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ScienceWeek
FUNDAMENTAL PHYSICS, COSMOLOGY, AND INFLATION
The following points are made by Frank Wilczek (Physics Today 2003 October):
1) A big reason for excitement and optimism among physicists is the emerging possibility of forging links between fundamental physics and cosmology through models of inflation. Several assumptions in our cosmological models, specifically uniformity, spatial flatness, and the Harrison-Zeldovich spectrum, were originally suggested on grounds of simplicity, expediency, or aesthetics. They can be supplanted with a single dynamical hypothesis: that very early in its history, the Universe underwent a period of superluminal expansion, or inflation. Such a period could have occurred while a matter field that was excited coherently out of its ground state permeated the universe.
2) Possibilities of that kind are easy to imagine in models of fundamental physics. For example, scalar fields are used to implement symmetry breaking even in the standard model and, theoretically, such fields can easily find themselves unable to shed energy quickly enough to stay close to their ground state as the Universe expands. Inflation will occur if the approach to the ground state is slow enough. Fluctuations will be generated because the relaxation process is not quite synchronized across the Universe.
3) Inflation is a wonderfully attractive, logically compelling idea, but very basic challenges remain. Can we be specific about the cause of inflation, and ground it in explicit, well-founded models of fundamental physics? To be concrete, can we calculate the correct amplitude of fluctuations convincingly? Existing implementations actually have a problem on this score; getting the amplitude sufficiently small takes some nice adjustment.
4) More promising, perhaps, than the difficult business of extracting hard quantitative predictions from the broadly flexible idea of inflation is to follow up on the essentially new and surprising possibilities it suggests. The violent restructuring of spacetime attending inflation should generate detectable gravitational waves, and the nontrivial dynamics of relaxation should generate some detectable deviation from a strictly scale-invariant spectrum of fluctuations. Future precision measurements of polarization in the microwave background radiation and of the large-scale distribution of matter will be sensitive to these effects.
5) There are many ideas for how an asymmetry between matter and antimatter might be generated in the early universe. Then after much mutual annihilation of particles and antiparticles, the asymmetry could be left as the present baryon density. Several of those ideas seem capable of accommodating the observed value. Unfortunately the answer generally depends on details of particle physics at energies that are unlikely to be accessible experimentally any time soon. So for a decision among them we may be reduced to waiting for a functioning Theory of (Nearly) Everything.
Physics Today http://www.physicstoday.org
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COSMOLOGY: ON QUINTESSENCE
The conceptual turmoil in current cosmology, a turmoil caused by significant apparent paradoxes and the lack of an acceptable broad theory of the evolution of the Universe that both explains observations and solves the paradoxes, portends important breakthroughs in our understanding of the Cosmos. Dramatic new insights may come soon or may require decades, but meanwhile the intellectual ferment continues without abatement.
Perhaps the central question in cosmology concerns the future: Will the Universe continue to expand indefinitely, or will the expansion eventually slow and stop and be replaced by a contraction? At present, attempts to answer this fundamental question involve two considerations: a) the total mass of the Universe, which will determine whether gravity can slow the expansion and produce a contraction; and b) the possible presence of special energy fields that may also influence the rate of expansion.
In this context, the term "dark matter" refers to material whose presence can be inferred from its effects on the motions of stars and galaxies, but which cannot be seen directly because it emits little or no radiation. It is believed that as much as 90 percent of the mass in the Universe may exist as some form or dark matter, although the proposed percentage of dark matter varies widely with different cosmological models.
In cosmology, what is called the "cosmological constant" refers to a mathematical term introduced by Einstein into the equations of general relativity, the purpose to obtain a solution of the equations corresponding to a "static Universe" (i.e., flat Universe). The term describes a pressure (if positive) or a tension (if negative) which can cause the Universe to expand or contract even in the absence of any matter (i.e., a "vacuum energy"). When the expansion of the Universe was discovered, Einstein apparently began to regard the introduction of this term as a mistake, and he described the cosmological constant as the "greatest mistake of my life". But the term has reappeared as the proposed source of apparent accelerated cosmic expansion.
Contrary to expectations, current evidence indicates that the Universe is expanding at approximately twice the velocity required to overcome the gravitational pull of all the matter the Universe contains. The implication of this is that in the past the greater density of mass in the Universe gravitationally slowed the expansion, while in the future the expansion rate will be close to constant or perhaps increasing under the influence of a new type of matter that some call "quintessence". Quintessence began theoretically as Einstein's cosmological constant, but in current theory it is more a time-variant parameter than a constant. It has negative gravitational mass: its gravity pushes things apart, and it is thus the origin of a repulsive force.
The following points are made by J.P. Ostriker and P.J. Steinhardt (Scientific American January 2001):
1) The authors point out that during the past 5 years, observations have convinced cosmologists that the chemical elements and the dark matter combined amount to less than half the content of the Universe. The bulk is a ubiquitous "dark energy" with a strange and remarkable feature: its gravity does not attract, it repels. "Whereas gravity pulls the chemical elements and dark matter into stars and galaxies, it pushes the dark energy into a nearly uniform haze that permeates space. The Universe is a battleground between the two tendencies, and repulsive gravity is winning."
