The Big Bang scenario with inflation is the prevailing standard model of cosmology, the theory that best accounts for the origin and evolution of the universe. It describes the early universe as arising from a singularity that at first expanded exponentially at super-light speeds, then slowed to the more moderate expansion that we presently observe. It has some problems that will be described below, but it is the best theory we have, and it fits the observations.
However, in the past year a new alternative to inflationary Big Bang cosmology has been proposed that is receiving considerable attention. It is called "ekpyrotic cosmology", and it describes an early universe in which there was no Big Bang at all, but instead a collision of higher-dimensional "branes".
This new cosmology uses as its starting point recent work on millimeter-scale gravity (see my column #98, "Millimeter Gravity and the Superstring Wall", in Analog, December-1999) that comes from "M-theory" (what the physics community used to call "superstring theory"). The millimeter-scale gravity ideas describe each 4-space location in our universe as confined to a thin extra-dimensional "D-brane wall" on which the strong, weak, and electromagnetic interactions are confined, while the gravitational interaction is allowed to expand away from this wall in two or more extra dimensions. Gravity, in this picture, is weak because most of its lines of force are dissipated into these extra dimensions. The other three forces, because they are confined to the D-brane wall, are not similarly weakened. Here the word "brane" is derived from the word “membrane” and means a thin membrane-like planar structure embedded in a multidimensional space.
This new ekpyrotic cosmology derives its name from the ancient Greek stoic philosopher’s notion that the universe was cyclically destroyed and re-created by fire. It suggests that our universe originated from the fiery collision of two of these “branes”. In other words, our universe, which perhaps had been cold and featureless for an indefinite time, was struck by another brane, like two hands coming together to produce a Big Clap. The effect of this collision of branes was to produce the energy, matter, and structure that we find in the present universe. In this scenario, the universe did not begin with an infinitely hot Planck-scale singularity. Instead, it started at a finite size and temperature that was initially static, but after the collision it expanded.
To understand the implications of these new ideas, let’s begin with a review of what is presently the “standard model” of cosmology, the inflationary Big Bang scenario. The simple Big Bang model describes our universe as having exploded from a “singularity”, a point-like region of space that was supersaturated with energy, forming an ultra-hot ultra-dense medium in which the gravity was so strong that it curved space back on itself in a distance of about 10-34 m. As the universe became larger it cooled from its ultra-dense ultra-hot origins. The fundamental interactions sorted themselves out into the strong, weak, electromagnetic and gravitational forces. The resulting mixed soup of matter, neutrinos, and radiation cooled and separated, and the components went their separate ways. The radiation component we see today as the cosmic microwave background. The matter congealed into dust clumps that became galaxies. Stars formed, exploded in supernova violence, and formed again, repeatedly recycling matter into heavier elements. Planets formed around some of the stars and became infested with life. We came along very late in the game to try to piece together all that had previously happened. The result is what we call the Big Bang.
However, this simple version of the Big Bang model has a number of problems which are called the problems of homogeneity, flatness, inhomogeneity, and monopoles. For example, a hypothetical observer looking at the sky immediately after the Big Bang would see the horizon of the visible universe (the distance at which light is Doppler shifted to zero energy) only about 10-34 m away. Immediately after the Big Bang, each spatial region of this size became causally disconnected from the many other similar regions. Today, however, the horizon of the visible universe combines 1090 of these disconnected regions, some of which are only now coming into causal contact with the rest of our universe. There is no particular reason why these 1090 regions should resemble one another at all, yet we know from the COBE microwave background measurements they vary at most by one part in 100,000. The remarkable smoothness of the universe is a fundamental mystery. Why are the 1090 independent pieces of today's universe so similar? This is the homogeneity problem.
The flatness problem is raised by the remarkable lack of curvature, either positive or negative, of the present universe. There is a nearly precise balance between the expansion energy and the gravitational pull on the universe. Gravity and expansion are within about 1% of perfect balance. The inhomogeneity problem concerns the origins of the structure observed in the cosmic microwave background radiation and in the large-scale structure of the universe. The monopole problem concerns the observed absence of magnetic monopoles in our universe, when they should have been produced in great numbers in the early Big Bang.
The currently accepted solution for these problems, the "inflation scenario", assumes that in the very early stages of the Big Bang, for reasons not well understood, the universe expanded at an exponentially increasing rate, with the radius growing much faster than the speed of light. The problem with the inflation scenario is that while it cures the ills of simple Big Bang cosmology, it seems rather contrived and raises unresolved questions of its own. (See my column #94, "Before the Big Bang", in Analog, March-1999).
