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Recent observations suggest that the accelerated expansion of the Universe at late times is caused by a temporally changing dark energy component, rather than the constant one in the standard $Λ$CDM scenario. In this context quintessence, i.e. a canonical scalar field minimally coupled to gravity, plays a prominent role. There are, however, three main types of quintessence models: thawing quintessence, scaling freezing quintessence, and tracking quintessence. Dynamical systems reformulations of the field equations for a broad set of scalar field potentials, including some new ones, allow us to use dynamical systems methods to derive global and asymptotic features, visualised in bounded state space pictures clearly illustrating the relationships and properties of the different types of quintessence, clarifying initial data issues, and yielding simple and accurate approximations.
Nations can be distinguished in terms of whether domestic or international research is cited. We analyzed the research output in natural sciences of three leading European research economies (Germany, the Netherlands, and the UK) and ask where their researchers look for the knowledge that underpins their most highly-cited papers. Is one internationally oriented or is citation limited to national resources? Do the citation patterns reflect a growing differentiation between the domestic and international research enterprise? To evaluate change over time, we include natural-sciences papers published in the countries from three publication years: 2004, 2009, and 2014. The results show that articles co-authored by researchers from Germany or the Netherlands are less likely to be among the globally most highly-cited articles if they also cite "domestic" research (i.e. research authored by authors from the same country). To put this another way, less well-cited research is more likely to stand on domestic shoulders and research that becomes more highly-cited is more likely to stand on international shoulders. A possible reason for the results is that researchers "over-cite" the papers fro
Tracking quintessence, in a spatially flat and isotropic space-time with a minimally coupled canonical scalar field and an asymptotically inverse power-law potential $V(\varphi)\propto\varphi^{-p}$, $p>0$, as $\varphi\rightarrow0$, is investigated. This is done by introducing a new three-dimensional \emph{regular} dynamical system, which enables a rigorous explanation of the tracking feature: 1) The dynamical system has a tracker fixed point $\mathrm{T}$ with a two-dimensional stable manifold that pushes an open set of nearby solutions toward a single tracker solution originating from $\mathrm{T}$. 2) All solutions, including the tracker solution and the solutions that track/shadow it, end at a common future attractor fixed point that depends on the potential. Thus, the open set of solutions that shadow the tracker solution share its properties during the tracking quintessence epoch. We also discuss similarities and differences of underlying mechanisms for tracking, thawing and scaling freezing quintessence, and, moreover, we illustrate with state space pictures that all of these types of quintessence exist simultaneously for certain potentials.
Despite its dominance in the present universe's energy budget, dark energy is the least understood component in the universe. Although there is a popular model for the dynamical dark energy, the quintessence scalar, the investigation is limited because of its highly elusive character. We present a model where the quintessence is gauged by an Abelian gauge symmetry. The quintessence is promoted to be a complex scalar whose real part is the dark energy field while the imaginary part is the longitudinal component of a new gauge boson. It brings interesting characters to dark energy physics. We study the general features of the model, including how the quintessence behavior is affected and how the solicited dark energy properties constrain its gauge interaction. We also note that while the uncoupled quintessence models are suffered greatly from the Hubble tension, it can be alleviated if the quintessence is under the gauge symmetry.
