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ScienceWeek
COSMOLOGY: GRAVITATIONAL MODELS AND THE ORIGIN OF THE UNIVERSE
The following points are made by Martin Bojowald (Nature 2005 436:920):
1) Among the deepest, borderline-philosophical questions in modern physics is that of the origin and formation of the Universe. Earlier attempts to formulate an answer that takes into account existing theories and observations have failed because of obstacles posed by gravity. New work[1] provides a "loop quantum gravitational" model that successfully merges current ideas, and which may enable us to overcome such difficulties.
2) The most important feature to bear in mind when considering the origin of the Universe is the radiation that was released when the Universe became transparent to light, the so-called cosmic microwave background[2]. Anisotropies in this radiation --slight variations in its temperature according to the direction in which you look at it -- carry information on the distribution of matter at the time of its release. Through backward evolution of theoretical models of the Universe, we can garner an idea of what the initial seeds for any structure we observe in the Cosmos might have been. The currently favored models are inflationary models, and postulate an accelerated expansion of the early Universe at the time when the initial seeds were being sown.
3) The trouble with these models is that they require a state at which space is not just tiny, but has no size at all, and where the amount of energy stored becomes infinite -- a situation impossible to deal with in the classical theory on which they rely. Mathematically, this is a "singularity", where the main equations and concepts of a theoretical framework become inapplicable. Quite often, this state of zero size is speculatively identified as the "initial" state of the Universe. However, it is simply ill-defined in the theory of general relativity, which is our current best description of the nature of space and time.
4) Near a singularity, we reach the limits of current theory. At extremely small sizes and high energies, quantum effects are expected to be significant, so a quantum theory of gravity is needed. The required combination of general relativity and quantum theory has so far resisted consistent formulation. We can, however, attempt to apply some promising candidate theories to the early Universe. Loop quantum gravity[3,4] is one such theory; it can deal with both strong gravity and a potentially vanishing space, and can be applied to cosmological situations in a framework known as loop quantum cosmology[5]. The theory gives rise to characteristic effects, such as the energy in matter in quantized space behaving differently, on small scales, from how it does in classical formulations. To some degree, quantum space can be considered as analogous to a crystal, which, through its atomic structure, changes the propagation of light relative to that through a vacuum.
References (abridged):
1. Mulryne, D. J., Tavakol, R., Lidsey, J. E. & Ellis, G. F. R. Phys. Rev. D 71, 123512 (2005)
2. Spergel, D. N. et al. Astrophys. J. Suppl. 148, 175-194 (2003)
3. Ashtekar, A. & Lewandowski, J. Class. Quantum Grav. 21, R53-R152 (2004)
4. Rovelli, C. Quantum Gravity (Cambridge Univ. Press, 2004)
5. Bojowald, M. in 100 Years of Relativity (ed. Ashtekar, A.) (World Scientific, Singapore, in the press)
Nature http://www.nature.com/nature
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Related Material:
THEORETICAL PHYSICS: ON THE WEAKNESS OF GRAVITY
The following points are made by Jonathan L. Feng (Science 2003 302:795):
1) Of the four known fundamental forces -- gravity, electromagnetism, and the weak and strong forces -- gravity is by far the weakest. The reasons for this weakness have long remained enigmatic. Recent proposals suggest, however, that the weakness of gravity may be evidence for extra spatial dimensions. Experiments ranging from tabletop tests of Newtonian gravity to searches for microscopic black holes in kilometer-scale detectors are now putting these ideas to the test.
2) The importance of gravity in everyday life results not from its strength but from its universality: Objects cannot be gravitationally neutral, and all bodies with mass attract. Yet as an interaction between elementary particles, gravity is extremely weak. For example, the gravitational attraction between two protons is 35 orders of magnitude weaker than their electromagnetic repulsion. This holds for protons separated by any distance r, because both gravitational and electromagnetic forces are proportional to 1/r^(2).
3) The observed weakness of gravity may, however, not be an intrinsic property of gravity, but may instead be an effect of extra spatial dimensions. This possibility is based on a simple consideration. Suppose that our three-dimensional (3D) world is merely a subspace of a higher-dimensional space, and that gravity propagates freely in all dimensions, but that all other forces are confined to our three dimensions. In contrast to the familiar three dimensions, the extra dimensions are curled up in small circles of circumference (L). Hence, moving a distance L in the direction of any of the extra dimensions brings one back to one's starting place.
4) Now suppose that at some separation distance (r < L), gravity is strong, that is, comparable to electromagnetism. As r increases, the electromagnetic force drops as 1/r^(2). However, the gravitational field spreads out in all available spatial dimensions, and the gravitational force therefore decreases much more rapidly as 1/r^(2+n), where n is the number of extra dimensions. This rapid drop continues until r > L, at which point the extra dimensions become less and less important and gravity recovers its 1/r^(2) behavior.
5) If this picture is correct, then gravity is not intrinsically weak: It is as strong as electromagnetism at small length scales. It appears weak at the relatively large distances of common experience only because its effects are diluted by propagation in extra dimensions. The distance at which the gravitational and electromagnetic forces might have equal strength is unknown, but a particularly interesting possibility is that it is 10^(-19) m, the distance at which the electromagnetic and weak forces are known to unify to form the electroweak force (1-5).
