One conjecture and two observations on de Sitter space

We propose that the state represented by the Nariai black hole inside de Sitter space is the ground state of the de Sitter gravity, while the pure de Sitter space is the maximal energy state. With this point of view, we investigate thermodynamics of de Sitter space, we find that if there is a dual field theory, this theory can not be a CFT in a fixed dimension. Near the Nariai limit, we conjecture that the dual theory is effectively an 1+1 CFT living on the radial segment connecting the cosmic horizon and the black hole horizon. If we go beyond the de Sitter limit, the "imaginary" high temperature phase can be described by a CFT with one dimension lower than the spacetime dimension. Below the de Sitter limit, we are approaching a phase similar to the Hagedorn phase in 2+1 dimensions, the latter is also a maximal energy phase if we hold the volume fixed.Comment: 12 pages, harvmac; references added; version for publication in JHE

Usage of Mobile Applications Amongst Older Adults: Retrospect and Prospect

Addressing age-specific issues related to IT usage is becoming increasingly significant. To consolidate the existing knowledge on the usage of mobile apps amongst older adults, this study synthesizes the research nature, patterns, and theoretical foundations in existing studies through a systematic literature review. Based on our investigation into the 22 identified articles, we find that previous studies on this topic focus mainly on app usage behaviors, app development and evaluation, and app usage learning and training. Prior studies are still fragmented and a more comprehensive and integrated framework is required to guide future research. Finally, we discuss the key antecedents of older adults’ app usage identified in reviewed articles and highlight seven heuristic topics with corresponding research questions for future research. Limitations of this review are also discussed

Comment on "Resolving the 180-deg Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions"

In a recent paper, Leka at al. (Solar Phys. 260, 83, 2009)constructed a synthetic vector magnetogram representing a three-dimensional magnetic structure defined only within a fraction of an arcsec in height. They rebinned the magnetogram to simulate conditions of limited spatial resolution and then compared the results of various azimuth disambiguation methods on the resampled data. Methods relying on the physical calculation of potential and/or non-potential magnetic fields failed in nearly the same, extended parts of the field of view and Leka et al. (2009) attributed these failures to the limited spatial resolution. This study shows that the failure of these methods is not due to the limited spatial resolution but due to the narrowly defined test data. Such narrow magnetic structures are not realistic in the real Sun. Physics-based disambiguation methods, adapted for solar magnetic fields extending to infinity, are not designed to handle such data; hence, they could only fail this test. I demonstrate how an appropriate limited-resolution disambiguation test can be performed by constructing a synthetic vector magnetogram very similar to that of Leka et al. (2009) but representing a structure defined in the semi-infinite space above the solar photosphere. For this magnetogram I find that even a simple potential-field disambiguation method manages to resolve the ambiguity very successfully, regardless of limited spatial resolution. Therefore, despite the conclusions of Leka et al. (2009), a proper limited-spatial-resolution test of azimuth disambiguation methods is yet to be performed in order to identify the best ideas and algorithms.Comment: Solar Physics, in press (19 pp., 5 figures, 2 tables

Generation of attosecond electron bunches in a laser-plasma accelerator using a plasma density upramp

Attosecond electron bunches and attosecond radiation pulses enable the study of ultrafast dynamics of matter in an unprecedented regime. In this paper, the suitability for the experimental realization of a novel scheme producing sub-femtosecond duration electron bunches from laser-wakefield acceleration in plasma with self-injection in a plasma upramp profile has been investigated. While it has previously been predicted that this requires laser power above a few hundred terawatts typically, here we show that the scheme can be extended with reduced driving laser powers down to tens of terawatts, generating accelerated electron pulses with minimum length of around 166. attoseconds and picocoulombs charge. Using particle-in-cell simulations and theoretical models, the evolution of the accelerated electron bunch within the plasma as well as simple scalings of the bunch properties with initial laser and plasma parameters are presented

De Sitter Holography with a Finite Number of States

We investigate the possibility that, in a combined theory of quantum mechanics and gravity, de Sitter space is described by finitely many states. The notion of observer complementarity, which states that each observer has complete but complementary information, implies that, for a single observer, the complete Hilbert space describes one side of the horizon. Observer complementarity is implemented by identifying antipodal states with outgoing states. The de Sitter group acts on S-matrix elements. Despite the fact that the de Sitter group has no nontrivial finite-dimensional unitary representations, we show that it is possible to construct an S-matrix that is finite-dimensional, unitary, and de Sitter-invariant. We present a class of examples that realize this idea holographically in terms of spinor fields on the boundary sphere. The finite dimensionality is due to Fermi statistics and an `exclusion principle' that truncates the orthonormal basis in which the spinor fields can be expanded.Comment: 23 pages, 1 eps figure, LaTe

Resonances in radiative hyperon decays

The importance of resonances for the radiative hyperon decays is examined in the framework of chiral perturbation theory. Low lying baryon resonances are included into the effective theory and tree contributions to these decays are calculated. We find significant contributions to both the parity-conserving and parity-violating decay amplitudes and a large negative value for the asymmetry parameter in polarized Sigma^+ -> p gamma is found, in agreement with the experimental result alpha(p Sigma^+) = -0.76 +/- 0.08.Comment: 14 pages, 2 figure

Health Misinformation on Social Media: A Literature Review

Health misinformation on social media is considered as a major public concern. This study evaluates the current state of this issue by conducting a systematic literature review. Based on a stepwise literature search and selection procedure, we have identified 21 articles relevant to the topic of health misinformation on social media. We find that health misinformation on social media is a new and emerging topic in multiple disciplines. One very important insight of this review is that most studies are theoretical and exploratory in nature. There is only a small number of studies have solid theoretical foundations. Finally, we discuss the implication of the literature review for future research

When Socialization Goes Wrong: Understanding the We-Intention to Participate in Collective Trolling in Virtual Communities

Although collective trolling poses a growing threat to both individuals and virtual community owners, the information systems (IS) literature lacks a rich theorization of this phenomenon. To address the research gaps, we introduce the concept of we-intention to capture the collective nature of collective trolling in virtual communities. We also integrate the social identity model of deindividuation effects (SIDE) and situational action theory to invoke the sociotechnical perspective in theorizing collective trolling in virtual communities. The objective of this study is to use the sociotechnical perspective to understand the we-intention to participate in collective trolling in virtual communities. We test our proposed model using data gathered from 377 Reddit users. Our moderated mediation analysis elaborates how technical elements (i.e., anonymity of self and anonymity of others) influence the we-intention to participate in collective trolling via individual-based social elements (i.e., perceived online disinhibition and social identity), with an environment-based social element (i.e., the absence of capable guardianship) as a boundary condition. We contribute to research by explaining collective trolling in virtual communities from the group-referent intentional action perspective and sociotechnical perspective. We also offer practical insights into ways to combat collective trolling in virtual communities

Design and implementation of a modified fourier analysis harmonic current computation technique for power active filters using DSPs

The design and implementation of a harmonic current computation technique based on a modified Fourier analysis, suitable for active power filters incorporating DSPs is presented. The proposed technique is suitable for the monitoring and control of load current harmonics for real-time applications. The derivation of the basic equations based on the proposed technique and the system implementation using the Analogue Devices SHARC processor are presented. The steady state and dynamic performance of the system are evaluated for a range of loading conditions