Perturbative analysis of generalized Einstein's theories

The hypothesis that the energy-momentum tensor of ordinary matter is not conserved separately, leads to a non-adiabatic expansion and, in many cases, to an Universe older than usual. This may provide a solution for the entropy and age problems of the Standard Cosmological Model. We consider two different theories of this type, and we perform a perturbative analysis, leading to analytical expressions for the evolution of gravitational waves, rotational modes and density perturbations. One of these theories exhibits satisfactory properties at this level, while the other one should be discarded.Comment: 14 pages, Latex fil

Thermodynamic Losses in a Gas Spring: Comparison of Experimental and Numerical Results

Reciprocating-piston devices can be used as high-efficiency compressors and/or expanders. With an optimal valve design and by carefully adjusting valve timing, pressure losses during intake and exhaust can be largely reduced. The main loss mechanism in reciprocating devices is then the thermal irreversibility due to the unsteady heat transfer between the compressed/expanded gas and the surrounding cylinder walls. In this paper, pressure, volume and temperature measurements in a piston-cylinder crankshaft driven gas spring are compared to numerical results. The experimental apparatus experiences mass leakage while the CFD code predicts heat transfer in an ideal closed gas spring. Comparison of experimental and numerical results allows one to better understand the loss mechanisms in play. Heat and mass losses in the experiment are decoupled and the system losses are calculated over a range of frequencies. As expected, compression and expansion approach adiabatic processes for higher frequencies, resulting in higher efficiency. The objective of this study is to observe and explain the discrepancies obtained between the computational and experimental results and to propose further steps to improve the analysis of the loss mechanisms

Different faces of the phantom

The SNe type Ia data admit that the Universe today may be dominated by some exotic matter with negative pressure violating all energy conditions. Such exotic matter is called {\it phantom matter} due to the anomalies connected with violation of the energy conditions. If a phantom matter dominates the matter content of the universe, it can develop a singularity in a finite future proper time. Here we show that, under certain conditions, the evolution of perturbations of this matter may lead to avoidance of this future singularity (the Big Rip). At the same time, we show that local concentrations of a phantom field may form, among other regular configurations, black holes with asymptotically flat static regions, separated by an event horizon from an expanding, singularity-free, asymptotically de Sitter universe.Comment: 6 pages, presented at IRGAC 2006, Barcelona, 11-15 July 200

Parameter evaluation in Michaelis-menten kinetics.

Parameter estimation reliability in enzyme kinetics depends upon the substrate range concentrations under assay. An inappropriate concentration set may lead to spurious values of km and Vmax in the Michaelis-Menten approach. In this paper, the theoretical arguments for a practical criterium concerning the best work range of substrate concentration are discussed on a velocity ratio basis (V1/Vn) as response to the pertinent substrate concentration ratio (S1/Sn)

Editorial: Student-teacher relationship quality research: Past, present and future

More than 20 years have passed since the publication of Pianta (2001) on the quality of the teacher-student relationship. Since then, several attempts have been made to elaborate theoretically the concept of teacher-student relationship quality and to provide empirical evidence of the impact that good teacher-student relationship quality might have on academic achievement, student psychological adjustment, and classroom climate. The teacher has been recognized as a “psychological parent” and defined as a secure base and safe heaven, following attachment theory (Verschueren and Koomen, 2012, 2021; Prino et al., 2022; Spilt et al., 2022). Several studies have shown that a relationship with the teacher characterized by affection, closeness, and respect predicts more favorable developmental outcomes and better adjustment to the classroom context in any school setting (Roorda et al., 2011, 2017; Longobardi et al., 2019, 2021; Lin et al., 2022). However, after 20 years, we saw the need to synthesize the current literature on the topic of teacher-learner relationship quality and to promote a collection of studies that provide new insights, ideas, and reflections to advance the research field and overcome current limitations.In this Research Topic, 16 publications were collected from different parts of the world. The Research Topic includes two literature reviews, several empirical works, some of which aim to develop and validate instruments to measure the quality of the teacher-student relationship, and others to promote new knowledge about the effects and mechanisms of action of the quality of the teacher-learner relationship on the psychological development and adjustment processes of children and adolescents. In addition, the Research Topic includes a contribution on possible intervention strategies on the quality of teacher-student relationship

The canavanine-pentacyanoamine ferrate complex formation.

