Improving the modelling of redshift-space distortions: I. A bivariate Gaussian description for the galaxy pairwise velocity distributions

As a step towards a more accurate modelling of redshift-space distortions in galaxy surveys, we develop a general description of the probability distribution function of galaxy pairwise velocities within the framework of the so-called streaming model. For a given galaxy separation $\vec{r}$, such function can be described as a superposition of virtually infinite local distributions. We characterize these in terms of their moments and then consider the specific case in which they are Gaussian functions, each with its own mean $\mu$ and dispersion $\sigma$. Based on physical considerations, we make the further crucial assumption that these two parameters are in turn distributed according to a bivariate Gaussian, with its own mean and covariance matrix. Tests using numerical simulations explicitly show that with this compact description one can correctly model redshift-space distorsions on all scales, fully capturing the overall linear and nonlinear dynamics of the galaxy flow at different separations. In particular, we naturally obtain Gaussian/exponential, skewed/unskewed distribution functions, depending on separation as observed in simulations and data. Also, the recently proposed single-Gaussian description of redshift-space distortions is included in this model as a limiting case, when the bivariate Gaussian is collapsed to a two-dimensional Dirac delta function. We also show how this description naturally allows for the Taylor expansion of $1+\xi_S(\vec{s})$ around $1+\xi_R(r)$, which leads to the Kaiser linear formula when truncated to second order, expliciting its connection with the moments of the velocity distribution functions. More work is needed, but these results indicate a very promising path to make definitive progress in our program to improve RSD estimators.Comment: 11 pages, 3 figures, 2 table

The CPV “toolbox”: New approaches to maximizing solar resource utilization with application-oriented concentrator photovoltaics

As the scaling of silicon PV cells and module manufacturing has driven solar energy penetration up and costs down, concentrator photovoltaic technologies, originally conceived as a cost-saving measure, have largely been left behind. The loss of market share by CPV is being locked in even as solar energy development encounters significant obstacles related to space constraints in many parts of the world. The inherently higher collection efficiency enabled by the use of concentrators could substantially alleviate these challenges, but the revival of CPV for this purpose requires substantial reinvention of the technology to actually capture the theoretically possible efficiency gains, and to do so at market-friendly costs. This article will discuss recent progress in key areas central to this reinvention, including miniaturization of cells and optics to produce compact, lightweight “micro-CPV” systems; hybridization of CPV with thermal, illumination and other applications to make use of unused energy streams such as diffuse light and waste heat; and the integration of sun-tracking into the CPV module architecture to enable greater light collection and more flexible deployment, including integration into built structures. Applications showing particular promise include thermal applications such as water heating, industrial processes and desalination; agricultural photovoltaics; building-integrated photovoltaics with dynamic daylighting capabilities; and chemical processes including photocatalysis and hydrogen production. By appropriately tailoring systems to the available solar resource and local energy demand, we demonstrate how CPV can finally achieve real-world efficiencies, or solar resource utilization factors, far higher than those of standard silicon-based PV systems. This makes the argument for sustained development of novel CPV designs that can be applied to the real-world settings where this efficiency boost will be most beneficial

Is the global response to Covid-19 justified?

Here we analyse the legitimacy of government actions during the Covid-19 pandemic (lockdowns, social distancing, and masks) and the way they restrict individual choice and derail society by appealing to the masses. Rather than focusing on scientific research on the actual virus, we focus on the set of arguments typically provided by pro-measures individuals to show that they are based on the concepts of cognitive misery, lazy option, and problem substitution. We discuss some of the most notorious slogans and provide counterarguments to show that governments, media, experts, and other institutions have been led by the inertia of a confusing problem based on arbitrary and entangled grounds. Our conclusion is that governments, media, and industry are forcing “one-liners” (slogans) upon the true minority under attack: the single individual. Only when the single individual manages to become a sufficiently strong power, consideration of personal choice is granted

Modeling and optimization of solar thermoelectric generators for terrestrial applications

In this paper we introduce a model and an optimization methodology for terrestrial solar thermoelectric generators (STEGs). We describe, discuss, and justify the necessary constraints on the STEG geometry that make the STEG optimization independent of individual dimensions. A simplified model shows that the thermoelectric elements in STEGs can be scaled in size without affecting the overall performance of the device, even when the properties of the thermoelectric material and the solar absorber are temperature-dependent. Consequently, the amount of thermoelectric material can be minimized to be only a negligible fraction of the total system cost. As an example, a Bi[subscript 2]Te[subscript 3]-based STEG is optimized for rooftop power generation. Peak efficiency is predicted to be 5% at the standard spectrum AM1.5G, with the thermoelectric material cost below 0.05 $/W[subscript p]. Integrating STEGs into solar hot water systems for cogeneration adds electricity at minimal extra cost. In such cogeneration systems the electric current can be adjusted throughout the day to favor either electricity or hot water production

