Epoch Of Acceleration Research Articles

Exponential quintessence: curved, steep and stringy?

We explore the possibility that our universe’s current accelerated expansion is explained by a quintessence model with an exponential scalar potential, V = V0e−λ ϕ, keeping an eye towards λ ≥ 2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{2} $$\\end{document} and an open universe, favorable to a string theory realisation and with no cosmological horizon. We work out the full cosmology of the model, including matter, radiation, and optionally negative spatial curvature, for all λ > 0, performing an extensive analysis of the dynamical system and its phase space. The minimal physical requirements of a past epoch of radiation domination and an accelerated expansion today lead to an upper bound λ ≲ 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{3} $$\\end{document}, which is driven slightly up in the presence of observationally allowed spatial curvature. Cosmological solutions start universally in a kination epoch, go through radiation and matter dominated phases and enter an epoch of acceleration, which is only transient for λ > 2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{2} $$\\end{document}. Field distances traversed between BBN and today are sub-Planckian. We discuss possible string theory origins and phenomenological challenges, such as time variation of fundamental constants. We provide theoretical predictions for the model parameters to be fitted to data, most notably the varying dark energy equation of state parameter, in light of recent results from DES-Y5 and DESI.

New non-conserved gravity theory and alleviate current acceleration epoch without dark energy field

Recently, a new modified theory of gravity was proposed in which Rastall's idea Tμ;νν=α,μ is reexamined. Although Rastall's idea shows potential for exploring field equations of gravity at high levels of energy, there are concerns about Rastall's proposal α=γR. In Ref. [1], it is suggested to find the viable form for α by reevaluating the first law of thermodynamics in a relativistic system. In this context, Israel and his colleagues have proposed a covariant equation linking the flux of the energy-momentum tensor and the 4-vector of entropy of the system, respecting the Gibbs-Duhem relation in flat spacetime [2,3]. Extending this idea to curved spacetime provides the theoretical form of α that supports Rastall's idea Tμ;νν=α,μ. However, this modified theory of gravity, known as ‘non-conserved gravity’, cannot present dark energy in FRW spacetime for the comoving observer without the cosmological constant. Through this study and considering the application of this gravity theory in cosmology, it was demonstrated that the model can explain the current acceleration epoch in the absence of extra fluid as dark energy. Therefore, the non-conserved theory of gravity attributes the matter field as the source of current accelerated expansion. This model also aligns with matter structure formations and thus does not conflict with observations in the past (high redshift) Universe.

Dynamical System Approach and Thermodynamical Perspective of Hořava‐Lifshitz Gravity

AbstractThe authors have examined a Friedmann Robertson Walker cosmological model in Hořava‐Lifshitz gravity by using a dynamical system approach. A set of autonomous equations is derived and their solutions are calculated. The critical points from these equations and find the characteristics values with the analysis of the physical interpretation of the phase space for this system are assessed. Three stable critical points are found and the values of the physical parameters and the scale factor's expressions at each critical points are displayed in Tables 1, 2, and 3. A hybrid scale factor to develop the model, which results in a phase transition from deceleration to acceleration is used. The suitable values of the parameters are governed by applying the Monte Chain Monte Carlo method technique to the Hubble 46 and joint Hubble 46 and Baryon Acoustic Oscillations 15 datasets. In contrast to the negative behavior of pressure, the positive behavior of energy density and illustrate the Universe's acceleration epoch and the model is represented by the EoS parameter . The authors investigated that the energy conditions and their model violates the strong energy condition. Utilizing diagnostic test, it is found that the model represents phantom behavior. The thermodynamical perspective for the model is also examined. The model accurately explained the Universe's propagation history and fits well with contemporary cosmic data.

Holographic dark energy in non-conserved gravity theory

Recently, reconsidering the Rastall idea Tμ;νν=a,μ through relativistic thermodynamics gives a new form for the scalar field a which led us to construct modern modified theory of gravity debugged ‘non-conserved gravity theory’ Fazlollahi 2023 Euro. Phys. J. C 83 923. This theory unlike other modified theories of gravity cannot directly explain the current acceleration expansion in the absence of the cosmological constant and or existence of other forms of dark energy. Hence, in this study we have reinvestigated holographic dark energy ρX∼H2 in the non-conserved theory of gravity. In this context, the density and pressure of dark energy depend on the non-conserved term and density of the dust matter field. As shown, due to non-conservation effects on large-scale structures, unlike the original holographic model, our model onsets an acceleration epoch for the current Universe satisfies observations. Moreover, the interaction and viscous scenarios are studied for this model.

