SSSara Saghafi
Sara Saghafi
*This is an algorithmically generated author record based on publicly available information.
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Sara Saghafi has a total of 34 co-authors. Here are the top co-authors by number of shared publications:
- Kourosh Nozari
University of Mazandaran
28 shared publications - Milad Hajebrahimi
Department of Theoretical Physics Faculty of Science University of Mazandaran P. O. Box 47416-95447 Babolsar Iran
6 shared publications Kourosh Nozari
3 shared publications- Ali Seidabadi
University of Mazandaran
2 shared publications - Fatemeh Aliyan
University of Mazandaran
2 shared publications K. Nozari
2 shared publications
Latest Publications
- Open Access
- 6 November 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (11) , 1273
Effective reheating in Gauss–Bonnet inflation with $\mu (\phi ,X)$ couplingWe study effective reheating in a scalar–Gauss–Bonnet inflationary model with a phase-space-dependent coupling $$\mu (\phi ,X),$$ in which a compact field-space feature is combined with a bounded kinetic gate. The modified inflationary background determines the pivot-scale quantities and the effective energy density at the end of inflation. These quantities are then used to derive the reheating duration $$N_{\textrm{re}}$$ and temperature $$T_{\textrm{re}}$$ through the thermal-history matching relation. We first perform two fixed-pivot reference scans by varying the overall Gauss–Bonnet strength $$\lambda _{\textrm{GB}}$$ and the kinetic-gate amplitude $$g_{_X}$$ separately. For the reference parameter choices, increasing either parameter increases $$N_{\textrm{re}}$$ and decreases $$T_{\textrm{re}}$$ for the selected reheating equations of state. Additional benchmark calculations clarify how these variations depend on the dynamical regime of the model. In the $$\lambda _{\textrm{GB}}$$ scan, the increase in $$N_{\textrm{re}}$$ and the decrease in $$T_{\textrm{re}}$$ persist, although both variations become strongly suppressed when the coupling is more localized or when the end of inflation is controlled more strongly by the E-model potential. In the $$g_{_X}$$ scan, stronger field-space localization and kinetic saturation can instead lead to a slight decrease in $$N_{\textrm{re}}$$ and an increase in $$T_{\textrm{re}}$$ as $$g_{_X}$$ is increased. When the bounded kinetic contribution is considered together with a weaker overall Gauss–Bonnet interaction, the resulting changes in the reheating quantities become nearly negligible. The fixed-pivot predictions of the representative and alternative benchmarks are compared with CMB constraints. These reheating constraints are then discussed for four representative values of the effective equation-of-state parameter, $$\overline{w}_{\textrm{re}}=-1/3,0,2/3,$$ and 1. Finally, the analysis is generalized to a time-dependent reheating equation of state $$w_{\textrm{re}}(N),$$ showing that the thermal-history matching relations depend only on its e-fold average $$\overline{w}_{\textrm{re}},$$ while the detailed evolution of $$w_{\textrm{re}}(N)$$ remains degenerate within the effective framework adopted here.- 6 November 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (11) , 1273
Open AccessEffective reheating in Gauss–Bonnet inflation with $\mu (\phi ,X)$ couplingWe study effective reheating in a scalar–Gauss–Bonnet inflationary model with a phase-space-dependent coupling $$\mu (\phi ,X),$$ in which a compact field-space feature is combined with a bounded kinetic gate. The modified inflationary background determines the pivot-scale quantities and the effective energy density at the end of inflation. These quantities are then used to derive the reheating duration $$N_{\textrm{re}}$$ and temperature $$T_{\textrm{re}}$$ through the thermal-history matching relation. We first perform two fixed-pivot reference scans by varying the overall Gauss–Bonnet strength $$\lambda _{\textrm{GB}}$$ and the kinetic-gate amplitude $$g_{_X}$$ separately. For the reference parameter choices, increasing either parameter increases $$N_{\textrm{re}}$$ and decreases $$T_{\textrm{re}}$$ for the selected reheating equations of state. Additional benchmark calculations clarify how these variations depend on the dynamical regime of the model. In the $$\lambda _{\textrm{GB}}$$ scan, the increase in $$N_{\textrm{re}}$$ and the decrease in $$T_{\textrm{re}}$$ persist, although both variations become strongly suppressed when the coupling is more localized or when the end of inflation is controlled more strongly by the E-model potential. In the $$g_{_X}$$ scan, stronger field-space localization and kinetic saturation can instead lead to a slight decrease in $$N_{\textrm{re}}$$ and an increase in $$T_{\textrm{re}}$$ as $$g_{_X}$$ is increased. When the bounded kinetic contribution is considered together with a weaker overall Gauss–Bonnet interaction, the resulting changes in the reheating quantities become nearly negligible. The fixed-pivot predictions of the representative and alternative benchmarks are compared with CMB constraints. These reheating constraints are then discussed for four representative values of the effective equation-of-state parameter, $$\overline{w}_{\textrm{re}}=-1/3,0,2/3,$$ and 1. Finally, the analysis is generalized to a time-dependent reheating equation of state $$w_{\textrm{re}}(N),$$ showing that the thermal-history matching relations depend only on its e-fold average $$\overline{w}_{\textrm{re}},$$ while the detailed evolution of $$w_{\textrm{re}}(N)$$ remains degenerate within the effective framework adopted here. - 10 October 2026
