In this column I want to describe a new possible description of the universe, a cosmological paradigm proposing a universe that is infinite, eternal, and that recycles itself from Big Bang to Big Crunch at regular time intervals. This new model of cosmology is the work of Paul J. Steinhardt of Princeton and Neil Turok of Cambridge, and is inspired by string theory and based on Steinhardt’s own ekpyrotic cosmology, as described in a previous AV column (“Brane Bashing: Big Bang or Big Clap?”, Analog, April-2002).
A cyclic universe, a world that goes through periodic cycles of destruction and renewal, is a classic theme of many mythologies and religions. There was also a decade or so in the mid 20th century when recycling universe models were quite fashionable in physics circles. This was after theoretical physics had taken on the job of describing possible scenarios of the origin and evolution of the universe using the mathematics of Einstein’s general relativity, but before the average mass-density of the universe had been measured with any accuracy. The cyclic cosmology hypothesized that our universe started with a point-like Big Bang singularity that expanded to its present dimensions. In the future, it would continue to expand for a while and then “stall”, as gravitational forces overcame its outward expansion momentum. From the stall point, it would re-collapse to a Big Crunch, bounce, and the whole process would repeat with regenerated and renewed matter and energy. There was a debate as to whether time would run forwards or backwards during the collapse phase. The recycling universe scenario was recognized to have a certain ecological appeal, because no matter how much garbage, mess, and pollution our civilization creates today, anything and everything will eventually be crunched together, renewed, and restored.
However, the cycles of the recycling model were discovered to be imperfect. Richard Tolman showed in the 1930s that a recycling universe would accumulate entropy (i.e., disorder), so that it would bounce to a larger size and would contain a different quantity of mass-energy with each cycle. Thus, extrapolating backwards, some past Big Bang would have to have been the first. Therefore, the cycling model is not eternal. Some cycle is must be the first, and the appeal of an eternally recycling universe is lost.
More recently, astrophysical estimates of the average mass density of the universe have shown that there is not enough mass in the universe (by roughly a factor of 3) to produce a closed recycling universe of the kind Tolman envisioned. Therefore, the fall-back to a Big Crunch and the cyclic universe models have been abandoned. The current inflationary Big Bang scenario, which has become our standard model of cosmology, describes a universe that starts from the Big Bang and continues to expand forever, apparently with the rate of expansion increasing with time.
The standard inflationary Big Bang scenario, while very successful in accommodating most of the astrophysical observations about our universe, has several problems. First, there is considerable difficulty with its starting point and the physics at the initial singularity, where the mass-energy density must be infinite and the laws of physics become “inoperative” (as they used to say in the Nixon Administration.) Contemporary physics is incapable of describing the starting point of the Big Bang. Even at the slightly later stage of the scenario, during which the universe is undergoing a superluminal “inflationary expansion”, the inflation does not follow naturally from the mathematics of the model, but rather must be put in “by hand”.
An additional problem for the inflationary Big Bang scenario comes from the recent observation from studies of distant supernovas, supported by small-angle variations found in the cosmic microwave background. These data imply that the expansion-rate of the universe is itself increasing, perhaps under the cumulative negative pressure of “dark energy” in the vacuum itself (see my AV columns in the May-1999 and January-2001 issues of Analog). Standard Big Bang cosmology did not anticipate this discovery, and it must undergo some meticulous fine tuning to accommodate it. For the Big Bang model, it’s a case of “Who ordered that?”, as Wolfgang Pauli asked when the mu lepton (a heavy-weight electron) was discovered unexpectedly.
The new Steinhardt-Turok cosmology offers several advantages over the standard Big Bang model in dealing with these problems: (1) it has no singularities, and therefore never reaches a Planck-scale energy density at which the laws of physics might fail, (2) it requires no inflation phase, and therefore needs no explanation for why inflation might have occurred, and (3) the observed dark matter, dark energy, and accelerated expansion of the universe are all natural consequences of the model.
How does it work? The trick is that the expansion and collapse of the universe does not occur in normal space, but in an extra (and unobserved) dimension occupied by gravity (see my AV column in the December-1999 Analog). The model has one time and four non-compact space dimensions (with any extra dimensions “compactified” into small loops). The four-space is delimited by two three-space boundaries or “branes”, one of which has positive tension (i.e., energy) and the other negative tension. Our universe is the positive-tension brane. All the matter in our universe is constrained to reside on that three-dimensional brane, and is not allowed to venture out into the 4th dimension, which separates the two branes. Only gravitational forces may cross the extra-dimensional gap. Therefore matter in our universe feels the gravitational influences of matter and energy on the other brane as well as ours, and is influenced by fields that fill the four-space.