The Surface Accelerations Reference—A Large-Scale, Interactive Catalog of Passenger Vehicle Accelerations

There is a need for a large-scale, real world, diverse, and context rich vehicle acceleration catalog that can be used to design, analyze, and compare various intelligent transportation systems. This paper fulfills three primary objectives. First, it provides such a catalog through the Surface Accelerations Reference, which is openly available as an interactive analytics tool as well as an open and downloadable dataset. The Surface Accelerations Reference statistically describes the driving profiles of about 3,500 individuals contributing 34 million miles of continuous driving data collected in the Second Strategic Highway Research Program Naturalistic Driving Study (SHRP 2 NDS). These profiles were created by summarizing billions of longitudinal and lateral acceleration epochs experienced by the participants. Second, this paper introduces a standardized methodology for creating such a catalog so that similar acceleration profiles can be produced for other human cohorts or automated driving systems. Finally, the data are used to analyze the effect of roadway speed category on the rates of lateral and longitudinal acceleration epochs at various thresholds. It is observed that, for the median driver, the rates of epochs are up to three orders of magnitude higher on low-speed roads as compared to high-speed roads. This catalog will facilitate intelligent vehicle system designers to compare and tune their systems for safer driving experiences. It will also allow agencies with similar data to create comparable catalogs facilitating safety and behavioral comparisons between populations.

Unified inflation and dark energy with accelerated particle holographic model

In this paper and in the follows of the holographic approach to describe the primary acceleration and the late-time acceleration eras, we have considered and unified inflation and dark energy while accelerated particle holographic (APHM) density is used (Fazlollahi in Chin Phys C 47:035101, 2023). As discussed, establishing holographic model for constant roll inflation during the very early Universe leads one to explicit form of Hubble parameter which satisfies inflationary era. The validity of such holographic constant roll inflation with respect to the Planck data puts a certain bound on the infrared cut-off at the time of horizon crossing. Beside the mere inflation, the Hubble parameter gives explicit form for density of accelerated particle holographic model. It is shown such density due to inflationary bound condition on infrared cut-off could present late-time acceleration epoch. Consequently, inflation and late time expansion era unified in unique infrared cut-off model. Moreover, such corresponding fractional density gives more matter regimes during matter era. Nevertheless, studying matter structure formation during matter era reveals the model is not in conflict with {Lambda }CDM model.

Revisiting the Epoch of Cosmic Acceleration

We revisit the epoch of cosmic speed-up characterized by the redshift of transition from a decelerated to an accelerated phase. This redshift is termed the transition redshift (z t ). We use the spatially flat and non-flat variants of the most common ΛCDM and XCDM models to put constraints on the transition redshift along with the other model parameters. The data for this analysis come from the recent and updated Pantheon+ supernova (SN) data set and the Hubble parameter measurements obtained from Cosmic Chronometers. We consider both data sets with their respective covariance matrices incorporating all statistical and systematic uncertainties. We observe that using the combined data sets of H(z) and SNe, the best fit value of transition redshift lies in the range 0.61 < z t < 0.79 for all four dark energy models. Incidentally, we observe a positive curvature for the non-flat models, correlations between several model parameters and a strong degeneracy between the curvature and the equation of state parameter.

Systematic exploration of heavy element nucleosynthesis in protomagnetar outflows

ABSTRACTWe study the nucleosynthesis products in neutrino-driven winds from rapidly rotating, highly magnetized and misaligned protomagnetars using the nuclear reaction network SkyNet. We adopt a semi-analytic parametrized model for the protomagnetar and systematically study the capabilities of its neutrino-driven wind for synthesizing nuclei and eventually producing ultra-high energy cosmic rays (UHECRs). We find that for neutron-rich outflows (Ye &amp;lt; 0.5), synthesis of heavy elements ($\overline{A}\sim 20-65$) is possible during the first $\sim 10\, {\rm s}$ of the outflow, but these nuclei are subjected to composition-altering photodisintegration during the epoch of particle acceleration at the dissipation radii. However, after the first $\sim 10\, {\rm s}$ of the outflow, nucleosynthesis reaches lighter elements ($\overline{A}\sim 10-50$) that are not subjected to subsequent photodisintegration. For proton-rich (Ye ≥ 0.5) outflows, synthesis is more limited ($\overline{A}\sim 4-15$). These suggest that while protomagnetars typically do not synthesize nuclei heavier than second r-process peak elements, they are intriguing sources of intermediate/heavy mass UHECRs. For all configurations, the most rapidly rotating protomagnetars are more conducive for nucleosynthesis with a weaker dependence on the magnetic field strength.

