Evolution of linear wave dark matter perturbations in the radiation-dominant era [CEA]


Linear perturbations of the wave dark matter, or $\psi$ dark matter ($\psi$DM), of particle mass $\sim 10^{-22}$eV in the radiation-dominant era are analyzed, and the matter power spectrum at the photon-matter equality is obtained. We identify four phases of evolution for $\psi$DM perturbations, where the dynamics can be vastly different from the counterparts of cold dark matter (CDM). While in late stages after mass oscillation long-wave $\psi$DM perturbations are almost identical to CDM perturbations, some subtle differences remain, let alone intermediate-to-short waves that bear no resemblance with those of CDM throughout the whole evolutionary history. The dissimilarity is due to quantum mechanical effects which lead to severe mode suppression. We also discuss the axion model with a cosine field potential. The power spectrum of axion models are generally almost identical to those of $\psi$DM, but in the extreme case when the initial axion angle is near the field potential top, this axion model predicts a higher spectral cutoff than $\psi$DM, which is equivalent to having a higher particle mass for $\psi$DM.

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U. Zhang and T. Chiueh
Fri, 24 Feb 17

Comments: 20 pages, 7 figures

The cool core state of Planck SZ-selected clusters versus X-ray selected samples: evidence for cool core bias [CEA]


We characterized the population of galaxy clusters detected with the SZ effect with Planck, by measuring the cool core state of the objects in a well-defined subsample of the Planck catalogue. We used as indicator the concentration parameter Santos et al. (2008). The fraction of cool core clusters is $29 \pm 4 \%$ and does not show significant indications of evolution in the redshift range covered by our sample. We compare the distribution of the concentration parameter in the Planck sample with the one of the X-ray selected sample MACS (Mann & Ebeling, 2011): the distributions are significantly different and the cool core fraction in MACS is much higher ($59 \pm 5 \%$). Since X-ray selected samples are known to be biased towards cool cores due to the presence of their prominent surface brightness peak, we simulated the impact of the “cool core bias” following Eckert et al. (2011). We found that it plays a large role in the difference between the fractions of cool cores in the two samples. We examined other selection effects that could in principle bias SZ-surveys against cool cores but we found that their impact is not sufficient to explain the difference between Planck and MACS. The population of X-ray under-luminous objects, which are found in SZ-surveys but missing in X-ray samples (Planck Collaboration 2016), could possibly contribute to the difference, as we found most of them to be non cool cores, but this hypothesis deserves further investigation.

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M. Rossetti, F. Gastaldello, D. Eckert, et. al.
Fri, 24 Feb 17

Comments: Accepted for publication in MNRAS

Large-Scale Clustering as a Probe of the Origin and the Host Environment of Fast Radio Bursts [CEA]


We propose to use degree-scale angular clustering of fast radio bursts (FRBs) to identify their origin and the host galaxy population. We study the information content in auto-correlation of the angular positions and dispersion measures (DM) and in cross-correlation with galaxies. We show that the cross-correlation with Sloan Digital Sky Survey (SDSS) galaxies will place stringent constraints on the mean physical quantities associated with FRBs. If ~10,000 FRBs are detected with <deg resolution in the SDSS field, the clustering analysis can constrain the global abundance of free electrons at $z<1$, the bias factor of FRB host galaxies, and the mean near-source dispersion measure, with fractional errors (with a $68\%$ confidence level) of $\sim5 \%, \sim 20 \%$, and $\sim70 \%$, respectively. The delay time distribution of FRB sources can be also determined by combining the clustering and the probability distribution function of dispersion measure. Our approach will be complementary to high-resolution ($\ll {\rm deg}$) event localization using e.g., VLA and VLBI for identifying the origin of FRBs and the source environment. We strongly encourage future observational programs such as CHIME, UTMOST, HIRAX to survey FRBs in the SDSS field.

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M. Shirasaki, K. Kashiyama and N. Yoshida
Fri, 24 Feb 17

Comments: 14 pages, 8 figures, 2 tables, To be submitted to Phys. Rev. D

Assessing the impact of bulk and shear viscosities on large scale structure formation [CEA]


It is analyzed the effects of both bulk and shear viscosities on the perturbations, relevant for structure formation in late time cosmology. It is shown that shear viscosity can be as effective as the bulk viscosity on suppressing the growth of perturbations and delaying the nonlinear regime. A statistical analysis of the shear and bulk viscous effects is performed and some constraints on these viscous effects are given.

