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arXiv:0802.1538 [ps, pdf, other] :
Title: The Intergalactic Propagation of Ultra-High Energy Cosmic Ray Nuclei: An Analytic Approach
Authors: Dan Hooper, Subir Sarkar, Andrew M. Taylor
Comments: -
It is likely that ultra-high energy cosmic rays contain a significant component of heavy or intermediate mass nuclei. The propagation of ultra-high energy nuclei through cosmic radiation backgrounds is more complicated than that of protons and its study has required the use of Monte Carlo techniques. We present an analytic method for calculating the spectrum and the composition at Earth of ultra-high energy cosmic rays which start out as heavy nuclei from their extragalactic sources. The results obtained are in good agreement with those obtained using numerical methods. |
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arXiv:0802.0733 [ps, pdf, other] :
Title: Optimal boarding method for airline passengers
Authors: Jason H. Steffen
Comments: 15 pages, 7 figures. Submitted to the Journal of Air Transportation Management. This study was motivated by an experience that occurred while traveling to a conference
Using a Markov Chain Monte Carlo optimization algorithm and a computer simulation, I find the passenger ordering which minimizes the time required to board the passengers onto an airplane. The model that I employ assumes that the time that a passenger requires to load his or her luggage is the dominant contribution to the time needed to completely fill the aircraft. The optimal boarding strategy may reduce the time required to board and airplane by over a factor of four and possibly more depending upon the dimensions of the aircraft. I explore some features of the optimal boarding method and discuss practical modifications to the optimal. Finally, I mention some of the benefits that could come from implementing and improved passenger boarding scheme. |
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arXiv:0802.1808 [ps, pdf, other] :
Title: Simulations of Baryon Acoustic Oscillations I: Growth of Large-Scale Density Fluctuations
Authors: Ryuichi Takahashi et al.
Comments: submitted to MNRAS, 8 pages, 8 figures
We critically examine how well the evolution of large-scale density perturbations is followed in cosmological $N$-body simulations. We first run a large volume simulation and perform a mode-by-mode analysis in three-dimensional Fourier space. We show that the growth of large-scale fluctuations significantly deviates from linear theory predictions. The deviations are caused by {\it nonlinear} coupling with a small number of modes at largest scales owing to finiteness of the simulation volume. We then develop an analytic model based on second-order perturbation theory to quantify the effect. Our model accurately reproduces the simulation results. For a single realization, the second-order effect appears typically as ``zig-zag'' patterns around the linear-theory prediction, which imprints artificial ``oscillations'' that lie on the real baryon-acoustic oscillations. Although an ensemble average of a number of realizations approaches the linear theory prediction, the dispersions of the realizations remain large even for a large simulation volume of several hundred megaparsecs on a side. For the standard $\Lambda$CDM model, the deviations from linear growth rate are as large as 10 percent for a simulation volume with $L = 500h^{-1}$Mpc and for a bin width in wavenumber of $\Delta k = 0.005h$Mpc$^{-1}$, which are comparable to the intrinsic variance of Gaussian random realizations. We find that the dispersions scales as $\propto L^{-3/2} \Delta k^{-1/2}$ and that the mean dispersion amplitude can be made smaller than a percent only if we use a very large volume of $L > 2h^{-1}$Gpc. The finite box size effect needs to be appropriately taken into account when interpreting results from large-scale structure simulations for future dark energy surveys using baryon acoustic oscillations. |
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arXiv:0802.1219 [ps, pdf, other] :
Title: On Minimally-Parametric Primordial Power Spectrum Reconstruction and the Evidence for a Red Tilt
Authors: Licia Verde, Hiranya V. Peiris
Comments: 18 pages, 8 figures, submitted. Version with full size figures available from this http URL
The latest cosmological data seem to indicate a significant deviation from scale invariance of the primordial power spectrum when parameterized either by a power law or by a spectral index with non-zero "running". This deviation, by itself, serves as a powerful tool to discriminate among theories for the origin of cosmological structures such as inflationary models. Here, we use a minimally-parametric smoothing spline technique to reconstruct the shape of the primordial power spectrum. This technique is well-suited to search for smooth features in the primordial power spectrum such as deviations from scale invariance or a running spectral index, although it would recover sharp features of high statistical significance. We use the WMAP 3 year results in combination with data from a suite of higher resolution CMB experiments (including the latest ACBAR 2008 release), as well as large-scale structure data from SDSS and 2dFGRS. We employ cross-validation to assess, using the data themselves, the optimal amount of smoothness in the primordial power spectrum consistent with the data. This minimally-parametric reconstruction supports the evidence for a power law primordial power spectrum with a red tilt, but not for deviations from a power law power spectrum. Smooth variations in the primordial power spectrum are not significantly degenerate with the other cosmological parameters. |
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arXiv:0802.1215 [ps, pdf, other] :
Title: TeVeS gets caught on caustics
Authors: Carlo R. Contaldi, Toby Wiseman, Benjamin Withers
Comments: 21 pages, 12 figures
TeVeS uses a dynamical vector field with timelike unit norm constraint to specify a preferred local frame. When matter moves slowly in this frame - the so-called quasi-static regime - Modified Newtonian Dynamics (MoND) results. Theories with such vectors (such as Einstein-aether) are prone to the vector dynamics form ing singularities which render their classical evolution problematic. Here we analyse the dynamics of the vector in TeVeS in various situations. We find that, quite generically, the vector field develops caustic singularities on time scales of order the gravitational in-fall time. Having shown singularity formation is generic with or without matter, Bekenstein's original formulation of TeVeS appears dynamically problematic. We argue that TeVeS might be saved as a relativistic theory with a MoND limit by modifying the vector field kinetic terms to the more general form used by Einstein-aether. |
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arXiv:0802.1710 [ps, pdf, other] :
Title: How accurately can 21 cm tomography constrain cosmology?