Subharmonic excitation in amplitude modulation atomic force microscopy in the presence of adsorbed water layers

In ambient conditions, nanometric water layers form on hydrophilic surfaces covering them and significantly changing their properties and characteristics. Here we report the excitation of subharmonics in amplitude modulation atomic force microscopy induced by intermittent water contacts. Our simulations show that there are several regimes of operation depending on whether there is perturbation of water layers. Single period orbitals, where subharmonics are never induced, follow only when the tip is either in permanent contact with the water layers or in pure noncontact where the water layers are never perturbed. When the water layers are perturbed subharmonic excitation increases with decreasing oscillation amplitude. We derive an analytical expression which establishes whether water perturbations compromise harmonic motion and show that the predictions are in agreement with numerical simulations. Empirical validation of our interpretation is provided by the observation of a range of values for apparent height of water layers when subharmonic excitation is predicted.Peer Reviewe

Fault-Tolerant Computing with Single Qudit Encoding

We present a general approach for the Fault Tolerant implementation of stabilizer codes with a logical qubit encoded into a single multi-level qudit, preventing the explosion of resources of multi-qubit codes. The proposed scheme allows for correction and universal quantum computation. We demonstrate its effectiveness by simulations on molecular spin qudits, finding an almost exponential suppression of logical errors with the qudit size. The resulting performance on a small qudit is remarkable when compared to qubit codes using thousands of units

Resin infiltrant for non-cavitated caries lesions: evaluation of color stability

The objective of this in vitro study was to evaluate the over time color stability of one resin infiltrant (Icon) upon exposure to staining solutions (coffee and wine) compared with one nano-hybrid sealant (Grandio Seal), one transparent fissure sealant with fluoride (Control Seal) and one nanofilled composite (Filtek Supreme XTE). All materials were polymerized according to manufacturers’ instructions into silicon rings (height 1 mm; internal diameter 6 mm; external diameter 8 mm) to obtain specimens identical in size. The specimens were immersed in staining solutions at room temperature over a 28-day test period. The control samples have not been subjected to the staining process. A colorimetric evaluation according to the CIE L*a*b* system was performed by a blind trained operator at 7, 14, 21, 28 days of the staining process. Shapiro Wilk test and Kruskal Wallis ANOVA were applied to assess significant differences among different materials. Means were compared with Scheffe’s multiple-comparison test at the 0.05 level of significance. In the case of all materials, immersion in solutions resulted in clinically perceivable color changes after 1 week (∆E 0.05). Color coordinate CIE b* varied similarly for all materials tested (P > 0.05). Immersion in coffee or red wine resulted in clinically perceivable color changes for all materials tested. Icon showed the highest color variations both after 1 week and 1 month. Icon can fix the initial esthetic problem associated with white spot lesions, but the resin may become more discolored than other materials over time

Oscillations:Linear theory and applications in AFM

The theory of oscillations can be studied from a mathematical point of view in terms of differential equations. The differential equation is written and then the solution or solutions worked out and mathematically analysed. Provided physical, economic, social, or other phenomena can be modelled in terms of equivalent differential equations, the solutions and results are applicable to all phenomena all the same. On the other hand, it is sometimes easier to learn a topic by having an experimental topic in mind. It is otherwise maybe surprising that a large body of phenomena in many fields of application will be easily understood if the equations are understood for a given case. The experimental analysis that forms the basis of this book is cantilever dynamics in dynamic atomic force microscopy (AFM). In a nutshell, the motion of the cantilever in dynamic AFM can be approximated to a perturbed driven oscillator. The generality of the analysis presented here can be confirmed by noting that much of what is covered in this book, particularly when dealing with the linear equation in section 1, is similar to what is covered in generic expositions such as that by Tipler and Mosca1 or the Feynman’s lectures on physics2 . Maybe the main advantage of this exposition is that the linear and nonlinear theories of oscillators, particularly phenomena that can be reduced to the analysis of a point-mass on a spring, are discussed in detail and differences in terminology that could lead to doubt, clarified. This means that this book can be used as a textbook to teach oscillation theory with a focus on applications. This is possible because oscillations are present generally in physics, engineering, biology, economics, sociology and so on. In summary, all phenomena dealing with oscillations can be reduced, to a first approximation, to a restoring parameter, i.e., force in mechanics, following Hooke’s law