The Ekpyrotic universe is one of the first credible alternatives to the cosmological inflation model of the very early universe. The theory has been constructed to make as few assumptions of the initial conditions of the universe as possible, and instead relies on the modern theory of unified physics, heterotic M-Theory, to allow the conditions to arise 'naturally'.
Although inflation has answered the questions raised by the more classical hot big bang theory, no real link has been made to the theory a lot of physicists believe will turn into a theory of everything, M-Theory.

The Ekpyrotic model relies on the universe consisting of a five dimensional space-time, that is bounded by two (3+1)-dimensional surfaces (3-branes) which are separated by a finite gap. This corresponds to the 11 dimension universe we live in now, (I believe) postulated by M-theory, albeit in a different topology.
One of these bounding branes, called the 'visible' brane corresponds to our own universe, in which ordinary particles and radiation can exist; the other is known as the 'hidden' brane. The gestation of our universe starts when a further 3-brane separates from the hidden brane and moves across the bulk 5d volume; and it's birth occurs when this moving 'bulk' brane hits the 'visible' brane. At this point a fraction of the kinetic energy from the bulk is converted into radiation and matter, and from this then on the growth of the universe proceeds in the same manner as predicted by the big bang model.

The universe could have been in the initial state for an arbitary, indefinite length of time before this, the clock of time only begins to tick when the branes interact.

The following diagrams depict the process....

|          |		|         ||
|          |		|         ||
|          |		|        _||
|          |		|       |  | 
|          |		|       |_ |
|          |		|      |  || 
|          |		|      V  ||
|          |		|         ||

    (A)                      (B)

|      |   |		|{         |
|     {    |		|{         |
|     {    |		|{         |
|     {    |		|{         | 
|     {    |		|{         |
|     {    |		|{         | 
|     {    |		|{         |
|      |   |		|{         |

    (C)                      (D)

A Here we have the two initial branes separated by a volume of warped 5 dimensional space. The warping is low near the 'hidden' brane on the right, and high near the 'visible' brane on the left.
B Now a region of the hidden brane peels off, forming a new 'bulk' brane in the shape of a terrace. The whole brane now slowly moves left, perhaps drawn by the exchange of virtual 2-branes between it and the visible brane. The edges of the terrace expand outwards at the speed of light.
C As the bulk brane moves, quantum fluctuations produce ripples in it's geometry over a wide range of length scales.
D Interaction. The energy released creates the substance of the universe we see today. The energies involved heat the universe to below the grand unification energy. As the ripples cause the bulk brane to interact at slightly different times, the density of the particles produced is slightly different over space.

So, this model offers solutions to the following problems, and can be compared to the solutions offered by the more standard cosmological inflation theory.
  1. The Horizon Problem The temperature of the universe at any one location is set by the exact dynamics of the collision of the bulk brane with the visible brane, at that location. As the branes are similar over their whole volume, and the universe we see is formed where these intersect, the universe has the same temperature over even (post collision) causally separate regions.
  2. The Flatness Problem The visible and the hidden brane are assumed to be in their ground state, which is a flat geometry. This is a fair enough assumption, the pre-big bang picture above could have existed for an arbitarily long time, long enough to reach the lowest energy state. When the bulk brane peels away, this flat geometry is kept over the branes passage through the 5 dimensional bulk space.
  3. The Inhomogeneity Problem The universe on the largest scales has the same temperature, and the same density, but on more local scales there are galaxy clusters, galaxies, you and me... This fine structure is explained by quantum fluctuations generating ripples in the bulk brane, causing it to impact with slightly different energies at different locations. This causes slight inhomogeneities in density, setting up gravitional waves, which eventually will cause clumping of matter as the universe cools.
  4. The Monopole Problem Monopoles and other topological defects are prevented from forming (and so being more widespread tha observation allows), as the universe cools (as cracks in ice form as water cools), because the temperature caused by the collision is in fact less than that needed to produce such artifacts. Unlike with standard models, where the temperature is essentially infinite; well E=mc2 where 'm' is the mass of the entire universe!

Another important, (and perhaps most importantly a testable) prediction of this model is that the gravitational wave background spectrum will be blue, whereas with inflation it will probably be red. There are experiments planned, such as the European Planck satelite that may be sensitive enough to detect the changes in microwave background radiation polarisation, caused by gravitional waves formed by inflation. Any effects from ekpyrotic inflation won't be detected, so if the experiments show any positive results, ekpyrotic inflation is probably wrong!

You may ask 'What stops the colliding branes from separating once again', well I don't know, and neither do (as of March 2001) the author's of the theory. Indeed this is where the 'ekpyrotic' term comes in... The Stoic model of the cosmos states the universe is consumed by fire at regular intervals, and then remade from this fire in a process termed ekpyrosis. If the branes making our universe separate...well that it's it, the universe ends! As the expansion of the universe is speeding up, perhaps they are already separating. Maybe we haven't seen any aliens because we've taken so long to evolve, all the clever species have found a way out of the universe!

Taken from the research paper 'The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang' by Justin Khoury, Princeton, NJ, USA, Paul J. Steinhardt, Pennsylania University, PA, USA and Neil Turok Cambridge University UK. 29 March 2001
Just a note, the 11 dimensional theory is reduced to a theory of 10 dimensional branes connected over a complex circle, this connection is the dimension along which the branes travel. The travelling branes are now 10 dimensional objects. Of these 10 dimensions 6 contain all of the majic of some modern particle physics (supersymmetric physics and the like). The visible brane (our brane) and the hidden brane live at opposite ends in a backgroud gravity. They live at the same potential and between them there is a potential drop. The Colliding brane falls down the potential and as it moves it gains extra energy from the background. When this brane collides with our brane the extra energy that it absorbed from the background is release as the "big bang" event in our universe.

An important point is that the brane has long enough to move to allow quantum fluctuations to equilibrate across the brane, these are then imprinted on our univese, given the appearerence of superluminal correlations on our sky. The Brane definitly moves across the gap at subluminal velocities and may move at substatially subluminal velocities. This is a large departure in method from inflation.

M-theory is an 11 dimesional theory. You need to get rid of 7 of these dimensions. The method for doing this is called compactification. The choice for compactification that the ekpyrotic universe makes is arbitrary and is chosen to agree with cosmological observations.

At a talk on the ekpyrotic universe that I attended reecently given by one of the theories inventors, Brian Greene made the comment that for a long time cosmologists said that boundry condtions for their theories would be justified from improved particle physics models, now it seems that the reverse is happeneing. There is not yet an ab initio criteria for these theories other than the antropomorphic one.

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