Authors: Yi Mao, Max Tegmark, Matthew McQuinn, Matias Zaldarriaga, Oliver Zahn
Comments: 20 PRD pages, 8 figs, 13 tables
There is growing interest in using 3-dimensional neutral hydrogen mapping with the redshifted 21 cm line as a cosmological probe, as it has been argued to have a greater long-term potential than the cosmic microwave background. However, its utility depends on many assumptions. To aid experimental planning and design, we quantify how the precision with which cosmological parameters can be measured depends on a broad range of assumptions. We cover assumptions related to modeling of the ionization power spectrum and associated nonlinearity, experimental specifications like array layout and noise, cosmological assumptions about reionization history and inter-galactic medium (IGM) evolution, and assumptions about astrophysical foregrounds. We derive simple analytic approximations for how various assumptions affect the results, and find that ionization power modeling is most important, followed by array layout (crudely, the more compact, the better). We also present an accurate yet robust method for measuring cosmological parameters in practice, separating the physics from the astrophysics by exploiting both gravitationally induced clustering anisotropy and the fact that the ionization power spectra are rather smooth functions that can be accurately fit by 7 phenomenological parameters. For example, a future square kilometer array optimized for 21 cm tomography could improve the sensitivity of the Planck CMB satellite to spatial curvature and neutrino masses by up to two orders of magnitude, to Delta-Omega_k ~ 0.0002 and Delta m_nu ~ 0.007 eV, and give a 4 sigma detection of the spectral index running predicted by the simpliest inflation models. |
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arXiv:0802.1523 [ps, pdf, other] :
Title: Confronting Lemaitre-Tolman-Bondi models with Observational Cosmology
Authors: Juan Garcia-Bellido, Troels Haugboelle
Comments: 27 pages, 8 figures. A general Fortran program for comparing LTB models with cosmological observations, that has been used to make the parameter scan in this paper is made public, and can be downloaded at this http URL
The possibility that we live in a special place in the universe, close to the center of a large void, seems an appealing alternative to the prevailing interpretation of the acceleration of the universe in terms of a LCDM model with a dominant dark energy component. In this paper we confront the asymptotically flat Lemaitre-Tolman-Bondi (LTB) models with a series of observations, from Type Ia Supernovae to Cosmic Microwave Background and Baryon Acoustic Oscillations data. We propose two concrete LTB models describing a local void in which the only arbitrary functions are the radial dependence of the matter density Omega_M and the Hubble expansion rate H. We find that all observations can be accommodated within 1 sigma, for our models with 4 or 5 independent parameters. The best fit models has a chi^2 very close to that of the LCDM model. We perform a simple Bayesian analysis and show that one cannot exclude the hypothesis that we live within a large local void of an otherwise Einstein-deSitter model. |
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arXiv:0802.1709 [ps, pdf, other] :
Title: AGILE detection of a strong gamma-ray flare from the blazar 3C 454.3
Authors: S. Vercellone et al.
Comments: Accepted by Astrophysical Journal Letters; 14 pages, 3 EPS Figures, 1 Table
We report the first blazar detection by the AGILE satellite. AGILE detected 3C 454.3 during a period of strongly enhanced optical emission in July 2007. AGILE observed the source with a dedicated repointing during the period 2007 July 24-30 with its two co-aligned imagers, the Gamma-Ray Imaging Detector and the hard X-ray imager Super-AGILE sensitive in the 30 MeV-50 GeV and 18-60 keV, respectively. Over the entire period, AGILE detected gamma-ray emission from 3C 454.3 at a significance level of 13.8-$\sigma$ with an average flux (E$>$100 MeV) of $(280 \pm 40) \times 10^{-8}$ photons cm$^{-2}$ s$^{-1}$. The gamma-ray flux appears to be variable towards the end of the observation. No emission was detected by Super-AGILE in the energy range 20-60 keV, with a 3-$\sigma$ upper limit of $2.3 \times 10^{-3}$ photons cm$^{-2}$ s$^{-1}$. The gamma-ray flux level of 3C 454.3 detected by AGILE is the highest ever detected for this quasar and among the most intense gamma-ray fluxes ever detected from Flat Spectrum Radio Quasars. |
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