- Published by Elsevier in Annals of Physics
- Vol. 495
Phenomenological rotating extension of black holes with primary scalar hair: Shadow signatures in beyond Horndeski gravity
- 10 October 2026
- Published by Elsevier in Annals of Physics
- Vol. 495
Phenomenological rotating extension of black holes with primary scalar hair: Shadow signatures in beyond Horndeski gravity
- Open Access
- 12 August 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (8) , 964
Probing quantum gravity through chaotic orbits and strong-field effects in Kerr black holes embedded in perfect fluid dark matter
We study the nonlinear photon dynamics in quantum improved rotating black hole surrounded by perfect fluid dark matter (PFDM) using several methods of analysis, including Poincaré sections, Lyapunov exponents, Kolmogorov–Sinai (KS) entropy and weighted Birkhoff averages (WBA). In particular, we examine the effect of quantum-improved parameter $$(\tilde{\omega })$$ and PFDM parameter $$(\zeta )$$ on the null geodesic motion hence stability of circular orbit. Poincaré sections illustrate the transition from regular to chaotic motion as these parameters increase, characterized by the deformation and fragmentation of invariant tori and the emergence of scattered chaotic regions in phase space. The stability properties of null circular orbits are quantified through the Lyapunov indicators, revealing that both the quantum improvement parameter and the PFDM parameter enhance the sensitivity of photon trajectories to initial conditions. The KS entropy provides an independent measure of dynamical complexity and confirms the growth of chaotic behavior with increasing quantum and PFDM corrections. Additionally, the WBA method offers a robust quantitative criterion for distinguishing regular and chaotic orbits and allows a detailed mapping of the phase-space structure. The results demonstrate that the combined effects of quantum gravity corrections and PFDM significantly modify the effective potential governing photon motion, leading to a rich mixed phase-space structure with coexisting regular and chaotic regions. These findings underscore the crucial role of quantum and dark matter contributions in shaping photon dynamics near rotating black holes and suggest possible observational implications on black hole shadows and gravitational lensing in strong-field regimes.- 12 August 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (8) , 964
Open AccessProbing quantum gravity through chaotic orbits and strong-field effects in Kerr black holes embedded in perfect fluid dark matter
We study the nonlinear photon dynamics in quantum improved rotating black hole surrounded by perfect fluid dark matter (PFDM) using several methods of analysis, including Poincaré sections, Lyapunov exponents, Kolmogorov–Sinai (KS) entropy and weighted Birkhoff averages (WBA). In particular, we examine the effect of quantum-improved parameter $$(\tilde{\omega })$$ and PFDM parameter $$(\zeta )$$ on the null geodesic motion hence stability of circular orbit. Poincaré sections illustrate the transition from regular to chaotic motion as these parameters increase, characterized by the deformation and fragmentation of invariant tori and the emergence of scattered chaotic regions in phase space. The stability properties of null circular orbits are quantified through the Lyapunov indicators, revealing that both the quantum improvement parameter and the PFDM parameter enhance the sensitivity of photon trajectories to initial conditions. The KS entropy provides an independent measure of dynamical complexity and confirms the growth of chaotic behavior with increasing quantum and PFDM corrections. Additionally, the WBA method offers a robust quantitative criterion for distinguishing regular and chaotic orbits and allows a detailed mapping of the phase-space structure. The results demonstrate that the combined effects of quantum gravity corrections and PFDM significantly modify the effective potential governing photon motion, leading to a rich mixed phase-space structure with coexisting regular and chaotic regions. These findings underscore the crucial role of quantum and dark matter contributions in shaping photon dynamics near rotating black holes and suggest possible observational implications on black hole shadows and gravitational lensing in strong-field regimes. - Open Access
- 12 June 2026
- Published by Elsevier in SSRN Electronic Journal