The complexation of pentacyanoamine ferrate with canavanine is not a specific one. Other amino acids, especially tryptophan and histidine may also react with proper analytical features. Se also do the indolyl and imidazolyl moieties of nonamino acids. Photoactivation does neither seem to occur by exposure of the pentacyanoamine ferrate reagent to daylight, nor is it a perequisite for a more sensitive response of canavanine complexation. On the other hand, oxidants and reducers may well interfere with the absorbance of the complex. In addition molar absorbance, stoichiometric ratio of the complex, and reation-order kinetics were determined. By mean of temperature programming equilibrion constant andreaction kinetics, enthalpy, Gibbs nenergy, entropy values, and activation energy of complex formation were stablished

Observational constraints on Rastall's cosmology

Rastall's theory is a modification of General Relativity, based on the non-conservation of the stress-energy tensor. The latter is encoded in a parameter $\gamma$ such that $\gamma = 1$ restores the usual $\nabla_\nu T^{\mu\nu} = 0$ law. We test Rastall's theory in cosmology, on a flat Robertson-Walker metric, investigating a two-fluid model and using the type Ia supernovae Constitution dataset. One of the fluids is pressureless and obeys the usual conservation law, whereas the other is described by an equation of state $p_x = w_x\rho_x$, with $w_x$ constant. The Bayesian analysis of the Constitution set does not strictly constrain the parameter $\gamma$ and prefers values of $w_x$ close to -1. We then address the evolution of small perturbations and show that they are dramatically unstable if $w_x \neq -1$ and $\gamma \neq 1$, i.e. General Relativity is the favored configuration. The only alternative is $w_x = -1$, for which the dynamics becomes independent from $\gamma$.Comment: Latex file, 14 pages, 6 figures in eps format. Substantial modifications performed, main conclusions change

About Starobinsky inflation

It is believed that soon after the Planck era, space time should have a semi-classical nature. According to this, the escape from General Relativity theory is unavoidable. Two geometric counter-terms are needed to regularize the divergences which come from the expected value. These counter-terms are responsible for a higher derivative metric gravitation. Starobinsky idea was that these higher derivatives could mimic a cosmological constant. In this work it is considered numerical solutions for general Bianchi I anisotropic space-times in this higher derivative theory. The approach is ``experimental'' in the sense that there is no attempt to an analytical investigation of the results. It is shown that for zero cosmological constant $\Lambda=0$, there are sets of initial conditions which form basins of attraction that asymptote Minkowski space. The complement of this set of initial conditions form basins which are attracted to some singular solutions. It is also shown, for a cosmological constant $\Lambda> 0$ that there are basins of attraction to a specific de Sitter solution. This result is consistent with Starobinsky's initial idea. The complement of this set also forms basins that are attracted to some type of singular solution. Because the singularity is characterized by curvature scalars, it must be stressed that the basin structure obtained is a topological invariant, i.e., coordinate independent.Comment: Version accepted for publication in PRD. More references added, a few modifications and minor correction

Attosecond sampling of arbitrary optical waveforms

Advances in the generation of ultrashort laser pulses, and the emergence of new research areas such as attosecond science, nanoplasmonics, coherent control, and multidimensional spectroscopy, have led to the need for a new class of ultrafast metrology that can measure the electric field of complex optical waveforms spanning the ultraviolet to the infrared. Important examples of such waveforms are those produced by spectral control of ultrabroad bandwidth pulses, or by Fourier synthesis. These are typically tailored for specific purposes, such as to increase the photon energy and flux of high-harmonic radiation, or to control dynamical processes by steering electron dynamics on subcycle time scales. These applications demand a knowledge of the full temporal evolution of the field. Conventional pulse measurement techniques that provide estimates of the relative temporal or spectral phase are unsuited to measure such waveforms. Here we experimentally demonstrate a new, all-optical method for directly measuring the electric field of arbitrary ultrafast optical waveforms. Our method is based on high-harmonic generation (HHG) driven by a field that is the collinear superposition of the waveform to be measured with a stronger probe laser pulse. As the delay between the pulses is varied, we show that the field of the unknown waveform is mapped to energy shifts in the high-harmonic spectrum, allowing a direct, accurate, and rapid retrieval of the electric field with subcycle temporal resolution at the location of the HHG