Isaac Newton famously said "If I saw further, it was because I stood on the shoulders of giants," referring to Galileo and Kepler. To make sense of a new theory of the Universe we must stand on the shoulders of preceding cosmologists, theoretical and observational. Steinhardt and Turok explain the key concepts of modern physics and cosmology needed to understand their new theory. (1) Likewise, this review summarizes those underlying ideas. (2)
Ever since Copernicus showed that the Cosmos doesn’t revolve around Earth, each discovery has found that the rest of the Universe is not basically different from our vicinity. Conditions may vary, but the Laws of Physics are the same everywhere. This cosmological principle is a necessary assumption for asserting universal laws of nature. A classic instance is Newton’s recognition that the moon is held in its orbit by the same force of gravity that pulls an apple to earth. The principle implies that the Universe cannot be infinite in both space and time, stable, and uniform in composition, due to Olber’s paradox: in an infinite Universe of uniform average density, we would receive an infinite amount of light from every direction.(3)
In the early 20th Century, the "great debate" was whether the Universe extended beyond the Milky Way galaxy. In the 1920s, Edwin Hubble measured the distance to spiral nebulae, showing that they are galaxies outside the Milky Way, then made an even greater discovery. By measuring the light spectra from distant galaxies, he found that the characteristic frequencies of known elements all appear to be shifted to the red. This is explained by the Doppler Effect: systematic color shifts indicate relative motion; red shifts imply motion away from the observer. Hubble found red shifts of galaxies directly proportional to their distances. In accord with the cosmological principle, astrophysicists account for this not by assuming that we are uniquely repulsive, but rather that the whole Universe is expanding: all the galaxies are moving away from each other. It is hard to avoid the implication that in the remote past, all the matter in the Universe was much closer together.
The Big Bang Theory - that our Universe originated in a cosmic explosion from an extremely hot, dense state – began in the 1920s with the cosmic models of Alexander Friedmann and George Lemaitre, who found solutions to Einstein’s General Relativity equations consistent with the Hubble expansion. Lemaitre called the initial state of the Universe the "primeval atom". Einstein himself, assuming that the Universe was neither expanding nor contracting, had reluctantly added a cosmological constant, an extra term for an antigravity field, to his equations. Einstein called that his "biggest blunder" once the Hubble expansion was discovered.
A fuller Big Bang theory was developed in the 1940s by George Gamow and his students Ralph Alpher, formerly of Union College (just deceased), and Robert Herman. Gamow included Hans Bethe as nominal co-author of the seminal paper On the Origin of the Elements to make it Alpher, Bethe, Gamow: Alpha, Beta, Gamma :-)
In the 1950s, the main rival to the Big Bang idea was the Steady State theory of Fred Hoyle, Thomas Gold and Hermann Bondi. They could not accept that the whole Universe emerged out of nothing. They postulated that new matter is created very gradually as space expands. That would maintain the average density of the Universe as it expands, consistent with the cosmological principle. The Steady State theory substituted a very large number of unexplained spontaneous creation events for a single humongous one. But the Big Bang hypothesis begs the question, what was before the Big Bang?
Stephen Hawking and others respond by noting that space and time are both aspects of a unified space-time continuum, according to Einstein’s theory of Relativity. Space-time need not be linear and Euclidian; it may be curved, like the surface of a globe, but in a higher dimension. General relativity explains gravity as the curvature of space-time in the vicinity of concentrated mass. At the birth of the Universe, all its mass was concentrated in a very small space, maybe even a point called a singularity, so the curvature of space-time was very great or even infinite. Depending on the true geometry of space-time, asking "what was before the Big Bang" may be as meaningless as asking "what is north of the North Pole?" That answer is logically and mathematically consistent, but as we will see, Steinhardt and Turok have a different answer.
