10,678 research outputs found
Reconstruction of mimetic gravity in a non-singular bouncing universe from quantum gravity
We illustrate a general reconstruction procedure for mimetic gravity.
Focusing on a bouncing cosmological background, we derive general properties
that must be satisfied by the function implementing the limiting
curvature hypothesis. We show how relevant physical information can be
extracted from power law expansions of in different regimes, corresponding
e.g. to the very early universe or to late times. Our results are then applied
to two specific models reproducing the cosmological background dynamics
obtained in group field theory and in loop quantum cosmology, and we discuss
the possibility of using this framework as providing an effective field theory
description of quantum gravity. We study the evolution of anisotropies near the
bounce, and discuss instabilities of scalar perturbations. Furthermore, we
discuss two equivalent formulations of mimetic gravity: one in terms of an
effective fluid with exotic properties, the other featuring two distinct
time-varying gravitational "constants" in the cosmological equations.Comment: Invited article for the special issue "Progress in Group Field Theory
and Related Quantum Gravity Formalisms" of the journal "Universe"; 21 pages,
2 figures; v2: matches published versio
Accelerated expansion of the Universe without an inflaton and resolution of the initial singularity from Group Field Theory condensates
We study the expansion of the Universe using an effective Friedmann equation
obtained from the dynamics of GFT (Group Field Theory) isotropic condensates.
The evolution equations are classical, with quantum correction terms to the
Friedmann equation given in the form of effective fluids coupled to the
emergent classical background. The occurrence of a bounce, which resolves the
initial spacetime singularity, is shown to be a general property of the model.
A promising feature of this model is the occurrence of an era of accelerated
expansion, without the need to introduce an inflaton field with an
appropriately chosen potential. We discuss possible viability issues of this
scenario as an alternative to inflation.Comment: 14 pages, 4 figures, v2 revised to match published versio
Recommended from our members
A comparative analysis of business process modelling techniques
Business process modelling is an increasingly popular research area for both organisations and academia due to its usefulness in facilitating human understanding and communication. Several modelling techniques have been proposed and used to capture the characteristics of business processes. However, available techniques view business processes from different perspectives and have different features and capabilities. Furthermore, to date limited guidelines exist for selecting appropriate modelling techniques based on the characteristics of the problem and its requirements. This paper presents a comparative analysis of some popular business process modelling techniques. The comparative framework is based on five criteria: flexibility, ease of use, understandability, simulation support and scope. The study highlights some of the major paradigmatic differences between the techniques. The proposed framework can serve as the basis for evaluating further modelling techniques and generating selection procedures
Recommended from our members
An ontology to model the research process in information systems
The IS community has relied mostly on two main paradigms to undertake IS research: positivist and interpretivist. This paper argues that the ongoing debate around which of these paradigms is better suited to undertake IS research has created confusion amongst IS researchers, particularly between those who are relatively inexperienced (e.g. PhD researchers). Inexperienced researchers tend to place emphasis on the justification of their research approaches in the context of existing paradigms without offering a clear description of how the chosen methods and paradigms are applied in the context of their own research, a key issue to assess and understand any research output. This paper does not attempt to give any suggestions as to which research methods/paradigms should be used for IS research, but to raise the awareness that the way we currently communicate our thoughts in the research methods domain may not be very effective. We argue that an initial step to undertake this challenge could be to take a more “practical” approach by focusing on the process of thinking and planning the research activity rather than focusing on the justification of the use of one or many research methods usually “loaned” from other discipline
Two-time Green's functions and spectral density method in nonextensive quantum statistical mechanics
We extend the formalism of the thermodynamic two-time Green's functions to
nonextensive quantum statistical mechanics. Working in the optimal Lagrangian
multipliers representation, the -spectral properties and the methods for a
direct calculation of the two-time % -Green's functions and the related
-spectral density ( measures the nonextensivity degree) for two generic
operators are presented in strict analogy with the extensive ()
counterpart. Some emphasis is devoted to the nonextensive version of the less
known spectral density method whose effectiveness in exploring equilibrium and
transport properties of a wide variety of systems has been well established in
conventional classical and quantum many-body physics. To check how both the
equations of motion and the spectral density methods work to study the
-induced nonextensivity effects in nontrivial many-body problems, we focus
on the equilibrium properties of a second-quantized model for a high-density
Bose gas with strong attraction between particles for which exact results exist
in extensive conditions. Remarkably, the contributions to several thermodynamic
quantities of the -induced nonextensivity close to the extensive regime are
explicitly calculated in the low-temperature regime by overcoming the
calculation of the grand-partition function.Comment: 48 pages, no figure
Spectral density method in quantum nonextensive thermostatistics and magnetic systems with long-range interactions
Motived by the necessity of explicit and reliable calculations, as a valid
contribution to clarify the effectiveness and, possibly, the limits of the
Tsallis thermostatistics, we formulate the Two-Time Green Functions Method in
nonextensive quantum statistical mechanics within the optimal Lagrange
multiplier framework, focusing on the basic ingredients of the related Spectral
Density Method. Besides, to show how the SDM works we have performed, to the
lowest order of approximation, explicit calculations of the low-temperature
properties for a quantum -dimensional spin-1/2 Heisenberg ferromagnet with
long-range interactions decaying as ( is the distance between
spins in the lattice)Comment: Contribution to Next-SigmaPhi conference in Kolymbari, Crete, Greece,
August 13-18, 2005, 9 page
Recommended from our members
A Semantic-based framework for discovering business process patterns
Patterns currently play an important role in modern information systems (IS) development and their use has mainly been restricted to the design and implementation phases of the development lifecycle. Given the increasing significance of business modeling in IS development, patterns have the potential of providing a viable solution for promoting reusability of recurrent generalized models in the very early stages of development. This paper focuses on business process patterns and proposes an initial framework for the discovery and reuse of business process patterns within the IS development lifecycle. The framework synthesizes the idea from the domain engineering literature and proposes the use of semantics to drive both the discovery of patterns as well as their reuse
Effective cosmological constant induced by stochastic fluctuations of Newton's constant
We consider implications of the microscopic dynamics of spacetime for the
evolution of cosmological models. We argue that quantum geometry effects may
lead to stochastic fluctuations of the gravitational constant, which is thus
considered as a macroscopic effective dynamical quantity. Consistency with
Riemannian geometry entails the presence of a time-dependent dark energy term
in the modified field equations, which can be expressed in terms of the
dynamical gravitational constant. We suggest that the late-time accelerated
expansion of the Universe may be ascribed to quantum fluctuations in the
geometry of spacetime rather than the vacuum energy from the matter sector.Comment: 10 pages, 1 figure, v2: added legend in Fig.1 and a referenc
Recommended from our members
Actor perception in business use case modeling
Mainstream literature recognizes the validity and effectiveness of use cases as a technique for gathering and capturing system requirements. Use cases represent the driver of various modern development methods, mainly of object-oriented extraction, such as the Unified Process. Although the adoption of use cases proliferated in the context of software systems development, they are not as extensively employed in business modeling . The concept of business use case is not a novelty, but only recently did it begin to re-circulate in the literature and in case tools.
This paper examines the issues involved in adopting business use cases for capturing the functionality of an organization and proposes guidelines for their identification, packaging, and mapping to system use cases. The proposed guidelines are based on the principle of actor perception described in the paper. The application of this principle is exemplified with a worked example aimed at demonstrating the utility of the proposed guidelines and at clarifying the application of the principle of actor perception. The worked example is based on a series of workshops run at a major UK financial institution
- …