ekpyrotic universe

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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!

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.