arXiv subjects

Mohsen Khodadi

Publications and source records attributed to Mohsen Khodadi.

At least 19 recordsLinked to original sources

arXiv2026

A New Window for Testing the PPN in the Strong-Field Regime: Stellar Rotation of S301

The recently discovered S301 star, with an orbital period of 8.7 years and eccentricity $e = 0.982$, reaches a pericenter of just $140\,R_{\mathrm{s}}$ --- approximately nine times closer than S2's $\sim 1200R_{\mathrm{s}}$ --- offering an unprecedented laboratory for testing strong-field gravity. We demonstrate that S301's stellar spin precession provides a novel probe of the PPN parameter $γ$, scaling as $(2γ_{\mathrm{PPN}} + 1)/3$ relative to general relativity (GR). For maximum projected velocity shifts of $46.1~\mathrm{km\,s^{-1}}$, next-generation spectrographs could constrain $γ_{\mathrm{PPN}}$ to $\sim 10\%$ precision. When combined with S301's orbital precession --- a substantial $\sim 1.95^\circ$ pericenter advance per orbit (GR prediction), approximately an order of magnitude larger than S2's $\sim 0.2^\circ$, constraining $(2β_{\mathrm{PPN}} + 2γ_{\mathrm{PPN}} - 1)/3$ --- the spin measurement breaks the $β$-$γ$ degeneracy. A MCMC analysis of projected S2 and S301 observations yields $σ_β\approx 2.3 \times 10^{-4}$, with the $γ$ precision limited by the spin precession measurement to $σ_γ\approx 0.1$. While the spin precession provides an independent consistency check, its true significance lies in probing gravity at $ϕ/c^2 \sim 10^{-4}$ and $v/c \sim 0.08$ --- four orders of magnitude stronger than Solar System tests and an order of magnitude stronger than current S-star constraints. This complementary approach opens a new window for testing the PPN framework in the strong-field regime and demonstrates the power of combining multiple stellar probes at the Galactic Center.

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arXiv2026

Non-minimally coupled Weyl connection gravity in the Solar System and at the Galactic Center

We explore the phenomenological viability of non-minimally coupled Weyl connection gravity by confronting its static, spherically symmetric black hole solutions with classical weak-field Solar System tests and stellar-orbit observations near Sgr A*. In this geometric framework, non-metricity is encoded via a Weyl vector field, giving rise to two distinct families of Schwarzschild-like vacuum solutions characterized by a free parameter $ω$ with dimensions of length. We compute the corrections to four classical observables---gravitational redshift, Mercury's perihelion advance, light deflection, and radar echo delay---and derive stringent lower bounds on $ω$ using current observational data. For Solution I (purely radial Weyl vector), the leading metric corrections scale as $1/ω$, yielding bounds as strong as $ω\gtrsim 10^{30}$ m from perihelion precession. For Solution II (time-radial Weyl vector), the corrections scale as $1/ω^2$, resulting in weaker constraints, with $ω\gtrsim 10^{20}$ m from the same test. This marked difference arises from the distinct behavior of the linear corrections in each solution: while Solution I exhibits an unsuppressed linear term $2r/ω$ that dominates in the Solar System regime, Solution II features a linear term suppressed by an additional factor of $M/ω$, making the quadratic term $-r^2/4ω^2$ dominant throughout. We then analyze stellar orbits near the Galactic Center, finding that current observations of the S2 star provide complementary constraints on both solutions. (...)

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arXiv2026

Atomic clocks and gravitational waves as probes of non-metricity

Non-metricity provides a natural extension of Riemannian geometry, yet its experimental signatures remain largely unexplored. In this work, we investigate how spacetime non-metricity can be probed through high-precision observations, focusing on atomic clocks and gravitational waves as complementary tools. Working within Weyl geometry as a minimal realization of vectorial non-metricity, we formulate observable effects in a gauge-invariant manner and show that they are associated with path-dependent length transport governed by the Weyl field strength. We derive constraints from atomic-clock experiments and demonstrate that, although gravitational waves do not directly source the Weyl field at linear order, its dynamical contribution induces a backreaction on gravitational-wave propagation, leading to an anomalous strain. As a result, the absence of deviations from General Relativity in current gravitational-wave observations already places meaningful and strong constraints on dynamical non-metric degrees of freedom, within the phenomenological classical framework considered.

