Nonlinear spherical perturbations in Quintessence Models of Dark Energy [CEA]

Observations have confirmed the accelerated expansion of the universe. The accelerated expansion can be modelled by invoking a cosmological constant or a dynamical model of dark energy. A key difference between these models is that the equation of state parameter $w$ for dark energy differs from $-1$ in dynamical dark energy (DDE). Further, the equation of state parameter is not constant for a general DDE model. Such differences can be probed using the variation of scale factor with time by measuring distances. Another significant difference between the cosmological constant and DDE models is that the latter must cluster. Linear perturbation analysis indicates that perturbations in quintessence models of dark energy do not grow to have a significant amplitude at small length scales. In this paper we study the response of quintessence dark energy to non-linear perturbations in dark matter. We use a fully relativistic model for spherically symmetric perturbations. In this study we focus on thawing models. We find that in response to non-linear perturbations in dark matter, dark energy perturbations grow at a faster rate than expected in linear perturbation theory. We find that dark energy perturbation remains localised and does not diffuse out to larger scales. The dominant drivers of the evolution of dark energy perturbations are the local Hubble flow and a supression of gradients of the scalar field. We also find that the equation of state parameter $w$ changes in response to perturbations in dark matter such that it also becomes a function of position. The variation of $w$ in space is correlated with matter density perturbation. Variation of $w$ and perturbations in dark energy is more pronounced in response to large scale perturbations in matter while the dependence on the amplitude of matter perturbations is much weaker.

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M. Rajvanshi and J. Bagla
Mon, 19 Feb 18

Comments: 23 pages, 15 figures. comments welcome

Gravitational waves from vacuum first-order phase transitions: from the envelope to the lattice [CEA]

We conduct large scale numerical simulations of gravitational wave production at a first order vacuum phase transition. We find a power law for the gravitational wave power spectrum at high wavenumber which falls off as $k^{-1.5}$ rather than the $k^{-1}$ produced by the envelope approximation. The peak of the power spectrum is shifted to slightly lower wave numbers from that of the envelope approximation. The envelope approximation reproduces our results for the peak power less well, agreeing only to within an order of magnitude. After the bubbles finish colliding the scalar field oscillates around the true vacuum. An additional feature is produced in the UV of the gravitational wave power spectrum, and this continues to grow linearly until the end of our simulation. The additional feature peaks at a length scale close to the bubble wall thickness and is shown to have a negligible contribution to the energy in gravitational waves, providing the scalar field mass is much smaller than the Planck mass.

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D. Cutting, M. Hindmarsh and D. Weir
Mon, 19 Feb 18

Comments: 15 pages, 15 figures

Projected WIMP sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment [IMA]

LUX-ZEPLIN (LZ) is a next generation dark matter direct detection experiment that will operate 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. Using a two-phase xenon detector with an active mass of 7 tonnes, LZ will search primarily for low-energy interactions with Weakly Interacting Massive Particles (WIMPs), which are hypothesized to make up the dark matter in our galactic halo. In this paper, the projected WIMP sensitivity of LZ is presented based on the latest background estimates and simulations of the detector. For a 1000 live day run using a 5.6 tonne fiducial mass, LZ is projected to exclude at 90% confidence level spin-independent WIMP-nucleon cross sections above $1.6 \times 10^{-48}$ cm$^{2}$ for a 40 $\mathrm{GeV}/c^{2}$ mass WIMP. Additionally, a $5\sigma$ discovery potential is projected reaching cross sections below the existing and projected exclusion limits of similar experiments that are currently operating. For spin-dependent WIMP-neutron(-proton) scattering, a sensitivity of $2.7 \times 10^{-43}$ cm$^{2}$ ($8.1 \times 10^{-42}$ cm$^{2}$) for a 40 $\mathrm{GeV}/c^{2}$ mass WIMP is expected. With construction well underway, LZ is on track for underground installation at SURF in 2019 and will start collecting data in 2020.

