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Cosmological inflation is a model used to describe the very early universe, only a small fraction of second after the big bang. After the inflation period has finished, the standard big bang model can take over. It was originally proposed by Alan Guth to explain several serious shortcomings in the 'classical' hot big bang model; namely the flatness problem and the horizon problem. Inflation now encompasses a variety of different models on the same theme, all with the same fundamental characteristic: - The universe underwent a very short period of great exponential expansion, expanding space from smaller than an atom to the size of a grapefruit or larger. One important point to note is this expansion is in fact, faster than the speed of light!
The main problem with the original big bang was it provided no mechanism to explain why the universe is basically so uniform in appearance. Firstly the universe appears to have much the same temperature; as measured by the cosmic microwave background radiation studies, (the horizon problem). Secondly in order that the universe didn't either a) expand to infinity, or b) collapse back in on itself, the density parameter value (omega) of the universe in it's first second has to be accurate to within 1 part in 1060! (The flatness problem)

Calculations show the amount of expansion predicted by inflation (1030 or more) can smooth out any irregularities, quickly making omega's value near one, pretty much whatever it's initial value. Also the 'faster than light' expansion allows the thermal equilibrium between all parts of the universe, no matter how far they are apart. (nb matter within the universe never travels faster than light, space only carries matter along for the ride).

The inflationary model has undergone many changes and refinements, partly as physicist's understanding of the fundamental equations, so called 'Grand Unified Theories' has progressed, this helps answer further questions... For instance, where did the universe actually come from, and if the universe was so dense at the start, how could it expand? Wouldn't it be a black hole, which nothing can escape from? Quantum mechanics and inflation provides the answer...

Quantum Inflation....

The quantum uncertaincy principle allows a bubble of energy or a particle and it's anti-particle to appear from 'nothing' provided they disappear within the Planck time. Now the energy in a gravitational field is in fact negative, whereas the mass 'generating' it has a positive energy associated with it, and the two numbers cancel out! So, theoretically, as there's no upper limit to the amount of energy that can be created by these quantum processes, the universe could have begun as a Planck sized super dense nugget, that just came into being.
Next, how to explain how this Planck sized (10-33) object didn't collapse and inflated to create things as we see today. Grand unified theories predict that at successively higher energies, the four fundamental forces unite. As higher densities have higher energies, this translates to at progressively later times after the appearance of the Planck sized nugget, the forces separated from the unified 'super-force'. This 'symmetry breaking' of one into four different forces releases a colossal amount of energy, which is dumped into space itself, blowing it up; the particles in the universe stayed in the same position, but the distances between them got larger as space expanded.
The symmetry breaking occurs because the initial conditions were in an unstable equilibrium, like a stack of golf balls, soon it's going to fall over and be unsymmetrical! These ideas, which come not from cosmology, but from experiments on sub-atomic particles in particle accelerators have at least been proved by solid experiments.

The exact behaviour of matter at such high energies still awaits new, or at least extended laws of physics to be written, but these laws will be constrained by observations of the universe at the largest scales. So you can't just say 'I've got a wonderful new grand unified theory, and it goes like this...' because someone else will work out that the symmetry will break in such and such a way at the birth of the universe, which inflation will blow up to encompass the whole universe, which an astronomer will then tell you is quite plainly wrong! Indeed observations made by the background radiation of the universe have shown a pattern of tiny variations in temperature which are predicted by inflationary models, as quantum fluctuations at early time are expanded to cover the whole universe.. This close marriage of physics in the quantum world, to physics on a universal scale is one of modern science's great achievements.

The current model is often termed 'chaotic inflation' and goes some way to answering such metaphysical questions such as 'What came before the big bang?'. This refinement came with the realisation that the original super-dense Planck nugget might not be special, and could just be a region of some 'super-space' where the conditions happen to be that which gave rise to inflation, producing our universe. This allows our universe to be one of many that arose from this super-space, and indeed allows our own universe to bud off new universes. As quantum mechanics say the initial starting conditions arise from uncertainty they can have basically any values, the term chaos was used.
Or, perhaps that's what happens when a black hole forms; this creates a singularity, which has enough energy density to trigger the inflation of a completely new and separate universe to our own! Physicists, and science fiction authors have played around with the theory so much, that it's sometimes hard to know what inflation actually means exactly sometimes...however the basic mechanism seems valid, and has an increasing amount of experimental evidence to support it. Also see :- Einstein Static Cosmology

Largely written on-the-fly from my memories of many books; I'll add some of my more read ones here shortly (29/3/01)
Any typos, mistakes, general editorialising or clarifications needed, please /msg me... !
Also see the web page :-
http://www.biols.susx.ac.uk/home/John_Gribbin/cosmo.htm

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