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arXiv2026

Constraining the Tilted Dipole Cosmology with Primordial Nucleosynthesis and Baryogenesis

The tilted dipole cosmology extends the standard model by incorporating a preferred spatial direction and separate bulk velocities for matter and radiation, offering a potential explanation for observed large-scale bulk flows and the CMB dipole. We conduct a comprehensive analysis of this anisotropic framework using Big Bang Nucleosynthesis (BBN) and Gravitational Baryogenesis (GB) to impose stringent constraints on the radiation tilt parameter $β_r$, which quantifies the magnitude of the radiation bulk flow. By deriving the modified expansion rate $H(T)$ and its impact on light element abundances, we find that the combined $2σ$ BBN limits from primordial helium-4 and deuterium are $|β_r| \lesssim 0.03$ for maximal shear (tightest bound) and $|β_r| \lesssim 0.2$ for minimal shear (weakest bound). These bounds are consistent with, but tighter than, those inferred from the effective neutrino species count $|ΔN_{\mathrm{eff}}| \lesssim 0.4$. Importantly, both bounds are upper limits; $β_r = 0$ is always allowed by the BBN constraints. The lithium-7 problem persists, as the $β_r$ values required to resolve it are excluded by helium-4 data. Furthermore, GB -- operating at decoupling temperatures $T_D \gtrsim 10^{12}$ GeV under the scaling $β_r \propto T$ -- yields far more severe constraints, limiting $|β_r^{\mathrm{BBN}}| \lesssim 10^{-8}$ for such high-scale baryogenesis scenarios.

astro-ph.CO↗

arXiv2026

A winding number analysis of Schwarzschild black hole stability in light of Planck-scale modified kinematics

Determining whether Planck-scale effects can stabilize black holes addresses fundamental questions about black hole evaporation and quantum gravity consistency. Here, we analyze the thermodynamic topology of Schwarzschild black holes under Planck-scale modified kinematics, using a cubic entropy correction derived from a well-known phenomenological MDR with leading correction $ηE^3/E_P$. Enforcing physical constraints ($S'(r_h) > 0$, $T > 0$) via the entropy-geometry correspondence, we find a single unstable branch with $w = -1$ and $W = -1$ for both signs of the correction parameter. A second root suggesting stability ($w = +1$) is excluded due to negative mass/temperature and lies outside the perturbative regime. Thus, this class of MDRs does not yield stable Schwarzschild black holes. However, MDRs with different leading-order corrections may behave otherwise, leaving the search for Planck-scale stabilization an open endeavor.

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arXiv2026

Primordial black hole in Lorentz-violating theories: Insights from Bumblebee gravity

The Bumblebee gravity (BG) model, featuring spontaneous Lorentz symmetry breaking via a vector field non-minimally coupled to curvature, has been widely used to explore Lorentz-violating effects in cosmology. We investigate primordial black hole (PBH) formation within this framework, deriving the complete set of modified perturbation equations. We demonstrate that BG, sourced by a timelike vector field, introduces three distinct enhancements to PBH abundance--modified expansion history, suppressed collapse threshold, and amplified power spectrum--which together render PBHs viable dark matter candidates across the asteroid-mass window for modest Lorentz-violating couplings. However, a systematic analysis of the quadratic action reveals that these phenomenological consequences emerge from a theoretically pathological foundation. The vector sector exhibits an intrinsic ghost instability, while the requirement of a stable symmetry-breaking minimum simultaneously induces a tachyonic instability on timescales far below cosmological scales. The model thus suffers from a fundamental inconsistency: the conditions for cosmological viability and spontaneous symmetry breaking are mutually exclusive within the minimal Bumblebee framework. Our results illustrate both the notable power of Lorentz violation to influence early Universe observables and the necessity of a consistent theoretical foundation for such predictions.

