Munch: Monday, October 16, 2006

                               


 

    usual time and place:  12:30, 6th floor conference room

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


FAQ

What is Munch?
       Munch Archive
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What's the trouble with anthropic reasoning?

Authors: Roberto Trotta (Oxford University), Glenn D. Starkman (Oxford University & Case Western Reserve University)
Comments: 8 pages, no figures. Contribution to the proceedings of the conference "The Dark Side of the Universe", Madrid, June 2006
Selection effects in cosmology are often invoked to "explain" why some of the fundamental constant of Nature, and in particular the cosmological constant, take on the value they do in our Universe. We briefly review this probabilistic "anthropic reasoning" and we argue that different (equally plausible) ways of assigning probabilities to candidate universes lead to totally different anthropic predictions, presenting an explicit example based on the total number of possible observations observers can carry out. We conclude that in absence of a fundamental motivation for selecting one weighting scheme over another the anthropic principle cannot be used to explain the value of Lambda. .

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21-cm Background Anisotropies Can Discern Primordial Non-Gaussianity from Slow-Roll Inflation

Authors: Asantha Cooray (Irvine)
Comments: 4 pages, 1 figure; PRL submitted
The non-Gaussianity of initial perturbations provides information on the mechanism that generated primordial density fluctuations. The expected non-Gaussianity for slow-roll inflationary models is well below the ultimate detection level with cosmic microwave background (CMB) anisotropies or large-large structure at low redshifts. We find that 21-cm background anisotropies of the low-frequency radio sky due to inhomogeneous neutral Hydrogen distribution at redshifts between 30 and 100 captures information on primordial non-Gaussianity better than any other cosmological probe. An all-sky 21-cm experiment over the frequency range from 14 MHz to 40 MHz using a bandwidth of 1 MHz and with angular information out to a multipole of 10^5 can limit the primordial non-Gaussianity parameter f_nl < 0.01. The 21-cm background should eventually reveal the non-Gaussianity associated with single field slow-roll inflation.

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Information content in the halo-model dark-matter power spectrum II: Multiple cosmological parameters

Authors: Mark C. Neyrinck, István Szapudi
Comments: 6 pages, 4 figures, submitted to MNRAS Letters
We investigate the cosmological Fisher information in the non-linear dark-matter power spectrum in the context of the halo model. We find that there is a plateau in information content on translinear scales which is generic to all cosmological parameters we tried. There is a rise in information on smaller scales, but we find that it is quite degenerate among different cosmological parameters (except, perhaps, the tilt). This suggests that the non-linear regime of the dark-matter power spectrum could be of little practical use to constrain cosmological parameters. Finally, we suggest ways to get around this problem, such as removing the largest haloes from consideration in survey analysis.

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Reconciling the CAST and PVLAS Results

Authors: R.N. Mohapatra, Salah Nasri
Comments: 4 pages, 2 figures; references updated
Report-no: UMD-PP-06-054; UFIFT-HEP-06-15
The PVLAS experiment has recently claimed evidence for an axion-like particle in the milli-electron-Volt mass range with a coupling to two photons that appears to be in contradiction with the negative results of the CAST experiment searching for solar axions. The simple axion interpretation of these two experimental results is therefore untenable and it has posed a challenge for theory. We propose a possible way to reconcile these two results by postulating the existence of an ultralight pseudo-scalar particle interacting with two photons and a scalar boson and the existence of a low scale phase transition in the theory.

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Understanding limitations in the determination of the diffuse Galactic gamma-ray emission

Authors: I. V. Moskalenko (Stanford), S. W. Digel (SLAC), T. A. Porter (UCSC), O. Reimer (Stanford), A. W. Strong (MPE)
Comments: 4 pages, to appear in proceedings of 7th UCLA symposium on Sources and Detection of Dark Matter and Dark Energy in the Universe (Marina del Rey, Feb 22-24, 2006); Nuclear Physics B, in press
We discuss uncertainties and possible sources of errors associated with the determination of the diffuse Galactic gamma-ray emission using the EGRET data. Most of the issues will be relevant also in the GLAST era. The focus here is on issues that impact evaluation of dark matter annihilation signals against the diffuse gamma-ray emission of the Milky Way.