The Big Bang theory made two other testable assertions in addition to explaining the Hubble expansion. One was that almost all the matter in the Universe would consist of hydrogen and helium in specific proportions which we do observe. Second, Alpher and Herman predicted Cosmic Microwave Background Radiation, practically uniform from all directions (isotropic). The Steady State model made no such prediction. When Arno Penzias and Robert W. Wilson of Bell Labs observed isotropic microwave radiation in 1964, a strong scientific consensus came to support the big Bang hypothesis. Penzias and Wilson were awarded the Nobel Prize, which many believe should also have gone to Gamow and Alpher. (4)
The Big Bang theory says that the background radiation was released some 380,000 years after the Big Bang, when the Universe had expanded and cooled below the temperature that ionizes atoms, which then remained stable. Before then, the Universe resembled an opaque, superheated fog, so hot and dense that radiation could not move through space without immediately being scattered by rapidly moving charged particles. When the atoms formed, space became transparent for the first time, and the whole Cosmos glowed like a red giant star. Since then, the Universe expanded by a factor of 1000 in every direction, stretching the wavelength of the radiation to the range of microwaves.
Ever since Hubble discovered the expansion of the Universe, models of cosmic evolution assumed that the observed expansion since the Big Bang would slow down due to the self gravitation of the entire Universe. Observation programs using the Hubble Space Telescope sought to measure the rate of expansion, to find whether that slowing effect would be sufficient to reverse the expansion in the remote future, leading to an eventual contraction and Big Crunch (a Big Bang in reverse), or insufficient, so that the expansion would continue forever, pulling all galactic clusters so far apart they would lose contact irreversibly in a Big Rip.
Observations of remote galaxies in the late 1990s used certain exploding stars as standard candles – objects of known brightness – to measure the distance to very remote galaxies, Billions of light-years away. Type 1A supernovae occur when very dense dwarf stars absorbing matter from companion stars reach a critical mass, collapse to form black holes, and explode with immense energy. Because the critical mass is the same for all 1A supernovae, they all emit about the same amount of energy. They can be distinguished from other types of supernovae - giant stars that burn out and implode - by how their brightness changes over time.
Supernovae are Billions of times as bright as normal stars like the Sun, so they can be seen over cosmic distances. By comparing the observed brightness of a 1A supernova to its known true brightness, the distance to the galaxy where it occurs can be accurately determined. The speed can be found from the red shift of the spectrum. If the Universe were expanding at a constant rate, the distance to a galaxy would be given by the formula: distance = velocity divided by "Hubble’s constant", the rate of expansion per unit distance. That estimated distance can be compared to the known distance of the supernova to calculate the changed expansion rate. Conversely, measuring both the distance and velocity allows Hubble’s constant to be calculated.
Two independent teams of astronomers measured the distances to remote galaxies in 1998 by the supernova method and found that they differ from the distances calculated using the velocity, so the velocity is changing over time: "Hubble’s constant" is actually a variable. But instead of the expected gravitational slowing of the cosmic expansion, they found that the expansion has been accelerating for about 5 Billion years! Before that, cosmic gravitation slowing the expansion was stronger than the weak but steady accelerating force. But as expansion increased all the cosmic distances, the force of gravity declined and acceleration took over.
But where does the energy come from to accelerate the expansion? Conservation of energy, the first law of thermodynamics, is one of the most fundamental and well established of all physical laws. It states that the total amount of energy (including mass, which Einstein showed is a form of energy) always remains constant in a closed system. To reconcile the observed acceleration with the conservation law, physicists proposed a mysterious new field called Dark Energy, acting like negative gravity. Dark Energy is a potential energy field which may be constant throughout all space and time, in which case it is called a Cosmological Constant, or else a variable field dubbed Quintessence, as in the Steinhardt-Turok model.
Discoveries in cosmology constrain the possibilities of quantum physics, and vice versa.(5) Cosmologists hope that earthly experiments will be able to account for the Dark Matter necessary to explain the observed gravitational behavior of galaxies and clusters. In the 1930s, Fritz Zwicky of Caltech used the Doppler shift to measure the speed of galaxies within clusters, and found that they are moving fast enough to escape the gravitational pull of all the visible matter. He said there must be "missing mass" to account for the fact that the clusters do not disperse. (6) Vera Rubin confirmed the existence of Dark Matter in the 1970s by measuring the orbits of stars within galaxies. She found that stars far from the centers of galaxies are moving faster than can be explained by the gravity of visible stars, gas and dust. Further evidence of Dark Matter is the gravitational lens effect, the bending of light in accordance with General Relativity in space curved by the gravitational fields of galaxies and clusters. Again this is more than can be explained by visible matter.