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arXiv2026

Illuminating the dark universe in the multi-messenger era

The precision era of multi-messenger astronomy, together with modern astrophysical, cosmological, and gravitational wave observations, increasingly points toward the existence of a ``dark" sector that cannot be explained within the framework of the Standard Model of particle physics and General Relativity. In this review, we explore extensions of standard physics and examine how observational data can be used to probe new particles and interactions. We consider a wide range of scales, from Solar System tests to galactic and cosmological observations, and investigate both conventional dark matter candidates, such as weakly interacting massive particles, and alternative scenarios including ultralight fields and primordial black holes. We discuss constraints derived from compact objects such as neutron stars, black holes, pulsars, and magnetars observations as well as from high-energy astrophysical phenomena. In addition, we analyze extensions of General Relativity involving additional scalar fields and their impact on gravitational wave signals and stochastic backgrounds from primordial black holes. We also study the capture and accumulation of dark matter in compact objects, which can alter properties such as mass, radius, and tidal deformability, and consider scenarios in which dark matter decays into Standard Model particles. While current observations already place significant limits on dark matter and modified-gravity models, upcoming experiments and observatories are expected to further probe or discover such new physics by improving constraints on particle masses and interaction strengths.

astro-ph.CO↗

arXiv2026

Multi-probe analysis of strong-field effects in $f(Q)$ gravity

Covariant $f(Q)$ gravity is a viable extension of General Relativity, however its strong-field predictions remain largely untested. Using the static, spherically symmetric black-hole solutions of the theory, we confront it with the most stringent probes available: black-hole shadows, Event Horizon Telescope (EHT) measurements, S2-star precession, and strong gravitational lensing. We show that the two admissible solution branches behave very differently: Case~I produces negligible deviations from Schwarzschild solution, whereas Case~II yields significant, potentially observable corrections to the photon sphere and shadow size. From the EHT shadow diameters of M87* and Sgr~A*, we obtain tight bounds, which are further strengthened by strong-lensing coefficients. These results provide the sharpest strong-field constraints on covariant $f(Q)$ gravity to date, and point toward future tests using next-generation horizon-scale imaging and precision Galactic-center astrometry.

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arXiv2026

Pathology of the Unified Dark Sectors in Modified General Relativity

This paper presents a comprehensive stability analysis of the black hole solution within Modified General Relativity (MGR), a theory proposing a unified geometric description of dark matter (DM) and dark energy (DE). A rigorous gauge-invariant formalism is employed to analyze gravitational perturbations of the extended Schwarzschild metric. The central finding is a critical pathology within the polar perturbation sector, where metric fluctuations couple to the theory's fundamental line element field. This coupling is governed by a factor that, while well-behaved at the horizon, diverges powerfully in the far-field limit as a direct consequence of the theory's non-asymptotically flat nature. This indicates a strong infrared instability that overwhelms perturbations at large distances. In stark contrast, the axial perturbation sector is found to be completely stable. This dichotomy proves that the instability is not inherent to the background metric but is specifically generated by the novel coupling mechanism encoding MGR's unified dark sectors. The results reveal a fundamental strong-coupling problem within the MGR framework, challenging its physical viability as an alternative to Einstein's General Relativity (EGR).

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arXiv2026

Primordial observables of explicit diffeomorphism violation in gravity

We investigate the potential for current and future gravitational-wave detectors to observe imprints of explicit diffeomorphism violation in primordial signals. Starting from a simple model with known effects, we derive the strain amplitude and power spectrum for primordial gravitational waves, both of which are affected by the symmetry breaking. Through this, we directly find predictions for the tensor spectral index and tensor-to-scalar which are different from general relativity. By considering the known sensitivity curves for NANOGrav, SKA, THEIA, $μ$-ARES, ASTROD-GW, LISA, BBO, DECIGO, CE, AION-km, AEDGE, ET, and aLIGO, we place observability limits on the parameters controlling the diffeomorphism violation. For instance, we find that aLIGO could observe signals for $s_{00} \lesssim -0.1$, while more sensitive future detectors like LISA and DECIGO could probe violations as small as $s_{00} \approx -5 \times 10^{-4}$ and $-3 \times 10^{-3}$, respectively. Finally, we consider the existing constraints on the number of relativistic degrees of freedom $ΔN_{\rm eff}$ which is tightly constrained by Big-Bang Nucleosynthesis (BBN) and we find that $ΔN_{\rm eff}$ only weakly depends on the symmetry breaking but places a lower bound on the coefficients which is consistent with available bounds from the speed of gravitational waves.