Noncommutativity, Sáez–Ballester Theory and Kinetic Inflation

This paper presents a noncommutative (NC) version of an extended Sáez–Ballester (SB) theory. Concretely, considering the spatially flat Friedmann–Lemaître–Robertson–Walker (FLRW) metric, we propose an appropriate dynamical deformation between the conjugate momenta and, applying the Hamiltonian formalism, obtain deformed equations of motion. In our model, the NC parameter appears linearly in the deformed Poisson bracket and the equations of the NC SB cosmology. When it goes to zero, we get the corresponding commutative counterparts. Even by restricting our attention to a particular case, where there is neither an ordinary matter nor a scalar potential, we show that the effects of the noncommutativity provide interesting results: applying numerical endeavors for very small values of the NC parameter, we show that (i) at the early times of the universe, there is an inflationary phase with a graceful exit, for which the relevant nominal condition is satisfied; (ii) for the late times, there is a zero acceleration epoch. By establishing an appropriate dynamical framework, we show that the results (i) and (ii) can be obtained for many sets of the initial conditions and the parameters of the model. Finally, we indicate that, at the level of the field equations, one may find a close resemblance between our NC model and the Starobinsky inflationary model.

A 3D Phase Space Analysis of Scalar Field Potentials

In this study, we present the phase-space analysis of Quintessence models specified by the choice of two potentials, namely the Recliner potential and what we call the broken exponential-law potential, which is a new proposal. Using a dynamical system analysis we provide a systematic study of the cosmological evolution of the two models and their properties. We find new scaling solutions characterised by a constant ratio between the energy density of the scalar field and that of the matter component. These solutions are of high interest in light of the possibility to alleviate the coincidence problem. Additionally, the models also show attractor solutions. We finally construct concrete models built using a double potential according to which one potential realises the early-time scaling regime and the second one allows to exit this regime and to enter in the epoch of cosmic acceleration driven by a scalar-field dominated attractor point.

Continuous cosmic evolution with diffusive barotropic fluid: First-order thermodynamic phase transition

In the background of homogeneous and isotropic flat FLRW model, a complete cosmic scenario from nonsingular emergent scenario to the present late time acceleration through inflationary era and matter-dominated epoch has been presented in this work with cosmic matter in the form of diffusive barotropic fluid. By proper choices of the diffusion parameter and using Friedmann equations, it is possible to show the transitions: Emergent scenario[Formula: see text]Inflationary era[Formula: see text]matter-dominated phase[Formula: see text]Late time acceleration epoch. In analogy with analytic continuation, it is found that the above evolution will be continuous for suitable values of the parameters involved. Finally, possible first-order thermodynamic phase transition has been analyzed for such cosmic evolution.

F(T) cosmology: from Pseudo-Bang to Pseudo-Rip

We investigate the complete universe evolution in the framework off(T) cosmology. We first study the requirements at thekinematic level and we introduce a simple scale factor with the necessaryfeatures. Performing a detailed analysis of the phaseportrait we show that the universe begins in the infinite past from aphase where thescale factor goes to zero but the Hubble parameter goes to a constant, and itsderivative to zero. Since these features resemble those of the Pseudo-Ripfate but in a revered way, we call this initial phaseas Pseudo-Bang. Then the universe evolves in a first inflationary phase, acosmological turnaround and a bounce, after which we have asecond inflationary regime with a successful exit. Subsequently we obtainthe standard thermal history and the sequence of radiation, matter andlate-time acceleration epochs, showing that the universe will result inan everlasting Pseudo-Rip phase. Finally, taking advantage of the fact thatthe field equations of f(T) gravity are of second order, and therefore thecorresponding autonomous dynamical system is one dimensional, we incorporate the aforementioned kinematicfeatures and we reconstruct the specific f(T)form that can dynamically generate the Pseudo-Bang cosmological scenario. Lastly, we examine the evolution of the primordial fluctuationsshowing that they are initially sub-horizon, and we show thatthe total fluid does not exhibit any singular behaviour at thephantom crossing points, while the torsional fluid experiencesthem as Type II singular phases.