Dark Matter is completely different from Dark Energy, though also mysterious. It seems to interact with known forms of matter through gravity, while not emitting light. Two possibilities for Dark Matter are WIMPs (Weakly Interacting Massive Particles) – heavy elementary particles which do not interact with light, and MACHOs (Massive Compact Halo Objects) -- non radiant objects in galactic halos such as failed stars, orphan planets or black holes. (Gamow was not the only scientist who couldn’t resist a pun.) Visible matter is estimated to account for only about 4 or 5 percent of the mass-energy in the Universe; Dark Matter is about another quarter and the rest is Dark Energy.
The Cyclic Universe model is a dramatic alternative to the prevailing Cosmic Inflation theory, an extension of the Big Bang model formulated in the 1980s by Alan Guth and modified by Andrei Linde, Steinhardt and others. Steinhardt in fact co-authored with Guth the May 1984 article in Scientific American that presented the inflation model to the public. (7)
According to the Inflation theory, the Universe popped into existence as a singularity, an infinitely dense point mass, and immediately went into an exponentially increasing expansion for the first billionth of a billionth of a trillionth of a second, with space expanding at many times the speed of light. (This doesn’t violate Einsteinian relativity, because matter would not move through space faster than light - space itself expands.) In this theory, the period of inflation ended with the Universe extremely hot and dense, many times hotter than the center of the sun, still expanding at very high speed.
Most cosmologists now accept the otherwise implausible inflation theory because it provides a coherent explanation for three observed facts about the Universe at large: flatness, uniformity, and scale invariant distribution of energy and matter in the early Universe, which any rival theory must explain.
Flatness means that, aside from the local gravitational effects of large masses, space is not curved, to a precision of many orders of magnitude. If space had significant positive curvature, like a sphere, the Universe would have collapsed long before its present age of about 13.7 Billion years; if the curvature were negative, like a saddle, the Universe would have expanded too rapidly for stars and galaxies to form. (8)
Uniformity means that the average distribution of matter and energy is the same in all directions, even though parts of the Universe are too far apart to have ever influenced each other and reached equilibrium. Observations of remote galaxies do show a roughly even distribution, with clusters of galaxies in all directions concentrated in similar cosmic bubbles separated by immense voids, on a scale of hundreds of millions of light years.
Scale invariance means that the spatial distribution of matter and energy in the early Universe has about the same amplitude at different frequencies. The quantitative predictions of the Inflation theory were verified by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite’s observations of the cosmic microwave background radiation, beginning in 2001.
WMAP also found tiny variations in the background radiation, about one part in a hundred thousand. The Inflation theory says that those variations are due to quantum fluctuations in the first instants of the Universe, which have now been inflated to intergalactic size. The regions of greater density gradually condensed to form cosmic bubbles rich in galaxies and clusters, while low density zones became voids.
The new cyclic model agrees with the Inflation theory in its description of the Universe from the age of about one second through the present. It gives the same attributes for the background radiation observed by WMAP, as well as the same cosmic flatness, uniformity and scale invariance. But the new theory predicts very different results for future observations, and for the ultimate fate of the Universe.
In the Steinhardt-Turok cyclic theory, the observable Universe we inhabit and experience is part of a larger multidimensional system. Additional dimensions are required by M-theory, an extension of supersymmetric string theory, (string theory for short) which is the leading candidate for a unified quantum theory to explain all the forces of nature, and all types of particle. Supersymmetry says that for each basic particle type there is a partner particle with a different quantum spin and different rules of behavior. Depending on the spin, clumping together of particles is either permitted or excluded.
Theoretical physicists hope that the Large Hadron Collider (LHC), scheduled to come online later this year, will detect particles to confirm supersymmetry and Higgs fields, required by currently prevailing theories, and identify WIMPs to account for the Dark Matter. Experimental physicists hope the LHC will confound the theorists by detecting phenomena not predicted by their theories, requiring "new physics" to explain.
