scholarly journals How a projectively flat geometry regulates F(R)-gravity theory?

2021 ◽  
Author(s):  
Tee-How Loo ◽  
Avik De ◽  
Sanjay Mandal ◽  
P. K. Sahoo
Keyword(s):  
Matter Field ◽  
Gravity Theory ◽  
Gravity Models ◽  
Spacetime Geometry ◽  
Projectively Flat ◽  
Expansion Scalar ◽  
Isotropic Pressure ◽  
Constant Energy Density ◽  
Projective Flatness

Abstract In the present paper we examine a projectively flat spacetime solution of F(R)-gravity theory. It is seen that once we deploy projective flatness in the geometry of the spacetime, the matter field has constant energy density and isotropic pressure. We then make the condition weaker and discuss the effects of projectively harmonic spacetime geometry in F(R)-gravity theory and show that the spacetime in this case reduces to a generalised Robertson-Walker spacetime with a shear, vorticity, acceleration free perfect fluid with a specific form of expansion scalar presented in terms of the scale factor. Role of conharmonic curvature tensor in the spacetime geometry is also briefly discussed. Some analysis of the obtained results are conducted in terms of couple of F(R)-gravity models.

scholarly journals ON p – q DUALITY AND EXPLICIT SOLUTIONS IN c ≤ 1 2D GRAVITY MODELS

1995 ◽  
Vol 10 (08) ◽  
pp. 1219-1236 ◽  
Author(s):  
S. KHARCHEV ◽  
A. MARSHAKOV
Keyword(s):  
String Theory ◽  
Exact Solutions ◽  
2D Gravity ◽  
Integral Formula ◽  
Integral Representations ◽  
Gravity Models ◽  
Explicit Solutions ◽  
String Equation ◽  
Duality Transformation

We study the role of integral representations in the description of nonperturbative solutions to c ≤ 1 string theory. A generic solution is determined by two functions, W(x) and Q(x), which behave at infinity like xp and xq respectively. The integral formula for arbitrary (p, q) models is derived, which explicitly realizes a duality transformation between (p, q) and (q, p) 2D gravity solutions. We also discuss the exact solutions to the string equation and reduction condition and present several explicit examples.

scholarly journals The role of networks for migration flows: an update

2016 ◽  
Vol 37 (7) ◽  
pp. 1154-1171 ◽  
Author(s):  
Michel Beine
Keyword(s):  
Specific Role ◽  
Gravity Models ◽  
Immigration Policies ◽  
Recent Experience ◽  
Data Set ◽  
Content Type ◽  
Driving Mechanisms ◽  
Migration Flows ◽  
Over Time

Purpose The purpose of this paper is to cover the literature on the role migrants networks in explaining aggregate migration flows between countries. The author emphasizes the specific role of family immigration policies. To that purpose, the author covers briefly the recent experience of seven receiving countries to highlight the importance of these policies in explaining part of the observed network elasticities. Design/methodology/approach The author first provides a small review of the literature and the issues at stake. The author then provides an update of the estimates of the network elasticities using the data set on migration stocks and flows from Ozden et al. (2011). Using micro-founded gravity models, the author estimates the network elasticities and discusses the key driving mechanisms explaining their size as well the variation in the amplitude across categories of destination and over time. The author accounts for the issue of multilateral resistance to migration. Findings The author obtains estimates that are in line with the ones documented previously in the literature. The author finds that the role of networks in attracting migrants has increased after the 1970s. The author emphasizes the specific role of family immigration policies. To that purpose, the author covers briefly the recent experience of a set of receiving countries to highlight the importance of these policies in explaining part of the observed network elasticities. Originality/value This paper covers the literature on the role migrants networks in explaining aggregate migration flows between countries and obtain new estimates of network elasticities that vary over time and across types of destination countries.

scholarly journals MULTIDIMENSIONAL INHOMOGENEOUS COSMOLOGY IN SCALAR TENSOR THEORY

