Gravity Model Research Articles

Causal Structure in Spin Foams

The metric field of general relativity is almost fully determined by its causal structure. Yet, in spin foam models of quantum gravity, the role played by the causal structure is still largely unexplored. The goal of this paper is to clarify how causality is encoded in such models. The quest unveils the physical meaning of the orientation of the two-complex and its role as a dynamical variable. We propose a causal version of the EPRL spin foam model and discuss the role of the causal structure in the reconstruction of a semiclassical space–time geometry.

Changes in Economic Network Patterns and Influencing Factors in the Urban Agglomeration of Guangdong–Hong Kong–Macao Greater Bay Area: A Comprehensive Study

The aim of this study is to comprehensively evaluate the economic network patterns and their influencing factors of the Guangdong–Hong Kong–Macao Greater Bay Area (GHMGBA) in China, considering population scale, economic development level, and land-resource endowment. By employing a modified gravity model and a social-network method, we quantitatively analyzed urban agglomeration integrity indices, such as network density, edge–core structure, cohesive-subgroup index, and urban individual index (e.g., centrality degree) of this region, encompassing nine cities in Guangdong Province and two special administrative regions. The results revealed significant changes in the economic network patterns within the GHMGBA over time. Furthermore, the quadratic assignment procedure correlation analysis index was used to identify the various factors affecting the strength of the economic interaction. The findings demonstrated an annual increase in the strength of economic interaction between cities and regions within the GHMGBA over the past 20 years, along with the emergence of a polycentric economic development pattern. The results also suggest that the spatial location and level of economic development are key determinants influencing the strength of economic linkages in this area. This study supports the conclusion that deepening exchanges and cooperation among core cities, bolstering economic development in sub-core cities, and facilitating the construction of an integrated regional transportation network will expedite the process of economic integration.

Brexit and foreign students in gravity

AbstractThis paper examines the impact of Brexit on international student migration. In a structural gravity model, we estimate student migration between 69 countries for counterfactual scenarios in which the United Kingdom leaves the European Union one year before the referendum. This exercise reveals a decrease in exchange students studying in the UK of around 3.8% to 4.9%. While the number of non‐EU students to the UK rises, a drop in EU student numbers drives this result. Similarly, 30% to 38% fewer UK students choose to study abroad. The estimated changes in international student stocks show that most other member countries lose international students and non‐EU countries host more than without Brexit. Our findings provide evidence that there may be hidden costs to Brexit affecting global student exchanges that we have yet to see.

EXPORT DEMAND FOR THAILAND'S KING OF FRUIT

This paper aims to find the determinant of Thailand’s king of fruit exports based on the gravity model. Several primary econometrics methods apply to the gravity model applying to Thailand’s durian export. The result indicates that the country-pair and time-fixed effect PML with interaction regarding the landlock and time dummy variables give the most logical results.  The empirical results suggest that enhancing labor productivity in durian cultivation is a strategy to increase durian cultivation and export. Durian exporters might use the lower price strategy to enhance their income. Thailand’s durian exporters should find a specific country in each income-level group where most people love durian.

Constructing massive particles surfaces in static spacetimes

Massive particle surface is a generalization of the photon surface, both of which translate the dynamical properties of geodesics orbiting the gravitating object onto the geometric description. The procedure for constructing the massive particle surfaces in static space-times is described in detail and the equivalence of the main results with the results of the geodesic approach is demonstrated for the following examples: Schwarzschild, Reissner–Nordström, dilatonic black holes, conformal gravity, and Culetu model.

Dark energy nature of logarithmic f(R, Lm)-Gravity models with Observational Constraints

Dark energy nature of logarithmic <i>f(R, L_m)</i>-Gravity models with Observational Constraints

COVID-19 Impacts on the Spatial Network of City Tourism in Fujian Province, China

This study examined the tourism spatial distribution of nine cities in the Fujian province and assessed the impacts of COVID-19. The modified gravity model found that it was widely dispersed, with uneven and relatively independent tourism development in different cities. The social network analysis showed that tourism connections across cities were significantly reduced after the pandemic. The impacts of brand awareness and transport accessibility on spatial networks were positive in the pre-pandemic period but became negative during the pandemic. In contrast, tourist volume had negative impacts on spatial networks pre-pandemic but had positive ones during the pandemic. Tourism resources and market performance had significantly positive impacts in the post-pandemic era. These findings provide advice on tourism recovery and destination management in coping with future critical events. In the spatial distribution network of the tourism economy in Fujian Province, cities have different positions and roles, so development strategies should be differentiated according to the characteristics of each city. For example, more supportive policies should be introduced to help the tourism development of cities with disadvantages. It also contributes to the theoretical gravity framework in tourism and the research scope of the social networks analysis at the city level.