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The supernova Hubble diagram for off-center observers in a spherically symmetric inhomogeneous universe

Authors: Havard Alnes, Morad Amarzguioui
Comments: 7 pages (REVTeX4), 5 figures
We have previously shown that spherically symmetric, inhomogeneous universe models can explain both the supernova data and the location of the first peak in the CMB spectrum without resorting to dark energy. In this work, we investigate whether it is possible to get an even better fit to the supernova data by allowing the observer to be positioned away from the origin in the spherically symmetric coordinate system. In such a scenario, the observer sees an anisotropic relation between redshifts and the luminosity distances of supernovae. The level of anisotropy allowed by the data will then constrain how far away from the origin the observer can be located, and possibly even allow for a better fit. Our analysis shows that the fit is indeed improved, but not by a significant amount. Furthermore, it shows that the supernova data do not place a rigorous constraint on how far off-center the observer can be located.

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How well can (renormalized) perturbation theory predict dark matter clustering properties?

Authors: Niayesh Afshordi (ITC, Harvard)
Comments: 3 pages, 3 figures, comments are welcome
There has been some recent activity in trying to understand the dark matter clustering properties in the quasilinear regime, through resummation of perturbative terms, otherwise known as the renormalized perturbation theory (astro-ph/0509418), or the renormalization group method (astro-ph/0606028). While it is not always clear why such methods should work so well, there is no reason for them to capture non-perturbative events such as shell-crossing. In order to estimate the magnitude of non-perturbative effects, we introduce a (hypothetical) model of sticky dark matter, which only differs from collisionless dark matter in the shell-crossing regime. This enables us to show that the level of non-perturbative effects in the dark matter power spectrum at k ~ 0.1 Mpc^{-1}, which is relevant for baryonic acoustic oscillations, is about a percent, but rises to order unity at k ~ 1 Mpc^{-1}.

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Non-Gaussianities in Multi-field Inflation

Authors: Thorsten Battefeld, Richard Easther
Comments: 23 pages
Report-no: Brown-HET-1472
We compute the amplitude of the non-Gaussianities in inflationary models with multiple, uncoupled scalar fields. This calculation thus applies to all models of assisted inflation, including N-flation, where inflation is driven by multiple axion fields arising from shift symmetries in a flux stabilized string vacuum. The non-Gaussianities are associated with nonlinear evolution of the field (and density) perturbations, characterized by the parameter $f_{NL}$. We derive a general expression for the nonlinear parameter, incorporating the evolution of perturbations after horizon-crossing. This is valid for arbitrary separable potentials during slow roll. To develop an intuitive understanding of this system and to demonstrate the applicability of the formalism we examine several cases with quadratic potentials: two-field models with a wide range of mass ratios, and a general N-field model with a narrow mass spectrum. We uncover that $f_{NL}$ is suppressed as the number of e-foldings grows, and that this suppression is increased in models with a broad spectrum of masses. On the other hand, we find no enhancement to $f_{NL}$ that increases with the number of fields. We thus conclude that the production of a large non-Gaussian signal in multi-field models of inflation is thus very unlikely. Finally, we compute a spectrum for the scalar spectral index that incorporates the nonlinear corrections to the fields' evolution.

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Search with EGRET for a Gamma Ray Line from the Galactic Center

Authors: Anthony R. Pullen, Ranga-Ram Chary, Marc Kamionkowski
Comments: 8 pages, 9 figures, 4 tables
We search data from the Energetic Gamma Ray Experiment Telescope (EGRET) for a gamma-ray line in the energy range 0.1-10 GeV from the 10 X 10 degree region around the Galactic center. Our null results lead to upper limits to the line flux from the Galactic center. Such lines may have appeared if the dark matter in the Galactic halo is composed of weakly-interacting massive particles (WIMPs) in the mass range 0.1-10 GeV. For a given dark-matter-halo model, our null search translates to upper limits to the WIMP two-photon annihilation cross section as a function of WIMP mass. We show that for a toy model in which Majorana WIMPs in this mass range annihilate only to electron-positron pairs, these upper limits supersede those derived from measurements of the 511-keV line and continuum photons from internal bremsstrahlung at the Galactic center.

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