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Time may come to an end

The end of the world as we know it cannot be avoided, but it can be predicted, according to a group of astrophysicists who see a 50 percent chance of the final countdown ending in 3.7 billion years.
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Rethinking Einstein: The end of space-time

It was a speech that changed the way we think of space and time. The year was 1908, and the German mathematician Hermann Minkowski had been trying to make sense of Albert Einstein's hot new idea - what we now know as special relativity - describing how things shrink as they move faster and time becomes distorted.
And so space-time - the malleable fabric whose geometry can be changed by the gravity of stars, planets and matter - was born. It is a concept that has served us well, but if physicist Petr Horava is right, it may be no more than a mirage. Horava, who is at the University of California, Berkeley, wants to rip this fabric apart and set time and space free from one another in order to come up with a unified theory that reconciles the disparate worlds of quantum mechanics and gravity - one the most pressing challenges to modern physics.

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Hubble confirms cosmic acceleration with weak lensing

A new study led by European scientists presents the most comprehensive analysis of data from the most ambitious survey ever undertaken by the NASA/ESA Hubble Space Telescope. These researchers have, for the first time ever, used Hubble data to probe the effects of the natural gravitational "weak lenses" in space and characterise the expansion of the Universe.
A group of astronomers, led by Tim Schrabback of the Leiden Observatory, conducted an intensive study of over 446 000 galaxies within the COSMOS field, the result of the largest survey ever conducted with Hubble. In making the COSMOS survey, Hubble photographed 575 slightly overlapping views of the same part of the Universe using the Advanced Camera for Surveys (ACS) onboard Hubble. It took nearly 1000 hours of observations.

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AdS/CFT correspondence

In physics, the AdS/CFT correspondence (anti-de-Sitter space/conformal field theory correspondence), sometimes called the Maldacena duality, is the conjectured equivalence between a string theory defined on one space, and a quantum field theory without gravity defined on the conformal boundary of this space, whose dimension  is lower by one or more. The name suggests that the first space is the product of anti de Sitter space (AdS) with some closed manifold like sphere, orbifold, or noncommutative space, and that the quantum field theory is a conformal field theory (CFT).
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Title: Information-theoretic natural ultraviolet cutoff for spacetime
Authors: Achim Kempf
(Version, v2)

Fields in spacetime could be simultaneously discrete and continuous, in the same way that information can: it has been shown that the amplitudes, \phi(x_n), that a field takes at a generic discrete set of points, x_n, can be sufficient to reconstruct the field \phi(x) for all x, namely if there exists a certain type of natural ultraviolet (UV) cutoff in nature, and if the average spacing of the sample points is at the UV cutoff scale. Here, we generalize this information-theoretic framework to spacetimes themselves. We show that samples taken at a generic discrete set of points of a Euclidean-signature spacetime can allow one to reconstruct the shape of that spacetime everywhere, down to the cutoff scale. The resulting methods could be useful in various approaches to quantum gravity.

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Spacetime Foam
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Title: A Cosmic Peek at Spacetime Foam
Authors: Wayne A. Christiansen, David J. E. Floyd, Y. Jack Ng, Eric S. Perlman

Plausibly spacetime is "foamy" on small distance scales, due to quantum fluctuations. We elaborate on the proposal to detect spacetime foam by looking for seeing disks in the images of distant quasars and AGNs. This is a null test in the sense that the continued presence of unresolved "point" sources at the milli-arc second level in samples of distant compact sources puts severe constraints on theories of quantised spacetime foam at the Planckian level. We discuss the geometry of foamy spacetime, and the appropriate distance measure for calculating the expected angular broadening. We then deal with recent data and the constraints they put on spacetime foam models. Thus far, images of high-redshift quasars from the Hubble Ultra-Deep Field (UDF) provide the most stringent test of spacetime foam theories. While random walk models (alpha = 1/2) have already been ruled out, the holographic (alpha=2/3) model remains viable. Here alpha~1 parametrises the different spacetime foam models according to which the fluctuation of a distance L is given by ~ L^(1 - alpha) l_P^alpha, with l_P being the Planck length. Indeed, we see a slight wavelength-dependent blurring in the UDF images selected for this study. Using existing data in the HST archive we find it is impossible to rule out the alpha=2/3 model, but exclude all models with alpha<0.65.

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Title: Spectral Dimension of the Universe in Quantum Gravity at a Lif****z Point
Authors: Petr Horava

We extend the definition of "spectral dimension" (usually defined for fractal and lattice geometries) to theories on smooth spacetimes with anisotropic scaling. We show that in quantum gravity dominated by a Lif****z point with dynamical critical exponent z in D+1 spacetime dimensions, the spectral dimension of spacetime is equal to d_s=1+D/z. In the case of gravity in 3+1 dimensions with z=3 in the UV which flows to z=1 in the IR, the spectral dimension changes from ds=4 at large scales to ds=2 at short distances.
Remarkably, this is the qualitative behaviour of d_s found numerically by Ambjorn, Jurkiewicz and Loll in their causal dynamical triangulations approach to quantum gravity.

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HoYava gravity
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Physicists have struggled to marry quantum mechanics with gravity for decades. In contrast, the other forces of nature have obediently fallen into line. For instance, the electromagnetic force can be described quantum-mechanically by the motion of photons. Try and work out the gravitational force between two objects in terms of a quantum graviton, however, and you quickly run into trouble - the answer to every calculation is infinity. But now Petr HoYava, a physicist at the University of California, Berkeley, thinks he understands the problem. Its all, he says, a matter of time.
The solution, HoYava says, is to snip threads that bind time to space at very high energies, such as those found in the early universe where quantum gravity rules.
At low energies, general relativity emerges from this underlying framework, and the fabric of spacetime restitches.

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Gamma-ray photon race ends in dead heat; Einstein wins this round

A pair of gamma-ray photons - one possessed of a million times the energy of the other - arrived at virtually the same instant at NASA's orbiting Fermi Gamma-ray Space Telescope, where the Large Area Telescope, for which Stanford's Peter Michelson is principal investigator, detected them after a 7.3 billion year race across the universe. Some proponents of alternatives to Einstein's theory of gravity would have predicted that the more energetic would have interacted with more matter along the way and thus been much farther behind the less energetic one. They were wrong - Einstein wins this round.


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Foamy Space-time
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Astronomers have used a high-energy burst of light from a distant galaxy to test the fabric of space and time. The work is the best test yet of attempts to create a 'theory of everything'.
At present, two separate theories dominate the world of physics. General relativity explains gravity and the motion of large objects such as planets, stars and galaxies, whereas quantum-mechanics explains the behaviour of very small things such as atoms.

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