Easing Tensions with Quartessence [CEA]


Tensions between cosmic microwave background (CMB) observations and the growth of the large-scale structure (LSS) inferred from late-time probes pose a serious challenge to the concordance $\Lambda$CDM cosmological model. State-of-the-art CMB data from the Planck satellite predicts a higher rate of structure growth than what preferred by low-redshift observables. Such tension has hitherto eluded conclusive explanations in terms of straightforward modifications to $\Lambda$CDM, e.g. the inclusion of massive neutrinos or a dynamical dark energy component. Here, we investigate `quartessence’ models, where a single dark component mimics both dark matter and dark energy. We show that such models greatly alleviate the tension between high and low redshift observations, thanks to the non-vanishing sound speed of quartessence that inhibits structure growth at late times on scales smaller than its corresponding Jeans’ length. In particular, the $3.4\sigma$ tension between CMB and LSS observables is thoroughly reabsorbed. For this reason, we argue that quartessence deserves further investigation and may lead to a deeper understanding of the physics of the dark Universe.

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S. Camera, M. Martinelli and D. Bertacca
Mon, 24 Apr 17

Comments: 7 pages, 5 figures, 1 table; comments are welcome

Constraining spatial variations of the fine-structure constant in symmetron models [CEA]


We introduce a methodology to test models with spatial variations of the fine-structure constant $\alpha$, based on the calculation of the angular power spectrum of these measurements. This methodology enables comparisons of observations and theoretical models through their predictions on the statistics of the $\alpha$ variation. Here we apply it to the case of symmetron models. We find no indications of deviations from the standard behavior, with current data providing an upper limit to the strength of the symmetron coupling to gravity ($\log{\beta^2}<-0.9$) when this is the only free parameter, and not able to constrain the model when also the symmetry breaking scale factor $a_{SSB}$ is free to vary.

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A. Pinho, M. Martinelli and C. Martins
Mon, 24 Apr 17

Comments: Phys. Lett. B (in press)

The extended Northern ROSAT Galaxy Cluster Survey (NORAS II) I. Survey Construction and First Results [CEA]


As the largest, clearly defined building blocks of our Universe, galaxy clusters are interesting astrophysical laboratories and important probes for cosmology. X-ray surveys for galaxy clusters provide one of the best ways to characterise the population of galaxy clusters. We provide a description of the construction of the NORAS II galaxy cluster survey based on X-ray data from the northern part of the ROSAT All-Sky Survey. NORAS II extends the NORAS survey down to a flux limit of 1.8 x 10^(-12) erg s^-1 cm^-2 (0.1 – 2.4 keV) increasing the sample size by about a factor of two. The NORAS II cluster survey now reaches the same quality and depth of its counterpart, the Southern REFLEX II survey, allowing us to combine the two complementary surveys. The paper provides information on the determination of the cluster X-ray parameters, the identification process of the X-ray sources, the statistics of the survey, and the construction of the survey selection function, which we provide in numerical format. Currently NORAS II contains 860 clusters with a median redshift of z = 0.102. We provide a number of statistical functions including the logN-logS and the X-ray luminosity function and compare these to the results from the complementary REFLEX II survey. Using the NORAS II sample to constrain the cosmological parameters, sigma_8 and Omega_m, yields results perfectly consistent with those of REFLEX II. Overall, the results show that the two hemisphere samples, NORAS II and REFLEX II, can be combined without problems to an all-sky sample, just excluding the Zone-of-Avoidance.

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H. Boehringer, G. Chon, J. Retzlaff, et. al.
Mon, 24 Apr 17

Comments: 14 pages, 13 figures

Iterative initial condition reconstruction [CEA]


Motivated by recent developments in perturbative calculations of the nonlinear evolution of large-scale structure, we present an iterative algorithm to reconstruct the initial conditions in a given volume starting from the dark matter distribution in real space. In our algorithm, objects are first iteratively moved back along estimated potential gradients until a nearly uniform catalog is obtained. The linear initial density is then estimated as the divergence of the cumulative displacement, with an optional second-order correction. This algorithm should undo non-linear effects up to one-loop order, including the higher-order infrared resummation piece. We test the method using dark matter simulations in real space. At redshift $z=0$, we find that after eight iterations the reconstructed density is more than $95\%$ correlated with the initial density at $k\le 0.35\; h\mathrm{Mpc}^{-1}$. The reconstruction also reduces the power in the difference between reconstructed and initial fields by more than two orders of magnitude at $k\le 0.2\; h\mathrm{Mpc}^{-1}$, and it extends the range of scales where the full broad-band shape of the power spectrum matches linear theory by a factor 2-3. As a specific application, we consider measurements of the Baryonic Acoustic Oscillation (BAO) scale that can be improved by reducing the degradation effects of large-scale flows. We find that the method improves the BAO signal-to-noise by a factor 2.7 at redshift $z=0$ and by a factor 2.5 at $z=0.6$ in our idealistic simulations. This improves standard BAO reconstruction by $70\%$ at $z=0$ and $30\%$ at $z=0.6$, and matches the optimal BAO signal and signal-to-noise of the linear density in the same volume.

