Archive for November, 2012

Articles

14 November, 2012

In High Energy Physics on November 19, 2012 by physthjc

1) The Universe as a simulation

  • Constraints on the Universe as a Numerical simulation
    abstract: Observable consequences of the hypothesis that the observed universe is a numerical simulation performed on a cubic space-time lattice or grid are explored. The simulation scenario is first motivated by extrapolating current trends in computational resource requirements for lattice QCD into the future. Using the historical development of lattice gauge theory technology as a guide, we assume that our universe is an early numerical simulation with unimproved Wilson fermion discretization and investigate potentially-observable consequences. Among the observables that are considered are the muon g-2 and the current differences between determinations of alpha, but the most stringent bound on the inverse lattice spacing of the universe, b^(-1) >~ 10^(11) GeV, is derived from the high-energy cut off of the cosmic ray spectrum. The numerical simulation scenario could reveal itself in the distributions of the highest energy cosmic rays exhibiting a degree of rotational symmetry breaking that reflects the structure of the underlying lattice.

 

2) Gamma rays

  • Constraining Extended Gamma-ray Emission from Galaxy Clusters
    abstract: Cold dark matter models predict the existence of a large number of substructures within dark matter halos. If the cold dark matter consists of weakly interacting massive particles, their annihilation within these substructures could lead to diffuse GeV emission that would dominate over the annihilation signal of the host halo. In this work we search for GeV emission from three nearby galaxy clusters: Coma, Virgo and Fornax. We first remove known extragalactic and galactic diffuse gamma-ray backgrounds and point sources from the Fermi 2-year catalog and find a significant residual diffuse emission in all three clusters. We then investigate whether this emission is due to (i) unresolved point sources; (ii) dark matter annihilation; or (iii) cosmic rays (CR). Using 45 months of Fermi-LAT data we detect several new point sources (not present in the Fermi 2-year point source catalogue) which contaminate the signal previously analyzed by Han et al.(arxiv:1201.1003). Including these and accounting for the effects of undetected point sources, we find no significant detection of extended emission from the three clusters studied. Instead, we determine upper limits on emission due to dark matter annihilation and cosmic rays. For Fornax and Virgo the limits on CR emission are consistent with theoretical models, but for Coma the upper limit is a factor of 2 below the theoretical expectation. Allowing for systematic uncertainties associated with the treatment of CR, the upper limits on the cross section for dark matter annihilation from our clusters are more stringent than those from analyses of dwarf galaxies in the Milky Way. We rule out the thermal cross section for supersymmetric dark matter particles for masses as large as 100 GeV (depending on the annihilation channel).