Dark Matter

Constraints on isocurvature models from the WMAP first-year data

Authors: K. Moodley, M. Bucher, J. Dunkley, P. G. Ferreira, C. Skordis
Comments: 20 pages, 24 figures. Submitted to PRD

We investigate the constraints imposed by the first-year WMAP CMB data extended to higher multipole by data from ACBAR, BOOMERANG, CBI and the VSA and by the LSS data from the 2dF galaxy redshift survey on the possible amplitude of primordial isocurvature modes. A flat universe with CDM and Lambda is assumed, and the baryon, CDM (CI), and neutrino density (NID) and velocity (NIV) isocurvature modes are considered. Constraints on the allowed isocurvature contributions are established from the data for various combinations of the adiabatic mode and one, two, and three isocurvature modes, with intermode cross-correlations allowed. Since baryon and CDM isocurvature are observationally virtually indistinguishable, these modes are not considered separately. We find that when just a single isocurvature mode is added, the present data allows an isocurvature fraction as large as 13+-6, 7+-4, and 13+-7 percent for adiabatic plus the CI, NID, and NIV modes, respectively. When two isocurvature modes plus the adiabatic mode and cross-correlations are allowed, these percentages rise to 47+-16, 34+-12, and 44+-12 for the combinations CI+NID, CI+NIV, and NID+NIV, respectively. Finally, when all three isocurvature modes and cross-correlations are allowed, the admissible isocurvature fraction rises to 57+-9 per cent. The sensitivity of the results to the choice of prior probability distribution is examined.

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Confronting LCDM with Gravitational Lensing Constraints on Small Scale Structure

Authors: R. Benton Metcalf (University of California, Santa Cruz)
Comments: submitted to ApJ, 17 pages, 7 figures

This paper primarily addresses the question of whether recent lensing observations probing the small scale structure in the universe are consistent with the LCDM model. Here a conservative approach is taken where only the most difficult to explain cases of image flux anomalies in strong lenses are considered. Numerical simulations are performed to compare predictions for the LCDM small scale mass function with observed flux ratios. It is found that all the cusp caustic lens anomalies and the disagreements between monochromatic flux ratios and simple lens models can be explained without any substructure in the primary lenses' dark matter halos. Extragalactic LCDM halos are enough to naturally explain these cases. However, spectroscopic gravitational lensing observations of Q2237+0305 - and, less conclusively, bent lensed radio jets - require more small mass halos (~ 10^6 Msun) than is expected in the LCDM model.

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Constraints on the Primordial Power Spectrum from High Resolution Lyman-alpha Forest Spectra and WMAP

Authors: Matteo Viel, Jochen Weller, Martin Haehnelt (Institute of Astronomy, Cambridge)
Comments: 6 pages, 3 Figures, 1 Table, submitted to MNRAS

The combined analysis of the cosmic microwave background on large scales and Lyman-alpha forest on small scales provides a sufficiently long lever arm to obtain strong constraints on the slope and curvature of the power spectrum of primordial density fluctuations. We present results from the combination of the first year WMAP data and the dark matter power spectrum inferred by Viel et al. (2004) for two different sets of high resolution and high signal-to-noise quasar absorption spectra: the Croft et al. (2002) sample with a median redshift z=2.72 and the LUQAS sample (Kim et al. 2004) with a median redshift z=2.125. The best fit value for the rms fluctuation amplitude of matter fluctuations is sigma8 =0.93+-0.05 and n=0.99+-0.03, if we do not include running of the spectral index. The best fit model with a running spectral index has parameters n=0.964+-0.031 and nrun=-0.034+- 0.025. The data is thus consistent with a scale-free primordial power spectrum with no running of the spectral index. We further include tensor modes and constrain the slow-roll parameters of inflation.

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Cosmology with a dynamically screened scalar interaction in the dark sector

Authors: Steven S. Gubser, P.J.E. Peebles
Comments: 26 pages, 3 figures
Report-no: PUPT-2123

Motivated in part by string theory, we consider a modification of the LambdaCDM cosmological model in which the dark matter has a long-range scalar force screened by light particles. Scalar forces can have interesting effects on structure formation: the main example presented here is the expulsion of dark matter halos from low density regions, or voids, in the galaxy distribution.

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Back-Reaction of Cosmological Perturbations in the Infinite Wavelength Approximation

Authors: Robert H. Brandenberger, C. S. Lam
Comments: 5 pages, no figures
Report-no: BROWN-HET-1405

Cosmological perturbations in an expanding universe back-react on the space-time in which they propagate. Calculations to lowest non-vanishing order in perturbation theory indicate that super-Hubble-scale fluctuations act as a negative and time-dependent cosmological constant and may thus lead to a dynamical relaxation mechanism for the cosmological constant. Here we present a simple model of how to understand this effect from the perspective of homogeneous and isotropic cosmology. Our analysis, however, also shows that an effective spatial curvature is induced, indicating potential problems in realizing the dynamical relaxation of the cosmological constant by means of back-reaction.

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The Revival of the Unified Dark Energy-Dark Matter Model ?

Authors: M.C.Bento, O. Bertolami, A.A. Sen
Comments: 6 pages, 4 eps figures, Revtex4 style. New References are added. Some typos are corrected. Conclusions remain the same

We consider the generalized Chaplygin gas (GCG) proposal for unification of dark energy and dark matter and show that it admits an unique decomposition into dark energy and dark matter components once phantom-like dark energy is excluded. Within this framework, we study structure formation and show that difficulties associated to unphysical oscillations or blow-up in the matter power spectrum can be circumvented. Furthermore, we show that the dominance of dark energy is related to the time when energy density fluctuations start deviating from the linear $\delta \sim a$ behaviour.