A High-Finesse Suspended Interferometric Sensor for Macroscopic Quantum Mechanics with Femtometre Sensitivity.

We present an interferometric sensor for investigating macroscopic quantum mechanics on a table-top scale. The sensor consists of a pair of suspended optical cavities with finesse over 350,000 comprising 10 g fused silica mirrors. The interferometer is suspended by a four-stage, light, in-vacuum suspension with three common stages, which allows for us to suppress common-mode motion at low frequency. The seismic noise is further suppressed by an active isolation scheme, which reduces the input motion to the suspension point by up to an order of magnitude starting from 0.7 Hz. In the current room-temperature operation, we achieve a peak sensitivity of 0.5 fm/Hz in the acoustic frequency band, limited by a combination of readout noise and suspension thermal noise. Additional improvements of the readout electronics and suspension parameters will enable us to reach the quantum radiation pressure noise. Such a sensor can eventually be utilized for demonstrating macroscopic entanglement and for testing semi-classical and quantum gravity models.

Analysis of the Spatial Correlation Network and Driving Mechanism of China’s Transportation Carbon Emission Intensity

From 2008 to 2021, this study analyzed the spatial correlation characteristics between provincial transportation carbon emission intensity and explored ways to reduce transportation carbon emissions. This study used the modified gravity model, social network analysis (SNA) method, and temporal exponential random graph model (TERGM) to analyze the spatial correlation network evolution characteristics and driving mechanism of China’s transportation carbon emission intensity. This study found that China’s transportation carbon emission intensity and spatial correlation network have unbalanced characteristics. The spatial correlation network of transportation carbon emission intensity revealed that Shanghai, Beijing, Tianjin, Guangdong, Fujian, and other provinces were at the center of the network, with significant intermediary effects. The spatial correlation of transportation carbon emission intensity was divided into four functional plates: “two-way spillover”, “net benefit”, “broker”, and “net spillover”. The “net benefit” plate was mainly located in developed regions, and the “net spillover” plate was primarily located in underdeveloped regions. Endogenous structural and exogenous mechanism variables were the main factors affecting the evolution of the spatial correlation network of provincial transportation carbon emission intensity.

Electrical Properties at Multi-Frequencies for Analysis of Physical and Anatomical Properties of Fast-Growing Standing Teak Trees at Various Ages

Fast-growing teak trees are cultivated extensively in Indonesia to meet the growing demand for teak wood. However, it is necessary to assess the conditions of teak stands throughout their growth period. The nondestructive testing of wood utilizing dielectric spectroscopy approaches based on electrical properties is currently under development, particularly for evaluating tree stands. This study aimed to analyze the dielectric values of fast-growing teak tree stands within a frequency range of 250 kHz to 60 MHz and to understand the relationship between their physical and anatomical properties. A capacitance measurement system was employed to collect dielectric spectroscopy data directly from trees aged 4, 5, and 7 years. Simultaneously, physical and anatomical samples were obtained using a 0.5 cm diameter increment borer. The results revealed significant differences in the fiber length, lumen diameter, and wall thickness at each age. The optimal dielectric frequency for distinguishing wood properties in standing trees was identified to be within a range of 18 MHz to 23 MHz. In the linear model, a moderate relationship was observed with a correlation coefficient of (r)0.403, although the coefficient of determination (r2) was weak at 0.162 for green density. However, a robust relationship was observed in the linear model for specific gravity with r = 0.826 and r2 = 0.682. A weak but significant relationship was also identified with r = 0.2, a coefficient of determination of r2 = 0.04, and a significance level &lt; 0.05 in the predictive model of wood anatomy properties (vessel diameter and fiber wall thickness). Models with low r2 but high significance indicate that the independent variables still noticeably contribute to explaining the dependent variable. Further analysis and data processing can be enhanced by identifying the crucial variables in the capacitance measurement system.

