Breaking the sound barrier in holography Hoyos, Carlos; Jokela, Niko; Rodríguez Fernández, David ...
Physical review. D,
11/2016, Letnik:
94, Številka:
10
Journal Article
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It has been conjectured that the speed of sound in holographic models with UV fixed points has an upper bound set by the value of the quantity in conformal field theory. If true, this would set ...stringent constraints for the presence of strongly coupled quark matter in the cores of physical neutron stars, as the existence of two-solar-mass stars appears to demand a very stiff equation of state. In this article, we present a family of counterexamples to the speed of sound conjecture, consisting of strongly coupled theories at finite density. The theories we consider include N=4 super Yang-Mills at finite R-charge density and nonzero gaugino masses, while the holographic duals are Einstein-Maxwell theories with a minimally coupled scalar in a charged black hole geometry. We show that for a small breaking of conformal invariance, the speed of sound approaches the conformal value from above at large chemical potentials.
Aggressive behavior is a growing problem across many contexts. Thus, searching for its predictors is important. The aim of the current study was to analyze the moderator effect of mentalization in ...the relationship between impulsiveness and both verbal and physical aggressive behavior, using a sample of 583 participants gathered from the general Spanish population (M
= 34.60, SD
= 12.99). In our sample, 182 were male and 401 were female. The results showed significant bivariate relationships among aggression, impulsiveness, and mentalization. Moderation structural equation modeling (MSEM) showed a significant moderation effect, so whereas the value of mentalization is not relevant in cases of people with low impulsiveness, high mentalization abilities allow those people with high impulsiveness to behave less aggressively than people with high impulsiveness and low mentalization abilities. Practical implications and limitations of the study are discussed.
The advantages and limitations of techniques for measuring the presence and amount of cavitation, and for quantifying the removal of contaminants, are provided. After reviewing chemical, physical, ...and biological studies, a universal cause for the cleaning effects of bubbles cannot yet be concluded. An “ideal sensor” with high spatial and temporal resolution is proposed. Such sensor could be used to investigate bubble jetting, shockwaves, streaming, and even chemical effects, by correlating cleaning processes with cavitation effects, generated by hydrodynamics, lasers or ultrasound.
The climatic variability hypothesis predicts the evolution of species with wide thermal tolerance ranges in environments with variable temperatures, and the evolution of thermal specialists in ...thermally stable environments. In caves, the extent of spatial and temporal thermal variability experienced by taxa decreases with their degree of specialization to deep subterranean habitats. We use phylogenetic generalized least squares to model the relationship among thermal tolerance (upper lethal limits), subterranean specialization (estimated using ecomorphological traits), and habitat temperature in 16 beetle species of the tribe Leptodirini (Leiodidae). We found a significant, negative relationship between thermal tolerance and the degree of subterranean specialization. Conversely, habitat temperature had only a marginal effect on lethal limits. In agreement with the climatic variability hypothesis and under a climate change context, we show that the specialization process to live in deep subterranean habitats involves a reduction of upper lethal limits, but not an adjustment to habitat temperature. Thermal variability seems to exert a higher evolutionary pressure than mean habitat temperature to configure the thermal niche of subterranean species. Our results provide novel insights on thermal physiology of species with poor dispersal capabilities and on the evolutionary process of adaptation to subterranean environments. We further emphasize that the pathways determining vulnerability of subterranean species to climate change greatly depend on the degree of specialization to deep subterranean environments.
C(sp3)-rich aliphatic motifs in drug molecules are strongly associated with clinical success. Historically, the availability of compound libraries based on C(sp3)-rich cores has been limited due to ...the challenging direct functionalization of aliphatic rings. Instead, most small molecule drug-like libraries are diversified around central aromatic rings. Herein, we present a general approach to the synthesis of diversified libraries featuring aliphatic core rings via photoredox catalysis under mild conditions.
Bubbles are known to influence energy and mass transfer in gas-evolving electrodes. However, we lack a detailed understanding on the intricate dependencies between bubble evolution processes and ...electrochemical phenomena. This review discusses our current knowledge on the effects of bubbles on electrochemical systems with the aim to identify opportunities and motivate future research in this area. We first provide a base background on the physics of bubble evolution as it relates to electrochemical processes. Then we outline how bubbles affect energy efficiency of electrode processes, detailing the bubble-induced impacts on activation, ohmic, and concentration overpotentials. Lastly, we describe different strategies to mitigate losses and how to exploit bubbles to enhance electrochemical reactions.
Electrochemical reactors will play a key role in the electrification of the chemical industry and can enable the integration of renewable electricity sources with chemical manufacturing. Most large-scale industrial electrochemical processes, including chloro-alkali and aluminum production, involve gas-evolving electrodes. The evolution of bubbles at the surface of redox reaction sites often leads to the reduction of the active electrode area, the increase of ohmic resistance in the electrolyte, and the formation of undesirable concentration gradients. All of these effects result in energy losses that reduce the efficiency of electrochemical systems. This review synthesizes our current understanding on the relationship between bubble evolution and energy losses in electrochemical reactors. By presenting a thorough account on the state of the research in this area, we aim to provide a common ground for the research community to improve our understanding on the complex processes involved in multiphase electrochemical systems. Increasing our knowledge on the relationship between bubbles and electrochemistry will lead to new strategies to mitigate and exploit bubble-induced phenomena leading to design guidelines for high-performing electrochemical reactors.
Bubbles influence energy and mass transfer in gas-evolving electrodes. However, we lack a detailed understanding on the intricate dependencies between bubble-evolution processes and electrochemical phenomena. This review discusses the current understanding on the effects of bubbles on electrochemical systems. We first provide a background on the physics of bubble evolution. Then we outline how bubbles affect energy efficiency of electrode processes. Lastly, we describe strategies to mitigate losses and how to exploit bubbles to enhance electrochemical reactions.
We present and analyze an entropy-stable semi-discretization of the Euler equations based on high-order summation-by-parts (SBP) operators. In particular, we consider general multidimensional SBP ...elements, building on and generalizing previous work with tensor–product discretizations. In the absence of dissipation, we prove that the semi-discrete scheme conserves entropy; significantly, this proof of nonlinear L2 stability does not rely on integral exactness. Furthermore, interior penalties can be incorporated into the discretization to ensure that the total (mathematical) entropy decreases monotonically, producing an entropy-stable scheme. SBP discretizations with curved elements remain accurate, conservative, and entropy stable provided the mapping Jacobian satisfies the discrete metric invariants; polynomial mappings at most one degree higher than the SBP operators automatically satisfy the metric invariants in two dimensions. In three-dimensions, we describe an elementwise optimization that leads to suitable Jacobians in the case of polynomial mappings. The properties of the semi-discrete scheme are verified and investigated using numerical experiments.
•Entropy-stable, SBP discretizations for non-tensor product, curved elements.•Polynomial mappings can be at most one degree higher than the SBP operators.•3D mapping Jacobian found by solving an elementwise optimization problem.•Numerical experiments verify entropy stability and accuracy.
We use a top-down holographic model for strongly interacting quark matter to study the properties of neutron stars. When the corresponding equation of state (EOS) is matched with state-of-the-art ...results for dense nuclear matter, we consistently observe a first-order phase transition at densities between 2 and 7 times the nuclear saturation density. Solving the Tolman-Oppenheimer-Volkov equations with the resulting hybrid EOSs, we find maximal stellar masses in excess of two solar masses, albeit somewhat smaller than those obtained with simple extrapolations of the nuclear matter EOSs. Our calculation predicts that no quark matter exists inside neutron stars.
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