* Astronomy

Title: Physical Theories, Eternal Inflation, and Quantum Universe
Authors:  Yasunori Nomura

We present a framework in which well-defined predictions are obtained in an eternally inflating multiverse, based on the principles of quantum mechanics. We show that the entire multiverse is described purely from the viewpoint of a single "observer," who describes the world as a quantum state defined on his/her past light cones bounded by the (stretched) apparent horizons. We find that quantum mechanics plays an essential role in regulating infinities. The framework is "gauge invariant," i.e. predictions do not depend on how spacetime is parameterised, as it should be in a theory of quantum gravity. Our framework provides a fully unified treatment of quantum measurement processes and the multiverse. We conclude that the eternally inflating multiverse and many worlds in quantum mechanics are the same. Other important implications include: global spacetime can be viewed as a derived concept; the multiverse is a transient phenomenon during the world relaxing into a supersymmetric Minkowski state. We also present a theory of "initial conditions" for the multiverse. By extrapolating our framework to the extreme, we arrive at a picture that the entire multiverse is a fluctuation in the stationary, fractal "mega-multiverse," in which an infinite sequence of multiverse productions occurs. The framework discussed here does not suffer from problems/paradoxes plaguing other measures proposed earlier, such as the youngness paradox, Boltzmann brain problem, and peculiar "end" of time.

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Title: The Topology and Size of the Universe from the Cosmic Microwave Background
Authors: Grigor Aslanyan, Aneesh V. Manohar

We study the possibility that the universe has compact topologies T³, T² x R¹, or S¹ x R² using the seven-year WMAP data. The maximum likelihood 95% confidence intervals for the size L of the compact direction are 1.7 < L/L_0 < 2.1, 1.8 < L/L_0 < 2.0, 1.2 < L/L_0 < 2.1 for the three cases, respectively, where L_0=14.4 Gpc is the distance to the last scattering surface. An infinite universe is compatible with the data at 4.3 sigma. We find using a Bayesian analysis that the most probable universe has topology T² x R¹, with L/L_0=1.9.

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Is space like a chessboard?

Physicists at UCLA set out to design a better transistor and ended up discovering a new way to think about the structure of space.
 Space is usually considered infinitely divisible - given any two positions, there is always a position halfway between. But in a recent study aimed at developing ultra-fast transistors using graphene, researchers from the UCLA Department of Physics and Astronomy and the California NanoSystems Institute show that dividing space into discrete locations, like a chessboard, may explain how point-like electrons, which have no finite radius, manage to carry their intrinsic angular momentum, or "spin."
 While studying graphene's electronic properties, professor Chris Regan and graduate student Matthew Mecklenburg found that a particle can acquire spin by living in a space with two types of positions - dark tiles and light tiles. The particle seems to spin if the tiles are so close together that their separation cannot be detected.
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Title: Limits on Spacetime Foam
Authors: Wayne A. Christiansen, David J. E. Floyd, Y. Jack Ng, Eric S. Perlman
(Version v2)

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. While time lags from distant pulsed sources such as GRBs have been posited as a possible test of spacetime foam models, we demonstrate that the time-lag effect is rather smaller than has been calculated, due to the equal probability of positive and negative fluctuations in the speed of light inherent in such 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 parameterises 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  Hubble Space Telescope (HST) archive we find it is impossible to rule out the \alpha=2/3 model, but exclude all models with \alpha<0.65. By comparison, current GRB time lag observations only exclude models with \alpha<0.3.

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