Germany has become a major country of immigration, as well as a research powerhouse in Europe. As Germany spends a higher fraction of its GDP on research and development than most countries with advanced economies, there is an expectation that Germany should be able to attract and retain international scholars who have high citation performance. Using an exhaustive set of over eight million Scopus publications, we analyze the trends in international migration to and from Germany among published researchers over the past 24 years. We assess changes in institutional affiliations for over one million researchers who have published with a German affiliation address at some point during the 1996-2020 period. We show that while Germany has been highly integrated into the global movement of researchers, with particularly strong ties to the US, the UK, and Switzerland, the country has been sending more published researchers abroad than it has attracted. While the balance has been largely negative over time, analyses disaggregated by gender, citation performance, and field of research show that compositional differences in migrant flows may help to alleviate persistent gender inequalities i
This research paper explores the interplay between quintessence and a cloud of strings, focusing on their influence on critical points, behaviors, and fractional-order phase transitions in AdS black holes. We analyze the thermodynamic properties of AdS black holes surrounded by quintessence and a cloud of strings, with particular attention to the effects of thermal fluctuations on their thermodynamics. Using critical conditions, we derive approximate analytical expressions for the critical points applicable to all types of these black holes. We further study the impact of the quintessence parameter and the intensity of the cloud of strings on critical behaviors, employing three-dimensional visualizations to depict these effects for different values of \(ω_q\). Our results show how these factors affect the locations of phase transitions and the regions where both phases coexist. In addition, we examine fractional-order phase transitions, which reveal significant transformations in the black hole system.
The correct detection of dense article layout and the recognition of characters in historical newspaper pages remains a challenging requirement for Natural Language Processing (NLP) and machine learning applications on historical newspapers in the field of digital history. Digital newspaper portals for historic Germany typically provide Optical Character Recognition (OCR) text, albeit of varying quality. Unfortunately, layout information is often missing, limiting this rich source's scope. Our dataset is designed to enable the training of layout and OCR modells for historic German-language newspapers. The Chronicling Germany dataset contains 693 annotated historical newspaper pages from the time period between 1852 and 1924. The paper presents a processing pipeline and establishes baseline results on in- and out-of-domain test data using this pipeline. Both our dataset and the corresponding baseline code are freely available online. This work creates a starting point for future research in the field of digital history and historic German language newspaper processing. Furthermore, it provides the opportunity to study a low-resource task in computer vision
Optical clocks have improved their frequency stability and estimated accuracy by more than two orders of magnitude over the best caesium microwave clocks that realise the SI second. Accordingly, an optical redefinition of the second has been widely discussed, prompting a need for the consistency of optical clocks to be verified worldwide. While satellite frequency links are sufficient to compare microwave clocks, a suitable method for comparing high-performance optical clocks over intercontinental distances is missing. Furthermore, remote comparisons over frequency links face fractional uncertainties of a few $10^{-18}$ due to imprecise knowledge of each clock's relativistic redshift, which stems from uncertainty in the geopotential determined at each distant location. Here, we report a landmark campaign towards the era of optical clocks, where, for the first time, state-of-the-art transportable optical clocks from Japan and Europe are brought together to demonstrate international comparisons that require neither a high-performance frequency link nor information on the geopotential difference between remote sites. Conversely, the reproducibility of the clocks after being transporte
We reanalyze a new quintessence scenario in a brane world model, assuming that a quintessence scalar field is confined in our 3-dimensional brane world. We study three typical quintessence models : (1) an inverse-power-law potential, (2) an exponential potential, and (3) kinetic-term quintessence ($k$-essence) model. With an inverse power law potential model ($V(φ) = μ^{α+ 4} φ^{- α}$), we show that in the quadratic dominant stage, the density parameter of a scalar field $Ω_φ$ decreases as $a^{-4(α-2)/(α+2)}$ for $2<α< 6$, which is followed by the conventional quintessence scenario. This feature provides us wider initial conditions for a successful quintessence. In fact, even if the universe is initially in a scalar-field dominant, it eventually evolves into a radiation dominant era in the $ρ^2$-dominant stage. Assuming an equipartition condition, we discuss constraints on parameters, resulting that $α\geq 4$ is required. This constraint also restricts the value of the 5-dimensional Planck mass, e.g. $4 \times 10^{-14}m_4 \lsim m_5 \lsim 3 \times 10^{-13}m_4$ for $α=5$. For an exponential potential model $V=μ^4\exp(-λφ/m_4)$, we may not find a natural and successful quintesse
