In the last 30 days

• Open/close Fractal analysis of the dark matter and gas distributions in the Mare-Nostrum universe

  José Gaite  JCAP03(2010)006 Tag this article

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  We develop a method of multifractal analysis of N-body cosmological simulations that improves on the customary counts-in-cells method by taking special care of the effects of discreteness and large scale homogeneity. The analysis of the Mare-Nostrum simulation with our method provides strong evidence of self-similar multifractal distributions of dark matter and gas, with a halo mass function that is of Press-Schechter type but has a power-law exponent -2, as corresponds to a multifractal. Furthermore, our analysis shows that the dark matter and gas distributions are indistinguishable as multifractals. To determine if there is any gas biasing, we calculate the cross-correlation coefficient, with negative but inconclusive results. Hence, we develop an effective Bayesian analysis connected with information theory, which clearly demonstrates that the gas is biased in a long range of scales, up to the scale of homogeneity. However, entropic measures related to the Bayesian analysis show that this gas bias is small (in a precise sense) and is such that the fractal singularities of both distributions coincide and are identical. We conclude that this common multifractal cosmic web structure is determined by the dynamics and is independent of the initial conditions.
• Open/close Cosmic chronometers: constraining the equation of state of dark energy. I: H(z) measurements

  Daniel Stern et al  JCAP02(2010)008 Tag this article

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  We present new determinations of the cosmic expansion history from red-envelope galaxies. We have obtained for this purpose high-quality spectra with the Keck-LRIS spectrograph of red-envelope galaxies in 24 galaxy clusters in the redshift range 0.2 < z < 1.0. We complement these Keck spectra with high-quality, publicly available archival spectra from the SPICES and VVDS surveys. We improve over our previous expansion history measurements in Simon et al. (2005) by providing two new determinations of the expansion history: H( z) = 97±62 km sec ⁻¹ Mpc ⁻¹ at z 0.5 and H( z) = 90±40 km sec ⁻¹ Mpc ⁻¹ at z 0.9. We discuss the uncertainty in the expansion history determination that arises from uncertainties in the synthetic stellar-population models. We then use these new measurements in concert with cosmic-microwave-background (CMB) measurements to constrain cosmological parameters, with a special emphasis on dark-energy parameters and constraints to the curvature. In particular, we demonstrate the usefulness of direct H( z) measurements by constraining the dark-energy equation of state parameterized by w ₀ and w _a and allowing for arbitrary curvature. Further, we also constrain, using only CMB and H( z) data, the number of relativistic degrees of freedom to be 4±0.5 and their total mass to be < 0.2 eV, both at 1σ.
• Open/close High-redshift cosmography

  Vincenzo Vitagliano et al  JCAP03(2010)005 Tag this article

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  We constrain the parameters describing the kinematical state of the universe using a cosmographic approach, which is fundamental in that it requires a very minimal set of assumptions (namely to specify a metric) and does not rely on the dynamical equations for gravity. On the data side, we consider the most recent compilations of Supernovae and Gamma Ray Bursts catalogues. This allows to further extend the cosmographic fit up to z = 6.6, i.e. up to redshift for which one could start to resolve the low z degeneracy among competing cosmological models. In order to reliably control the cosmographic approach at high redshifts, we adopt the expansion in the improved parameter y = z/(1+ z). This series has the great advantage to hold also for z > 1 and hence it is the appropriate tool for handling data including non-nearby distance indicators. We find that Gamma Ray Bursts, probing higher redshifts than Supernovae, have constraining power and do require (and statistically allow) a cosmographic expansion at higher order than Supernovae alone. Exploiting the set of data from Union and GRBs catalogues, we show (for the first time in a purely cosmographic approach parametrized by deceleration q ₀, jerk j ₀, snap s ₀) a definitively negative deceleration parameter q ₀ up to the 3σ confidence level. We present also forecasts for realistic data sets that are likely to be obtained in the next few years.
• Open/close Probing eV-scale axions with CAST

  E. Arik et al  JCAP02(2009)008 Tag this article

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  We have searched for solar axions or other pseudoscalar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup. Whereas we previously have reported results from CAST with evacuated magnet bores (Phase I), setting limits on lower mass axions, here we report results from CAST where the magnet bores were filled with ⁴He gas (Phase II) of variable pressure. The introduction of gas generates a refractive photon mass m _γ, thereby achieving the maximum possible conversion rate for those axion masses m _a that match m _γ. With 160 different pressure settings we have scanned m _a up to about 0.4 eV, taking approximately 2 h of data for each setting. From the absence of excess x-rays when the magnet was pointing to the Sun, we set a typical upper limit on the axion-photon coupling of g _aγ 2.2 × 10 ⁻¹⁰ GeV ⁻¹ at 95% CL for m _a 0.4 eV, the exact result depending on the pressure setting. The excluded parameter range covers realistic axion models with a Peccei-Quinn scale in the neighborhood of f _a ~ 10 ⁷ GeV. Currently in the second part of CAST Phase II, we are searching for axions with masses up to about 1.2 eV using ³He as a buffer gas.
• Open/close Dark matter detection in the BMSSM

