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....

| | | ||
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| | | _||
| | | | |
| | | |_ |
| | | | ||
| | | 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.

**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.
**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.
**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.
**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=mc^{2} 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 www.arXiv.org/abs/hep-th/0103239