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D. Akerib, C. Akerlof, S. Alsum, et. al.
Mon, 19 Feb 18

Comments: 14 pages, 11 figures

Quantitative Constraints on the Reionization History from the IGM Damping Wing Signature in Two Quasars at z > 7 [CEA]

During reionization, neutral hydrogen in the intergalactic medium (IGM) imprints a damping wing absorption feature on the spectrum of high-redshift quasars. A detection of this signature provides compelling evidence for a significantly neutral Universe, and enables measurements of the hydrogen neutral fraction $x_{\rm HI}(z)$ at that epoch. Obtaining reliable quantitative constraints from this technique, however, is challenging due to stochasticity induced by the patchy inside-out topology of reionization, degeneracies with quasar lifetime, and the unknown unabsorbed quasar spectrum close to rest-frame Ly$\alpha$. We combine a large-volume semi-numerical simulation of reionization topology with 1D radiative transfer through high-resolution hydrodynamical simulations of the high-redshift Universe to construct models of quasar transmission spectra during reionization. Our state-of-the-art approach captures the distribution of damping wing strengths in biased quasar halos that should have reionized earlier, as well as the erosion of neutral gas in the quasar environment caused by its own ionizing radiation. Combining this detailed model with our new technique for predicting the quasar continuum and its associated uncertainty, we introduce a Bayesian statistical method to jointly constrain the neutral fraction of the Universe and the quasar lifetime from individual quasar spectra. We apply this methodology to the spectra of the two highest redshift quasars known, ULAS J1120+0641 and ULAS J1342+0928, and measured volume-averaged neutral fractions $\langle x_{\rm HI} \rangle(z=7.09)=0.48^{+0.26}{-0.26}$ and $\langle x{\rm HI} \rangle(z=7.54)=0.60^{+0.20}{-0.23}$ (posterior medians and 68% credible intervals) when marginalized over quasar lifetimes of $10^3 \leq t{\rm q} \leq 10^8$ years.

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F. Davies, J. Hennawi, E. Banados, et. al.
Mon, 19 Feb 18

Comments: 18 pages, 13 figures. Submitted to ApJ

Anisotropies in the stochastic gravitational wave background: formalism and the cosmic string case [CEA]

We develop a powerful analytical formalism for calculating the energy density of the stochastic gravitational wave background, including a full description of its anisotropies. This is completely general, and can be applied to any astrophysical or cosmological source. As an example, we apply these tools to the case of a network of Nambu-Goto cosmic strings. We find that the anisotropies are relatively insensitive to the choice of model for the string network, but that they are very sensitive to the value of the string tension $G\mu$.

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A. Jenkins and M. Sakellariadou
Mon, 19 Feb 18

Comments: 29 pages, 8 figures

Inflation in a scale invariant universe [CEA]

A scale-invariant universe can have a period of accelerated expansion at early times: inflation. We use a frame-invariant approach to calculate inflationary observables in a scale invariant theory of gravity involving two scalar fields – the spectral indices, the tensor to scalar ratio, the level of isocurvature modes and non-Gaussianity. We show that scale symmetry leads to an exact cancellation of isocurvature modes and that, in the scale-symmetry broken phase, this theory is well described by a single scalar field theory. We find the predictions of this theory strongly compatible with current observations.

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P. Ferreira, C. Hill, J. Noller, et. al.
Mon, 19 Feb 18

Comments: 9 pages, 2 figures

AKARI NEP-Deep: galaxy clustering through the AKARI IRC filters [GA]

We present a preliminary analysis of clustering of galaxies luminous in the near- and mid-infrared as seen by seven various ilters of the AKARI IRC instrument from 2 $\mu$m to 24 $\mu$m in the the AKARI NEP-Deep field. We compare populations of galaxies detected in different filters and their clustering properties. We conclude that different AKARI filters allow to trace different populations composed mainly of star-forming galaxies located in different environments. In particular, the mid-infrared filters at redshift z $\sim$ 0.8 and higher trace a population of strongly evolving galaxies located in massive haloes which might have ended as elliptical galaxies today.

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A. Pollo and A. Solarz
Mon, 19 Feb 18

Comments: To be published in AKARI 2017 Conference proceedings in JAXA Repository / AIREX (JAXA-SP series)