The starting point of a given cosmological cycle is what I called in a previous column the Big Clap. There is a force between the two separated branes that causes them to move with respect to one another, and when they momentarily come together, the resulting “clap” fills both branes with mass-energy. The resulting energy density is large, but is not large enough to cause problems with the laws of physics. As the branes move apart, the matter and energy expands, the fundamental forces sort themselves out, matter and radiation form, nuclei are synthesized, radiation decouples from matter, stars and galaxies form and cycle through supernova explosions, planets coalesce, life forms and evolves, and so, here we are.
In the Steinhardt-Turok cosmology, the field responsible for the forces between the branes is a form of energy that permeates the space of the branes, so dark energy and accelerated expansion are direct consequences of the model. The Steinhardt-Turok universe expands at an ever-increasing rate under the influence of the dark energy, and as it does, the average mass-energy density drops. Black holes that have formed during the high density era decay into photons and electrons by Hawking radiation, and perhaps protons and nuclei eventually decay to the same final products. Ultimately, mass-energy in the cold and ancient universe reaches the level of less than one particle per horizon, and the universe is a cold, dark, dead, and empty place.
Then, in the Steinhardt-Turok model, a remarkable thing happens. The restoring force betweens the branes becomes dominant, and the branes begin to move together again. One might think that this would make space on the brane re-contract in a time-reverse of the expansion, but it does not. Instead, the space remains cold and empty, but the coupling constants of the fundamental forces, which had been constant since the Big Clap, begin to change and become stronger. An observer would experience some fairly weird physics during the collapse phase as the forces change, but the universe is empty and so nobody notices. Finally, the branes clap together again, filling the almost empty branes with a flood of new mass-energy, and the cycle repeats.
That’s the model. And remarkably, it can probably be tested against the standard Big Bang model in the next decade. The Big Bang model, as a result of its turbulent super-high density initial phase, should produce in its early stages a cosmic flood of gravitational radiation, and this should show up as polarization of the electromagnetic cosmic background radiation. The Steinhardt-Turok model predicts no such gravity waves and polarization. If that polarization of the microwave background is found, the Big Bang model gains strong experimental support. On the other hand, if no such polarization is present, the Steinhardt-Turok cyclic model receives similar support. The crucial measurements should be accomplished with space- or balloon-based instruments in the next decade.
Since this is a science fiction magazine, let’s consider some of the SF implications of this new cosmology. My friend, the late (and deeply missed) Poul Anderson, wrote Tau Zero, a marvelous hard-SF novel set aboard a runaway spaceship that accelerates ever faster, edging so close to the speed of light that relativistic time dilation permits the crew to view and experience the expansion, reversal, and re-collapse of a Tolman-type cyclic universe. They ultimately pass through the climactic Big Crunch and emerge in a brand new universe, in which they are to become the Elder Race. Assuming the ship technology could be worked out, would that scenario be possible in a Steinhardt-Turok universe?
I’m afraid not. Up to the recollapse, things might work about as described in Tau Zero. However, no human observer could survive the era when the force coupling constants begin to change. The atomic and nuclear physics implicit in the chemistry of life works only because of a delicate choreography that precisely balances the strengths of the forces. A slight shift in this balance, and human (or non-human) observers would die from a variety of system failures.
The key concept used in Paul Preuss’ hard-SF novel Re-Entry is that a spaceship hyperdrive that appears to provide faster-than-light travel (and even tine travel) is actually transporting the ship from one cycle of a cyclic universe to the next, but arriving at identical space-time locations that could not be reached by sub-lightspeed travel.
Preuss’ delightful notion, while requiring a fully deterministic universe, is a lovely solution to the paradoxes of faster-than light travel and time travel, and it fits well with the Steinhardt-Turok universe. However, I think its requirement that each universe cycle produces exactly the same worlds, down to the same individual humans thinking exactly the same thoughts, is a show stopper. The intrinsic randomness that permeates the quantum level of physics guarantees that each cycle of a Steinhardt-Turok universe will be very different than the others. Score one for variety.
So the field is wide open for new hard SF based on the Steinhardt-Turok universe. Any takers?
AV Columns On-line: Electronic reprints of over 110 "The Alternate View" columns by John G. Cramer, previously published in Analog, are available on-line at: http://www.npl.washington.edu/av.
"A Cyclic Model of the Universe”, Paul J. Steinhardt and Neil Turok, Phys. Rev. D65, 126003 (2002); preprint hep-th/0111098, available at http://arxiv.org/ ;
"The Cyclic Universe: An Informal Introduction”, Paul J. Steinhardt and Neil Turok, preprint astro-ph/0204479, available at http://arxiv.org/ .