Munch: Monday, September 25, 2006
		usual time and place:  12:30, 6th floor conference room
FAQ What is Munch? Munch Archive --------- 18 Sep 2006 11 Sep 2006 05 Sep 2006 26June 2006 19 June 2006 12 June 2006 5 June 2006 22 May 2006 15 May 2006 8 May 2006 1 May 2006 24 Apr 2006 17 Apr 2006 10 Apr 2006 03 Apr 2006 27 Mar 2006 13 Mar 2006 6 Mar 2006	On the Origin of the Type Ia Supernova Width-Luminosity Relation Authors: Daniel Kasen, S.E. Woosley Comments: 6 pages, submitted to ApJ Brighter Type Ia supernovae (SNe Ia) have broader, more slowly declining B-band light curves than dimmer SNe Ia. We study the physical origin of this width-luminosity relation (WLR) using detailed radiative transfer calculations of Chandrasekhar mass SN Ia models. We find that the luminosity dependence of the diffusion time (emphasized in previous studies) is in fact of secondary relevance in understanding the model WLR. Instead, the essential physics involves the luminosity dependence of the spectroscopic/color evolution of SNe Ia. Following maximum-light, the SN colors are increasingly affected by the development of numerous Fe II/Co II lines which blanket the B-band and, at the same time, increase the emissivity at longer wavelengths. Because dimmer SNe Ia are generally cooler, they experience an earlier onset of Fe III to Fe II recombination in the iron-group rich layers of ejecta, resulting in a more rapid evolution of the SN colors to the red. The faster B-band decline rate of dimmer SNe Ia thus reflects their faster ionization evolution. Full-text: PostScript, PDF, or Other formats Importance of Supernovae at z<0.1 for Probing Dark Energy Authors: Eric V. Linder Comments: 8 pages, 6 figures Supernova experiments to characterize dark energy require a well designed low redshift program; we consider this for both ongoing/near term (e.g. Supernova Legacy Survey) and comprehensive future (e.g. SNAP) experiments. The derived criteria are: a supernova sample centered near z=0.05 comprising 150-500 (in the former case) and 300-900 (in the latter case) well measured supernovae. Low redshift Type Ia supernovae play two important roles for cosmological use of the supernova distance-redshift relation: as an anchor for the Hubble diagram and as an indicator of possible systematics. An innate degeneracy in cosmological distances implies that 300 nearby supernovae nearly saturate their cosmological leverage for the first use, and their optimum central redshift is z=0.05. This conclusion is strengthened upon including velocity flow and magnitude offset systematics. Limiting cosmological parameter bias due to supernova population drift (evolution) systematics plausibly increases the requirement for the second use to less than about 900 supernovae. Full-text: PostScript, PDF, or Other formats Observation of the GZK Cutoff Using the HiRes Detector Authors: D.R. Bergman (presented on behalf of the High Resolution Fly's Eye Collaboration) Comments: 8 pages, 12 figures. Proceedings submission for CRIS 2006, Catania, May/June 2006 The High Resolution Fly's Eye (HiRes) experiment has observed the GZK cutoff. HiRes observes two features in the ultra-high energy cosmic ray (UHECR) flux spectrum: the Ankle at an energy of $4\times10^{18}$ eV and a high energy suppression at $6\times10^{19}$ eV. The later feature is at exactly the right energy for the GZK cutoff according to the $E_{1/2}$ criterion. HiRes cannot claim to observe a third feature at lower energies, the Second Knee. The HiRes monocular spectra are presented, along with data demonstrating our control and understanding of systematic uncertainties affecting the energy and flux measurements. Full-text: PostScript, PDF, or Other formats Observation of the GZK Cutoff by the HiRes Experiment Authors: G.B. Thomson, for the High Resolution Fly's Eye (HiRes) Collaboration Comments: 15 pages, 5 figures, submitted to Proceedings of the Quarks'06 Conference The High Resolution Fly's Eye (HiRes) experiment has observed the GZK cutoff. HiRes' measurement of the flux of cosmic rays shows a sharp suppression at an energy of 6 x 10^{19} eV, exactly the expected cutoff energy. We observe the ``ankle'' of the cosmic ray spectrum as well, at an energy of 4 x 10^{18} eV. We describe the experiment, data collection, analysis, and estimate the systematic uncertainties. The results are presented and the calculation of a five standard deviation observation of the GZK cutoff is described. Full-text: PostScript, PDF, or Other formats CMB Anisotropies from Outflows in Lyman Break Galaxies Authors: Daniel Babich (Caltech), Abraham Loeb (Harvard) Comments: 5 pages, 2 figures; to be submitted to MNRAS Thomson scattering of the Cosmic Microwave Background (CMB) on moving electrons in the outflows of Lyman Break Galaxies (LBGs) at redshifts 2-8 contributes to the small-scale CMB anisotropies. The net effect produced by each outflow depends on its level of deviation from spherical symmetry, caused either by an anisotropic energy injection from the nuclear starburst or quasar activity, or by an inhomogeneous