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ScienceWeek
COSMOLOGY: ON DARK ENERGY
The following points are made by Lawrence M. Krauss (Nature 2004 431:519):
1) The nature of the "dark energy" that is causing the apparent accelerated expansion of the Universe is, without doubt, the biggest mystery in physics and astronomy. Although it was astrophysical observations of the acceleration that led to the discovery of dark energy, there are precious few tests that can be performed to work out what dark energy is -- whether it is simply the rebirth of Einstein's cosmological constant, or whether it might stem from something even weirder. All the evidence so far is consistent with the existence of a cosmological constant, which, in modern language, is understood to be the quantum-mechanical energy associated with otherwise empty space. Kunz et al(1) suggest, however, that by comparing data on a range of astrophysical phenomena, it might be possible to rule out a cosmological constant as the origin of dark energy.
2) Dark energy is perplexing. Physical theory currently has no explanation of why the energy of empty space should be precisely zero (quantum-mechanical effects combined with relativity in fact predict quite the opposite). But it also gives no explanation of why that energy should not instead be so huge that it would dwarf all of the energy in anything else (making galaxy formation impossible). Yet arguments based on a host of different cosmological observations -- even before the direct observation of the accelerated expansion -- implied that the energy in empty space could not be more than three to four times greater than the energy contained in the matter and radiation of the Universe. To decide on what physics might be associated with dark energy, we have to rely on experiments and observations. No laboratory experiment we can imagine would be sensitive enough to do the job, so we are left with astrophysical probes. Which is where Kunz et al(1) come in.
3) Kunz et al(1) propose a three-way comparison of data: of the expansion rate of the Universe as it changes with distance (from measurements made using type-Ia supernovae, which originally led to the discovery of dark energy(2,3)); with measurements(4) of the temperature fluctuations in the cosmic microwave background (the relic radiation of the Big Bang); and with measurements of the clustering of galaxies on large scales. Studies of the cosmic microwave background (CMB) have provided remarkably precise constraints on most major cosmological parameters, and are in some sense complementary to the limits derived using type-Ia supernovae. To describe the different possibilities for dark energy, an "equation-of-state" parameter, (w), is defined. This is the ratio of the pressure to the energy of the material. For the cosmological constant, (w) is exactly -1; any measured difference from this value would signal the need for another explanation. Data from the CMB, in combination with those from supernovae, currently limit w to the range -1.2 < w < -0.8, consistent with the value for a cosmological constant(4,5). For comparison, (w) for matter is 0, and for radiation it is 1/3.
4) But Kunz et al(1) point out that allowance should be made for a possible dynamical variation of (w) over time. The key new ingredient they introduce is a comparison between the observed clustering of matter on large scales across the Universe and the predicted level of such clustering based on observations of the fluctuations of the CMB. It turns out that, because of the way that the dark energy comes to dominate the expansion of the Universe, the CMB temperature fluctuations should change on the largest angular scales (spanning more than about ten degrees across the sky) in a way that is sensitive to the dark-energy equation of state.
References (abridged):
1. Kunz, M., Corasaniti, P. -S., Parkinson, D. & Copeland, E. J. Phys. Rev. D 70, 041301 (2004)
2. Schmidt, B. P. et al. Astrophys. J. 507, 46-63 (1998)
3. Perlmutter, S. J. et al. Astrophys. J. 517, 565-586 (1999)
4. Spergel, D. N. et al. Astrophys. J. Suppl. Ser. 148, 175-194 (2003)
5. Krauss, L. M. Astrophys. J. 604, 481-483 (2004)
Nature http://www.nature.com/nature
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THEORETICAL PHYSICS: ON THE SPACE-TIME VACUUM
The following points are made by R. B. Laughlin (Science 2004 303:1475):
1) In discussing cosmic matters it is impossible not to draw analogies with science fiction from time to time, for the issues are as large as those depicted in science fiction and equally mysterious, despite being experimentally constrained.(1) Our knowledge of the cosmos is still very primitive, and much of our thinking about it correspondingly speculative, more along the lines of what might plausibly have been than what is so. Plausibility is an interesting concept in theoretical physics, usually amounting to either a physical analogy with something observed to occur elsewhere in nature or a mathematical extrapolation of microscopic law. The latter, however, is actually a shibboleth, for the things that matter are nearly always collective organizational phenomena that cannot be reliably predicted from microscopics. The shapes of galaxies and the behavior of cosmic jets are simple cases in point, but the observation also applies to the grandest issues of modern cosmology: inflationary expansion and the hierarchical consolidation of matter after the big bang (2-4). The absence of predictive power is actually self-evident, because there would be no point in measuring these things if one could calculate them. As a practical matter, all plausibility arguments that count are analogies.
2) It may seem shocking to speak of the vacuum of space-time as an organizational phenomenon, but this is actually just a matter of semantics. The idea behind the words is mainstream and fully consistent with the facts. It has been known since the 1950s, and routinely verified by accelerator experiments since then, that empty space is a kind of matter quantum-mechanically similar to a rock (5). The standard model of elementary particles is grounded firmly on the idea of space as a phase. A multiplicity of such phases and a complex sequence of transitions among them in the early universe are corner-stones of modern particle cosmology. The existence of such phases is implicated in the structure one sees on intergalactic scales, and the heat released in the transition between two of them is the ostensible power source of inflation. Inflation itself is partly motivated by these phases, because they make the observed uniformity of the universe unnatural and something requiring explanation.
3) The semantic incongruity, however, like the sublimated worries about modern life that give us science fiction nightmares, belies something important -- unfinished business of the 1970s that has been slowly and systematically tearing physics apart. Stripped of their confusing mathematical descriptions, the phases of the vacuum boil down to physical analogies with phases of ordinary matter, natural phenomena observed to exhibit universality. That means that their properties at long length and time scales, where we normally do experiments, do not depend on microscopic details at all, and thus do not constrain them when measured. A simple example of emergent universality would be sound propagation in fluids and solids, an effect perfectly well accounted for as the motion of atoms, but also a generic property of the phases not requiring atoms to make sense. Sound is an especially pertinent example because it has a second identity at low temperatures as an emergent elementary particle with properties identical to those of particles of light. Insensitivity to microscopic detail thus turns the concept of fundamental on its head, in that it makes principles of self-organization the truly important thing, rendering the quantum underpinnings of the Universe, whatever they are, unknowable in the absence of experiments that reach shorter scales and irrelevant to behavior we presently see. Little wonder that physicists remain bitterly divided over full acceptance of the vacuum as a phase.
References (abridged):
1. Akira, 124 min, directed by Katsuhiro Otomo (Kodansha Ltd., Japan, 1988)
2. S. Weinberg, The First Three Minutes: A Modern View of the Origin of the Universe (Basic Books, New York, 1994)
3. M. Rees, New Perspectives in Astrophysical Cosmology (Cambridge Univ. Press, Cambridge, 2000)
4. A. H. Guth, A. P. Lightman, The Inflationary Universe: The Quest for a New Theory of Cosmic Origins (Perseus, New York, 1998)
5. M. E. Peskin, D. E Schroeder, An Introduction to Quantum Field Theory (Westview, Boulder, CO, 1995)
Science http://www.sciencemag.org
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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 by ScienceWeek:
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.
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.
In general, the term "latent heat" refers to the quantity of heat absorbed or released when a substance changes its physical phase (e.g., solid to liquid) at constant temperature.
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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