String theory asserts that at the smallest scale, the elemental components of the Universe are not pointlike particles, but tiny one-dimensional strings, which can vibrate only in a limited set of discrete frequencies in a space of ten dimensions. Each type of particle is a distinct vibration mode of the string. Most of the dimensions are too small to observe, but three dimensions of space and one of time are extended
as we experience them.
Five different string models were developed in the 1980s, but then Edward Witten and others formulated M-theory to unify them. M-theory proposes lower-dimensional subspaces of an eleven dimensional space, called "branes", generalizing from two dimensional membranes. The different string models just represent different lower-dimensional cross sections of the same space. (9)
This all sounds utterly artificial, like the epicycles used to get Ptolemy’s Earth-centered model of the solar system to match the data. It seems to violate the great scientific injunction known as Occam’s razor: avoid the unnecessary proliferation of hypothetical entities, choose the simplest explanation that covers the phenomena. But string theory and M-theory were developed to solve a very serious problem in the foundations of physics: the incompatibility of general relativity with quantum mechanics. No simpler theory seemed able to resolve those contradictions.
General relativity is the only comprehensive and extremely well verified theory of matter, energy, space, and time at the largest scales; quantum theory is likewise the only extremely powerful and well verified explanation for phenomena at the smallest scales of elementary particles and their interactions. But as presently formulated they don’t fit together; if the Universe is lawful, a more general theory is required that includes both. String and M-theories are attempts to combine them into a single consistent system covering the entire physical world at all scales, under all conditions. (There are a few rival theories to solve the same problem, including Loop Quantum Gravity and the Twistor theory of Roger Penrose; some top physicists argue that all of this is speculation, not really physics, until the theories make testable predictions).
Steinhardt and Turok, attending a lecture on M-theory, asked what would happen if two branes collided, and realized that it would take the form of a Big Bang: matter and radiation would appear at extremely high temperature and density, and explode in all directions. In contrast with the inflation model, the density, though extremely high, would remain finite, there would be no singularity, and the known laws of physics would apply everywhere. They called this model ekpyrotic, "out of fire" in Greek.
In the ekpyrotic model, our Universe exists in a ten-dimensional brane, associated with another Universe on another brane separated along an eleventh dimension. (10) That Universe can only affect ours by its gravity, which in this model might explain the Dark Matter. No other interaction occurs between the two brane Universes, but Dark Energy – which the theory identifies with potential energy in the gap between them - can move the two branes together or apart along the added dimension. As they developed their model, Steinhardt and Turok realized that it could describe a Universe that goes through endlessly repeated cycles of expansion and reconvergence – a cyclic Universe.
The Inflation model postulates a weird episode of exponentially accelerating cosmic expansion, in the
first instants after the singularity, with the field strength a hundred orders of magnitude greater than that driving the weird episode of accelerating cosmic expansion which has actually been observed in progress now. The Inflation model has no explanation of the Dark Energy driving this acceleration, just as the Steady State model had no explanation of the Cosmic Background Radiation.
Instead, the new cyclic theory explains both initial expansion and current acceleration with one hypothesis – the dark energy field in the extra dimension. In this model, the acceleration we now observe will not continue exponentially forever. Instead, as accelerating expansion stretches the space within our Universe on its brane to an almost perfect vacuum, the extra dimension between the two brane universes contracts.
In about a Trillion years the two branes will converge in a Big Crunch, to a state of extremely high but finite density and temperature. Another Big Bang or Big Bounce will then occur, and the entire cycle of rapid expansion, formation of galaxies, stars and planets, acceleration, and convergence will occur in Trillion year cycles continuing beyond count.
Dark energy, rather than posing a mind bending problem, plays a central role in this model helping to solve several critical problems. The theory identifies the dark energy as potential energy in the extra dimension, between the two brane universes. It accelerates the expansion of our Universe, as observed, and the accelerated expansion stretches space in each cycle uniformly to account for the observed flatness in the next cycle.
The potential energy is converted to kinetic energy of the two moving branes, and when the potential energy goes negative the branes start to converge. When they hit, kinetic energy is converted to matter and radiation. Finally, the dark energy damps the collision of the two branes so that they don’t collide too violently, and generates very high pressure which smoothes out ripples in the branes. The cyclic model avoids postulating creation out of nothing, consistent with the law of conservation of energy.