Every year many scholars are funded by the China Scholarship Council (CSC). The CSC is a funding agency established by the Chinese government with the main initiative of training Chinese scholars to conduct research abroad and to promote international collaboration. In this study, we identified these CSC-funded scholars sponsored by the China Scholarship Council based on the acknowledgments text indexed by the Web of Science. Bibliometric data of their publications were collected to track their scientific mobility in different fields, and to evaluate the performance of the CSC scholarship in promoting international collaboration by sponsoring the mobility of scholars. Papers funded by the China Scholarship Council are mainly from the fields of natural sciences and engineering sciences. There are few CSC-funded papers in the field of social sciences and humanities. CSC-funded scholars from mainland China have the United States, Australia, Canada, and some European countries, such as Germany, the UK, and the Netherlands, as their preferential mobility destinations across all fields of science. CSC-funded scholars published most of their papers with international collaboration during
The behavior of the quintessence field is studied during inflation. In order to have a satisfactory model of dark energy, the quintessence field value today should be as insensible to the initial conditions as possible. Usually, only the dependence on the initial conditions specified at the end of inflation or, equivalently, at the beginning of the radiation dominated era, is considered. Provided the quintessence field is initially within a large but, crucially, finite interval, its present value becomes independent of the initial value it started from. The question as to whether inflation naturally drives the quintessence field to the above-mentioned interval is addressed. Since the quantum effects turn out to be important, the formalism of stochastic inflation is used in order to calculate the evolution of the quintessence field. Moreover, the quantum effects originating from the inflaton field are also taken into account and are proved to be sub-dominant in most cases. Finally, the requirement that the quintessence field is on tracks today is shown to imply quite tight constraints on the initial values of the quintessence and inflaton fields at the beginning of inflation. In par
Several recent works suggested the possibility of describing inflation by means of a renormalization group equation. In this paper we discuss the application of these methods to models of quintessence. In this framework a period of exponential expansion corresponds to the slow evolution of the scalar field in the neighborhood of a fixed point. A minimal set of universality classes for models of quintessence is defined and the transition from a matter dominated to quintessence dominated universe is studied. Models in which quintessence is non-minimally coupled with gravity are also discussed. We show that the formalism proves to be extremely convenient to describe quintessence and moreover we find that in most of the models discussed in this work quintessence naturally takes over ordinary matter.
In this work we investigate corrections of the quintessence regime of the dark energy on the Joule-Thomson (JT) effect of the Reissner Nordström anti de Sitter (RNAdS) black hole. The quintessence dark energy has equation of state as $p_q=ωρ_q$ in which $-1<ω<-\frac{1}{3}.$ Our calculations are restricted to ansatz: $ω=-1$ (the cosmological constant regime) and $ω=-\frac{2}{3}$ (quintessence dark energy). To study the JT expansion of the AdS gas under the constant black hole mass, we calculate inversion temperature $T_i$ of the quintessence RNAdS black hole where its cooling phase is changed to heating phase at a particular (inverse) pressure $P_i.$ Position of the inverse point $\{T_i,P_i\}$ is determined by crossing the inverse curves with the corresponding Gibbons-Hawking temperature on the T-P plan. We determine position of the inverse point verse different numerical values of the mass $M$ and the charge $Q$ of the quintessence AdS RN black hole. The cooling-heating phase transition (JT effect) is happened for $M>Q$ in which the causal singularity is still covered by the horizon. Our calculations show sensitivity of the inverse point $\{T_i,P_i\}$ position on the T-P p
Extended quintessence is obtained by coupling a normal scalar field to the Ricci scalar defined in the metric formalism. In this paper, we propose a new extended quintessence dark energy by introducing a non-minimal coupling between the quintessence and gravity, but with the Ricci scalar given from the Palatini formalism rather than the metric one. We find that the equation of state of the new extended quintessence can cross the phantom divide line, and moreover, it oscillates around the -1 line. We also show that the universe driven by the new extended quintessence will enter a dark energy dominated de Sitter phase in the future.
