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ScienceWeek
COSMOLOGY: WHAT IS DARK ENERGY?
The following points are made by Gia Dvali (Nature 2004 432:567):
1) The discovery that the rate of expansion of the Universe is accelerating has created a confusing situation in cosmology and particle physics. Although the standard cosmological model has been confirmed by data from the WMAP space telescope and by other telescope surveys of the large-scale structure of the Universe, nobody knows why the cosmic expansion is accelerating. The effect has been attributed to some mysterious gravitational source called "dark energy", but this name is merely a codeword for something unknown. Theoretical explanations have been postulated, some of which should be testable through precise cosmological measurements and gravitational interactions. But recently Kaplan et al[1], and Fardon et al[2] propose that innocuous subatomic particles -- neutrinos -- might offer a nongravitational means of probing the nature of dark energy.
2) The accelerating rate of expansion has been inferred directly from surveys of distant supernovae[3,4], but it also fits well with all other existing cosmological data. Supernovae, the cataclysmic explosions of dying stars, are often used in astronomical surveys as "standard candles", because their brightness is believed to be well understood. But very distant supernovae seem to be strikingly dimmer -- and thus farther away -- than expected. This suggests that, at large scales, the expansion of the Universe is accelerating, not slowing down; some mysterious repulsive force is making the Universe fly apart. The puzzling thing, however, is that this new force cannot possibly arise from either matter or radiation, because the gravitational force exerted by such sources could only make the expansion slow down (just as the Earth's gravity slows a stone thrown into the air).
3) Perhaps the accelerating expansion is due to the fact that gravity itself is modified at cosmologically large distances[5]. But if we assume that gravity remains "normal" at reasonably large distances, then we are left with the option that the Universe must be filled with a mysterious source of gravity, whose properties are strikingly different from those of standard matter and radiation -- "dark energy". To generate a repulsive gravitational force, the new source must have an unusual "equation of state" describing its behavior, and must exert a large negative pressure. Also, as the Universe expands, the new source must be diluted at a much slower rate than any known form of matter or radiation.
4) What could this new, almost undilutable source be? Apart from modified gravity, the only such source considered until recently was the potential energy of a spatially uniform, scalar (Bose-Einstein) condensate, known as "quintessence". This energy acts in just the same way as Einstein's cosmological constant, inducing the accelerated expansion of space. In particular, this observation is the basis of so-called inflationary cosmology --the commonly accepted cosmological paradigm for the early history of our Universe that includes a period of exponential expansion, or inflation. Perhaps, 13 billion years after the Big Bang, we are once again entering an inflationary period driven by some scalar condensate.
References (abridged):
1. Kaplan, D. B., Nelson, A. E. & Weiner, N. Phys. Rev. Lett. 93, 091801 (2004)
2. Fardon, R., Nelson, A. E. & Weiner, N. J. Cosmol. Astropart. Phys. 10(2004)005 (2004)
3. Perlmutter, S. et al. Astrophys. J. 517, 565-586 (1999)
4. Riess, A. G. et al. Astron. J. 116, 1009-1038 (1998)
5. Dvali, G. Sci. Am. 290, no. 2, 68-75 (2004)
Nature http://www.nature.com/nature
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Related Material:
ASTROPHYSICS: DARK MATTER AND DARK ENERGY
The following points are made by Sean Carroll (Nature 2004 429:27):
1) Humans seem to be extremely unimportant in the grand scheme of the Universe. This insight is often associated with Copernicus (1473-1543), who suggested (although not for the first time) that the Earth was not the center of the Solar System. A bigger step towards calibrating our insignificance was taken by Edwin Hubble (1889-1953), who determined that astrophysical nebulae are really separate galaxies in their own right. We now think there are about one hundred billion such galaxies in the observable Universe, with perhaps one hundred billion stars per galaxy.
2) But a metaphysically distinct blow to our importance came with the introduction of the idea of dark matter -- we are not even made of the same stuff that comprises most of the Universe. The need for dark matter, in the sense of "matter we cannot see", was noticed in 1933 by Fritz Zwicky (1898-1974), when studying the dynamics of the Coma cluster of galaxies. When galaxies are orbiting each other, their typical velocities will depend on the total mass involved, but when we observe clusters of galaxies, the velocities are consistently much higher than we would expect from the mass we actually see in stars and gas. Vera Rubin and others have driven the point home by examining individual galaxies. As we move away from the central galactic region, the velocity of orbiting gas becomes systematically higher than it should be. These observations imply the existence of an extended, massive halo of dark matter. Indeed, the picturesque galaxies we see in astronomical images are really just splashes of visible matter collected at the bottom of these more substantial, yet invisible, halos.
3) Of course, the air we breathe is invisible and transparent, just like dark matter. A sensible first guess might be that the extra mass we infer is ordinary matter, just in some form we cannot see. But we have independent ways to measure the amount of ordinary matter, through its influence on the early-Universe processes of primordial nucleosynthesis and the evolution of density perturbations. These constraints imply that ordinary matter falls far short of what is needed to explain galaxies and clusters (perhaps one-fifth of the total). Not only is dark matter "dark", it is a completely new kind of particle --something outside the standard model of particle physics, something not yet detected in any laboratory here on Earth.
4) And we have not even mentioned dark energy -- the mysterious form of energy that is smoothly distributed throughout space and (at least approximately) constant through time. Independent observations of high-redshift supernovae, the microwave background radiation, and the distribution of large-scale structure all require the existence of dark energy. The featureless, persistent nature of dark energy convinces us that it is not even a particle at all. About 70% of our current Universe is dark energy and 25% is dark matter. This leaves all the stuff we have directly observed at a paltry 5% of the whole Universe.
References:
1. Krauss, L. Quintessence: The Mystery of the Missing Mass (Basic Books, New York, 2001)
2. Peebles, P. J. E. From Precision Cosmology to Accurate Cosmology online at http://arxiv.org/abs/astro-ph/0208037
3. Rees, M. Our Cosmic Habitat (Princeton Univ. Press, Princeton, 2003)
Nature http://www.nature.com/nature
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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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