2) Where does the dark energy come from? The best-known possibility is that the energy is inherent in the fabric of space (vacuum energy as represented by Einstein's cosmological constant). Many cosmologists, however, are now leaning towards a different idea -- quintessence. In 1997, R.R. Caldwell, R. Dave, and P.J. Steinhardt introduced the term to refer to a dynamical quantum field, not unlike an electrical or magnetic field, that gravitationally repels.
3) Vacuum energy is completely inert, maintaining the same density for all time. Consequently, to explain the amount of dark energy present today, the value of the cosmological constant would have to be fine-tuned at the creation of the Universe, which is conceptually unsatisfactory and reinforces the criticism of the cosmological constant as a fudge factor. In contrast, quintessence interacts with matter and evolves with time, so it might naturally adjust itself to reach the value observed today.
4) In general, according to the current conception, the main ingredient of the Universe is dark energy, which consists of either vacuum energy or quintessence. The other ingredients are dark matter composed of exotic elementary particles, ordinary matter (both nonluminous and visible) and a trace amount of radiation.
5) In general, the Universe expands at different rates depending on which form of energy predominates. Matter causes the expansion to decelerate, whereas vacuum energy causes it accelerate. Quintessence is intermediate: quintessence forces an accelerating expansion, but one less rapid than that produced by vacuum energy.
6) Supernova data may be a way to decide between quintessence and vacuum energy: for supernova at a given distance (given redshift), cosmic expansion results in a dimming of apparent luminosity; measurements can thus distinguish an acceleration of expansion due to a vacuum energy model from an acceleration of expansion due to a quintessence model. Existing telescopes cannot resolve the two cases, but the proposed Supernova Acceleration Probe should be able to accomplish this.
Scientific American http://www.sciam.com
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ON QUINTESSENCE AND THE EVOLUTION OF THE COSMOLOGICAL CONSTANT
The following points are made by P.J.E. Peebles ((Nature 1999 398:25):
1) Contrary to expectations, the evidence is that the Universe is expanding at approximately twice the velocity required to overcome the gravitational pull of all the matter the Universe contains. The implication of this is that in the past the greater density of mass in the Universe gravitationally slowed the expansion, while in the future the expansion rate will be close to constant or perhaps increasing under the influence of a new type of matter that some call "quintessence".
2) Quintessence began as Einstein's cosmological constant, Lambda. It has negative gravitational mass: its gravity pushes things apart.
3) Particle physicists later adopted Einstein's Lambda as a good model for the gravitational effect of the *active vacuum of quantum physics, although the idea is at odds with the small value of Lambda indicated by cosmology.
4) Theoretical cosmologists have noted that as the Universe expands and cools, Lambda tends to decrease. As the Universe cools, *symmetries among forces are broken, particles acquire masses, and these processes tend to release an analogue of *latent heat. The vacuum energy density accordingly decreases, and with it the value of Lambda. Perhaps an enormous Lambda drove an early rapid expansion that smoothed the primeval chaos to make the near uniform Universe we see today, with a decrease in Lambda over time to its current value. This is the cosmological *inflation concept.
5) The author suggests that the recent great advances in detectors, telescopes, and observatories on the ground and in space have given us a rough picture of what happened as our Universe evolved from a dense, hot, and perhaps quite simple early state to its present complexity. Observations in progress are filling in the details, and that in turn is driving intense debate on how the behavior of our Universe can be understood within fundamental physics.
Nature http://www.nature.com/nature
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Notes:
active vacuum of quantum physics: This refers to the idea that the vacuum state in quantum mechanics has a zero-point energy (minimum energy) which gives rise to vacuum fluctuations, so the vacuum state does not mean a state of nothing, but is instead an active state.
symmetries among forces are broken: If a theory or process does not change when certain operations are performed on it, the theory or process is said to possess a symmetry with respect to those operations. For example, a circle remains unchanged under rotation or reflection, and a circle therefore has rotational and reflection symmetry. The term "symmetry breaking" refers to the deviation from exact symmetry exhibited by many physical systems, and in general, symmetry breaking encompasses both "explicit" symmetry breaking and "spontaneous" symmetry breaking. Explicit symmetry breaking is a phenomenon in which a system is not quite, but almost, the same for two configurations related by exact symmetry. Spontaneous symmetry breaking refers to a situation in which the solution of a set of physical equations fails to exhibit a symmetry possessed by the equations themselves.
latent heat: In general, this is the quantity of heat absorbed or released when a substance changes its physical phase (e.g., solid to liquid) at constant temperature.
inflation concept: The inflationary model, first proposed by Alan Guth in 1980, proposes that quantum fluctuations in the time period 10^(-35) to 10^(-32) seconds after time zero were quickly amplified into large density variations during the "inflationary" 10^(50) expansion of the universe in that time frame.
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INFLATION IN A LOW-DENSITY UNIVERSE
There is an apparent consensus among cosmologists that recent observational evidence is not consistent with the current "*inflation theory" of the early evolution of the Universe, and that to keep this theory relevant requires either the postulate of an exotic form of energy or the addition of "a layer of complexity" to inflation theory.
