Munch

We describe the Dark Energy Survey (DES), a proposed optical-near infrared survey of 5000 sq. deg of the South Galactic Cap to ~24th magnitude in SDSS griz, that would use a new 3 sq. deg CCD camera to be mounted on the Blanco 4-m telescope at Cerro Telolo Inter-American Observatory (CTIO). The survey data will allow us to measure the dark energy and dark matter densities and the dark energy equation of state through four independent methods: galaxy clusters, weak gravitational lensing tomography, galaxy angular clustering, and supernova distances. These methods are doubly complementary: they constrain different combinations of cosmological model parameters and are subject to different systematic errors. By deriving the four sets of measurements from the same data set with a common analysis framework, we will obtain important cross checks of the systematic errors and thereby make a substantial and robust advance in the precision of dark energy measurements.

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Constraining Dark Energy with the Dark Energy Survey: Theoretical Challenges

Authors: James Annis, Sarah Bridle, Francisco J. Castander, August E. Evrard, Pablo Fosalba, Joshua A. Frieman, Enrique Gaztanaga, Bhuvnesh Jain, Andrey V. Kravtsov, Ofer Lahav, Huan Lin, Joseph Mohr, Albert Stebbins, Terence P. Walker, Risa H. Wechsler, David H. Weinberg, Jochen Weller
Comments: 5 pages. White paper submitted to the Dark Energy Task Force

The Dark Energy Survey (DES) will use a new imaging camera on the Blanco 4-m telescope at CTIO to image 5000 square degrees of sky in the South Galactic Cap in four optical bands, and to carry out repeat imaging over a smaller area to identify and measure lightcurves of Type Ia supernovae. The main imaging area overlaps the planned Sunyaev-Zel'dovich survey of the South Pole Telescope. The idea behind DES is to use four distinct and largely independent methods to probe the properties of dark energy: baryon oscillations of the power spectrum, abundance and spatial distribution of clusters, weak gravitational lensing, and Type Ia supernovae. This white paper outlines, in broad terms, some of the theoretical issues associated with the first three of these probes (the issues for supernovae are mostly different in character), and with the general task of characterizing dark energy and distinguishing it from alternative explanations for cosmic acceleration. A companion white paper discusses the kind of numerical simulations and other theoretical tools that will be needed to address the these issues and to create mock catalogs that allow end-to-end tests of analysis procedures. Although we have been thinking about these problems in the specific context of DES, many of them are also relevant to other planned dark energy studies.

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Cosmological Constraints from Weak Lensing Surveys

Authors: Dipak Munshi, Patrick Valageas
Comments: 19 pages, 24 figs., submitted to MNRAS

Focusing on the well motivated aperture mass statistics $\Map$, we study the possibility of constraining cosmological parameters using future space based SNAP class weak lensing missions. Using completely analytical results we construct the covariance matrix for estimators based on two-point and three-point statistics. Our approach incorporates an accurate modelling of higher-order statistics to describe cosmic variance as well as various sources of discrete noise at small angular scales. These results are then fed into a Fisher matrix based analysis to study cosmological parameter degeneracies. Joint and independent analysis, with or without redshift binning, for various parameter combinations are presented. An analytical modelling of the covariance matrix opens up the possibility of testing various approximations which are often used in derivations of semi-analytical results. These include how inclusion of full non-Gaussian terms in covariance matrix affects parameter estimation. Inclusion of three-point information and how such information can enhance the accuracy with which certain parameters can be estimated is also studied in detail. It is shown that broad correlation structure among various angular scales in such circumstances implies reduction in number of available angular scales which carry completely independent information. On the other hand, the effect of theoretical inaccuracies, in modelling either the power-spectrum or bi-spectrum evolution, onto the measure of cosmological parameters from weak lensing surveys is also considered. Several cosmological parameters, $\Om$, $\sigma_8$, spectral index $n_s$, running of spectral index $\alpha_s$ and equation of state of the dark energy $\wde$ are included in the analysis.