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arXiv2025

Confronting dark energy in Harada's Conformal Killing Gravity with observational data

Based on a comprehensive analysis of recent observational data-a combination of DESI DR1, Planck CMB, and Pantheon+ SN Ia-this study critically evaluates the two dark energy (DE) proposals within Harada's Conformal Killing Gravity (CKG) model. The model in question predicts either a dominant phantom-type effective DE component with EoS $ω= -5/3$ or a hybrid scenario combining a cosmological constant ($ω= -1$) with a subdominant $ω= -5/3$ fluid (around $5\%$) to address the Hubble tension (HT) and late-time acceleration. An analysis based on the Trans-Planckian Censorship Conjecture (TCC) demonstrates that the pure CKG fluid scenario $ω= -5/3$ is excluded, whereas the hybrid model remains only marginally compatible. Our Markov Chain Monte Carlo (MCMC) analysis constrains the effective DE density parameter to $Ω_{\text{eff}} = 0.009^{+0.006}_{-0.007}$ ($68\%$ CL), consistent with zero and ruling out the around $5\%$ contribution required by Harada's CKG. The resulting Hubble expansion history $H(z)$ and effective EoS $ω_{\text{eff}}(z)$ are indistinguishable from those of $Λ$CDM. Bayesian model comparison via the Akaike Information Criterion (AIC) shows no statistical preference for CKG over $Λ$CDM ($Δ\text{AIC} = +2.6$), disfavoring the additional complexity of the CKG model. The key output of this study is that both DE proposals in Harada's CKG are ruled out by current cosmological data, and HT remains unresolved.

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arXiv2025

The Weyl Geometric Gravity black hole in light of the Solar System tests

The Weyl geometric gravity theory, in which the gravitational action is constructed from the square of the Weyl curvature scalar and the strength of the Weyl vector, has been intensively investigated recently. The theory admits a scalar-vector-tensor representation, obtained by introducing an auxiliary scalar field, and can therefore be reformulated as a scalar-vector-tensor theory in a Riemann space, in the presence of a nonminimal coupling between the Ricci scalar and the scalar field. By assuming that the Weyl vector has only a radial component, an exact spherically symmetric vacuum solution of the field equations can be obtained, which depends on three integration constants. As compared to the Schwarzschild solution, the Weyl geometric gravity solution contains two new terms, linear and quadratic in the radial coordinate, respectively. In the present work we consider the possibility of testing and obtaining observational restrictions on the Weyl geometric gravity black hole at the scale of the Solar System, by considering six classical tests of general relativity (gravitational redshift, the Eötvös parameter and the universality of free fall, the Nortvedt effect, the planetary perihelion precession, the deflection of light by a compact object, and the radar echo delay effect, respectively) for the exact spherically symmetric black hole solution of the Weyl geometric gravity. All these gravitational effects can be fully explained and are consistent with the vacuum solution of the Weyl geometric gravity. Moreover, the study of the classical general relativistic tests also allows to constrain the free parameter of the solution.

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arXiv2025

Could regular primordial black holes be dark matter?

The recent proposal proposed by Paul Davies and colleagues [Phys. Rev. D \textbf{111} (2025) no.10, 103512] that regular primordial black holes (RPBHs) form stable, zero-temperature remnants and could thereby constitute dark matter is critically examined. While the introduction of a fundamental length scale indeed regulates the Hawking temperature, preventing its divergence, we show that the evaporation timescale for such RPBHs is infinite. This result holds generically for analytic regular black hole spacetimes under standard adiabatic and quasi-static evolution. Consequently, RPBHs never actually reach a true remnant state within any finite time, but instead persist as slowly evaporating objects with a non-zero luminosity. When the combined emission from a cosmological population of these near-remnants is considered, the resulting radiation is found to violate stringent observational constraints from the cosmic microwave background and extragalactic gamma-ray backgrounds. Therefore, low-mass RPBHs are not viable dark matter candidates.

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arXiv2025

Primordial gravitational waves from spontaneous Lorentz symmetry breaking

We study the effect of Spontaneous Lorentz Symmetry Breaking (SLSB) on Primordial Gravitational Waves (PGWs) generated during inflation. The SLSB is induced by a time-like Bumblebee vector field which is non-minimally coupled to the Ricci tensor in the Friedmann-Lemaître-Robertson-Walker background. The power spectrum and GW amplitude are computed to investigate how Lorentz violation leaves observable imprints. We calculate the GW strain amplitude over frequencies $(10^{-10}~\mathrm{Hz}, 10^4~\mathrm{Hz})$, for a range of the dimensionless Lorentz-violating parameter, $ -10^{-3} \leq l \leq 10^{-4} $, which essentially comes from a slight sensitivity to the equation of state for dark energy. For positive $ l $ values, the amplitude of GW shows a mild suppression compared to the standard cosmological scenario $( l = 0) $. This effect could be observable with detectors like SKA, $μ$-Ares, and BBO. Conversely, negative $ l $ values amplify the GW amplitude, enhancing detectability by both SKA, $μ$-Ares, and BBO, as well as by THEIA and DECIGO. Notably, the GW strain amplitude increases by an order of magnitude as $ l $ moves from 0 to $ -10^{-3} $, improving prospects for detection in high-sensitivity detectors like THEIA and DECIGO.

