Accretion Process as a Probe of Extra Dimensions in MOG Compact Object Spacetimes

TL;DR

This work analyzes accretion around a higher-dimensional, regular MOG compact object within Scalar-Tensor-Vector Gravity (STVG). By examining neutral test-particle motion and hydrodynamic accretion, it derives the effective potential, ISCO, radiant flux, and disk temperature, highlighting how extra dimensions (higher $D$) and the MOG parameter $\alpha$ jointly shape disk energetics. The results show that increasing $D$ generally shrinks ISCO and increases flux and $T_{\text{eff}}$, while larger $\alpha$ enhances gravity and radially outward disk emission but can suppress the flux depending on the regime. A comparison with EHT observations of Sgr A* suggests that the predicted deviations from the Schwarzschild case lie near current detectability, offering a potential avenue to constrain or reveal MOG+extra-dimensional effects with future high-precision VLBI data. The study also provides analytical expressions for the four-velocity and energy density of accreting perfect fluids, and demonstrates that the transonic (critical) structure of the flow is markedly affected by $\alpha$ and $D$, reinforcing the prospects for observational tests of modified gravity in higher dimensions.

Abstract

The idea of extra spatial dimensions arises from attempts to unify gravity with other fundamental interactions, develop a consistent theory of quantum gravity, and address open problems in particle physics and cosmology. Considerable attention has been devoted to understanding how such dimensions modify gravitational theories. One way to probe their impact is through the analytical study of astrophysical processes such as black hole accretion. Since accretion efficiently converts gravitational energy into radiation, this makes it a powerful tool to test modified gravity (MOG) theories and higher-dimensional frameworks via the behavior of dark compact objects like black holes, neutron stars, and white dwarfs. In this work, we investigate the dynamics of neutral particles around a higher-dimensional, regular, spherically symmetric MOG compact object, focusing on the innermost stable circular orbit (ISCO), energy flux, temperature, and differential luminosity. We further analyze the accretion of a perfect fluid onto the same object, deriving analytical expressions for the four-velocity and proper energy density of the inflowing matter. Our findings show that extra dimensions reduce the ISCO radius while enhancing the corresponding flux and temperature. Finally, by comparing the effective disk temperature $T_{\text{eff}}$ with Event Horizon Telescope (EHT) observations of Sgr A*, we argue that MOG and higher-dimensional corrections to the accretion disk properties could be close to the current threshold of detectability.

Figures (9)

• Figure 1: Plots of $E^2$ versus $r$ for different $\alpha$ values and spacetime dimensions. (a) illustrates $E^2$ for different values of $\alpha$ with fixed spacetime dimensions, and (b) shows the variation of $E^2$ for different spacetime dimensions with fixed $\alpha$.
• Figure 2: Plots of $L^2$ versus $r$ for different $\alpha$ values and spacetime dimensions. (a) illustrates $L^2$ for different values of $\alpha$ with fixed spacetime dimensions, and (b) shows the variation of $L^2$ for different spacetime dimensions with fixed $\alpha$.
• Figure 3: Plots of ${{\Omega_\varphi}}^2$ versus $r$ for different $\alpha$ values and spacetime dimensions. (a) illustrates ${\Omega_{\varphi}}^2$ for different values of $\alpha$ with fixed spacetime dimensions, and (b) shows the variation of ${\Omega_{\varphi}}^2$ for different spacetime dimensions with fixed $\alpha$.
• Figure 4: The behavior of $\mathcal{F}(r)$ versus $r$ for different values of $\alpha$ at $D=4$ and $D=5$. For the case $D=5$, the values of $\mathcal{F}(r)$ are multiplied by ${10}^{16}$.
• Figure 5: The behavior of $T_{eff}$ versus $r$ for different values of $\alpha$ at $D=4$ and $D=5$. For the case $D=5$, the values of $T$ are multiplied by ${10}^{5}$.