The problem with the inflationary scenario is that it is "put in by hand". In other words, it is inserted into the theory without explanation or physical mechanism in order to solve certain problems. The origin of the enormous force that produced the exponential expansion is not explained, nor is the requirement that it operated for a time and then stopped. Further, the inflationary Big Bang model compels us to try to understand the laws of physics at the Planck Scale near the initial singularity, where energy densities are so enormous that we have no usable theories and no potential experiments.
The ekpyrotic scenario provides an interesting alternative to inflation. It describes our universe as a "visible brane", the D-brane hyper-surface that we live in and experience. It also hypothesizes the existence of a nearby “hidden brane”, another universe on a D-brane that is parallel to ours but separated at a constant distance from our universe across two or more of the extra dimensions. Initially, perhaps for a very long time, the visible brane remains cold, static, and empty. At some time, however, the hidden brane sloughs off a lighter “bulk brane” which travels across the extra dimensional separation and violently collides with our visible brane. There are ripples in the bulk brane, so that the collision happens at slightly different times in different regions of the visible brane. There are gravitational and other forces acting between the branes before and after collision, and these cause the length scale in the visible brane to contract before the collision and expand after the collision. This shrinking, collision, and expansion produced the expanding universe that we presently observe.
The colliding branes are initially flat (in the sense of space curvature,) and this flatness is retained by our universe after the collision. The ripples in the bulk brane produced the balance between homogeneity and large-scale structure that we presently observe. And the ekpyrotic universe, even at the instant of the Big Clap, was never hot enough or close enough to a singularity to produce the flood of monopoles that the simple Big Bang model predicts. Therefore, the ekpyrotic scenario deals with all of the problems of the simple Big Bang scenario without invoking inflation. It has the advantage that no Planck-scale physics or mysterious forces that switch on and off are involved. It becomes identical to the Big Bang model at some extremely high temperature, so that the subsequent evolution of the universe, in particular the differentiation of the four forces, the synthesis of light elements, and the production of the background radiation, is the same in both models.
The crucial test of any theory is whether it makes predictions that can be tested experimentally. In the present case, the question is whether there are tests that can distinguish inflationary cosmology from ekpyrotic cosmology. The answer seems to be "perhaps". The main differences between the models are in their production of primordial gravity waves. Inflation tends to produce a "red" gravity-wave spectrum that decreases in strength with decreasing wavelength, while ekpyrotic cosmology produces a "blue" gravity-wave spectrum that increases in strength with decreasing wavelength. Therefore, a study of the primordial spectrum of gravity waves would be a crucial test of the models.
Unfortunately, theses effects are too small to be detected by the NSF's LIGO gravity detector that is now coming into operation in Louisiana and Washington State, or by the space-based LISA detector project planned by the European Space Agency. The remaining possibility is the detection of optical polarization in the cosmic microwave background radiation induced by long wavelength gravity-wave effects. If such polarization were detected, it would tend to support inflationary cosmology and to falsify ekpyrotic cosmology. However, no such polarization has yet been observed.
Finally, since this is a science fiction magazine, let's consider the science-fictional applications and implications of this new cosmology. If our universe was indeed produced by a collision with a traveling brane in a Big Clap, there's no particular reason why that would happen only once. That has the makings of a universe-scale disaster scenario, with unexpected forces first detected weakly, then building up as the bulk brane approaches our universe through the extra dimensions. The Clap when the branes collide would restart the Big Bang and bring an end to all life and civilizations that did not have an extra-dimensional escape hatch.
Another scenario might involve travel to another brane, a parallel universe that is separated from ours only by a small distance in the extra dimensions (as in my hard SF novel Twistor.) Are there many of these brane universes? Would each brane-universe resemble ours, or would it be different? Are its laws of physics the same? Is it dominated by matter (as is ours) or by antimatter (a convenient energy source if one could fish some antimatter across the inter-dimensional gap)? Does time run in the same direction? Is the speed of light the same, or can one go faster over there and then return to our universe at a different location? Do the other branes contain stars? Planets? Life? Civilizations?
All of these questions depend on whether the Big Clap of ekpyrotic cosmology and M-theory actually describe our universe. Stay tuned for further developments.
"The Ekpyrotic Universe: Colliding Branes and the Origins of the Hot Big Bang", J. Khoury, B. O. Ovrut, P. J. Steinhardt, and N. Turok, preprint hep-th/0103239 v3
"From Big Crunch to Big Bang", J. Khoury, B. O. Ovrut, N. Seiberg, P. J. Steinhardt, and N. Turok, preprint hep-th/0108187 v3.
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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