Munch

We propose a fast and efficient bispectrum statistic for Cosmic Microwave Background (CMB) temperature anisotropies to constrain the amplitude of the primordial non-Gaussian signal measured in terms of the non-linear coupling parameter f_NL. We show how the method can achieve a remarkable computational advantage by focussing on subsets of the multipole configurations, where the non-Gaussian signal is more concentrated. The detection power of the test, increases roughly linearly with the maximum multipole, as shown in the ideal case of an experiment without noise and gaps. The CPU-time scales as l_{max}^3 instead of l_{max}^5 for the full bispectrum which for Planck resolution l_{max} \sim 3000 means an improvement in speed of a factor 10^7 compared to the full bispectrum analysis with minor loss in precision. We find that the introduction of a galactic cut partially destroys the optimality of the configuration, which will then need to be dealt with in the future. We find for an ideal experiment with l_{max}=2000 that upper limits of f_{NL}<8 can be obtained at 1 sigma. For the case of the WMAP experiment, we would be able to put limits of |f_{NL}|<40 if no galactic cut were present. Using the real data with galactic cut, we obtain an estimate of -80<f_{NL}<80 and -160<f_{NL}<160 at 1 and 2 sigma respectively.

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The Good and Evil of Gamma Rays Bursts

What Can Gamma Ray Bursts Teach Us About Dark Energy?

Authors: Dan Hooper, Scott Dodelson
Comments: 5 pages, 9 figures
Report-no: FERMILAB-PUB-05-532-A

It has been suggested that Gamma Ray Bursts (GRB) may enable the expansion rate of our Universe to be measured out to very high redshifts ($z \gsim 5$) just as type Ia supernovae have done at $z \sim$1--1.5. We explore this possibility here, and find that GRB have the potential to detect dark energy at high statistical significance, but they are unlikely to be competitive with future supernovae missions, such as SNAP, in measuring the properties of the dark energy. The exception to this conclusion is if there is appreciable dark energy at early times, in which case the information from GRB's will provide an excellent complement to the $z\sim 1$ information from supernovae.

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Implications for the Cosmic Reionization from the Optical Afterglow Spectrum of the Gamma-Ray Burst 050904 at z = 6.3

Authors: Tomonori Totani (1), Nobuyuki Kawai (2), George Kosugi (3), Kentaro Aoki (3), Toru Yamada (3), Masanori Iye (3), Kouji Ohta (1), Takashi Hattori (3) ((1) Kyoto, (2) TITech, (3) NAO)
Comments: The version with full resolution figures is available at this http URL This is the second paper from the Subaru GRB team for GRB 050904, following Kawai et al. (astro-ph/0512052)

The gamma-ray burst (GRB) 050904 at z = 6.3 provides the first opportunity of probing the intergalactic medium (IGM) by GRBs at the epoch of the reionization. Here we present a spectral modeling analysis of the optical afterglow spectrum taken by the Subaru Telescope, aiming to constrain the reionization history. The spectrum shows a clear damping wing at wavelengths redward of the Lyman break, and the wing shape can be fit either by a damped Ly alpha system with a column density of log N_HI ~ 21.6 at a redshift close to the detected metal absorption lines (z_{metal} = 6.295), or by almost neutral IGM extending to a slightly higher redshift of z_{IGM,u} ~ 6.36. In the latter case, the difference from z_{metal} may be explained by acceleration of metal absorbing shells by the activities of the GRB or its progenitor. However, we exclude this possibility by using the light transmission feature around the Ly beta resonance, leading to a firm upper limit of z_{IGM,u} < 6.314. We then show an evidence that the IGM was largely ionized already at z=6.3, with the best-fit neutral fraction of IGM, x_HI = 0.00 +- 0.17, and an upper limit of x_HI < 0.60 (95% C.L.). This is the first quantitative upper limit on x_HI at z > 6. Various systematic uncertainties are examined, but none of them appears large enough to change this conclusion. To get further information on the reionization, it is important to increase the sample size of z >6 GRBs, to find GRBs with low column densities (log N_HI <~ 20) within their host galaxies, and for statistical studies of Ly alpha line emission from host galaxies.