Rotating Black Holes with Primary Scalar Hair: Shadow Signatures in Beyond Horndeski Gravity
The Event Horizon Telescope (EHT) image of M87* provides a direct test of strong-field gravity, measuring an angular shadow diameter $\theta_{d}=42\pm 3~\mu\mathrm{as}$ and a circularity deviation $\Delta C\leq 0.1$. Such observations allow quantitative tests of the Kerr paradigm and of possible deviations from the no-hair theorem. In scalar-tensor extensions of gravity, black holes may possess primary scalar hair, introducing an additional independent parameter beyond mass and spin. In this work, we {\bf{construct a rotating configuration inspired by}} black hole {\bf{solutions}} with primary scalar hair in beyond Horndeski gravity and analyze their photon regions and shadow formation. We show that the scalar hair parameter $Q$ induces characteristic modifications of the shadow, and in particular negative $Q$ enlarges the shadow and reduces its oblateness, while positive $Q$ shrinks and enhances its distortion. {\bf{Adopting}} M87* {\bf{as a representative case}} within this framework and imposing the EHT bounds on $\theta_{d}$ and $\Delta C$, we {\bf{identify the viable}} $(a,Q)$ parameter space. We find that current observations do not exclude rotating black holes with primary scalar hair, although the allowed region is significantly restricted for $Q>0$. Finally, the scalar-hair-induced deviations are of order $\mathcal{O}(\mu\mathrm{as})$, placing them near the sensitivity threshold of present instruments and within reach of next-generation horizon-scale imaging.- 12 June 2026
- Published by Elsevier in SSRN Electronic Journal
Open AccessRotating Black Holes with Primary Scalar Hair: Shadow Signatures in Beyond Horndeski Gravity
The Event Horizon Telescope (EHT) image of M87* provides a direct test of strong-field gravity, measuring an angular shadow diameter $\theta_{d}=42\pm 3~\mu\mathrm{as}$ and a circularity deviation $\Delta C\leq 0.1$. Such observations allow quantitative tests of the Kerr paradigm and of possible deviations from the no-hair theorem. In scalar-tensor extensions of gravity, black holes may possess primary scalar hair, introducing an additional independent parameter beyond mass and spin. In this work, we {\bf{construct a rotating configuration inspired by}} black hole {\bf{solutions}} with primary scalar hair in beyond Horndeski gravity and analyze their photon regions and shadow formation. We show that the scalar hair parameter $Q$ induces characteristic modifications of the shadow, and in particular negative $Q$ enlarges the shadow and reduces its oblateness, while positive $Q$ shrinks and enhances its distortion. {\bf{Adopting}} M87* {\bf{as a representative case}} within this framework and imposing the EHT bounds on $\theta_{d}$ and $\Delta C$, we {\bf{identify the viable}} $(a,Q)$ parameter space. We find that current observations do not exclude rotating black holes with primary scalar hair, although the allowed region is significantly restricted for $Q>0$. Finally, the scalar-hair-induced deviations are of order $\mathcal{O}(\mu\mathrm{as})$, placing them near the sensitivity threshold of present instruments and within reach of next-generation horizon-scale imaging. - 26 May 2026
- Published by Elsevier in Journal of High Energy Astrophysics
- Vol. 53
Thin accretion disks around rotating charged black holes in an effective higher-curvature spacetime
- 26 May 2026
- Published by Elsevier in Journal of High Energy Astrophysics
- Vol. 53
Thin accretion disks around rotating charged black holes in an effective higher-curvature spacetime
- Open Access
- 2 May 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (5) , 442
Beyond $f(\phi ){\mathcal {G}}$: Gauss–Bonnet inflation with $\mu (\phi ,X)$Gauss–Bonnet inflation typically affects the dynamics over an extended portion of the trajectory, making it difficult to isolate a controlled imprint at CMB scales. We consider a trajectory-selective coupling $$\mu (\phi ,X)$$ that gates the Gauss–Bonnet sector in phase space, enabling the higher-curvature contribution to be localized within a finite e-fold window while remaining negligible elsewhere. We identify stable inflationary solutions consistent with this localization and enforce standard ghost and gradient stability conditions for both scalar and tensor perturbations. For these viable backgrounds we compute pivot-scale observables and examine their dependence on the overall Gauss–Bonnet strength and on the kinetic gating. The framework offers a controlled route for realizing localized higher-curvature effects with predictable consequences for CMB-scale measurements.- 2 May 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (5) , 442