Quantifying the effect of roadway, driver, vehicle, and location characteristics on the frequency of longitudinal and lateral accelerations

The purpose of this study is to understand and quantify the simultaneous effects of roadway speed category, driver age, driver gender, vehicle class, and location on the rates of longitudinal and lateral acceleration epochs. The rate of usual as well as harsh acceleration epochs are used to extract insights on driving risk and driver comfort preferences. However, an analysis of acceleration rates at multiple thresholds incorporating various effects while using a large-scale and diverse dataset is missing. This analysis will fill this research gap. Data from the 2nd Strategic Highway Research Program Naturalistic Driving Study (SHRP2 NDS) was used for this analysis. The rate of occurrence of acceleration epochs was modeled using negative binomial distribution based generalized linear mixed effect models. Roadway speed category, driver age, driver gender, vehicle class, and location were used as the fixed effects and the driver identifier was used as the random effect. Incidence rate ratios were then calculated to compare subcategories of each fixed effect. Roadway speed category has the strongest effect on longitudinal and lateral accelerations of all magnitudes. Acceleration epoch rates consistently decrease as the roadway speed category increases. The difference in the rates depends on the threshold and is up to three orders of magnitude. Driver age is another significant factor with clear trends for longitudinal and lateral acceleration epochs. Younger and older drivers experience higher rates of longitudinal accelerations and decelerations. However, the rate of lateral accelerations consistently decreases with age. Vehicle class also has a significant effect on the rate of harsh accelerations with minivans consistently experiencing lower rates.

Cosmological dynamics of dark energy in scalar-torsion f(T,phi ) gravity

It is investigated the cosmological dynamics of scalar-torsion f(T,phi ) gravity as a dark energy model, where T is the torsion scalar of teleparallel gravity and phi is a canonical scalar field. In this context, we are concerned with the phenomenology of the class of models with non-linear coupling to gravity and exponential potential. We obtain the critical points of the autonomous system, along with the stability conditions of each one of them and their cosmological properties. Particularly, we show the existence of new attractors with accelerated expansion, as well as, new scaling solutions in which the energy density of dark energy scales as the background fluid density, thus, defining the so-called scaling radiation and scaling matter epochs. The scaling solutions are saddle points, and therefore, the system exits these solutions to the current epoch of cosmic acceleration, towards an attractor point describing the dark energy-dominated era.

Cosmological viable models in f(T, B) theory as solutions to the H0 tension

In this work we present a further investigation about teleparallel gravity cosmology. We demonstrate that according to the current astrophysical data (CC + Pantheon + BAO samplers with late Universe measurements SH0ES + H0LiCOW), an f(T, B) theory can provide another interpretation to the oscillatory behaviour of the dark energy equation of state when applied to late times. The four f(T, B) cosmological viable models proposed here can undergo an epoch of late-time acceleration and reproduce quintessence and phantom regimes with a transition along the phantom-divided line, making this theory a good approach to modify the standard ΛCDM model.

Dynamic Functional Connectivity between order and randomness and its evolution across the human adult lifespan

Functional Connectivity (FC) during resting-state or task conditions is not static but inherently dynamic. Yet, there is no consensus on whether fluctuations in FC may resemble isolated transitions between discrete FC states rather than continuous changes. This quarrel hampers advancing the study of dynamic FC. This is unfortunate as the structure of fluctuations in FC can certainly provide more information about developmental changes, aging, and progression of pathologies. We merge the two perspectives and consider dynamic FC as an ongoing network reconfiguration, including a stochastic exploration of the space of possible steady FC states. The statistical properties of this random walk deviate both from a purely “order-driven” dynamics, in which the mean FC is preserved, and from a purely “randomness-driven” scenario, in which fluctuations of FC remain uncorrelated over time. Instead, dynamic FC has a complex structure endowed with long-range sequential correlations that give rise to transient slowing and acceleration epochs in the continuous flow of reconfiguration. Our analysis for fMRI data in healthy elderly revealed that dynamic FC tends to slow down and becomes less complex as well as more random with increasing age. These effects appear to be strongly associated with age-related changes in behavioural and cognitive performance.

Swampland conjectures and late-time cosmology

We discuss the cosmological implications of the string swampland conjectures for late-time cosmology, and test them against a wide range of state of the art cosmological observations. The refined de Sitter conjecture constrains either the minimal slope or the curvature of the scalar potential, and depends on two dimensionless constants. For constants of size one or larger, tension exists between observations, especially the Hubble constant, and the slope and curvature conjectures at a level of 4.5 sigma and 2.3 sigma, respectively. Smaller values of the constants are permitted by observations, and we determine upper bounds at varying confidence levels. We also derive and constrain the relationship between cosmological observables and the scalar field excursion during the acceleration epoch, thereby testing the distance conjecture.