2005 ◽  
Vol 14 (06) ◽  
pp. 1083-1094
Author(s):  
D. PANIGRAHI ◽  
S. CHATTERJEE ◽  
Y. Z. ZHANG
Keyword(s):  
Scalar Field ◽  
Dimensional Reduction ◽  
Big Bang ◽  
Matter Field ◽  
Fluid Distribution ◽  
Alternative Mechanism ◽  
Inhomogeneous Cosmology ◽  
Fifth Dimension ◽  
Exponential Expansion ◽  
Isotropic Pressure

Exact cosmological solutions are obtained for a five-dimensional inhomogeneous fluid distribution along with a Brans–Dicke type of scalar field. The set includes varied forms of matter field including ρ+p = 0, where p is the 3D isotropic pressure. Depending on the signature of 4-space curvature, our solutions admit of indefinite expansion in the usual 3-space and dimensional reduction of the fifth dimension. Due to the presence of the scalar field, the case p = -ρ does not yield an exponential expansion of the scale factor, which strikingly differs from our earlier investigations without scalar field. The effective four-dimensional values of entropy and matter are calculated and possible consequences of entropy and matter generation in the 4D world as a result of dimensional reduction of the extra space are also discussed. It is encouraging to point out that aside from the well known big bang singularity, our inhomogeneous cosmology is spatially regular everywhere. Further, our model seems to suggest an alternative mechanism pointing to a smooth pass over from a primordial, inhomogeneous cosmological phase to a 4D homogeneous one.

scholarly journals EXPANSIONFREE FLUID EVOLUTION AND SKRIPKIN MODEL IN f(R) THEORY

2011 ◽  
Vol 20 (11) ◽  
pp. 2239-2252 ◽  
Author(s):  
M. SHARIF ◽  
H. RIZWANA KAUSAR
Keyword(s):  
General Relativity ◽  
The Self ◽  
Spherically Symmetric ◽  
Junction Conditions ◽  
Dynamical Equations ◽  
Constant Energy ◽  
Isotropic Pressure ◽  
Symmetric Star ◽  
Constant Energy Density ◽  
General Influence

We consider the modified f(R) theory of gravity whose higher-order curvature terms are interpreted as a gravitational fluid or dark source. The gravitational collapse of a spherically symmetric star, made up of locally anisotropic viscous fluid, is studied under the general influence of the curvature fluid. Dynamical equations and junction conditions are modified in the context of f(R) dark energy and by taking into account the expansionfree evolution of the self-gravitating fluid. As a particular example, the Skripkin model is investigated which corresponds to isotropic pressure with constant energy density. The results are compared with corresponding results in General Relativity.

scholarly journals Cosmological model with fermion and tachyon fields interacting via Yukawa-type potential

2016 ◽  
Vol 31 (06) ◽  
pp. 1650039 ◽  
Author(s):  
Marlos O. Ribas ◽  
Fernando P. Devecchi ◽  
Gilberto M. Kremer
Keyword(s):  
Dark Energy ◽  
Cosmological Model ◽  
Matter Field ◽  
Fermionic Field ◽  
Tachyonic Field ◽  
Fermionic Fields ◽  
The Universe ◽  
Type Potential

A model for the universe with tachyonic and fermionic fields interacting through a Yukawa-type potential is investigated. It is shown that the tachyonic field answers for the initial accelerated regime and for the subsequent decelerated regime so that it behaves as an inflaton at early times and as a matter field at intermediate times, while the fermionic field has the role of a dark energy constituent, since it leads to an accelerated regime at later times. The interaction between the fields via a Yukawa-type potential controls the duration of the decelerated era, since a stronger coupling makes a shorter decelerated period.

scholarly journals Observational Role of Dark Matter in f(R) Models for Structure Formation