Open AccessBeyond $f(\phi ){\mathcal {G}}$: Gauss–Bonnet inflation with $\mu (\phi ,X)$Gauss–Bonnet inflation typically affects the dynamics over an extended portion of the trajectory, making it difficult to isolate a controlled imprint at CMB scales. We consider a trajectory-selective coupling $$\mu (\phi ,X)$$ that gates the Gauss–Bonnet sector in phase space, enabling the higher-curvature contribution to be localized within a finite e-fold window while remaining negligible elsewhere. We identify stable inflationary solutions consistent with this localization and enforce standard ghost and gradient stability conditions for both scalar and tensor perturbations. For these viable backgrounds we compute pivot-scale observables and examine their dependence on the overall Gauss–Bonnet strength and on the kinetic gating. The framework offers a controlled route for realizing localized higher-curvature effects with predictable consequences for CMB-scale measurements. - Open Access
Rotating Black Holes with Primary Scalar Hair: Shadow Signatures in Beyond Horndeski Gravity
The Event Horizon Telescope (EHT) image of M87* provides a direct test of strong-field gravity, measuring an angular shadow diameter $θ_d = 42 \pm 3~μ\mathrm{as}$ and a circularity deviation $ΔC \leq 0.1$. Such observations allow quantitative tests of the Kerr paradigm and of possible deviations from the no-hair theorem. In scalar-tensor extensions of gravity, black holes may possess primary scalar hair, introducing an additional independent parameter beyond mass and spin. In this work, we construct rotating black hole solutions with primary scalar hair in beyond Horndeski gravity and analyze their photon regions and shadow formation. We show that the scalar hair parameter $Q$ induces characteristic modifications of the shadow, and in particular negative $Q$ enlarges the shadow and reduces its oblateness, while positive $Q$ shrinks and enhances its distortion. Modeling M87* within this framework and imposing the EHT bounds on $θ_d$ and $ΔC$, we determine the viable $(a,Q)$ parameter space. We find that current observations do not exclude rotating black holes with primary scalar hair, although the allowed region is significantly restricted for $Q>0$. Finally, the scalar-hair-induced deviations are of order $\mathcal{O}(μ\mathrm{as})$, placing them near the sensitivity threshold of present instruments and within reach of next-generation horizon-scale imaging.Open AccessRotating Black Holes with Primary Scalar Hair: Shadow Signatures in Beyond Horndeski Gravity
The Event Horizon Telescope (EHT) image of M87* provides a direct test of strong-field gravity, measuring an angular shadow diameter $θ_d = 42 \pm 3~μ\mathrm{as}$ and a circularity deviation $ΔC \leq 0.1$. Such observations allow quantitative tests of the Kerr paradigm and of possible deviations from the no-hair theorem. In scalar-tensor extensions of gravity, black holes may possess primary scalar hair, introducing an additional independent parameter beyond mass and spin. In this work, we construct rotating black hole solutions with primary scalar hair in beyond Horndeski gravity and analyze their photon regions and shadow formation. We show that the scalar hair parameter $Q$ induces characteristic modifications of the shadow, and in particular negative $Q$ enlarges the shadow and reduces its oblateness, while positive $Q$ shrinks and enhances its distortion. Modeling M87* within this framework and imposing the EHT bounds on $θ_d$ and $ΔC$, we determine the viable $(a,Q)$ parameter space. We find that current observations do not exclude rotating black holes with primary scalar hair, although the allowed region is significantly restricted for $Q>0$. Finally, the scalar-hair-induced deviations are of order $\mathcal{O}(μ\mathrm{as})$, placing them near the sensitivity threshold of present instruments and within reach of next-generation horizon-scale imaging. - 4 November 2025
- Published by World Scientific Pub Co Pte Ltd in International Journal of Geometric Methods in Modern Physics
- Vol. 23 (16)
Shadow of extreme compact charged objects in consistent 4-dimensional Einstein–Gauss–Bonnet gravity
In order to better describe gravitational phenomena on both very small and cosmological scales, there have been constant attempts to generalize and expand the theory of General Relativity (GR) since its inception. The Einstein–Gauss–Bonnet (EGB) theory is one such extension that adds spacetime corrections related to curvature. Since the standard Gauss–Bonnet term is purely topological, it does not contribute to the field equations in four dimensions. To get around this restriction, however, an invariant four-dimensional limit [ ] has been developed. In this work, we study Extreme Compact Charged Objects (ECCOs), which can resemble black holes, in a gravity framework that is compatible with Einstein–Gauss–Bonnet in four dimensions. Our main goal is to compare theoretical predictions with Event Horizon Telescope (EHT) observational data in order to constrain the Gauss–Bonnet coupling constant [ ]. In order to achieve this, we investigate important optical characteristics like the shadow, light-bending angle, and other associated observables, as well as the geodesic structure of ECCO spacetimes in EGB gravity. Finally, we apply these findings to constrain the Gauss–Bonnet constant.