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Dark Energy: The Observational Challenge

Authors: David H. Weinberg
Comments: 13 pages, to appear in proceedings of Wide Field Imaging From Space, New Astronomy Reviews, eds. T. McKay, A. Fruchter, and E. Linder

Nearly all proposed tests for the nature of dark energy measure some combination of four fundamental observables: the Hubble parameter H(z), the distance-redshift relation d(z), the age-redshift relation t(z), or the linear growth factor D_1(z). I discuss the sensitivity of these observables to the value and redshift history of the equation of state parameter w, emphasizing where these different observables are and are not complementary. Demonstrating time-variability of w is difficult in most cases because dark energy is dynamically insignificant at high redshift. Time-variability in which dark energy tracks the matter density at high redshift and changes to a cosmological constant at low redshift is {\it relatively} easy to detect. However, even a sharp transition of this sort at z_c=1 produces only percent-level differences in d(z) or D_1(z) over the redshift range 0.4 < z < 1.8$, relative to the closest constant-w model. Estimates of D_1(z) or H(z) at higher redshift, potentially achievable with the Ly-alpha forest, galaxy redshift surveys, and the CMB power spectrum, can add substantial leverage on such models, given precise distance constraints at z < 2. The most promising routes to obtaining sub-percent precision on dark energy observables are space-based studies of Type Ia supernovae, which measure d(z) directly, and of weak gravitational lensing, which is sensitive to d(z), D_1(z), and H(z).

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SHELS: The Hectospec Lensing Survey

Authors: Margaret J. Geller, Ian P. Dell'Antonio, Michael J. Kurtz, Massimo Ramella, Daniel G. Fabricant, Nelson Caldwell, J. Anthony Tyson, David Wittman
Comments: Submitted to Astrophysical Journal Letters 9pages, 3 figures

The Smithsonian Hectospec Lensing Survey (SHELS) combines a large deep complete redshift survey with a weak lensing map from the Deep Lens Survey (Wittman et al. 2002; 2005). We use maps of the velocity dispersion based on systems identified in the redshift survey to compare the three-dimensional matter distribution with the two-dimensional projection mapped by weak lensing. We demonstrate directly that the lensing map images the three-dimensional matter distribution obtained from the kinematic data.

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Cosmological Structure Evolution and CMB Anisotropies in DGP Braneworlds

Authors: Ignacy Sawicki, Sean M. Carroll
Comments: 22 pages, 10 figures
Report-no: EFI-0518

The braneworld model of Dvali, Gabadadze and Porrati (DGP) provides an intriguing modification of gravity at large distances and late times. By embedding a three-brane in an uncompactified extra dimension with separate Einstein-Hilbert terms for both brane and bulk, the DGP model allows for an accelerating universe at late times even in the absence of an explicit vacuum energy. We examine the evolution of cosmological perturbations on large scales in this theory. At late times, perturbations enter a DGP regime in which the effective value of Newton's constant increases as the background density diminishes. This leads to a suppression of the integrated Sachs-Wolfe effect, bringing DGP gravity into slightly better agreement with WMAP data than conventional LCDM. However, we find that this is not enough to compensate for the significantly worse fit to supernova data and the distance to the last-scattering surface in the pure DGP model. LCDM is, therefore, a better fit.

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The Cosmological Unimportance of Low Surface Brightness Galaxies

Authors: C. C. Hayward, J. A. Irwin, J. N. Bregman
Comments: 17 pages, 4 figures, to be published in The Astrophysical Journal

We have searched for Type Ia supernovae (SNe Ia) in the local (d < 60 Mpc) Universe using Northern Sky Variability Survey (NSVS) data collected from the nightly optical surveys of the Robotic Optical Transient Search Experiment} (ROTSE) Telescope. It was hoped that SNe Ia would provide a means to find previously-unknown low surface brightness (LSB) galaxies or displaced stars that would otherwise be very difficult to detect. The ROTSE data allowed us to survey 19,000 square degrees at declinations north of 0 degrees, but we did not find a single SN Ia in a period of time covering roughly one year. Using known SNe Ia rates in bright galaxies, we set an upper limit on the optical luminosity density, L_B, of LSBs in the local Universe. Using mean LSB baryonic and dynamical mass-to-light ratios, we find 95% upper limits for LSBs of L_B \le 2.53 x 10^8 L_{B, solar} Mpc^{-3}, Omega_b \le 0.0040, and Omega_m \le 0.036. We conclude that LSBs and displaced stars are not a major constituent of matter in the local Universe.