astro-ph.CO↗

arXiv2024

Event Horizon Telescope observations exclude compact objects in baseline mimetic gravity

Mimetic gravity has gained significant appeal in cosmological contexts, but static spherically symmetric space-times within the baseline theory are highly non-trivial: the two natural solutions are a naked singularity and a black hole space-time obtained through an appropriate gluing procedure. We study the shadow properties of these two objects, finding both to be pathological. In particular, the naked singularity does not cast a shadow, whereas the black hole casts a shadow which is too small. We argue that the Event Horizon Telescope images of M87$^{\star}$ and Sgr A$^{\star}$ rule out the baseline version of mimetic gravity, preventing the theory from successfully accounting for the dark sector on cosmological scales. Our results highlight an interesting complementarity between black hole imaging observations and modified gravity theories of cosmological interest.

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arXiv2024

A note on the stability of the Cauchy horizon in regular black holes without mass inflation

Unlike generic models of regular black holes (BHs) with nonzero surface gravity on both Cauchy and event horizons, an inner-degenerate counterpart with zero Cauchy horizon surface gravity was recently proposed. For this regular BH solution with spherical symmetry, we examine the stability of the Cauchy horizon from both classic and quantum mechanics viewpoints. We find that the classical perturbations do not lead to mass inflation at the Cauchy horizon, indicating classically stability, while quantum fluctuations cause it to be unstable quantum mechanically.

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arXiv2024

Anisotropic Generalized Polytropic Spheres: Regular 3D Black Holes

We model gravitating relativistic 3D spheres composed of an anisotropic fluid in which the radial and transverse components of the pressure correspond to the vacuum energy and a generalized polytropic equation-of-state, respectively. By using the generalized Tolman-Oppenheimer-Volkoff (TOV) equation, and solving the complete system of equations for these anisotropic generalized polytropic spheres, for a given range of model parameters, we find three novel classes of asymptotically AdS black hole solutions with regular core. We show the regularity of the solutions using curvature scalars and the formalism of geodesic completeness. Then, using the eigenvalues of the Riemann curvature tensor, we consider the effects of repulsive gravity in the three static 3D regular black holes, concluding that their regular behavior can be explained as due to the presence of repulsive gravity near the center of the objects. Finally, we study the stability of the regular black holes under the flow of the energy through the Cauchy horizon.

gr-qc↗

arXiv2023

Harvesting energy driven by Comisso-Asenjo process from Kerr-MOG black holes

Magnetic reconnection is a process that plays a critical role in plasma astrophysics by converting magnetic energy into plasma particle energy. Recently, Comisso and Asenjo demonstrated that rapid magnetic reconnection within a black hole's ergosphere can efficiently extract energy from a rotating black hole. In this paper, by considering a Kerr black hole in the MOdified gravity (MOG) framework, we investigate the impact of the MOG parameter $α$ on the rotational energy extraction via the Comisso-Asenjo process (CAP). To model energy extraction from supermassive black holes located in the center of galaxies, we set the value of $α$ within the range inferred from the recent observation of Sgr A* by the Event Horizon Telescope (EHT). Our results indicate that the Kerr-MOG black hole is a more efficient host for CAP-based rotational energy extraction compared to the Kerr black hole, since it amplifies the power of energy extraction and efficiency of the plasma energization process. We show that, from the energy extraction viewpoint, the CAP is more efficient than the Blandford-Znajek process (BZP). The latter is another magnetic field-based energy extraction model which is widely believed to be an engine for powering the high-energy astrophysics jets emerging from the supermassive black holes at active galactic nuclei. In particular, we show that the ratio of the energy extraction power of CAP to BZP in the presence of the MOG parameter is greater than that of the Kerr black hole. Our results promise this phenomenological message that the MOG-induced correction on the Kerr black hole background plays an important role in favor of energy extraction via the CAP.

astro-ph.HE↗