A New 2D Formulation of Modified General Relativity

AbstractIt is well known that the Einstein–Hilbert action in two dimensions is topological and yields an identically vanishing Einstein tensor. Consequently one is faced with difficulties when formulating a non‐trivial gravity model. The authors present a new, intrinsically two‐dimensional, approach to this problem based on the Einstein action. This yields a well defined variational approach which results in new field equations that break diffeomorphism invariance. This proposed approach does not require the introduction of additional scalar fields, nor the use of conformal transformations. However, how including conformal counter terms leads to equivalent results is shown. In doing so, an explanation for why previous approaches worked can be provided. Solutions to the field equations are briefly discussed.

How business environment of countries along the Belt and Road impacts China’s OFDI efficiency: a stochastic frontier gravity model approach

The Belt and Road Initiative (BRI) provides a new opportunity for the continued deepening of economic and trade cooperation between China and countries along the BRI route. However, different business environments among countries have led to the problem of a low and uneven distribution of China’s overall trade and investment efficiency in the countries along the route. Using the stochastic frontier gravity model, this study evaluates the efficiency of China’s outward foreign direct investment (OFDI) based on the data of 47 countries along the Belt and Road route from 2013 to 2019. The empirical results indicate that the efficiency level of China’s OFDI in countries located along the Belt and Road route is 43.39%, which suggests a regional imbalance. In terms of business environment factors, regulatory governance, civic discourse, government accountability, and regulatory quality in host countries have a positive impact on China’s OFDI. The positive effects of the BRI have enabled Chinese enterprises to better face factors such as political instability, corruption and imperfections in the legal system when investing abroad. The findings and suggestions could help the governments and enterprises of the countries along the route improve the business environment in a more targeted manner, enhance the space for economic and trade cooperation, and promote the common development of the countries along the route.

Determinants of Ethiopian sesame and coffee exports to its major trade partners: application of the gravity model

Determinants of Ethiopian sesame and coffee exports to its major trade partners: application of the gravity model

Assessing the spatial distributions of public electric vehicle charging stations with emphasis on equity considerations in King County, Washington

This study assessed the availability and accessibility of public electric vehicle (EV) charging stations at the census tract level in King County, Washington, USA. A gravity model was used to measure the accessibility of stations in each tract while considering the competition among tracts for charging infrastructure. Unlike prior studies, we evaluate current distribution from various equity viewpoints, clearly defining these perspectives and investigating their implications for charging distribution. The findings revealed significantly higher accessibility of public stations in areas with a lower share of single-family homes, a higher share of commercial areas, and proximity to major roadways, irrespective of population and EV adoption rates. Moreover, we found that the current distribution is consistent with utilitarian and capabilities-based theories of equity. Spatial tools such as Moran's I and spatial regression were used to analyze the spatial disparity of the proposed accessibility measure, showing the existence of a highly clustered spatial distribution. This study offers valuable insights into integrating equity considerations in assessing accessibility to transportation facilities, especially EV charging stations. Such an approach can facilitate the promotion of EV adoption across diverse groups, a critical factor in achieving sustainable city development goals by mitigating carbon emissions and enhancing mobility.

Peering Through the Fog of Uncertainty: Out-of-Sample Forecasts of Post-Pandemic Tourism

Abstract This paper uses an augmented gravity model framework to investigate the historical impact of infectious diseases on international tourism and develops an out-of-sample prediction model. Using bilateral tourism flows among 38,184 pairs of countries during the period 1995–2017, I compare the forecasting performance of alternative specifications and estimation methods. These computations confirm the statistical and economic significance of infectious-disease episodes in forecasting international tourism flows. Including infectious diseases in the model brings a significant improvement in forecast accuracy relative to the standard gravity model. The magnitude of these effects, however, is likely to be much greater in the case of COVID-19, which is a highly contagious virus that has spread fast throughout populations across the world.

Climate, conflict and internal migration in Colombia

Robust empirical evidence on the potential causal linkages between environmental change, conflict and migration is scarce. We evaluate this relationship in the context of internal migration in Colombia for the period from 2000 to 2005. Using municipalitylevel data in a gravity model that considers the issue of endogenous selection regarding both the outbreak of conflicts and the existence of non-zero migration flows, we establish an empirical causal link between droughts, conflict and migration. Our results show a positive relationship between the severity of droughts and the likelihood of conflicts, as well as between conflicts and human mobility, suggesting an indirect effect of climate on internal migration in Colombia.