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High-energy Cosmic Rays

Authors: Thomas K. Gaisser, Todor Stanev
Comments: 16 pages, 5 fugures, to be published in Nuclear Physics A

After a brief review of galactic cosmic rays in the GeV to TeV energy range, we describe some current problems of interest for particles of very high energy. Particularly interesting are two features of the spectrum, the `knee' above $10^{15}$ eV and the `ankle' above $10^{18}$ eV. An important question is whether the highest energy particles are of extra-galactic origin and, if so, at what energy the transition occurs. A theme common to all energy ranges is use of nuclear abundances as a tool for understanding the origin of the cosmic radiation.

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Dark matter search experiment with CaF2(Eu) scintillator at Kamioka Observatory

Authors: Y. Shimizu, M. Minowa, W. Suganuma, Y. Inoue
Comments: 11 pages, 4 figures

We report recent results of a WIMP dark matter search experiment using 310g of CaF2(Eu) scintillator at Kamioka Observatory. We chose a highly radio-pure crystal, PMTs and radiation shields, so that the background rate decreased considerably. We derived limits on the spin dependent WIMP-proton and WIMP-neutron coupling coefficients, a_p and a_n. The limits excluded a part of the parameter space allowed by the annual modulation observation of the DAMA NaI experiment.

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Devaluation: a dynamical mechanism for a naturally small cosmological constant

Authors: Katherine Freese, James T. Liu, Douglas Spolyar
Categories: hep-ph
Comments: 6 pages and prepared in ReV-TeX
Report-no: MCTP-05-94

We propose a natural solution to the cosmological constant problem consistent with the standard cosmology and successful over a broad range of energies. This solution is based on the existence of a new field, the devaluton, with its potential modeled on a tilted cosine. After inflation, the universe reheats and populates the devaluton's many minima. As the universe cools, domain walls form between different regions. The domain wall network then evolves and sweeps away regions of higher vacuum energy in favor of lower energy ones. Gravitation itself provides a cutoff at a minimum vacuum energy, thus leaving the universe with a small cosmological constant comparable in magnitude to the present day dark energy density.

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Radiative Transfer Effects on the Lya Forest

Authors: A. Maselli (1), A. Ferrara (2) ((1)MPA, Garching (2)SISSA, Trieste)
Comments: 13 pages, 10 figures, Accepted for publication in MNRAS

Strong observational evidence for a fluctuating ultraviolet background (UVB) has been accumulating through a number of studies of the HI and HeII Lya forest as well as accurate IGM metallicity measurements. UVB fluctuations could arise both from the inhomogeneous distribution of the ionizing sources and/or from radiative transfer (RT) through the filamentary IGM. In this study we investigate, via numerical simulations, the role of RT effects such as shadowing, self-shielding and filtering of the ionizing radiation, in giving raise to a fluctuating UVB. We focus on possible detectable signatures of these effects on quantities derived from Lya forest spectra, as photoionization rate fluctuations, eta parameter (the HeII to HI column density ratio) distributions and the IGM temperature at redshift about 3. We find that RT induces fluctuations up to 60% in the UVB, which are tightly correlated to the density field. The UVB mean intensity is progressively suppressed toward higher densities and photon energies above 4 Ryd, due to the high HeII opacity. Shielding of overdense regions (Delta > 5) from cosmic HeII ionizing radiation, produces a decreaseing trend of eta with overdensity. Furthermore we find that the mean eta value inferred from HI-HeII Lya forest observations can be explained only by properly accounting for the actual IGM opacity. We outline and discuss several implications of our findings.

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