Physical constraints on interacting dark energy models

Physical limits on the equation-of-state (EoS) parameter of a dark energy component non-minimally coupled with the dark matter field are examined in light of the second law of thermodynamics and the positiveness of entropy. Such constraints are combined with observational data sets of type Ia supernovae, baryon acoustic oscillations and the angular acoustic scale of the cosmic microwave background to impose restrictions on the behaviour of the dark matter/dark energy interaction. Considering two EoS parameterisations of the type $w = w_0 + w_a\zeta(z)$, we derive a general expression for the evolution of the dark energy density and show that the combination of thermodynamic limits and observational data provide tight bounds on the $w_0 - w_a$ parameter space.Comment: 7 pages, 4 figures. Accepted for publication in European Physical Journal

Perturbation propagation in random and evolved Boolean networks

We investigate the propagation of perturbations in Boolean networks by evaluating the Derrida plot and modifications of it. We show that even small Random Boolean Networks agree well with the predictions of the annealed approximation, but non-random networks show a very different behaviour. We focus on networks that were evolved for high dynamical robustness. The most important conclusion is that the simple distinction between frozen, critical and chaotic networks is no longer useful, since such evolved networks can display properties of all three types of networks. Furthermore, we evaluate a simplified empirical network and show how its specific state space properties are reflected in the modified Derrida plots.Comment: 10 pages, 8 figure

Mg/Ti multilayers: structural, optical and hydrogen absorption properties

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a "spinodal-like" microstructure with a small degree of chemical short-range order in the atoms distribution. In the present study we artificially engineer short-range order by depositing Pd-capped Mg/Ti multilayers with different periodicities and characterize them both structurally and optically. Notwithstanding the large lattice parameter mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. Upon exposure to H2 gas a two-step hydrogenation process occurs, with the Ti layers forming the hydride before Mg. From in-situ measurements of the bilayer thickness L at different hydrogen pressures, we observe large out-of-plane expansions of the Mg and Ti layers upon hydrogenation, indicating strong plastic deformations in the films and a consequent shortening of the coherence length. Upon unloading at room temperature in air, hydrogen atoms remain trapped in the Ti layers due to kinetic constraints. Such loading/unloading sequence can be explained in terms of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by diffusion calculations on a model multilayered systems. Absorption isotherms measured by hydrogenography can be interpreted as a result of the elastic clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces

Matter sound waves in two-component Bose-Einstein condensates

The creation and propagation of sound waves in two-component Bose-Einstein condensates (BEC) are investigated and a new method of wave generation in binary BEC mixtures is proposed. The method is based on a fast change of the inter-species interaction constant and is illustrated for two experimental settings: a drop-like condensate immersed into a second large repulsive condensate, and a binary mixture of two homogeneous repulsive BEC's. A mathematical model based on the linearized coupled Gross-Pitaevskii equations is developed and explicit formulae for the space and time dependence of sound waves are provided. Comparison of the analytical and numerical results shows excellent agreement, confirming the validity of the proposed approach.Comment: 16 pages, 9 figure

The Star Formation Histories of z ~ 2 Dust-obscured Galaxies and Submillimeter-selected Galaxies

The Spitzer Space Telescope has identified a population of ultraluminous infrared galaxies (ULIRGs) at z ~ 2 that may play an important role in the evolution of massive galaxies. We measure the stellar masses (M_*) of two populations of Spitzer-selected ULIRGs that have extremely red R – [24] colors (dust-obscured galaxies, or DOGs) and compare our results with submillimeter-selected galaxies (SMGs). One set of 39 DOGs has a local maximum in their mid-infrared (mid-IR) spectral energy distribution (SED) at rest frame 1.6 μm associated with stellar emission ("bump DOGs"), while the other set of 51 DOGs have power-law mid-IR SEDs that are typical of obscured active galactic nuclei ("power-law DOGs"). We measure M_* by applying Charlot & Bruzual stellar population synthesis models to broadband photometry in the rest-frame ultraviolet, optical, and near-infrared of each of these populations. Assuming a simple stellar population and a Chabrier initial mass function, we find that power-law DOGs and bump DOGs are on average a factor of 2 and 1.5 more massive than SMGs, respectively (median and inter-quartile M_* values for SMGs, bump DOGs, and power-law DOGs are log(M_*/M_☉) = 10.42^(+0.42)_(–0.36), 10.62^(+0.36)_(–0.32), and 10.71^(+0.40)_(–0.34), respectively). More realistic star formation histories drawn from two competing theories for the nature of ULIRGs at z ~ 2 (major merger versus smooth accretion) can increase these mass estimates by up to 0.5 dex. A comparison of our stellar masses with the instantaneous star formation rate (SFR) in these z ~ 2 ULIRGs provides a preliminary indication supporting high SFRs for a given M_*, a situation that arises more naturally in major mergers than in smooth accretion-powered systems

Internal thermal noise in the LIGO test masses : a direct approach

The internal thermal noise in LIGO's test masses is analyzed by a new technique, a direct application of the Fluctuation-Dissipation Theorem to LIGO's readout observable, $x(t)=$(longitudinal position of test-mass face, weighted by laser beam's Gaussian profile). Previous analyses, which relied on a normal-mode decomposition of the test-mass motion, were valid only if the dissipation is uniformally distributed over the test-mass interior, and they converged reliably to a final answer only when the beam size was a non-negligible fraction of the test-mass cross section. This paper's direct analysis, by contrast, can handle inhomogeneous dissipation and arbitrary beam sizes. In the domain of validity of the previous analysis, the two methods give the same answer for $S_x(f)$, the spectral density of thermal noise, to within expected accuracy. The new analysis predicts that thermal noise due to dissipation concentrated in the test mass's front face (e.g. due to mirror coating) scales as $1/r_0^2$, by contrast with homogeneous dissipation, which scales as $1/r_0$ ($r_0$ is the beam radius); so surface dissipation could become significant for small beam sizes.Comment: 6 pages, RevTex, 1 figur

A_4 flavour symmetry breaking scheme for understanding quark and neutrino mixing angles

We propose a spontaneous A_4 flavour symmetry breaking scheme to understand the observed pattern of quark and neutrino mixing. The fermion mass eigenvalues are arbitrary, but the mixing angles are constrained in such a way that the overall patterns are explained while also leaving sufficient freedom to fit the detailed features of the observed values, including CP violating phases. The scheme realises the proposal of Low and Volkas to generate zero quark mixing and tribimaximal neutrino mixing at tree-level, with deviations from both arising from small corrections after spontaneous A_4 breaking. In the neutrino sector, the breaking is A_4 --> Z_2, while in the quark and charged-lepton sectors it is A_4 --> Z_3 = C_3. The full theory has A_4 completely broken, but the two different unbroken subgroups in the two sectors force the dominant mixing patterns to be as stated above. Radiative effects within each sector are shown to deviate neutrino mixing from tribimaximal, while maintaining zero quark mixing. Interactions between the two sectors -- "cross-talk" -- induce nonzero quark mixing, and additional deviation from tribimaximal neutrino mixing. We discuss the vacuum alignment challenge the scenario faces, and suggest three generic ways to approach the problem. We follow up one of those ways by sketching how an explicit model realising the symmetry breaking structure may be constructed.Comment: 14 pages, no figures; v3: Section 5 rewritten to correct an error; new section added to the appendix; added references; v4: minor change to appendix C, version to be published by JHE