This AV column is about a new alternative to standard Big Bang cosmology that reaches back in time to the era before the Big Bang in an effort to remove some of the arbitrary assumptions from the model. It's in part the work of Gabriele Veneziano, a theorist at CERN, and it is called pre-Big-Bang cosmology. We'll begin by reviewing the standard scenario of the origin of the universe.
The standard Big Bang model of cosmology describes our universe as having precipitated out of some prior cosmos, rather like a bubble forming in a newly poured glass of champagne. About 8 billion years ago an irregularity in that cosmos, like the grain of dust at the heart of a rain drop, caused a tiny region of space to begin expansion, an expanding bubble that became our universe. Within the bubble was normal space, while outside was the energy-saturated medium of the initial cosmos. The outward-expanding walls of the bubble were driven by the energy liberated in changing space at the interface. 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 mixed soup of neutrinos, matter, and light cooled and separated, and the components went their separate ways. 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, and we came along very late in the game to try to piece together all that had happened. This is what we call the Big Bang model.
The problems with this form of the Big Bang model are ones of "tuning". A hypothetical observer looking at the sky a few Planck times after the Big Bang would see the horizon of the visible universe (the distance at which light is Doppler shifted to zero energy) only a few Planck lengths away (about 10-34 m). Starting at the Big Bang, each spatial region of this size was causally disconnected from 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 obvious 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?
A similar problem is raised by the remarkable "flatness" of the universe, the nearly precise balance between expansion energy and gravitational pull, which are within about 15% of perfect balance. Consider the mass of the universe as a cannonball fired upward against gravity at the Big Bang, a cannonball that for the past 8 billion years has been rising ever more slowly against the pull. The extremely large initial kinetic energy has been nearly cancelled by the extremely large gravitational energy debt. The remaining expansion velocity is only a tiny fraction of the initial velocity. The very small remaining expansion kinetic energy and gravitational potential energy are still within 15% of one another. To accomplish this the original energy values at one second after the Big Bang must have matched to one part in 1015. That two independent variables should match to such unimaginably high precision seems unlikely.
The now-standard solution for these problems is to introduce "inflation", to assume that in the very early stages of the Big Bang the universe expanded at an exponentially increasing rate, the radius growing much faster than the speed of light. The superluminal inflation brought all the 1090 regions into causal contact and homogenized them. The inflation scenario assumes a special force field (the inflaton field) which allows the universe to roll downhill in potential energy, driving the exponential expansion and simultaneously creating the mass-energy and expansion in the early universe, thereby locking expansion energy and gravitational potential together because they have the same source. This strikes the observed expansion-gravity balance that produces our "flat" universe.
The problem with all of this is that the inflation scenario seems rather contrived and raises many unresolved questions. Why is the universe created with the inflaton field displaced from equilibrium? Why is the displacement the same everywhere? What are the initial conditions that produce inflation? How can the inflationary phase be made to last long enough to produce our universe? Thus, the inflation scenario which was invented to eliminate the contrived initial conditions of the Big Bang model apparently needs contrived initial conditions of its own.
For this reason,Veneziano has recently been looking in another direction in accounting for our present universe. He has attempted to apply to the universe itself the techniques of superstring theory, which attempts to describe a fundamental particle (electron, quark, …) not as a mathematical point but as a tiny loop of string embedded in a multidimensional space. Superstring theory is mathematically interesting but experimentally untested (and perhaps untestable). Veneziano's starting point is the so-called "scale-factor duality" of superstring theory, a symmetry in the time domain. If one knows the behavior of a system for positive times, its behavior at negative times can be predicted. He applies this duality to the universe before and after the Big Bang, reflecting the universe in the duality mirror to examine its condition before the Big Bang
The universe behind the mirror looks very different from ours. It starts as a nearly empty, cold, flat space. It is very simple, lacking in structure, and describable by a minimal set of parameters, but it is dynamically unstable. A small patch of this simple universe fluctuates slightly, and that patch begins to expand exponentially. As it approaches t=0 (the Big Bang) from the back side this pre-universe has a rapidly increasing Hubble parameter (or expansion rate). It squeezes through t=0 point with a maximum expansion rate, emerging as the small, hot, dense system, the Big Bang from which our universe evolved.
Another way to play this scenario is to run it backwards, starting with some familiar universe-model on this side of the Big Bang and extrapolating back to the pre-universe on the other side. When this is done, it is found that closed universes (with too much mass and an omega-parameter greater than 1) must begin with a singularity, while open universes (with too little mass and an omega-parameter less than 1) evolve backwards to a flat cold universe as described above. The preponderance of evidence suggests that our universe is open, and therefore it is plausible that our universe started as the flat cold system of the latter scenario.
Therefore, pre-Big-Bang cosmology appears to be a very promising line of theoretical development. It offers the possibility that the fine-tuning problem can be avoided and the related problems of horizon and flatness solved without theoretical gymnastics by extending superstring theory, developed to explain particle behavior at the smallest distance scales, to the grand scale of the universe itself.
As I observed at the beginning of this column, one of the regrettable features of superstring theory is its lack of experimentally testable predictions. It is therefore fair to ask if pre-Big-Bang cosmology has the same problem. The answer to this question seems to be "No". There seem to be at least two feasible experimental tests of the model. The first is that the fluctuations at the Planck scale which triggered the exponential expansion of the pre-universe will be preserved by the process and stretched out in size by the red-shift accompanying the pre- and post-Big-Bang expansion. These fluctuations should be observable in small-angle variations in the cosmic microwave background. Fortunately the small-angle structure is the cosmic microwave background is already of great interest, and satellite microwave measurements are currently being implemented..
Another test involves LIGO, the gravity wave detector (see my Alternate View column in the April-1998 issue of Analog) scheduled completion and first operation is sometime in the year 2000. LIGO is a pair of L-shaped gravity-wave detectors, one located in Eastern Washington and the other in Central Louisiana. The pre-Big-Bang scenario predicts an unusual spectrum for gravity-waves, giving them an energy-density that rises rapidly with frequency. This effect should be detectable from correlations between the two LIGO detectors for plausible ranges of parameters of the underlying superstring theory.
In a few years we should know if the initial promise of the new pre-Big-Bang cosmology can be supported with experimental data. If this turns out to be the case, the same data will also indirectly provide the first experimental evidence for superstring theory, which has thusfar been sustained only by its mathematical elegance.
Thus, we have the prospect of seeing further back in time than we had ever imagined, not only back to the kernel from which our universe exploded, but much further back than that, back beyonf the Big Bang to an era when the universe was cold, flat, empty, and unstable.
"A Simple/Short Introduction to Pre-Big-Bang Physics/Cosmology", Gabriele Venetian, preprint hep-th/9802057 v2 in ArXiv, (March 2, 1998);
"Detecting relic gravitational radiation from string cosmology with LIGO", Bruce Allen and Ram Brustein, Phys.Rev. D55 (1997) 3260-3264, preprint gr-qc/9609013 in ArXiv, (Sept. 5, 1996);
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