2018 ◽  
Vol 46 ◽  
pp. 1860045
Author(s):  
Murli Manohar Verma ◽  
Bal Krishna Yadav
Keyword(s):  
Dynamical System ◽  
Dark Matter ◽  
Fixed Points ◽  
Gravity Models ◽  
Stability Conditions ◽  
Cosmological Observations ◽  
General Relativistic ◽  
The Stability ◽  
The Universe

The fixed points for the dynamical system in the phase space have been calculated with dark matter in the [Formula: see text] gravity models. The stability conditions of these fixed points are obtained in the ongoing accelerated phase of the universe, and the values of the Hubble parameter and Ricci scalar are obtained for various evolutionary stages of the universe. We present a range of some modifications of general relativistic action consistent with the [Formula: see text]CDM model. We elaborate upon the fact that the upcoming cosmological observations would further constrain the bounds on the possible forms of [Formula: see text] with greater precision that could in turn constrain the search for dark matter in colliders.

Halal food export and Malaysia’s potential: the applicability of the gravity theory of trade

2020 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Md. Yunus Ali ◽  
Puteri Zahrah Aminan Abdul Ghaffar ◽  
Shahriar Kabir ◽  
Sa'adiah Munir
Keyword(s):  
Gross Domestic Product ◽  
Design Methodology ◽  
Gravity Theory ◽  
Content Type ◽  
Trading Partners ◽  
Food Market ◽  
Halal Food ◽  
Food Commodities ◽  
Behavioural Aspects

Purpose The gravity theory of trade explains the potential for trade between nations, but its application to trade in halal food has been questioned by previous studies. This study aims to investigate this issue and the role of trading partners’ economic strength and their distance from one another to identify Malaysia’s potential to export food to key halal markets. Design/methodology/approach The gravity theory of trade was used to examine Malaysia’s top 10 food exports to key halal markets from 2000–2017. The gravity panels were estimated using the Hausman-Taylor modelling technique to control for endogeneity within the model. Findings The application of the gravity theory of trade to a halal market context provides mixed results. Although the high economic strength (gross domestic product) of the trading partners enhances halal trade, the distance between the partners does not affect the volume of halal food exports. Moreover, the study identifies Malaysia’s potential to export only a few food commodities to key halal markets. Originality/value This study challenges the applicability of the gravity theory of trade to the halal food market. The study extends the model with additional controls for behavioural aspects and applies it to commodity-specific segregated trade in halal food. The findings underscore the need to extend the theory beyond its current focus when explaining trade opportunities in halal markets.

scholarly journals Globalization and export flows between Eurasian Economic Union countries: a gravity model approach

2020 ◽  
Vol 74 ◽  
pp. 06001
Author(s):  
Juliet Abakumova ◽  
Olena Primierova
Keyword(s):  
Gravity Model ◽  
Positive Impact ◽  
Gravity Models ◽  
Trade Integration ◽  
Transport Costs ◽  
Integration Model ◽  
Time Estimates ◽  
Eurasian Economic Union ◽  
Model Approach

In the empirical tools for the research of trade integration a special place is occupied by gravity models, insofar as these models have rather high accuracy in explaining mutual trade flows. Recently, however, the gravity model approach has been subjected to critical rethinking: globalization brought considerable changes not only in economic growth, but also in international trade, what allowed speaking about a “Death of Distance”. At the same time, estimates based on gravity models almost always demonstrated an increase in the coefficient of distance as a proxy for transport costs, which contradicts the general perception of the phenomenon of globalization. The paper is devoted to testing the validity of inclusion of the globalization index in the model, which would allow consider the role of globalization in bilateral cross-country trade, as well as testing the hypothesis of reducing the coefficient of distance. Based on the annual panel data over the period 2000-2016 the trade integration model for the EEU countries was estimated. To test the hypothesis of a decrease in the estimated coefficient of distance over time, the gravity model was also evaluated at different time intervals. And the positive impact of the globalization factor on the volume of exports is revealed.

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