Colliding brane cosmology is actually a ‘cyclic model’, i.e., it does not require a beginning of time, because the two (or more) branes must have existed before our current cosmic expansion started. In fact the ekpyrotic model says that it could be an ‘eternal’ process of collisions and ‘heat death’ due to expansion, because the two branes would move only a short distance apart before approaching again for another collision. The ‘short distance’ could however take billions of years to accomplish, by which time each brane has expanded a significant amount.
Colliding Branes Cosmic Model
by Jorrie
http://cr4.globalspec.com/blogentry/15640/Colliding-Branes-Cosmic-Model
The idea of ‘branes’ (or membranes) originated in quantum physics, more specifically as an extension of superstring theory. Superstrings are one dimensional entities that operate in a ten dimensional spacetime (three space, one time and six compacted extra dimensions). They avoid singularities, because their minimum size is the Planck length (~10-35 m), not point particles as in ‘standard’ particle physics. However, there were a few problems.
From http://en.wikipedia.org/wiki/Superstring_theory: “Theoretical physicists were troubled by the existence of five separate string theories. A possible solution for this dilemma was suggested at the beginning of what is called the second superstring revolution in the 1990s, which suggests that the five string theories might be different limits of a single underlying theory, called M-theory.” It is today more or less accepted that the “M” stands for “membrane”, although it was never explicitly stated by its originator, Edward Witten of the Institute for Advanced Study. It essentially boils down to adding an extra (eleventh) dimension to superstring theory, giving a string a two dimensional character, like a membrane or simple surface in a three dimensional world. The five different superstring theories are thought to be just different aspects or views of the same thing, which lives on a brane.
The Ekpyrotic Cosmic Model
This rather weird name came out of “work by Neil Turok and Paul Steinhardt and maintains that the universe did not start in a singularity, but came about from the collision of two branes. The name comes from a Stoic term “ekpyrosis”, meaning conflagration or “conversion into fire” (http://en.wikipedia.org/wiki/Ekpyrotic)(a). Weird as it may sound, the model essentially makes the same predictions as the standard ΛCDM cosmic model (without inflation(b)) and as a bonus, it offers some degree of explanation for dark matter and dark energy. The squares represent two parallel branes in normal three dimensional space (x,y,z) with one dimension (x) suppressed. The normal x-direction is replaced by the fourth space dimension (w), also call ‘the bulk’. The branes are embedded into the bulk and can move in that fourth dimension.
Of even more importance, this ‘conversion-into-fire’ model predicts some potentially observable effects that differ from the standard cosmic model and hence it is potentially falsifiable by observation, something considered a plus point for any good scientific model or theory. The best news is that both predictions have to do with the Cosmic Microwave Background (CMB), which is currently under renewed scrutiny by the ESA Planck Mission.
Observations that may distinguish between the ekpyrotic and inflationary models include polarization of the CMB and the frequency distribution and strength of the CMB gravitational wave spectrum. The inflationary scenario of the standard BB theory makes specific predictions about these observables and all one needs is a detailed enough map of the CMB to verify it. By detailed enough is meant that very small temperature fluctuations must be detectable so that effects like polarization and gravitational wave signatures can be extracted. The ekpyrotic model does not predict the same level of gravitational waves as the inflationary model; in fact the amplitude may be extremely low. More about that in the next section.
Since the very first Planck data have just been released, we may have to wait a number of years for the finer details to be analyzed, but this is surely exiting stuff. It is not often that a new cosmological model arrives that satisfies all the present observations and makes predictions about relatively easily measurable parameters that differ from the accepted theory.
How does ‘Conversion-into-Fire’ differ from the ‘Big Bang’?