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Biological Effects of Gamma-Ray Bursts: Critical distances for severe damage on the biota

Authors: Douglas Galante, Jorge Ernesto Horvath
Comments: 27 pages, 3 figure, submitted to Astrobiology

We present in this work a unified, quantitative synthesis of analytical and numerical calculations of the effects caused on an Earth-like planet by a Gamma-Ray Burst (GRB), considering atmospheric and biological implications. The main effects of the illumination by a GRB are classified in four distinct ones and analyzed separately, namely the direct gamma radiation transmission, UV flash, ozone layer depletion and cosmic rays. The effectiveness of each of these effects is compared and lethal distances for significant biological damage are given for each one. We find that the first three effects have potential to cause global environmental changes and biospheric damages, even if the source is located at great distances (perhaps up to ~ 100 kpc). Instead, cosmic rays would only be a serious threat for very close sources. As a concrete example of a recorded similar event, the effects of the giant flare from SGR1806-20 of Dec 27, 2004 could cause on the biosphere are addressed. In spite of not belonging to the so-called 'classical' GRBs, most of the parameters of this recent flare are well-known and serve as a calibration for our study. We find that giant flares are not a threat for life in all practical situations on Earth, mainly because it is not as energetic, in spite of being much more frequent than GRBs.

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Does Dark Energy Exist and How Can We Find It?

Correlated Fluctuations in Luminosity Distance and the (Surprising) Importance of Peculiar Motion in Supernova Surveys

Authors: Lam Hui (Columbia University), Patrick B. Greene (University of Texas, San Antonio)
Comments: 25 pages, 9 figures, references updated

Large scale structure introduces two different kinds of errors in the luminosity distance estimates from standardizable candles such as supernovae Ia (SNe) - a Poissonian scatter for each SN and a coherent component due to correlated fluctuations between different SNe. Increasing the number of SNe helps reduce the first type of error but not the second. The coherent component has been largely ignored in forecasts of dark energy parameter estimation from upcoming SN surveys. For instance it is commonly thought, based on Poissonian considerations, that peculiar motion is unimportant, even for a low redshift SN survey such as the Nearby Supernova Factory (SNfactory; z = 0.03 - 0.08), which provides a useful anchor for future high redshift surveys by determining the SN zero-point. We show that ignoring coherent peculiar motion leads to an underestimate of the zero-point error by about a factor of 2, despite the fact that SNfactory covers almost half of the sky. More generally, there are four types of fluctuations: peculiar motion, gravitational lensing, gravitational redshift and what is akin to the integrated Sachs-Wolfe effect. Peculiar motion and lensing dominates at low and high redshifts respectively. Taking into account all significant luminosity distance fluctuations due to large scale structure leads to a degradation of up to 60% in the determination of the dark energy equation of state from upcoming high redshift SN surveys, when used in conjunction with a low redshift anchor such as the SNfactory. The most relevant fluctuations are the coherent ones due to peculiar motion and the Poissonian ones due to lensing, with peculiar motion playing the dominant role. We also discuss to what extent the noise here can be viewed as a useful signal, and whether corrections can be made to reduce the degradation.

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An inhomogeneous alternative to dark energy?

Authors: Havard Alnes, Morad Amarzguioui, Oyvind Gron
Comments: 8 pages (REVTeX4), 4 figures

Recently, there have been suggestions that the apparent accelerated expansion of the universe is not caused by repulsive gravitation due to dark energy, but is rather a result of inhomogeneities in the distribution of matter. In this work, we investigate the behaviour of a dust dominated inhomogeneous Lemaitre-Tolman-Bondi universe model, and confront it with various astrophysical observations. We find that such a model can easily explain the observed luminosity distance-redshift relation of supernovae without the need for dark energy, when the inhomogeneity is in the form of an underdense bubble centered near the observer. With the additional assumption that the universe outside the bubble is approximately described by a homogeneous Einstein-de Sitter model, we find that the position of the first CMB peak can be made to match the WMAP observations. Whether or not it is possible to reproduce the entire CMB angular power spectrum in an inhomogeneous model without dark energy, is still an open question.