- 4 November 2025
- Published by World Scientific Pub Co Pte Ltd in International Journal of Geometric Methods in Modern Physics
- Vol. 23 (16)
Shadow of extreme compact charged objects in consistent 4-dimensional Einstein–Gauss–Bonnet gravity
In order to better describe gravitational phenomena on both very small and cosmological scales, there have been constant attempts to generalize and expand the theory of General Relativity (GR) since its inception. The Einstein–Gauss–Bonnet (EGB) theory is one such extension that adds spacetime corrections related to curvature. Since the standard Gauss–Bonnet term is purely topological, it does not contribute to the field equations in four dimensions. To get around this restriction, however, an invariant four-dimensional limit [ ] has been developed. In this work, we study Extreme Compact Charged Objects (ECCOs), which can resemble black holes, in a gravity framework that is compatible with Einstein–Gauss–Bonnet in four dimensions. Our main goal is to compare theoretical predictions with Event Horizon Telescope (EHT) observational data in order to constrain the Gauss–Bonnet coupling constant [ ]. In order to achieve this, we investigate important optical characteristics like the shadow, light-bending angle, and other associated observables, as well as the geodesic structure of ECCO spacetimes in EGB gravity. Finally, we apply these findings to constrain the Gauss–Bonnet constant.
- Open Access
Accretion Process as a Probe of Extra Dimensions in MOG Compact Object Spacetimes
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.Open AccessAccretion Process as a Probe of Extra Dimensions in MOG Compact Object Spacetimes
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. - Open Access
Shadow of Extreme Compact Charged Objects in Consistent 4-Dimensional Einstein-Gauss-Bonnet Gravity
In order to better describe gravitational phenomena on both very small and cosmological scales, there have been constant attempts to generalize and expand the theory of General Relativity (GR) since its inception. The Einstein Gauss Bonnet (EGB) theory is one such extension that adds spacetime corrections related to curvature. Since the standard Gauss Bonnet term is purely topological, it does not contribute to the field equations in four dimensions. To get around this restriction, however, an invariant four dimensional limit has been developed. In this work, we study Extreme Compact Charged Objects (ECCOs), which can resemble black holes, in a gravity framework that is compatible with Einstein Gauss Bonnet in four dimensions. Our main goal is to compare theoretical predictions with Event Horizon Telescope (EHT) observational data in order to constrain the Gauss Bonnet coupling constant α. In order to achieve this, we investigate important optical characteristics like the shadow, light bending angle, and other associated observables, as well as the geodesic structure of ECCO spacetimes in EGB gravity. Finally, we apply these findings to constrain the Gauss Bonnet constant.
Open AccessShadow of Extreme Compact Charged Objects in Consistent 4-Dimensional Einstein-Gauss-Bonnet Gravity
In order to better describe gravitational phenomena on both very small and cosmological scales, there have been constant attempts to generalize and expand the theory of General Relativity (GR) since its inception. The Einstein Gauss Bonnet (EGB) theory is one such extension that adds spacetime corrections related to curvature. Since the standard Gauss Bonnet term is purely topological, it does not contribute to the field equations in four dimensions. To get around this restriction, however, an invariant four dimensional limit has been developed. In this work, we study Extreme Compact Charged Objects (ECCOs), which can resemble black holes, in a gravity framework that is compatible with Einstein Gauss Bonnet in four dimensions. Our main goal is to compare theoretical predictions with Event Horizon Telescope (EHT) observational data in order to constrain the Gauss Bonnet coupling constant α. In order to achieve this, we investigate important optical characteristics like the shadow, light bending angle, and other associated observables, as well as the geodesic structure of ECCO spacetimes in EGB gravity. Finally, we apply these findings to constrain the Gauss Bonnet constant.