The Big Bang is usually associated with a theorized cosmic epoch from about t=10-35 to 10-32 seconds, also simply called cosmic inflation, during which the universe expanded by a factor of at least 1026. This means a region of space the size of an atomic nucleus (about 10-14 m) ‘blew up’ to at least 1012 m, about the distance of Jupiter from the Sun. The temperature immediately after inflation was immense, about 1027 K. The colliding brane scenario does not produce that high a temperature, but ‘only’ about 1020 K, hot enough to make all the particles predicted by the inflationary cosmos. Still a very hot start, but not from the near-infinite energy density required when inflation started.
Because branes must have quantum fluctuations in energy, a collision between two of them should produce the pattern of cold and hot spots that is observed in the CMB today. However, the colliding branes should produce a different polarization and a much lower level of gravitational waves than inflation predicts. An ‘explosive’ inflation must produces a large amount of energetic gravitational waves, but a brane collision should happen over million or even billions of years and will produce much less, if any, gravitational waves. The Planck mission should be able to detect the level of gravitational waves (as ‘ripples’ in the CMB) that standard inflation should have produced. If not observed, it could perhaps have implications for the inflationary epoch and the ‘shares’ of theories like brane cosmology will improve.
No Beginning of Time?
Colliding brane cosmology is actually a ‘cyclic model’, i.e., it does not require a beginning of time, because the two (or more) branes must have existed before our current cosmic expansion started. In fact the ekpyrotic model says that it could be an ‘eternal’ process of collisions and ‘heat death’ due to expansion, because the two branes would move only a short distance apart before approaching again for another collision. The ‘short distance’ could however take billions of years to accomplish, by which time each brane has expanded a significant amount. Author John Gribbin describes(c) the force between the branes as a “spring-like force”, i.e., when they get too far apart, they are pulled together and if they are getting very close, they are pushed apart again – an oscillation in the fourth space dimension.
The ‘collision’ between two branes does not have to proceed all the way – the spring-like force should stop them from actually colliding, similar to ‘bounce cosmology’ in ordinary three dimensional space. Hence, no singularities, even in the fourth space dimension. It is however not completely clear if there must still be a beginning of time, when the oscillations started. If so, how did they get started?
Dark Stuff explained?
The oscillations have the same effect on the expansion of each brane as dark energy in the standard ΛCDM model. It causes the branes to expand at an accelerating rate after an initial deceleration. Before the next collision, the density of our universe thins out to be virtually zero and it is near absolute zero in temperature. Then a lot of positive energy is dumped into both branes by the collision and the cycle repeats. Because of the negative value of resulting gravitational energy, there is no net energy gain – the total energy remains precisely zero, just like in the standard flat cosmic model, where it starts with zero net energy.
Dark matter of the ΛCDM model is explained as the effect that each brane has on its next-door neighbors. Unlike other particles, gravitons (the particles carrying the gravitational effect) are not constrained to just the surface of each brane. Gravitons can move between branes and so one brane can enhance the gravitational effects in another brane. This is possibly what we observe as the ‘missing matter’ that prevents galaxies from disintegrating under their own (too-fast) rotation. It is also exactly what is required to make the universe spatially ‘flat’.
Another claim of the theory is an explanation for the extreme weakness of gravity. It is only 10-38 times as strong as the strong nuclear force, making it somewhat of a puzzle. The other force carrying particles move in the brane only, but gravitons cannot move there. They must first go into the bulk and then, according to the theory, only some of them ever return to the brane, to some other spot.(d) It may be this “leaking” of gravitons out of the brane and into the bulk that makes gravity such a weak force.
Conclusions
Is this all too good to be true? Maybe it is. The main criticism against the ekpyrotic model is the speculative nature of inter-brane forces in the fourth space dimension. There is no good mechanism in string theory (or any other solid theory) for the ‘spring-like force’, so it may be just another disguise for mysterious dark energy and dark matter. Another (weaker) criticism is that it still needs a beginning somewhere in the past, where the branes were created.
Nevertheless, it would be very interesting if Planck does not detect the gravitational wave signatures in the CMB that are predicted by inflation. It may obviously then mean that they are just below the threshold detectable by Planck and will be detected by the next space observatory, but still…