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Monitoring lensed starlight emitted close to the Galactic Center

Authors: Adi Nusser, Tom Broadhurst
Comments: 4 pages, 3 figures, submitted to MNRAS

We describe the feasibility of detecting the gravitational deflection of light emitted by stars moving under the influence of the massive object at the Galactic center. Light emitted by a star orbiting behind the central mass has a smaller impact parameter than the star itself, and suffers the effect of gravitational lensing, providing a closer probe of the central mass distribution and hence a stricter test of the black hole hypothesis. A mass of $4.3\times 10^{6} M_{\odot}$ causes a $0.1-2\rm mas$ deviation in the apparent position of orbiting stars projected within $10^{\circ}$ of the line of sight to the galactic center. In addtion, we may uniquely constrain the distance to the center of the galaxy because lensing deflections constrain the ratio $\rg/R_{0}$ of the Schwarzschild radius to the distance to the black hole, $R_{o}$, whereas the ratio $\rg/R_{o}^{3}$ is obtained by fitting the orbit.

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Redshift-Space Distortions, Pairwise Velocities and Nonlinearities

Authors: Roman Scoccimarro
Comments: 30 pages, 7 figures

We derive the exact relationship, including all non-linearities, between real-space and redshift-space two-point statistics through the pairwise velocity distribution function. We show using numerical simulations that the pairwise velocity PDF is strongly non-Gaussian at all scales, and explain why this is so. We caution that a commonly used ansatz to model the redshift-space power spectrum gives rise to an unphysical distribution of pairwise velocities, and show that it is in general impossible to derive the distribution from measurements of redshift-space clustering. Methods that claim to do this obtain instead something else, whose properties we derive.
We provide a general derivation of the large-scale limit of the redshift-space power spectrum and show that it differs from the Kaiser formula by terms that depend on Gaussian and non-Gaussian contributions to the velocity dispersion of large-scale flows. We also show that the large-scale evolution of velocity fields is not well described by linear theory and discuss how this impacts the redshift-space power spectrum. Finally, we stress that using the monopole of the redshift-space power as an indicator of the real-space power spectrum shape can lead to systematic effects in the determination of cosmological parameters; nevertheless a simple procedure is able to recover the large-scale real-space power spectrum rather well.

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Cosmic microwave background constraints on the strong equivalence principle

Authors: V. Boucher (1), J.-M. Gerard (1), P. Vandergheynst (2), Y. Wiaux (2) ((1) Universite catholique de Louvain, Louvain-la-Neuve, Belgium, (2) Swiss Federal Institute of Technology, Lausanne, Switzerland)
Comments: 15 pages

We study the effect of a violation of the strong equivalence principle (SEP) on the cosmic microwave background (CMB). Such a violation would modify the weight of baryons in the primordial gravitational potentials and hence their impact in the establishment of the photon-baryon plasma acoustic oscillations before recombination. This cosmological Nordtvedt effect alters the odd peaks height of the CMB temperature anisotropy power spectrum. A gravitational baryonic mass density of the universe may consequently be inferred at the first peak scale from the analysis of WMAP data. Experimental constraints on a primordial SEP violation are suggested by comparison with independent measurements of the universe's inertial baryonic mass density. At present, the corresponding best constraint is obtained through the analysis of light element abundances in the framework of the standard big bang nucleosynthesis (BBN).

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Testing cosmological models and understanding cosmological parameter determinations with metaparameters

Authors: Mike Chu, Lloyd Knox (UC Davis)
Comments: 5 pages, submitted to ApJ

Cosmological parameters affect observables in physically distinct ways. For example, the baryon density, omega_b, affects the ionization history and also the pressure of the pre-recombination fluid. To investigate the relative importance of different physical effects to the determination of omega_b, and to test the cosmological model, we artificially split omega_b into two `metaparameters': omega_{be} which controls the ionization history and omega_{bp} which plays the role of omega_b for everything else. In our demonstration of the technique we find omega_b = .0229 +/- .0012 (with no parameter splitting), omega_{bp} = .0238 +/- .0021, omega_{be}= .0150 +/- .0034 and omega_{bp}-omega_{be} = .0088 +/- .0039.

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Interacting Quintessence, Cosmic Acceleration and the Coincidence Problem

Authors: Greg Huey, Benjamin D. Wandelt (UIUC)
Comments: 4 pages and 3 figures, submitted to PRL on March 25 2004

Faced by recent evidence for a flat universe dominated by dark energy, cosmologists grapple with deep cosmic enigmas such as the cosmological constant problem, extreme fine-tuning and the cosmic coincidence problem. The extent to which we observe the dimming of distant supernovae suggests that the cosmic acceleration is as least as severe as in cosmological constant models. Extrapolating this to our cosmic future implies terrifying visions of either a cold and empty universe or an explosive demise in a ``Big Rip.'' We construct a class of dynamical scalar field models of dark energy and dark matter. Within this class we can explain why supernovae imply a cosmic equation of state $w\lesssim-1$, address fine tuning issues, protect the universe from premature acceleration and predict a constant fraction of dark energy to dark matter in the future (thus solving the coincidence problem), satisfy the dominant energy condition, and ensure that gravitationally bound objects remain so forever (avoid a Big Rip). This is achieved with a string theory inspired Lagrangian containing standard kinetic terms, exponential potentials and couplings, and parameters of order unity.