Title: Fractal Space Time and Variation of Fine Structure Constant Authors: A. Bhattacharya, A. Chandra, B. Chakrabarti

The effect of fractal space time of the quantum particles on the variation of the fine structure constant \alpha has been studied. The variation of fine structure constant has been investigated around De Broglie length \lambda and compton length \lambda_{c} and it has been suggested that the variation may be attributed to the dimensional transition of the particle trajectories between these two quantum domains. Considering the Fractal universe with a small inhomogeneity in the mass distribution in the early universe, the variation of the fine structure constant have been investigated between matter and radiation dominated era. The fine structure constant shows a critical behaviour with critical exponent which is fractional and shows a discontinuity. It has been suggested that the variation of the fine structure constant may be attributed to the intrinsic scale dependence of the fundamental constants of nature.

Title: No quantum gravity signature from the farthest quasars Authors: Fabrizio Tamburini (1), Carmine Cuofano (2), Massimo Della Valle (3,4), Roberto Gilmozzi (5) ((1) Dept. of Astronomy, University of Padova, Italy, (2) Dept. of Physics, University of Ferrara, Italy, (3) INAF - Osservatorio Astronomico di Capodimonte, Naples, Italy, (4) International Center for Relativistic Astrophysics Network, Pescara, Italy, (5) European Southern Observatory, Garching bei Muenchen, Germany)

Context: Strings and other alternative theories describing the quantum properties of space-time suggest that space-time could present a foamy structure and also that, in certain cases, quantum gravity (QG) may manifest at energies much below the Planck scale. One of the observable effects could be the degradation of the diffraction images of distant sources. Aims: We searched for this degradation effect, caused by QG fluctuations, in the light of the farthest quasars (QSOs) observed by the Hubble Space Telescope with the aim of setting new limits on the fluctuations of the space-time foam and QG models. Methods: We developed a software that estimates and compares the phase variation in the interference patterns of the high-redshift QSOs, taken from the snapshot survey of HST-SDSS, with those of stars that are expected to not be affected by QG effects. We used a two-parameter function to determine, for each test star and QSO, the maximum of the diffraction pattern and to calculate the Strehl ratio. Results: Our results go far beyond those already present in the literature. By adopting the most conservative approach where the correction terms, that describe the possibility for space-time fluctuations cumulating across long distances and partially compensate for the effects of the phase variations, are taken into account. We exclude the random walk model and most of the holographic models of the space-time foam. Without considering these correction terms, all the main QG scenarios are excluded. Finally, our results show the absence of any directional dependence of QG effects and the validity of the cosmological principle with an independent method; that is, viewed on a large scale, the properties of the Universe are the same for all observers, including the effects of space-time fluctuations.

Title: No quantum gravity signature from the farthest quasars Authors: F. Tamburini, C. Cuofano, M. Della Valle and R. Gilmozzi

Context. Strings and other alternative theories describing the quantum properties of space-time suggest that space-time could present a foamy structure and also that, in certain cases, quantum gravity (QG) may manifest at energies much below the Planck scale. One of the observable effects could be the degradation of the diffraction images of distant sources. Aims. We searched for this degradation effect, caused by QG fluctuations, in the light of the farthest quasars (QSOs) observed by the Hubble Space Telescope with the aim of setting new limits on the fluctuations of the space-time foam and QG models. Methods. We developed a software that estimates and compares the phase variation in the interference patterns of the high-redshift QSOs, taken from the snapshot survey of HST-SDSS, with those of stars that are expected to not be affected by QG effects. We used a two-parameter function to determine, for each test star and QSO, the maximum of the diffraction pattern and to calculate the Strehl ratio. Results. Our results go far beyond those already present in the literature. By adopting the most conservative approach where the correction terms, that describe the possibility for space-time fluctuations cumulating across long distances and partially compensate for the effects of the phase variations, are taken into account. We exclude the random walk model and most of the holographic models of the space-time foam. Without considering these correction terms, all the main QG scenarios are excluded. Finally, our results show the absence of any directional dependence of QG effects and the validity of the cosmological principle with an independent method; that is, viewed on a large scale, the properties of the Universe are the same for all observers, including the effects of space-time fluctuations.

The theory that our universe is contained inside a bubble, and that multiple alternative universes exist inside their own bubbles - making up the 'multiverse' - is, for the first time, being tested by physicists. Two research papers published in Physical Review Letters and Physical Review D are the first to detail how to search for signatures of other universes. Physicists are now searching for disk-like patterns in the cosmic microwave background (CMB) radiation - relic heat radiation left over from the Big Bang - which could provide tell-tale evidence of collisions between other universes and our own. Read more

We propose a mechanism capable to provide a natural solution to two major cosmological problems, first cosmic acceleration and second the coincidence problem. Analysing a specific brane-bulk energy exchange mechanism through astrophysical black holes it is possible first to understand and realize the natural interrelation between dark energy and matter density and second explain why dark energy can be of the same order of the matter density for a wide range of the involved parameters. Furthermore, the model can lead to a crossing of the phantom divide recently.

Cosmologists have been looking for signs of quantum gravity in the distant universe. One prediction of both string theory and LQG is that space-time is not smooth but "grainy" at extremely small scales. In theory, this graininess can be observed by studying particles from the same source but with different energies, to see if they are affected differently by the structure of space-time. Now a team has used data from the Integral satellite, run by the European Space Agency (ESA), to study an entirely different effect: the polarisation of light of different energies from a GRB. Philippe Laurent and his colleagues have used the combined data to look for effects of quantum gravity. One idea is that if space-time is composed of indivisible grains, then this would polarise photons in a way that depends on their energy. This effect would accumulate over cosmological distances and so be observable from Earth. Yet the team found no such effect. Read more

ESA's Integral gamma-ray observatory has provided results that will dramatically affect the search for physics beyond Einstein. It has shown that any underlying quantum 'graininess' of space must be at much smaller scales than previously predicted. Einstein's General Theory of Relativity describes the properties of gravity and assumes that space is a smooth, continuous fabric. Yet quantum theory suggests that space should be grainy at the smallest scales, like sand on a beach. One of the great concerns of modern physics is to marry these two concepts into a single theory of quantum gravity. Read more

Title: On the measure of spacetime and gravity Authors: Naresh Dadhich

By following the general guiding principle that nothing should be prescribed or imposed on the universal entity, spacetime, we establish that it is the homogeneity (by which we mean homogeneity and isotropy of space and homogeneity of time) that requires not only a universally constant invariant velocity but also an invariant length given by its constant curvature, \Lambda and spacetime is completely free of dynamics. Thus c and \Lambda are the only two true constants of the spacetime structure and no other physical constant could claim this degree of fundamentalness. When matter is introduced, the spacetime becomes inhomogeneous and dynamic, and its curvature then determines by the Bianchi differential identity the equation of motion for the Einstein gravity. The homogeneity thus demands that the natural state of free spacetime is of constant curvature and the cosmological constant thus emerges as a clear prediction which seems to be borne out by the observations of accelerating expansion of the Universe. However it has no relation to the vacuum energy and it could be envisioned that in terms of the Planck area the Universe measures 10^{120} units!

He always denied there's any limit on how much stuff you can pack into a certain volume. It was just a question of ingenuity. Alas, the theoretical physicists speaking at A Thin Sheet of Reality: The Universe as a Hologram at this year's World Science Festival have bad news: there is a limit. If you exceed it, the gravitational force exerted by the contents of your suitcase will become so intense that the suitcase collapses into a black hole, and you'll never see your stuff again. Read more