We introduce a natural model of quintessence in string theory where the light rolling scalar is radiatively stable and couples to Standard Model matter with weaker-than- Planckian strength. The model is embedded in an anisotropic type IIB compactification with two exponentially large extra dimensions and TeV-scale gravity. The bulk turns out to be nearly supersymmetric since the scale of the gravitino mass is of the order of the observed value of the cosmological constant. The quintessence field is a modulus parameterising the size of an internal four-cycle which naturally develops a potential of the order (gravitino mass)^4, leading to a small dark energy scale without tunings. The mass of the quintessence field is also radiatively stable since it is protected by supersymmetry in the bulk. Moreover, this light scalar couples to ordinary matter via its mixing with the volume mode. Due to the fact that the quintessence field is a flat direction at leading order, this mixing is very small, resulting in a suppressed coupling to Standard Model particles which avoids stringent fifth-force constraints. On the other hand, if dark matter is realised in terms of Kaluza-Klein states, unsuppr
We describe a model of hybrid quintessence in which in addition to the tracker field there is a trigger field which is responsible for ending quintessence. As a result, hybrid quintessence does not suffer from the problems associated with the eternal acceleration of the universe. We derive the hybrid quintessence potential on branes from the interbrane interaction in string theory and show that it requires TeV scale strings and two millimeter size dimensions. This scenario predicts a dark energy density of $O(mm^{-4})$ and relates the smallness of this energy to the large size of the extra dimensions.
We study a new class of quintessence models, in which the scalar field possesses a O(\textit{N}) internal symmetry. We give a critical condition of instability for the potential against Q ball formation. We find that the most widely used potentials of quintessence don't satisfy the above condition, and therefore the O(\textit{N}) quintessence with these potential will not lead to the Q ball formation. It is worth noting that O(\textit{N}) quintessence with cosine-type potential is especially interesting in that the angular contribution is not negligible.
We assess the impact of COVID-19 response measures implemented in Germany and Switzerland on cumulative COVID-19-related hospitalization and death rates. Our analysis exploits the fact that the epidemic was more advanced in some regions than in others when certain lockdown measures came into force, based on measuring health outcomes relative to the region-specific start of the epidemic and comparing outcomes across regions with earlier and later start dates. When estimating the effect of the relative timing of measures, we control for regional characteristics and initial epidemic trends by linear regression (Germany and Switzerland), doubly robust estimation (Germany), or synthetic controls (Switzerland). We find for both countries that a relatively later exposure to the measures entails higher cumulative hospitalization and death rates on region-specific days after the outbreak of the epidemic, suggesting that an earlier imposition of measures is more effective than a later one. For Germany, we also evaluate curfews (as introduced in a subset of states) based on cross-regional variation. We do not find any effects of curfews on top of the federally imposed contact restriction that
If CDM particles decay and their lifetime is comparable to the age of the Universe, they can modify its equation of state. By comparing the results of numerical simulations with high redshift SN-Ia observations we show that this hypothesis is consistent with present data. Fitting the simplest quintessence models with constant $w_q$ to data leads to $w_q \lesssim -1$. We show that a universe with a cosmological constant or quintessence matter with $w_q \sim -1$ and a decaying Dark Matter has an effective $w_q < -1$ and fits SN data better than stable CDM or quintessence models with $w_q > -1$.
The archetypal theory of dark energy is quintessence: a minimally coupled scalar field with a canonical kinetic energy and potential. By studying random potentials we show that quintessence imposes a restricted set of priors on the equation of state of dark energy. Focusing on the commonly-used parametrisation, $w(a)\approx w_0+w_a(1-a)$, we show that there is a natural scale and direction in the $(w_0, w_a)$ plane that distinguishes quintessence as a general framework. We calculate the expected information gain for a given survey and show that, because of the non-trivial prior information, it is a function of more than just the figure of merit. This allows us to make a quantitative case for novel survey strategies. We show that the scale of the prior sets target observational requirements for gaining significant information. This corresponds to a figure of merit FOM$\gtrsim 200$, a requirement that future galaxy redshift surveys will meet.