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Post Info TOPIC: Space-time
Blobrana


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Posts: 131433
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Homogeneity
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Title: Testing the homogeneity of the Universe using gamma-ray bursts
Author: Ming-Hua Li, Hai-Nan Lin

In this paper, we study the homogeneity of the GRB distribution using a subsample of the Greiner GRB catalogue, which contains 314 objects with redshift 0<z<2.5 (244 of them discovered by the Swift GRB Mission).

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Blobrana


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Wainwright-Marshman space-times
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Title: Inhomogeneity effect in Wainwright-Marshman space-times
Authors: Sebastian J. Szybka, Krzysztof Gód, Micha J. Wyrbowski, Alicja Konieczny

Green and Wald have presented a mathematically rigorous framework to study, within general relativity, the effect of small scale inhomogeneities on the global structure of space-time. The framework relies on the existence of a one-parameter family of metrics that approaches the effective background metric in a certain way. Although it is not necessary to know this family in an exact form to predict properties of the backreaction effect, it would be instructive to find explicit examples. In this paper, we provide the first example of such a family of exact non-vacuum solutions to the Einstein's equations. It belongs to the Wainwright-Marshman class and satisfies all of the assumptions of the Green-Wald framework.

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Blobrana


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RE: Space-time
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New mathematical model links space-time theories

Researchers at the University of Southampton have taken a significant step in a project to unravel the secrets of the structure of our Universe.
Professor Kostas Skenderis, Chair in Mathematical Physics at the University, comments: "One of the main recent advances in theoretical physics is the holographic principle. According to this idea, our Universe may be thought of as a hologram and we would like to understand how to formulate the laws of physics for such a holographic Universe."
A new paper released by Professor Skenderis and Dr Marco Caldarelli from the University of Southampton, Dr Joan Camps from the University of Cambridge and Dr Blaise Goutéraux from the Nordic Institute for Theoretical Physics, Sweden published in the Rapid Communication section of Physical Review D, makes connections between negatively curved space-time and flat space-time.
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Blobrana


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Posts: 131433
Date:
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Title: What if Planck's Universe isn't flat?
Authors: Philip Bull, Marc Kamionkowski

Inflationary theory predicts that the observable Universe should be very close to flat, with a spatial-curvature parameter |Omega_K| < 10^-4. The WMAP satellite currently constrains |Omega_K| < 0.01, and the Planck satellite will be sensitive to values near 10^-3. Suppose that Planck were to find Omega_K to be non-zero at this level. Would this necessarily be a serious problem for inflation? We argue that an apparent departure from flatness could be due either to a local (wavelength comparable to the observable horizon) inhomogeneity, or a truly superhorizon departure from flatness. If there is a local inhomogeneity, then secondary CMB anisotropies distort the CMB frequency spectrum at a level potentially detectable by a next-generation experiment. We discuss how these spectral distortions would complement constraints on the Grishchuk-Zel'dovich effect from the CMB power spectrum at large angular scales in discovering the source of the departure from flatness.

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Blobrana


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Posts: 131433
Date:
Quantum foam
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Title: Can quantum gravity be exposed in the laboratory?: A tabletop experiment to reveal the quantum foam
Authors: Jacob D. Bekenstein

I propose an experiment that may be performed, with present low temperature and cryogenic technology, to reveal Wheeler's quantum foam. It involves coupling an optical photon's momentum to the center of mass motion of a macroscopic transparent block with parameters such that the latter is displaced in space by approximately a Planck length. I argue that such displacement is sensitive to quantum foam and will react back on the photon's probability of transiting the block. This might allow determination of the precise scale at which quantum fluctuations of space-time become large, and so differentiate between the brane-world and the traditional scenarios of spacetime.

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Blobrana


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RE: Space-time
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Title: Cosmology with a spin
Authors: Joao Magueijo, T.G. Zlosnik, T.W.B. Kibble

Using the chiral representation for spinors we present a particularly transparent way to generate the most general spinor dynamics in a theory where gravity is ruled by the Einstein-Cartan-Holst action. In such theories torsion need not vanish, but it can be re-interpreted as a 4-fermion self-interaction within a torsion-free theory. The self-interaction may or may not break parity invariance, and may contribute positively or negatively to the energy density, depending on the couplings considered. We then examine cosmological models ruled by a spinorial field within this theory. We find that while there are cases for which no significant cosmological novelties emerge, the self-interaction can also turn a mass potential into an upside-down Mexican hat potential. Then, as a general rule, the model leads to cosmologies with a bounce, for which there is a maximal energy density, and where the cosmic singularity has been removed. These solutions are stable, and range from the very simple to the very complex.

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Blobrana


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Posts: 131433
Date:
Space-time foam
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Single photon could detect quantum-scale black holes

Space is not smooth: physicists think that on the quantum scale, it is composed of indivisible subunits, like the dots that make up a pointillist painting. This pixellated landscape is thought to seethe with black holes smaller than one trillionth of one trillionth of the diameter of a hydrogen atom, continuously popping in and out of existence.
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Blobrana


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Posts: 131433
Date:
RE: Space-time
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Ripping a hole in space and time

In an unassuming laboratory at Heriot-Watt University in Edinburgh, they are ripping a hole in the fabric of space and time.
Not too far away, they are working with information that can mean two things at once.
They are two separate studies, but what they have in common is the interaction of light and matter.
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Blobrana


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Posts: 131433
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Title: A flat space-time model of the Universe
Authors: Pankaj Jain

We propose a model of the Universe based on Minkowski flat space-time metric. In this model the space-time does not evolve. Instead the matter evolves such that all the mass parameters increase with time. We construct a field theoretic model to show how this can be accomplished within the framework of flat space-time. We show that the model predicts the Hubble law if the masses increase with time. Furthermore we show that it fits the high z supernova data in a manner almost identical to the standard Big Bang model. Furthermore we show that at early times the Universe is dominated by radiative energy density. The phenomenon of recombination also arises in our model and hence predicts the existence of CMBR. However a major difference with the standard Big Bang is that the radiative temperature and energy density does not evolve in our model.

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Blobrana


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Posts: 131433
Date:
Foamy Space-time
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Spacetime: A Smoother Brew Than We Knew

Spacetime may be less like beer and more like sipping whiskey.
Or so an intergalactic photo finish would suggest.
Physicist Robert Nemiroff of Michigan Technological University reached this heady conclusion after studying the tracings of three photons of differing wavelengths that had been recorded by NASA's Fermi Gamma-ray Space Telescope in May 2009.
The photons originated about 7 billion light years away from Earth in one of three pulses from a gamma-ray burst and arrived at the orbiting telescope just one millisecond apart, in a virtual tie.
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