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Large-scale galaxy correlations as a test for dark energy

Authors: Alain Blanchard (LATT, Toulouse), Marian Douspis (LATT, Toulouse), Michael Rowan-Robinson (Imperial College, London), Subir Sarkar (Oxford University)
Comments: 4 pages, 2 figures, submitted to A&A

We have shown earlier that, contrary to popular belief, Einstein--de Sitter models can still fit the {\sl WMAP} data on the cosmic microwave background provided one adopts a low Hubble constant and relaxes the usual assumption that the primordial density perturbation is scale-free. The recent {\sl SDSS} measurement of the large-scale correlation function of luminous red galaxies has however provided a new constraint by detecting a baryonic 'acoustic' peak. Our best-fit E--deS models do possess a baryonic feature at a similar physical scale as the best-fit $\Lambda$CDM concordance model, but do not fit the new observations as well as the latter. In particular the shape of the correlation function in the range $\sim 10-100 h^{-1}$ Mpc cannot be reproduced properly without violating the CMB angular power spectrum in the multipole range $l \sim 100-1000$. Thus, the combination of the CMB fluctuations and the shape of the correlation function up to $\sim 100 h^{-1}$Mpc, if confirmed, does seem to require dark energy for a cosmological model based on (adiabatic) inflationary perturbations.

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Detecting dark energy in long baseline neutrino oscillations

Authors: Pei-Hong Gu, Xiao-Jun Bi, Bo Feng, Bing-Lin Young, Xinmin Zhang
Comments: 9 pages, 6 figures
Report-no: BIHEP-TH-2005-16, RESCEU-38/05

In this paper, we discuss a possibility of studying properties of dark energy in long baseline neutrino oscillation experiments. We consider two types of models of neutrino dark energy. For one type of models the scalar field is taken to be quintessence-like and for the other phantom-like. In these models the scalar fields couple to the neutrinos to give rise to a spatially varying neutrino masses. We will show that the two types of models predict different behaviors of the spatial variation of the neutrino masses inside Earth and consequently result in different signals in long baseline neutrino oscillation experiments.

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Possible evidence for "dark radiation" from Big Bang Nucleosynthesis Data

Authors: V.V. Flambaum, E.V. Shuryak

We address the emerging discrepancy between the Big Bang Nucleosynthesis data and standard cosmology, which asks for a bit longer evolution time. If this effect is real, one possible implication (in a framework of brane cosmology model) is that there is a ``dark radiation'' component which is negative and makes few percents of ordinary matter density. If so, all scales of this model can be fixed, provided brane-to-bulk leakage problem is solved.

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Two proposals for new particles

Minimal Dark Matter

Authors: Marco Cirelli, Nicolao Fornengo, Alessandro Strumia
Comments: 16 pages, 2 figures
Report-no: DFTT40/2005, IFUP-TH/2005-34

A few multiplets that can be added to the SM contain a lightest neutral component which is automatically stable and provides allowed DM candidates with a non-standard phenomenology. Thanks to coannihilations, a successful thermal abundance is obtained for well defined DM masses. The best candidate seems to be a SU(2)_L fermion quintuplet with mass 4.5 TeV, accompanied by a charged partner 166 MeV heavier with life-time 1.8 cm, that manifests at colliders as charged tracks disappearing in pi^\pm with 97.7% branching ratio. Its cross section for usual NC direct DM detection is sigma_SI = f^2 1.0 10^-43 cm^2 where f ~ 1 is a nucleon matrix element. We study prospects for CC direct detection and for indirect detection.