New scaling solutions in cubic Horndeski theories

We propose a viable dark energy scenario in the presence of cubic Horndeski interactions and a standard scalar-field kinetic term with two exponential potentials. We show the existence of new scaling solutions along which the cubic coupling $G_3$ provides an important contribution to the field density that scales in the same way as the background fluid density. The solutions finally exit to the epoch of cosmic acceleration driven by a scalar-field dominated fixed point arising from the second exponential potential. We clarify the viable parameter space in which all the theoretically consistent conditions including those for the absence of ghost and Laplacian instabilities are satisfied on scaling and scalar-field dominated critical points. In comparison to Quintessence with the same scalar potential, we find that the cubic coupling gives rise to some novel features: (i) the allowed model parameter space is wider in that a steeper potential can drive the cosmic acceleration; (ii) the dark energy equation of state $w_{\phi}$ today can be closer to $-1$ relative to Quintessence; (iii) even if the density associated with the cubic coupling dominates over the standard field density in the scaling era, the former contribution tends to be suppressed at low redshifts. We also compute quantities associated with the growth of matter perturbations and weak lensing potentials under the quasi-static approximation in the sub-horizon limit and show that the cubic coupling leads to the modified evolution of perturbations which can be distinguished from Quintessence.

Acceleration Profile of Elite Gaelic Football Match Play.

Ryan, M, Malone, S, and Collins, K. An acceleration profile of elite Gaelic football match play. J Strength Cond Res 32(3): 812-820, 2018-The use of global positioning system (GPS) technology in Gaelic football is the primary source of quantifying game demands. The aim of this study was to quantify the acceleration profile of elite Gaelic football. Thirty-six elite male Gaelic football players (mean ± SD, age: 24 ± 6 years; height: 180 ± 7 cm; mass: 81 ± 7 kg) across 5 playing positions took part in a multiple study (n = 154 observations). Player movement was recorded during 19 (n = 19) competitive games over 2 seasons using 4-Hz GPS (VXSport, New Zealand). The average total distance (m), high-speed running distance (m; ≥17 km·h), and very high-speed running distance (m; ≥22 km·h) were recorded. In addition, the number (n), distance (m), and the duration of accelerations were quantified. Accelerations were subdivided into 14 equal parts of 5-minute epochs (E1 = 0-5 minutes, E2 = 5-10 minutes, E3 = 10-15 minutes etc). Players performed 166 ± 41 accelerations. High-speed running distance and very high-speed running distance was 1563 ± 605 and 524 ± 190 m, respectively. The mean acceleration distance was 267 ± 45 m distributed between 12 ± 5 accelerations per 5-minute epoch. The maximum acceleration epoch classified as the greatest distance covered accelerating during a predetermined 5-minute epoch was 296 ± 134 m. The PEAK epoch resulted in a significant reduction of acceleration distance covered in the period before and in the subsequent epoch. An understanding of the acceleration profile in Gaelic football can inform the prescription of appropriate training regimen.

Statefinder diagnostic and constraints on the Palatini f(R) gravity theories

We focus on a series of f(R) gravity theories in Palatini formalism to investigate the probabilities of producing late-time acceleration for the flat Friedmann-Robertson-Walker (FRW) universe. We apply a statefinder diagnostic to these cosmological models for chosen series of parameters to see if they can be distinguished from one another. The diagnostic involves the statefinder pair {r, s}, where r is derived from the scale factor a and its higher derivatives with respect to the cosmic time t, and s is expressed by r and the deceleration parameter q. In conclusion, we find that although two types of f(R) theories: (i) f(R) = R + αRm – βR−n and (ii) f(R) = R + α ln R – β can lead to late-time acceleration, their evolutionary trajectories in the r – s and r – q planes reveal different evolutionary properties, which certainly justify the merits of the statefinder diagnostic. Additionally, we utilize the observational Hubble parameter data (OHD) to constrain these models of f(R) gravity. As a result, except for m = n = 1/2 in case (i), α = 0 in case (i) and case (ii) allow the ΛCDM model to exist in the 1σ confidence region. After applying the statefinder diagnostic to the best-fit models, we find that all the best-fit models are capable of going through the deceleration/acceleration transition stage with a late-time acceleration epoch, and all these models turn to the de Sitter point ({r, s} = {1, 0}) in the future. Also, the evolutionary differences between these models are distinct, especially in the r – s plane, which makes the statefinder diagnostic more reliable in discriminating cosmological models.