In 1929 the American astronomer Edwin Hubble made the most significant of all discoveries in cosmology: the galaxies are moving apart from one another, as thought the Universe is expanding like a balloon being inflated. However, clusters of galaxies such as the Local Group do not expand they are held together by their natural gravitational attraction.
Hubble's discovery that the Universe is expanding came from a study of the spectrum of each galaxy's light. This revealed that the light form the galaxies was being lengthened in wavelength as result of high-speed recession (this called the Dopper effect). Such a lengthening of wavelength is called a redshift, because the light from the galaxy is moved towards the red (longer wavelength) end of the spectrum.
Hubble found that the amount of redshift in a galaxy's light becomes greater with distance. Therefore, by measuring the redshift of a galaxy astronomers can tell how far away it is. Quasars, of instance, exhibit such as enormous redshifts that they must be the most distance objects visible in the Universe, over 10,000 million light years away.
Since the Universe is expanding, it is logical conclude that it was once smaller and more densely packed than it is now. According to the most widely accepted theory, the entire Universe was originally a compressed superdense blob which, for some unknown reason, exploded in a cataclysm know as the Big Bang. The galaxies are the fragments form that explosion, still flying outwards as the space between them expands.
As far as anyone can tell at present, the Universe will continue to expand for ever. According to the best current estimate the Big Bang took place 13,700 billion years ago; that is the age of Universe as we know it. It is impossible to tell what, if anything, happened before the Big Bang.
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