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Ultra High Energy Cosmic Rays from Sequestered X Bursts

Authors: Peter L. Biermann, Paul H. Frampton
Comments: 12 pages LaTeX

Assuming that there is no GZK (Greisen-Zatsepin-Kuzmin) cut-off and that super-GZK cosmic rays correlate with AGN (Active Galactic Nuclei) at cosmological distances, it is speculated that a relic superheavy particle (X) has its lifetime enhanced by sequestration in an extra dimension. This sequestration is assumed to be partially liberated by proximity of merging supermassive black holes in an AGN, temporarily but drastically reducing the lifetime, thus stimulating an X burst. Based on sequestration of the decay products of X, a speculative explanation of the observed $\gamma/N$ ratio is proposed.

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Simulating Cosmic Reionization at Large Scales I: the Geometry of Reionization

Authors: Ilian T. Iliev (1), Garrelt Mellema (2,3), Ue-Li Pen (1), Hugh Merz (1), Paul R. Shapiro (4), Marcelo A. Alvarez (4) ((1) CITA, (2) ASTRON, Dwingeloo, (3) Leiden Observatory, (4) Univ. Texas at Austin)
Comments: Comments: 16 pages, 16 figures, submitted to MNRAS. Movies and higher resolution figures can be found at this http URL

We present the first large-scale radiative transfer simulations of cosmic reionization, in a simulation volume of (100/h Mpc)^3, while at the same time capturing the dwarf galaxies which are primarily responsible for reionization. We achieve this by combining the results from extremely large, cosmological, N-body simulations with a new, fast and efficient code for 3D radiative transfer, C^2-Ray. The resulting electron-scattering optical depth is in good agreement with the first-year WMAP polarization data. We show that reionization clearly proceeded in an inside-out fashion, with the high-density regions being ionized earlier, on average, than the voids. Ionization histories of smaller-size (5 to 10 comoving Mpc) subregions exibit a large scatter about the mean and do not describe the global reionization history well. The minimum reliable volume size for such predictions is ~30 Mpc. We derive the power-spectra of the neutral, ionized and total gas density fields and show that there is a significant boost of the density fluctuations in both the neutral and the ionized components relative to the total at arcminute and larger scales. We find two populations of HII regions according to their size, numerous, mid-sized (~10 Mpc) regions and a few, rare, very large regions tens of Mpc in size. We derive the statistical distributions of the ionized fraction and ionized gas density at various scales and for the first time show that both distributions are clearly non-Gaussian. (abridged)

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Acoustic oscillations in the SDSS DR4 Luminous Red Galaxy sample power spectrum

Authors: Gert Huetsi
Comments: Extended version of the previous work astro-ph/0507678, now including new data from the SDSS DR4. 17 pages, 18 figures, accepted for publication in A&A

We calculate the redshift-space power spectrum of the Sloan Digital Sky Survey (SDSS) Data Release 4 (DR4) Luminous Red Galaxy (LRG) sample, finding evidence for a full series of acoustic features down to the scales of \sim 0.2 hMpc^{-1}. This corresponds up to the 7th peak in the CMB angular power spectrum. The acoustic scale derived, (105.4 \pm 2.3) h^{-1}Mpc, agrees very well with the ``concordance'' model prediction and also with the one determined via the analysis of the spatial two-point correlation function by Eisenstein et al. (2005). The models with baryonic features are favored by 3.3 \sigma over their ``smoothed-out'' counterparts without any oscillatory behavior.

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Cluster Masses from CMB and Galaxy Weak Lensing

Authors: Antony Lewis, Lindsay King
Comments: 17 pages, 7 figures

Gravitational lensing can be used to directly constrain the projected density profile of galaxy clusters. We discuss possible future constraints from lensing of the CMB temperature and polarization, and compare to results from galaxy weak lensing. We model the moving lens and kinetic SZ signals that confuse the temperature CMB lensing when cluster velocities and angular momenta are unknown, and show how they degrade parameter constraints. The CMB polarization cluster lensing signal is ~1 micro-Kelvin for massive clusters and challenging to detect; however it should be significantly cleaner than the temperature signal and may provide the most robust constraints at low noise levels. Galaxy lensing is likely to be much better for constraining cluster masses at low redshift, but for clusters at redshift z >~ 1 future CMB lensing observations may be able to do better.

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