129 research outputs found

    On the streaming model for redshift-space distortions

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    The streaming model describes the mapping between real and redshift space for 2-point clustering statistics. Its key element is the probability density function (PDF) of line-of-sight pairwise peculiar velocities. Following a kinetic-theory approach, we derive the fundamental equations of the streaming model for ordered and unordered pairs. In the first case, we recover the classic equation while we demonstrate that modifications are necessary for unordered pairs. We then discuss several statistical properties of the pairwise velocities for DM particles and haloes by using a suite of high-resolution NN-body simulations. We test the often used Gaussian ansatz for the PDF of pairwise velocities and discuss its limitations. Finally, we introduce a mixture of Gaussians which is known in statistics as the generalised hyperbolic distribution and show that it provides an accurate fit to the PDF. Once inserted in the streaming equation, the fit yields an excellent description of redshift-space correlations at all scales that vastly outperforms the Gaussian and exponential approximations. Using a principal-component analysis, we reduce the complexity of our model for large redshift-space separations. Our results increase the robustness of studies of anisotropic galaxy clustering and are useful for extending them towards smaller scales in order to test theories of gravity and interacting dark-energy models.Comment: 22 pages, 20 figures, accepted for publication in MNRA

    Cosmology with the kinetic Sunyaev-Zeldovich effect: Independent of the optical depth and σ8\sigma_8

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    The cosmological constraints from the kinetic Sunyaev-Zeldovich experiments are degenerate with the optical depth measurement, which is commonly known as the optical-depth degeneracy. In this work, we introduce a new statistic based on the first moment of relative velocity between pairs in a triplet, which is capable of constraining cosmological parameters independent of the optical depth, and σ8\sigma_8. Using 22,000 NN-body simulations from the Quijote suite, we quantify the information content in the new statistic using Fisher matrix forecast. We find that it is able to obtain strong constraints on the cosmological parameters, particularly on the summed neutrino mass. The constraints have a factor of 6.2-12.9, and 2.3-5.7 improvement on all cosmological model parameters when compared to those obtained from the mean pairwise velocity, and the redshift-space halo power spectrum, respectively. Thus the new statistic paves a way forward to constrain cosmological parameters independent of the optical depth and σ8\sigma_8 using data from future kinetic Sunyaev-Zeldovich experiments alone.Comment: 8 pages, 6 figures, submitted to A&A, comments welcom

    Cosmology with cosmic web environments I. Real-space power spectra

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    We undertake the first comprehensive and quantitative real-space analysis of the cosmological information content in the environments of the cosmic web (voids, filaments, walls, and nodes) up to non-linear scales, k=0.5k = 0.5 hh/Mpc. Relying on the large set of NN-body simulations from the Quijote suite, the environments are defined through the eigenvalues of the tidal tensor and the Fisher formalism is used to assess the constraining power of the power spectra derived in each of the four environments and their combination. Our results show that there is more information available in the environment-dependent power spectra, both individually and when combined all together, than in the matter power spectrum. By breaking some key degeneracies between parameters of the cosmological model such as MνM_\nu--σ8\sigma_\mathrm{8} or Ωm\Omega_\mathrm{m}--σ8\sigma_8, the power spectra computed in identified environments improve the constraints on cosmological parameters by factors 15\sim 15 for the summed neutrino mass MνM_\nu and 8\sim 8 for the matter density Ωm\Omega_\mathrm{m} over those derived from the matter power spectrum. We show that these tighter constraints are obtained for a wide range of the maximum scale, from kmax=0.1k_\mathrm{max} = 0.1 hh/Mpc to highly non-linear regimes with kmax=0.5k_\mathrm{max} = 0.5 hh/Mpc. We also report an eight times higher value of the signal-to-noise ratio for the combination of spectra compared to the matter one. Importantly, we show that all the presented results are robust to variations of the parameters defining the environments hence suggesting a robustness to the definition we chose to define them.Comment: 15 pages, 11 figure

    Testing decaying dark matter models as a solution to the S8S_8 tension with the thermal Sunyaev-Zel'dovich effect