References (abridged):
1. N. Arkani-Hamed, S. Dimopoulos, G. R. Dvali, Phys. Lett. B 429, 263 (1998)
2. E. G. Adelberger et al, http://arXiv.org/abs/hep-ex/0202008 (2002)
3. S. Cullen, M. Perelstein, Phys. Rev. Lett. 83, 268 (1999)
4. L. J. Hall, D. R. Smith, Phys. Rev. D 60, 085008 (1999)
5. S. B. Giddings, S. Thomas, Phys. Rev. D 65, 056010 (2002)
Science http://www.sciencemag.org
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THEORETICAL PHYSICS: THE WEAKNESS OF GRAVITY
Notes by ScienceWeek:
According to Newton's law of gravitation, there is a force of attraction between any two massive particles in the Universe. This force of attraction, as stated by Newton, may be expressed as a simple relationship involving the masses of the two particles and the distance between them. When the two masses are unit masses and the distance between their centers of mass is unit distance, then the force of attraction is equal to what is called the "gravitational constant", usually denoted as "G". The gravitational constant is usually regarded as a true universal constant independent of place or time, but in some cosmological models it is proposed that the gravitational constant decreases with time as the Universe expands.
The following points are made by Frank Wilczek (Physics Today 2001 June):
1) Gravity dominates the large-scale structure of the Universe, but only by default: matter arranges itself to cancel electromagnetism, and the *strong and weak forces are intrinsically short range. At a more fundamental level, gravity is extremely weak: acting between protons, gravitational attraction is approximately 10^(-36) times weaker than electrical repulsion. The author asks: "Where does this outlandish disparity come from? What does it mean?"
2) The author points out that these questions greatly disturbed Richard Feynman (1918-1988). In 1963, in Feynman's famous paper on quantizing general relativity, the paper in which he first described his discovery of the "ghost particles" that eventually played a crucial role in understanding modern *gauge field theories, Feynman noted that the correct problem is to understand what determines the size of gravitation.
3) Wilczek points out that the same question drove Paul Dirac (1902-1984) to consider, 30 years before Feynman, the radical idea that the fundamental "constants" of nature are time dependent, so that the weakness of gravity could be related to the great age of the Universe. Dirac's argument was that the expansion rate of the Universe suggests that it began with a bang approximately 10^(17) seconds ago. On the other hand, the time it takes light to cross the diameter of a proton is approximately 10^(-24) seconds, which provides a ratio, 10^(-41), which is not so far from the mysterious 10^(-36). But the age of the Universe, of course, changes with time, so if the numerological coincidence is to abide, something else -- the relative strength of gravity or the size of protons -- will have to change in proportion. Since there are powerful experimental constraints on such effects, Dirac's idea is not easy to reconcile with standard modern theories of cosmology and fundamental interactions, theories which are extremely successful.
4) Wilczek discusses the dimensionless number N = Gm^(2)/hc, where (G) is Newton's constant, (m) is the mass of the proton, (h) is Planck's constant, and (c) is the speed of light. Substituting measured values, we find N is approximately 3 x 10^(-39), and Wilczek notes: "This is what we mean when we say gravity is extravagantly feeble." But the real problem, Wilczek suggests, is to understand the smallness of (N).
5) Wilczek then proposes that an understanding of the smallness of N can be derived from an understanding of the smallness of the mass of the proton, and in particular from the constraints imposed by the *theory of quantum chromodynamics on the coupling constants between quarks, the fundamental components of the proton. Essentially, the inter-quark coupling force increases with distance between quarks, which provides a powerful constraint on the size and mass of the proton. The smallness of N, therefore, is an apparent natural consequence of the theory of quantum chromodynamics.
Physics Today http://www.physicstoday.org
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Notes:
strong and weak forces: The weak forces are the forces responsible for the change of neutrons and protons into each other in radioactive processes and in the stars. The strong forces are the forces that hold quarks together inside protons and neutrons, and that hold protons and neutrons together inside atomic nuclei.
gauge field theories: Quantum field theory is the mathematical fusion of quantum mechanics with special relativity theory, and there are essentially 2 branches: quantum electrodynamics (applicable to charged particles involved in electromagnetic interactions) and quantum chromodynamics (applicable to nuclear particles involved in strong force interactions). In this context, a "gauge theory" is any quantum field theory which has the property of "gauge symmetry", i.e., the equations describing the field do not change when some operation is applied to all particles everywhere in space. In general, fields with gauge symmetry can be remeasured (regauged) from different baselines without affecting their properties. Quantum electrodynamics and quantum chromodynamics are examples of gauge theories.
theory of quantum chromodynamics: Quantum chromodynamics (QCD) is a theory that describes the strong interaction (strong nuclear force) in terms of quarks and antiquarks and the exchange of massless "gluons" between them. The "chromo-" in chromodynamics derives from the use of designated "color" attributes of quarks, the various "colors" labels for various quark properties.
ScienceWeek http://scienceweek.com
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