Millions of words have been written on this subject and thousands of references could be quoted. However we believe that the type of enlightened reader that you undoubtedly are can do no better than to study Professor Stephen Hawking’s book The Illustrated ‘A Brief History of Time’ (Bantam, 1996. ISBN 0-593-04059-7). It is the much-improved, easier-to-comprehend (entertaining, even), updated and expanded version of his previous publication A Brief History of Time. Mind you, we still can’t understand how the esteemed professor failed to notice the features which led to the ACU theory! The following synopsis is loosely based on Hawking’s chapter, which is stunningly illustrated and recommended reading.
For aeons the Earth
was thought to be flat Aristotle had the Earth as the centre of the universe with the moving stars (planets) in circular orbits. Ptolemy had the (fixed) stars on an outer sphere, so that they all rotate around the Earth together. Despite problems such as “What lies outside the star-sphere?” (don’t ask!) it kept the Christian Church happy - they could put heaven and hell there. In 1514 Copernicus proposed the Sun to be stationary at the Centre of the Universe with everything else in circular orbits. This idea was later modified by Kepler to have the planets in elliptical orbits. At last theoretical predictions appeared to match observations. But Kepler had a problem: He thought that magnetic forces kept everything in place, something that was incompatible with elliptical orbits. It was Sir Isaac Newton who sorted that one. In 1687 he solved the hard maths and put forward his Theory of Universal Gravitation, in which a body attracts any other body with a force proportional to:- their masses divided by their separation. Newton’s mechanics predicted elliptical orbits. The universe was generally thought to be static. Newton also thought the stars should attract each other and fall together. Unless, that is, there were an infinite number of stars in an infinite space (and hence no Centre), but this turns out to be impossible. No-one had thought “Is the Universe expanding or contracting?” And even though Newton’s ideas suggested it might be contracting, still no-one suggested it could be expanding! Some crazy ideas about gravity were proposed to get round this. Also some lovely ideas as to why the whole night sky isn’t as bright as the sun! (You’ve just got to read Stephen’s book! He also goes on to discuss the early theories of the origin of the Universe - fascinating reading!!!). It was in 1929 that Edwin Hubble discovered that distant galaxies are receding and that the further away they are, the faster they go, hence the Universe is expanding. So backtracking would lead to the Universe having all been at a single point once upon a time - the Big Bang - the start of time and everything. Sounds crazy? Experiments in 1965 discovered the Cosmic Background Radiation, left over from the Big Bang. It comes almost equally from everywhere, but the COBE (Cosmic Background Explorer) satellite showed irregularities in it which are the reason the galaxies formed. The Hubble Space telescope has taken some amazing photos, showing thousands of galaxies and things, of an age only 10,000,000,000 years after the Big Bang! (See "Journey to the Centre of the Universe".) Theories cannot be proven correct, but they can be proven to be wrong and Newton’s theory fails to predict the exact orbit of Mercury. However, a theory of gravity proposed by Einstein, the General Theory of Relativity, gets it right. It’s a bit more complicated, but if you really want to know what’s going on you can’t ignore it. The General Theory of Relativity is based on a constant velocity for light. Anybody who measures the speed of light in a vacuum gets the same answer, wherever they are and whatever they are doing. (You don’t believe me? Go try it!) The trouble (and the bit that most people have a hard time getting their heads round!) is that this demonstrable constant velocity of light requires that everyone has his own “time” - two people needn’t agree on how much time has elapsed, and that also can be experimentally verified. No one said it was easy! Newton’s theory is an approximation of Einstein’s. In Einstein’s General Theory of Relativity, gravity is a result of a curving of space by the masses in it. It works with “massive” objects at large distances, i.e. the large scale structure of the Universe, but it does nothing for things on the atomic scale. Here it is appropriate to use the Quantum Theory, developed by Max Planck, et.al., but quantum mechanics doesn’t work on the large scale. The two theories are mutually exclusive; neither is universally correct. By now you should have given up on this and bought the book. And why not visit Stephen Hawkings home page to read all about his world?
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Photo,
right: Professor Stephen W.Hawking, CH, CBE, FRS,
What Professor Hawking and others are trying to do is come up with a
Unified Theory that incorporates both and describes everything - a
Quantum Theory of Gravity. Work
in Progress.... 
In
fact, our nucular expert tells us that things get harder to summarise
after this and need much more room than the odd HTML page, so
you might as well read the book anyhow. 
http://www.g4uxd.talktalk.net/acu