But the fit of the cyclic model with the current cosmic acceleration is a "retrodiction", an explanation of previous observations, like the Alpher-Gamow explanation of the abundances of the light elements. The scientific community will not be convinced of the new model unless it can predict phenomena which are observed afterward, as the cosmic background radiation verified Alpher’s prediction.
Previous cyclic universe theories came to grief on the rocks of the Second Law of Thermodynamics, considered as certain as any law of nature can be. This law requires that in any closed system, such as the whole Universe, the entropy, or randomness of energy distribution, can never decrease, can only increase. Richard Tolman used this law to knock down previous cyclic models. He showed that each time the Universe cycled, it would emerge into a more random state than the previous one. Reasoning backward, the nonrandom order of previous Universes would have been greater and greater. That would eventually lead to a state of maximum order a finite time ago, contradicting the assumption that the cycles had no beginning.
Steinhardt and Turok respond to the Second Law argument by saying that the problem would only be fatal if the entropy density increased from cycle to cycle. Since the branes are extremely stretched out as they collide at the end of each cycle, the entropy, though increasing, is diluted below the threshold of detection. The added entropy would be pushed out beyond the horizon of observation. So in each new convergence, the new matter and energy in the next Universe start in the same observable state as the previous cycle. I am not yet convinced that this argument really enables the cycles to extend forever in both directions; maybe the authors explain this more rigorously in their technical papers. (11)
Anyway, they argue that their theory, despite the extra dimensions and invisible second Universe, is more plausible than cosmic Inflation. The current Inflation models require that our Universe is just one of a vast, possibly infinite number of other universes constituting a Multiverse, all mutually inaccessible, each of which may have different laws of physics. How is that for proliferation of entities?
Steinhardt and Turok say that although their Cyclic model and Inflation both give the same answers for what we now observe, the theories diverge sharply in predicting possible future observations. The cosmic background radiation would be polarized in the inflation model, but not in the cyclic model. The WMAP data shows no polarization, but its sensitivity is insufficient to verify or refute either theory. Proposed experiments could refine the sensitivity by about a factor of forty.
If polarization is found, that would refute the Cyclic model; if it is not found; the Inflationary theory would be in big trouble. Likewise, the Inflation model, but not Steinhardt-Turok, predicts that cosmic gravitational waves generated in the Big Bang may be strong enough to detect with experiments now under development.
Furthermore, if the gravity wave and cosmic background polarization observations support the Steinhardt-Turok theory, that would also lend support to the M-Theory which seeks to unify all of physics, which has been criticized as unverifiable.
Stay tuned. Ω
_________________NOTES_________________
(1) The terms theory and model both mean a systematic scientific explanation of a wide range of observed phenomena, not just a speculative hypothesis. The term "model" may also mean a specific version of a more general theory.
(2) See The Big Bang by Simon Singh, an excellent history of cosmology, from Eratosthenes and Aristarchus measuring the Earth and Moon, through WMAP measuring the cosmic background radiation, confirming the hot origin of this Universe. Singh clearly explains the scientific ideas as they evolved, and vividly brings the scientists’ personalities to life as he dramatizes the problems and conflicts.
(3) This can be proved mathematically by considering spherical shells around us, like layers of an onion, each one light year thick. The volume of each shell is proportional to the square of the distance from Earth. The intensity of light reaching us decreases with the square of the distance, so the light from the Nth shell would be a constant independent of N. As N, the distance to each shell, increases to infinity, so does the total amount of light from all N shells, N times the constant light per shell. That is not observed, so the hypothesis is disproved.
The objection that planets or dark dust clouds between us and the distant stars could intercept the light is refuted as follows: those dark bodies would absorb the radiant energy and heat up until they reach thermal equilibrium, at which point they would radiate as much energy as they absorb, so they would be as hot as stars, and would radiate like stars; every line of sight would be at least as bright as the sun. The only ways out of the paradox are for the Universe to be finite in size, finitely old, or (as in some models) for space beyond some finite distance, called a horizon, to be expanding faster than the speed of light, so the light can’t reach us.