  Nicolás Bernal and Andreas Goudelis  JCAP03(2010)007 Tag this article

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  The addition of non-renormalizable terms involving the Higgs fields to the MSSM (BMSSM) ameliorates the little hierarchy problem of the MSSM. For neutralino dark matter, new regions for which the relic abundance of the LSP is consistent with WMAP (as the bulk region and the stop coannihilation region) are now permitted. In this framework, we analyze in detail the direct dark matter detection prospects in a XENON-like experiment. On the other hand, we study the capability of detecting gamma-rays, antiprotons and positrons produced in the annihilation of neutralino LSPs in the Fermi and oncoming AMS-02 experiments.
• Open/close Global interpretation of direct Dark Matter searches after CDMS-II results

  Joachim Kopp et al  JCAP02(2010)014 Tag this article

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  We perform a global fit to data from Dark Matter (DM) direct detection experiments, including the recent CDMS-II results. We discuss possible interpretations of the DAMA annual modulation signal in terms of spin-independent and spin-dependent DM-nucleus interactions, both for elastic and inelastic scattering. We find that for the spin-dependent inelastic scattering off protons a good fit to all data is obtained. We present a simple toy model realizing such a scenario. In all the remaining cases the DAMA allowed regions are disfavored by other experiments or suffer from severe fine tuning of DM parameters with respect to the galactic escape velocity. Finally, we also entertain the possibility that the two events observed in CDMS-II are an actual signal of elastic DM scattering, and we compare the resulting CDMS-II allowed regions to the exclusion limits from other experiments.
• Open/close A note on second-order perturbations of non-canonical scalar fields

  Corrado Appignani et al  JCAP03(2010)010 Tag this article

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  We study second-order perturbations for a general non-canonical scalar field, minimally coupled to gravity, on the unperturbed FRW background, where metric fluctuations are neglected a priori. By employing different approaches to cosmological perturbation theory, we show that, even in this simplified set-up, the second-order perturbations to the stress tensor, the energy density and the pressure display potential instabilities, which are not present at linear order. The conditions on the Lagrangian under which these instabilities take place are provided. We also discuss briefly the significance of our analysis in light of the possible linearization instability of these fields about the FRW background.
• Open/close The dark side of curvature

  Gabriela Barenboim et al  JCAP03(2010)008 Tag this article

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  Geometrical tests such as the combination of the Hubble parameter H( z) and the angular diameter distance d _A ( z) can, in principle, break the degeneracy between the dark energy equation of state parameter w( z), and the spatial curvature Ω _k in a direct, model-independent way. In practice, constraints on these quantities achievable from realistic experiments, such as those to be provided by Baryon Acoustic Oscillation (BAO) galaxy surveys in combination with CMB data, can resolve the cosmic confusion between the dark energy equation of state parameter and curvature only statistically and within a parameterized model for w( z). Combining measurements of both H( z) and d _A ( z) up to sufficiently high redshifts z ~ 2 and employing a parameterization of the redshift evolution of the dark energy equation of state are the keys to resolve the w( z)−Ω _k degeneracy.
• Open/close Hard X-ray emission from accretion shocks around galaxy clusters

  Doron Kushnir and Eli Waxman  JCAP02(2010)025 Tag this article

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  We show that the hard X-ray (HXR) emission observed from several galaxy clusters is consistent with a simple model, in which the nonthermal emission is produced by inverse Compton scattering of cosmic microwave background photons by electrons accelerated in cluster accretion shocks: The dependence of HXR surface brightness on cluster temperature is consistent with that predicted by the model, and the observed HXR luminosity is consistent with the fraction of shock thermal energy deposited in relativistic electrons being 0.1. Alternative models, where the HXR emission is predicted to be correlated with the cluster thermal emission, are disfavored by the data. The implications of our predictions to future HXR observations (e.g. by NuStar, Simbol-X) and to (space/ground based) γ-ray observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed.
• Open/close An accelerating cosmology without dark energy

  G. Steigman et al  JCAP06(2009)033 Tag this article

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  The negative pressure accompanying gravitationally-induced particle creation can lead to a cold dark matter (CDM) dominated, accelerating Universe (Lima et al. 1996 [1]) without requiring the presence of dark energy or a cosmological constant. In a recent study, Lima et al. 2008 [2] (LSS) demonstrated that particle creation driven cosmological models are capable of accounting for the SNIa observations [3] of the recent transition from a decelerating to an accelerating Universe, without the need for Dark Energy. Here we consider a class of such models where the particle creation rate is assumed to be of the form Γ = β H+γ H ₀, where H is the Hubble parameter and H ₀ is its present value. The evolution of such models is tested at low redshift by the latest SNe Ia data provided by the Union compilation [4] and at high redshift using the value of z _eq, the redshift of the epoch of matter — radiation equality, inferred from the WMAP constraints on the early Integrated Sachs-Wolfe (ISW) effect [5]. Since the contributions of baryons and radiation were ignored in the work of LSS, we include them in our study of this class of models. The parameters of these more realistic models with continuous creation of CDM are constrained at widely-separated epochs ( z _eq   3000 and z 0) in the evolution of the Universe. The comparison of the parameter values, {β, γ}, determined at these different epochs reveals a tension between the values favored by the high redshift CMB constraint on z _eq from the ISW and those which follow from the low redshift SNIa data, posing a potential challenge to this class of models. While for β = 0 this conflict is only at  2σ, it worsens as β increases from zero.