The Origins Of The Universe
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 know that


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.

The Big Bang
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.

Evidence 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.

The COBE satellite

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. 

Link Bar

The best site on the COBE satellite

More information about the Big Bang theory

NASA site with information about the Big Bang

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