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C. Barbosa, H. Velten, J. Fabris, et. al.
Fri, 24 Feb 17

Comments: 21 pages, 19 figures

Two- and Three-dimensional Probes of Parity in Primordial Gravity Waves [CEA]


We show that three-dimensional information is critical to discerning the effects of parity violation in the primordial gravity-wave background. Helical gravity waves would induce parity-violating correlations in the cosmic microwave background (CMB) between parity-odd polarization $B$-modes and parity-even temperature anisotropies ($T$) or polarization $E$-modes. Unfortunately, $EB$ correlations are much weaker than would be naively expected, which we show is due to an approximate symmetry resulting from the two-dimensional nature of the CMB. The detectability of parity-violating correlations is exacerbated by the fact that the handedness of individual modes cannot be discerned in the two-dimensional CMB, leading to a noise contribution from scalar matter perturbations. In contrast, the tidal imprints of primordial gravity waves fossilized into the large-scale structure of the Universe will provide a three-dimensional probe of parity violation. Using such fossils the handedness of gravity waves may be determined on a mode-by-mode basis, permitting future surveys to probe helicity at the percent level if the amplitude of primordial gravity waves is near current observational upper limits.

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K. Masui, U. Pen and N. Turok
Thu, 23 Feb 17

Comments: 6 pages, 1 figure + 4 page supplement. To be submitted to PRL

Evolution of mass and velocity filed in comic web: comparison between baryonic and dark matter [CEA]


We investigate the evolution of cosmic web since $z=5$ in grid based cosmological hydrodynamical simulations, focusing on the mass and velocity field of both baryonic and cold dark matter. The tidal tensor of density is used as the main method for web identification, with $\lambda_{th}=0.2-1.2$. The evolution trends in baryonic and dark matter are similar, while moderate differences are observed. Sheets appeared early and their large scale pattern may have been set up by $z=3$. In term of mass, filaments superseded sheets as the primary collapsing structures at $z\sim2-3$. Tenuous filaments assembled with each other to form prominent ones at $z<~2$. In accordance with the construction of the frame of sheets, the cosmic divergence velocity, $v_{div}$, had been well developed above 2-3 Mpc by z=3. Afterwards, curl velocity, $v_{curl}$, grown dramatically along with the rising of filaments, become comparable to $v_{div}$, for $<2-3 Mpc$ at $z=0$. The scaling of $v_{curl}$ can be described by the hierarchical turbulence model. The alignment between vorticity and eigenvectors of shear tensor in baryonic matter field resembles dark matter, and is even moderately stronger between $\vec{\omega}$ and $\vec{e}_1$, and $\vec{e}_3$. Compared with dark matter, mildly less baryonic matter is found residing in filaments and clusters, and its vorticity has been developed more significantly below $2-3 Mpc$. These differences may be underestimated due to the limited resolution and lack of star formation in our simulation. The impact of the change of dominant structures in over-dense regions at $z\sim2-3$ on galaxy formation and evolution is shortly discussed.

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W. Zhu and L. Feng
Thu, 23 Feb 17

Comments: 28pages, 24 figures, accepted for publication in The Astrophysical Journal

Mimicking Dark Energy with the backreactions of gigaparsec inhomogeneities [CEA]


Spatial averaging and time evolving are non-commutative operations in General Relativity, which questions the reliability of the FLRW model. The long standing issue of the importance of backreactions induced by cosmic inhomogeneities is addressed for a toy model assuming a peak in the primordial spectrum of density perturbations and a simple CDM cosmology. The backreactions of initial Hubble-size inhomogeneities are determined in a fully relativistic framework, from a series of simulations using the BSSN formalism of numerical relativity. In the FLRW picture, these backreactions can be effectively described by two so-called morphon scalar fields, one of them acting at late time like a tiny cosmological constant. Initial density contrasts ranging from $10^{-2}$ down to $10^{-4}$, on scales crossing the Hubble radius between $z\sim 45 $ and $z\sim 1000$ respectively, i.e. comoving gigaparsec scales, mimic a Dark Energy (DE) component that can reach $\Omega_{\mathrm{DE}} \approx 0.7$ when extrapolated until today. A similar effect is not excluded for lower density contrasts but our results are then strongly contaminated by numerical noise and thus hardly reliable. A potentially detectable signature of this scenario is a phantom-like equation of state $w< -1$, at redshifts $z\gtrsim 4$ for a density contrast of $10^{-2}$ initially, relaxing slowly to $w \approx -1$ today. This new class of scenarios would send the fine-tuning and coincidence issues of Dark energy back to the mechanism at the origin of the primordial power spectrum enhancement, possibly in the context of inflation.

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S. Clesse, A. Roisin and A. Fuzfa
Thu, 23 Feb 17

Comments: 14 pages, 8 figures, comments welcome