4,992 research outputs found
Low redshift constraints on energy-momentum-powered gravity models
There has been recent interest in the cosmological consequences of
energy-momentum-powered gravity models, in which the matter side of Einstein's
equations is modified by the addition of a term proportional to some power,
, of the energy-momentum tensor, in addition to the canonical linear term.
In this work we treat these models as phenomenological extensions of the
standard CDM, containing both matter and a cosmological constant. We
also quantitatively constrain the additional model parameters using low
redshift background cosmology data that are specifically from Type Ia
supernovas and Hubble parameter measurements. We start by studying specific
cases of these models with fixed values of which lead to an analytic
expression for the Friedmann equation; we discuss both their current
constraints and how the models may be further constrained by future
observations of Type Ia supernovas for WFIRST complemented by measurements of
the redshift drift by the ELT. We then consider and constrain a more extended
parameter space, allowing to be a free parameter and considering scenarios
with and without a cosmological constant. These models do not solve the
cosmological constant problem per se. Nonetheless these models can
phenomenologically lead to a recent accelerating universe without a
cosmological constant at the cost of having a preferred matter density of
around instead of the usual . Finally we
also briefly constrain scenarios without a cosmological constant, where the
single component has a constant equation of state which needs not be that of
matter; we provide an illustrative comparison of this model with a more
standard dynamical dark energy model with a constant equation of state.Comment: 13+2 pages, 12+1 figures; A&A (in press
Robustness of bipartite Gaussian entangled beams propagating in lossy channels
Subtle quantum properties offer exciting new prospects in optical
communications. Quantum entanglement enables the secure exchange of
cryptographic keys and the distribution of quantum information by
teleportation. Entangled bright beams of light attract increasing interest for
such tasks, since they enable the employment of well-established classical
communications techniques. However, quantum resources are fragile and undergo
decoherence by interaction with the environment. The unavoidable losses in the
communication channel can lead to a complete destruction of useful quantum
properties -- the so-called "entanglement sudden death". We investigate the
precise conditions under which this phenomenon takes place for the simplest
case of two light beams and demonstrate how to produce states which are robust
against losses. Our study sheds new light on the intriguing properties of
quantum entanglement and how they may be tamed for future applications.Comment: To be published - Nature Photonic
On reference frames in spacetime and gravitational energy in freely falling frames
We consider the interpretation of tetrad fields as reference frames in
spacetime. Reference frames may be characterized by an antisymmetric
acceleration tensor, whose components are identified as the inertial
accelerations of the frame (the translational acceleration and the frequency of
rotation of the frame). This tensor is closely related to
gravitoelectromagnetic field quantities. We construct the set of tetrad fields
adapted to observers that are in free fall in the Schwarzschild spacetime, and
show that the gravitational energy-momentum constructed out of this set of
tetrad fields, in the framework of the teleparallel equivalent of general
relatrivity, vanishes. This result is in agreement with the principle of
equivalence, and may be taken as a condition for a viable definition of
gravitational energy.Comment: 19 pages, no figures, accepted by Classical and Quantum Gravit
A constructivist model of bank branch front-office employee evaluation: an FCM-SD-based approach
The banking sector is one of the primary drivers of economic development. This sector has been affected by various crises throughout its history – most recently, the 2008 financial and economic crisis. In response, banking institutions have had to make diverse changes to their procedures and deal with new concerns related to changes within markets. One of the main recent developments in this sector is the new commercial function assigned to bank branch front-office employees, who have become responsible for selling financial products and services, as well as recruiting and retaining clients. As a result, the sector needs new employee performance evaluation methods in line with banks and staff members’ requirements. This study combined fuzzy cognitive mapping techniques and the system dynamics (SD) approach to develop a well-informed performance analysis system for assessing bank branch front-office employees. The proposed system was validated by the Business Process Management Competence Center director at Millennium BCP – a Portuguese private banking corporation. The main difference between the model constructed in the present research and current evaluation practices is that the criteria were collected directly from multiple specialists working at different commercial banks, who deal daily with this decision problem. The model’s theoretical and practical implications are also discussed.info:eu-repo/semantics/publishedVersio
Comparative yield loss estimates due to blast in some upland rice cultivars.
Leaf and panicle blast severities and grain yield of some upland rice cultivars were measured in three successive years in field plots unprotected or protected with fungicides. The variation in disease severities in different plots was used to establish relationships between severity of leaf and panicle blast and yield. Linear multiple regression equations for each cultivar by year were developed to estimate the yield decrease in different cultivars per unit increase in disease. Leaf blast severities at maximum tillering or booting stage and panicle blast 25 days after heading accounted for variation in grain yield in most of the cultivars. General equations combining five early and eight medium-duration rice cultivars were developed. The estimated percentage losses in grain yield due to blastwere 2.7 and 1.5 for one percent increase in blast in the early and medium-duration cultivars, respectively
Disentanglement in Bipartite Continuous-Variable Systems
Entanglement in bipartite continuous-variable systems is investigated in the
presence of partial losses, such as those introduced by a realistic quantum
communication channel, e.g. by propagation in an optical fiber. We find that
entanglement can vanish completely for partial losses, in a situa- tion
reminiscent of so-called entanglement sudden death. Even states with extreme
squeezing may become separable after propagation in lossy channels. Having in
mind the potential applications of such entangled light beams to optical
communications, we investigate the conditions under which entanglement can
survive for all partial losses. Different loss scenarios are examined and we
derive criteria to test the robustness of entangled states. These criteria are
necessary and sufficient for Gaussian states. Our study provides a framework to
investigate the robustness of continuous-variable entanglement in more complex
multipartite systems.Comment: Phys. Rev. A (in press
Variation of the speed of light with temperature of the expanding universe
From an extended relativistic dynamics for a particle moving in a cosmic
background field with temperature T, we aim to obtain the speed of light with
an explicit dependence on the background temperature of the universe. Although
finding the speed of light in the early universe much larger than its current
value, our approach does not violate the postulate of special relativity.
Moreover, it is shown that the high value of the speed of light in the early
universe was drastically decreased before the beginning of the inflationary
period. So we are led to conclude that the theory of varying speed of light
should be questioned as a possible solution of the horizon problem.Comment: 3 pages and 1 figure; Phys. Rev. D86, 027703 (2012
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