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M. Schmittfull, T. Baldauf and M. Zaldarriaga
Mon, 24 Apr 17

Comments: 26 pages, 14 figures, comments welcome

Shape of the acoustic gravitational wave power spectrum from a first order phase transition [CEA]


We present results from large-scale numerical simulations of a first order thermal phase transition in the early universe, in order to explore the shape of the acoustic gravitational wave and the velocity power spectra. We compare the results with the predictions of the recently proposed sound shell model. For the gravitational wave power spectrum, we find that the predicted $k^{-3}$ behaviour, where $k$ is the wavenumber, emerges clearly for detonations. The power spectra from deflagrations show similar features, but exhibit a steeper high-$k$ decay and an extra feature not accounted for in the model. There are two independent length scales: the mean bubble separation and the thickness of the sound shell around the expanding bubble of the low temperature phase. It is the sound shell thickness which sets the position of the peak of the power spectrum. The low wavenumber behaviour of the velocity power spectrum is consistent with a causal $k^{3}$, except for the thinnest sound shell, where it is steeper. We present parameters for a simple broken power law fit to the gravitational wave power spectrum for wall speeds well away from the speed of sound where this form can be usefully applied. We examine the prospects for the detection, showing that a LISA-like mission has the sensitivity to detect a gravitational wave signal from sound waves with an RMS fluid velocity of about $0.05c$, produced from bubbles with a mean separation of about $10^{-2}$ of the Hubble radius. The shape of the gravitational wave power spectrum depends on the bubble wall speed, and it may be possible to estimate the wall speed, and constrain other phase transition parameters, with an accurate measurement of a stochastic gravitational wave background.

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M. Hindmarsh, S. Huber, K. Rummukainen, et. al.
Fri, 21 Apr 17

Comments: 16 pages, 9 figures

Anisotropy of the astrophysical gravitational wave background I: analytic expression of the angular power spectrum and correlation with cosmological observations [CEA]


Unresolved sources of gravitational waves are at the origin of a stochastic gravitational wave background. While the computation of its mean density as a function of frequency in a homogeneous and isotropic universe is standard lore, the computation of its anisotropies requires to understand the coarse graining from local systems, to galactic scales and then to cosmology. An expression of the gravitational wave energy density valid in any general spacetime is derived. It is then specialized to a perturbed Friedmann-Lema\^itre spacetime in order to determine the angular power spectrum of this stochastic background as well as its correlation with other cosmological probes, such as the galaxy number counts and weak lensing. Our result for the angular power spectrum also provides an expression for the variance of the gravitational wave background.

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G. Cusin, C. Pitrou and J. Uzan
Fri, 21 Apr 17

Comments: 20 pages, 1 figure

Search for magnetic inelastic dark matter with XENON100 [CEA]


We present the first search for dark matter-induced delayed coincidence signals in a dual-phase xenon time projection chamber, using the 224.6\,live days of the XENON100 science run~II. This very distinct signature is predicted in the framework of magnetic inelastic dark matter which has been proposed to reconcile the modulation signal reported by the DAMA/LIBRA collaboration with the null results from other direct detection experiments. No candidate event has been found in the region of interest and upper limits on the WIMP’s magnetic dipole moment are derived. The scenarios proposed to explain the DAMA/LIBRA modulation signal by magnetic inelastic dark matter interactions of WIMPs with masses of 58.0\,GeV/c$^2$ and 122.7\,GeV/c$^2$ are excluded at 3.3\,$\sigma$ and 9.3\,$\sigma$, respectively.

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XENON. collaboration, E. Aprile, J. Aalbers, et. al.
Thu, 20 Apr 17

Comments: 8 pages, 4 figures