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    Considering possible solutions to the S8S_8 tension between the Planck cosmic microwave background (CMB) measurement and low-redshift probes, we extended the standard Λ\LambdaCDM cosmological model by including decay of dark matter (DDM). We first tested the DDM model in which dark matter decays into a form of noninteracting dark radiation. Under this DDM model, we investigated the impacts of DDM on the Sunyaev Zel'dovich (SZ) effect by varying the decay lifetime, Γ1\Gamma^{-1}, including the background evolution in cosmology and the nonlinear prescription in the halo mass function. We performed a cosmological analysis under the assumption of this extended cosmological model by combining the latest high-redshift Planck CMB measurement and low-redshift measurements of the SZ power spectrum as well as the baryonic acoustic oscillations (BAO) and luminosity distances to type Ia supernovae (SNIa). Our result shows a preference for Γ1220\Gamma^{-1} \sim 220 Gyr with a lower bound on the decay lifetime of \sim 38 Gyr at 95\% confidence level. Additionally, we tested the other DDM model in which dark matter decays into warm dark matter and dark radiation. This model supports Γ1137\Gamma^{-1} \sim 137 Gyr to resolve the S8S_8 tension with a lower bound on the decay lifetime of \sim 24 Gyr at 95\% confidence level. Comparing these two models, we find that the second leads to slightly better reconciliation of the S8S_8 tension.Comment: 9 pages, 5 figures, Accepted for publication in A&

    Stress relaxation behavior of organically modified montmorillonite filled natural rubber/nitrile rubber nanocomposites

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    International audienceTo reduce material consumption, it is important to have reinforced material with longer life time. Incorporation of nanoparticles to reinforce and compatibilize polymer blends is one of the widely undergoing research areas in polymer science technology. A series of natural rubber and nitrile rubber (NR/NBR) nanocomposite vulcanazite, reinforced with two different organically modified clay (OMt) were prepared. To predict the performance of a material over long periods of time, stress relaxation studies with both the reinforced systems were done. The effects of loading, blend composition, filler polarity and temperature on stress relaxation of OMt reinforced NR/NBR nanocomposites were carefully measured. Based on the stress relaxation measurements, it was observed that due to its polarity difference, O1Mt (Mt modified with dimethyl, benzyl, HT modification provided by Southern Clay Products) was preferentially located at the NBR phase while O2Mt (Mt modified with mercapto silane provided by English India Clay) had more affinity with natural rubber in the NR/NBR nanocomposites. The preferential localization of OMt has been analyzed by HRTEM. The nature of interaction of the nanoclay was found to influence the stress relaxation rate. NR/NBR nanocomposites with higher filler loading showed higher rates of relaxation rate due to the presence of more filler-filler interactions. At 70 degrees C, the viscosity ratio was found to influence the reinforcement, and consequently relaxation rate of the 50/50 NR/NBR nanocomposites. It was found that the rearrangements of the polymer chains are dependent on the blend composition, temperature, filler/polymer interactions etc. To explain and predict observed phenomena, the stretched-exponential Kohlrausch equation and Maxwell-Weichert model were used. For both models, the experimental curve fitted well with the theoretical models. (C) 2013 Elsevier B.V. All tights reserved

    High Performance In-Situ Composites Developed from Polypropylene/Nylon 6/Carbon Nanotube Blend Systems

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    In the present work, microfibrillar composites (MFCs) based on polypropylene (PP) /Nylon 6 (NY) blends, along with multi walled carbon tubes (MWCNT) were prepared by melt processing technique. The blending of the fibre forming polymers was carried out in a twin screw extruder with varying concentrations of MWCNT. The drawing of the extruded strands was accomplished in a stretching unit followed by isotropization by compression moulding at a processing temperature below the melting point of NY. At an optimized fixed composition of PP/NY (70/30 w/w %), the influence of stretch ratio on the properties of nanofiller incorporated MFCs was investigated. The morphology development of the MFC samples was observed using high resolution scanning electron microscopy (HRSEM). The static mechanical studies signify the constructive effect of microfibrils and MWCNTs in reinforcing PP matrix. Dynamic rheological studies support the microfibrils contribution towards the stiffness of the syste