temperature of about 3K.
In 1989 a space astronomy satellite called the Cosmic Background Explorer (COBE) was launched. It measured the distribution of microwave radiation in more detail and confirmed a blackbody curve with a peak wavelength corresponding to a temperature of 2.7K. The cosmic microwave background represents photons which started their journey when the universe was only a few 100,000 years old and in this way shows us what its nature was like when the universe was very young.
C.O.B.E BIG BANG
The best site on the COBE satellite
More information about the Big Bang theory
NASA site with information about the Big Bang