intergalactic environment. We find that for plausible outflow parameters consistent with spectroscopic observations of LBGs, the induced CMB anisotropies on arcminute scales reach up to $\sim 1 \mu$K, comparable to the level produced during the epoch of reionization. Full-text: PostScript, PDF, or Other formats Dark energy rest frame and the CMB dipole Authors: Antonio L. Maroto Comments: 7 pages, 2 figures. Contribution to the Proceedings of the International Workshop The Dark Side of the Universe, Madrid, June 20-24, 2006 If dark energy can be described as a perfect fluid, then, apart from its equation of state relating energy density and pressure, we should also especify the corresponding rest frame. Since dark energy is typically decoupled from the rest of components of the universe, in principle such a frame could be different from that of matter and radiation. In this work we consider the potential observable effects of the motion of dark energy and the possibility to measure the dark energy velocity relative to matter. In particular we consider the modification of the usual interpretation of the CMB dipole and its implications for the determination of matter bulk flows on very large scales. We also comment on the possible origin of a dark energy flow and its evolution in different models. Full-text: PostScript, PDF, or Other formats Testing X-ray Measurements of Galaxy Clusters with Cosmological Simulations Authors: Daisuke Nagai (1), Alexey Vikhlinin (2,3), Andrey V. Kravtsov (4) ((1) Caltech, (2) SAO, Harvard, (3) IKI, Moscow, (4) KICP, U. Chicago) Comments: 11 pages, 6 figures, submitted to ApJ X-ray observations of galaxy clusters potentially provide powerful cosmological probes if systematics due to our incomplete knowledge of the intracluster medium (ICM) physics are understood and controlled. In this paper, we present mock Chandra analyses of cosmological cluster simulations and assess X-ray measurements of galaxy cluster properties using a model and procedure essentially identical to that used in real data analysis. We show that reconstruction of three-dimensional ICM density and temperature profiles is excellent for relaxed clusters, but still reasonably accurate for unrelaxed systems. The total ICM mass is measured quite accurately (<6%) in all clusters, while the hydrostatic estimate of the gravitationally bound mass is biased low by about 5%-20% through the virial region, primarily due to additional pressure support provided by subsonic bulk motions in the ICM, ubiquitous in our simulations even in relaxed systems. Gas fraction determinations are therefore biased high; the bias increases toward cluster outskirts and depends sensitively on its dynamical state, but we do not observe significant trends of the bias with cluster mass or redshift. We also find that different average ICM temperatures, such as the X-ray spectroscopic Tspec and gas-mass-weighted Tmg, are related to each other by a constant factor with a relatively small object-to-object scatter and no systematic trend with mass, redshift or the dynamical state of clusters. We briefly discuss direct applications of our results for different cluster-based cosmological tests. Full-text: PostScript, PDF, or Other formats Cosmology with X-ray Cluster Baryons Authors: Eric V. Linder Comments: 6 pages, 5 figures X-ray cluster measurements interpreted with a universal baryon/gas mass fraction can theoretically serve as a cosmological distance probe. We examine issues of cosmological sensitivity for current (e.g. Chandra X-ray Observatory) and next generation (e.g. Con-X) observations, along with systematic uncertainties and biases. Astrophysical uncertainties degrade the cosmological leverage and even in the absence of systematics the non-Gaussianity of the translation from observations to distance measures can distort the cosmological conclusions. Accounting for systematic effects, even with 1000 well measured clusters out to z=1.7 the determination of dark energy parameters is modest. Full-text: PostScript, PDF, or Other formats Joining the Hubble Flow: Implications for Expanding Space Authors: Luke A. Barnes, Matthew J. Francis, J. Berian James, Geraint F. Lewis Comments: Accepted for publication in MNRAS Report-no: gfl-001 The concept of expanding space has come under fire recently as being inadequate and even misleading in describing the motion of test particles in the universe. Previous investigations have suffered from a number of shortcomings, which we seek to correct. We study the motion of test particles in the universe in detail, solving the geodesic equations of General Relativity for a number of cosmological models. In particular, we use analytic methods to examine whether particles removed from the Hubble flow asymptotically rejoin the Hubble flow, a topic that has caused confusion because of differing definitions and invalid reasoning. We conclude that particles in eternally expanding but otherwise arbitrary universes do not in general rejoin the Hubble flow. Full-text: PostScript, PDF, or Other formats