Emergent modified gravity

A complete canonical formulation of general covariance makes it possible to construct new modified theories of gravity that are not of higher-curvature form, as shown here in a spherically symmetric setting. The usual uniqueness theorems are evaded by using a crucial and novel ingredient, allowing for fundamental fields of gravity distinct from an emergent space-time metric that provides a geometrical structure to all solutions. As specific examples, there are new expansion-shear couplings in cosmological models, a form of modified Newtonian dynamics can appear in a space-time covariant theory without introducing extra fields, and related effects help to make effective models of canonical quantum gravity fully consistent with general covariance.

Integrating gravity models and network analysis in logistical strategic planning: a case of the India Middle-East Europe Economic Corridor (IMEC)

Integrating gravity models and network analysis in logistical strategic planning: a case of the India Middle-East Europe Economic Corridor (IMEC)

The Permian Cornubian granite batholith, SW England; Part 1: Field, structural, and petrological constraints

The Permian Cornubian granite batholith (295−275 Ma) in SW England includes seven major plutons and numerous smaller stocks extending for ∼250 km from the Isles of Scilly in the WSW to Dartmoor in the ENE. The granites are peraluminous and classified as crustal melt S-type, predominantly two-mica granites, and biotite or tourmaline monzo- and syenogranites, with subordinate minor topaz granite and lithium mica granite. The granites and their host rocks are pervasively mineralized with tin (cassiterite), tungsten (wolframite, ferberite), copper (chalcopyrite, chalcocite, bornite), arsenic (arsenopyrite), and zinc (sphalerite) mineralized lodes. Quartz-muscovite selvedges (greisen-bordered) also contain enrichment of lithophile elements such as boron (tourmaline), fluorine (fluorite), and lithium (lithium-micas such as lepidolite and zinnwaldite). They are derived from both muscovite and biotite dehydration melting of pelitic-psammitic rocks and intruded from a common source along the length of the batholith. Pressure estimates from andalusite and cordierite-bearing hornfels in the contact metamorphic aureole (150 ± 100 MPa) show that the granites intruded to 3 km depth. Cupolas around the Land’s End and Tregonning granites show aplite-pegmatite dikes and tourmaline + quartz + muscovite veins (greisen) that are frequently mineralized. Synchronous intrusions of lamprophyre dikes suggest an additional heat source for crustal melting may have been from underplating of alkaline magmas. The lack of significant erosion means that the source region is not exposed. In an accompanying paper (Part 2; Watts et al., 2024), gravity modeling reveals possible solutions for the shape and depth of the granite and the structure of the lower crust. We present a new model for the Land’s End, Tregonning, and Carnmenellis granites showing a mid-crustal source composed of amphibolite facies migmatites bounded by prominent seismic reflectors, with upward expanding dikes feeding inter-connected granite laccoliths that show inflated cupolas with shallow contact metamorphism. The Cornubian granites intruded &amp;gt;90 m.y. after obduction of the Lizard ophiolite complex, and after Upper Devonian−Carboniferous Variscan compressional, and later extensional, deformation of the surrounding Devonian country rocks. Comparisons are made between the Cornubian batholith and the Patagonian batholith in Chile, the Himalayan leucogranites, and the Baltoro granite batholith along the Karakoram range in northern Pakistan.

Dynamical analysis of coupled curvature-matter scenario in viable f(R) dark energy models at de Sitter phase

We explore the interaction between dark matter and curvature-driven dark energy within viable f(R) gravity models, employing the phase-space analysis approach of linear stability theory. By incorporating an interacting term, denoted as Q=αHρ~m(κ23H2ρcurv+1) , into the continuity equations of both sectors, we examine dynamics of two f(R) gravity models that adhere to local gravity constraints and fulfill cosmological viability criteria. In de Sitter phase, our investigation reveals modifications of critical points compared to the conventional form, attributed to the introduced interaction. Through a comprehensive phase-space analysis, we illustrate the trajectories near critical points and outline the constraints on the phase space based on cosmological and cosmographic parameters. Furthermore, we can also observe stable behavior of this interacting system during the de Sitter phase of the Universe.