(4) Even Hoyle, who could not accept creation out of nothing, had to abandon his original Steady State model. Instead, he argued for a "Quasi-steady state" – an unending series of Big Bangs, expansions, collapses, and Big Crunches. He never came up with a convincing cyclic model, as Steinhardt and Turok have now done. But as Singh argues, like Gamow and Alpher, Hoyle deserved the Nobel for his brilliant theory of the creation of the heavy elements in massive stars and supernovae, complementing their theory of the light elements.
If the Steinhardt-Turok cyclic model is confirmed, it would vindicate both the great antagonists Hoyle and Gamow, who never said that his Big Bang model necessarily implied creation ex nihilo. In the second edition of his book, The Creation of the Universe, Gamow said that it could be taken in the sense of "the latest creation of Paris fashion": order from chaos rather than something from nothing.
- The Big Bang links cosmology, the physics of the largest scale, with the quantum physics of the smallest scale. Right after the Big Bang, density was so great that both general relativity – the space-warping effect of huge mass – and quantum mechanics – the strange behavior of waves and particles at the smallest scales - both must be taken into account. The extreme conditions in the baby Universe accelerated particles far beyond the energies that can be reached by even the most powerful accelerators we could hope to build, so cosmic observations can shed light on quantum phenomena, and quantum level insights can help understand the Universe. Similarly, this review tries to pack maximum information into minimum space.
(6) Zwicky, a prescient genius and an irascible curmudgeon, also argued that the red shift observed by Hubble was caused by "tired light" losing energy to gravitational fields rather than by cosmic expansion. That hypothesis has been refuted as inconsistent with the conservation of energy. Light loses some energy, but not enough to account for the observed red shifts.
(7) Alan Guth and Paul Steinhardt, The Inflationary Universe, anthologized in The Scientific American Book of Astronomy. See also Guth’s book , The Inflationary Universe,1997.
(8) The precise flatness of space-time on the cosmic scale is one of a number of conditions that make the Universe compatible with conscious life. Several other basic constants of physics seem to be finely tuned to allow atoms to form, matter to aggregate into stars, heavy elements to be built up from hydrogen, and those conditions to continue long enough for life to evolve. The book Just Six Numbers by Martin Rees explains the specific conditions making the Universe just right for life. The remarkable "Goldilocks" character of this Universe is called the anthropic principle: the Universe is such that beings like us can exist to observe it.
There are two versions: the "weak anthropic principle" is the tautological idea that any Universe observed by conscious beings must be compatible with their existence. There might be many other universes with different characteristics incompatible with life, perhaps an infinite number, but they would not be observable. The "strong anthropic principle" interprets the amazing luck making this Universe friendly to life as evidence that it must have been designed that way by a Creator. But that begs the question – what conditions made a "Meta-Universe" compatible with the existence of a Creator?
Such questions may be unanswerable by science, but scientists hope to discover basic principles to explain at least some of the life-friendly conditions. For example, both the Cosmic Inflation and Cyclic Universe theories explain the flatness of space-time, as a natural consequence of inflation and dark energy respectively. Maybe some of the other apparently arbitrary "fine-tunings" will follow from a more complete theory of fundamental physics.
(9) Higher dimensional spaces are more fully explained in Lisa Randall’s popular book Warped Passages (2005). Steinhardt and Turok cite her Randall-Sudrum model as a basis for the cyclic theory.
(10) The Steinhardt-Turok theory is compatible with other higher dimensional models than the 11 dimensions of current M-theory, though the authors use that model for purposes of explication. They do require at least one extra dimension to hold the Dark Energy field and separate the two brane universes.
(11) Eg Steinhardt and Turok: A Cyclic Model of
the Universe, Science vol. 296, p.1436 (2002); www.sciencemag.org/cgi/content/full/296/5572/1436;
Cosmic Evolution in a Cyclic Universe, Physical Review D, March 2002, arXiv:hep-th/0111098 v2
The Cyclic Universe, an Informal Introduction arXiv:astro-ph/0204479 v1
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