The following points are made by M.A. Bucher and D.N. Spergel ((Scientific American January 1998):
1) Despite its success, the standard *Big Bang theory cannot answer several important questions: Why is the density and temperature of the present Universe so uniform? Why did the early Universe have any density variations at all? Why is the rate of cosmic expansion just enough to counteract the collective gravity of all the matter in the Universe?
2) The failure of the standard Big Bang theory to answer these questions provoked, in the 1980s, the formulation of the theory of inflation by Guth, Sato, Linde, Albrecht, Steinhardt and others.
3) Inflation theory predicts a flat (i.e., Euclidean) and uniform Universe, with an observed value of the *Omega parameter either exactly 1 or so close to 1 that the deviation is not detectable. The implication of an Omega value of 1 is that the cosmic gravitational energy exactly equals the cosmic kinetic energy (i.e., the energy contained in the motion of matter as space expands). The problem is that a wide variety of recent astronomical observations involving galaxy clusters and distant supernovae suggest that gravity is too weak to combat cosmic expansion, that the density of matter must be much less than predicted, and that the value of the Omega parameter is equal to approximately 0.3.
4) The authors propose there are 3 ways to interpret this result: a) Standard inflation theory is completely wrong. Or b) Standard inflation theory is correct: the Universe is flat, but an additional new form of energy exists, and this is responsible for what appears to be an accelerating expansion. Or c) Standard inflation theory is partially correct, and its assumption of the inevitability of a flat Universe needs to be revised.
5) The focus of the authors is on the 3rd option. They review a revision of standard inflation theory, the revision involving the introduction of a "*false-vacuum decay" preceding the standard inflation, this false-vacuum decay producing nonuniform "bubbles" of expansion [*Note #1]. The new conception is called "open inflationary theory".
6) The authors state that at the current levels of precision, observations cannot distinguish between the predictions of the 2 theories of inflation. The authors suggest the "moment of truth" will come with the planned deployment late next year of the *Microwave Anisotropy Probe, and the launch in 2007 of its European counterpart, *Planck. These satellites will perform observations similar to those of the *Cosmic Microwave Background Explorer (COBE) nearly a decade ago, but at a much higher resolution. The authors suggest these new satellites will be able to resolve which of the 3 theoretical options is correct: a) an abandonment of any inflation theory; b) standard inflation theory with a new form of energy; c) open inflation theory.
Scientific American http://www.sciam.com
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Notes:
inflation theory: The inflationary model, first proposed by Alan Guth in 1980, proposes that quantum fluctuations in the time period 10^(-35) to 10^(-32) were quickly amplified into large density variations during the "inflationary" 10^(50) expansion of the universe in that time frame.
Big Bang theory: The Big Bang theory is the general cosmological model that proposes that all matter and radiation in the universe originated in an explosion at a finite time in the past.
Omega parameter: Another approach to the Omega parameter is to define it as the ratio of the density of matter (or energy) in the Universe to the theoretical density required for flatness. An Omega with a value of greater than 1 implies a closed Universe; a value less than 1 implies an open Universe; a value equal to 1 implies a flat Universe.
false-vacuum decay: A "false vacuum" is a peculiar state of matter which has never been observed but whose properties are unambiguously predicted by *quantum field theory. Essentially, the idea of a false vacuum refers to a miniature energy minimum above the true minimum, a saddle "trap". The most peculiar property of the false vacuum is probably its pressure, which is both large and negative. The term "false-vacuum decay" refers to a breaking out of the trap, in this case via *quantum mechanical tunneling through the miniature energy barrier, and then a fall to the true zero-point (minimum vacuum energy). The application of the idea of false vacuum to the inflation model was already well underway in the late 1980s by Guth and others.
quantum field theory: Quantum field theory is the mathematical fusion of quantum mechanics with special relativity theory.
quantum mechanical tunneling: "Tunneling" is a quantum mechanical phenomenon involving an effective penetration of an energy barrier resulting from the width of the barrier being less than the wavelength of the particle.
Note #1: It should be noted that this idea was already described in the late 1980s by Alan Guth and others.
Microwave Anisotropy Probe: Information on this satellite project can be found at URL [http://map.gsfc.nasa.gov].
Planck: Information on this satellite project can be found at [http://astro.esctec.esa.nl/SA-general/Projects/Planck].
Cosmic Microwave Background Explorer (COBE): A NASA orbiting satellite launched in 1989 and dedicated to the study of the *cosmic microwave background radiation. The most important results were the discoveries of irregularities in the cosmic background radiation on the level of one part in 10^(5), and the confirmation that the spectrum of the cosmic background radiation is that of a black body with a temperature of 2.73 degrees kelvin.
cosmic microwave background radiation: The cosmic microwave background is black-body radiation (the emission radiation of a perfect absorber of radiation) at a present temperature of 2.73 degrees Kelvin, and has an almost equal intensity in all directions in space. The deviations from isotropic intensity, however, are of extreme importance in theoretical cosmology. The cosmic background radiation is predicted by the Big Bang theory and is considered one of the most important pieces of evidence for it.
ScienceWeek http://www.scienceweek.com
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