arXiv subjects

Waleed El Hanafy

Publications and source records attributed to Waleed El Hanafy.

15 recordsLinked to original sources

arXiv2026

A Six-Parameter Teleparallel Cosmology Beyond $Λ$CDM: Sign-Changing Torsional Dark Energy and Implications for $H_0$

We investigate a particular case of the extended exponential infrared f(T) teleparallel gravity, in which the geometric sector naturally produces an effective dark-energy density that evolves from negative values in the past to positive values at late times. This behaviour could be motivated by observational results providing a compelling motivation for a geometric, sign-changing dark-energy scenario within modified gravity. We demonstrate that the parameter space of the present model contains only six parameters similar to $Λ$CDM. A Markov Chain Monte Carlo (MCMC) analysis using Planck, DESI, and Type Ia supernova data yields a well-constrained transition redshift of $z_{\rm tr} \gtrsim 1.62$, accompanied by a transition in the effective equation of state to the phantom regime ($w < -1$) for $z < z_{\rm tr}$. Since the effective dark energy originates from modified geometric degrees of freedom, no instabilities or violations of energy conditions arise. The model naturally accounts for the $H_0$ tension, where Planck+DESI data combination gives $H_0 = 72.13 \pm 0.28 \text{ km s}^{-1} \text{ Mpc}^{-1}$ in a better agreement with local measurements than $Λ$CDM which gives $H_0 = 68.46 \pm 0.30 \text{ km s}^{-1} \text{ Mpc}^{-1}$. However, the model is disfavored in comparison to $Λ$CDM in terms of the values of $χ^2$ of the bestfit parameters. We discuss the result among other issues related to CMB-BAO tension.

physics.gen-ph↗

arXiv2026

Cosmological Viability of Exponential Infrared $f(T)$ Gravity

We investigate the cosmological viability of exponential infrared $f(T)$ teleparallel gravity using current cosmological observations. This framework realizes late-time cosmic acceleration through torsional modifications of gravity without enlarging the six-parameter cosmological parameter space of spatially flat $Λ$CDM, and admits two distinct solution branches: a phantom-like model (Model I) and a model featuring a negative-to-positive transition in the effective torsional dark-energy density (Model II). We constrain both branches using CMB observations from Planck, ACT, and SPT together with DESI BAO and Pantheon+ Type Ia supernovae. We find that the principal branch (Model I) alleviates the Hubble tension relative to $Λ$CDM, but remains statistically disfavoured by the combined dataset. The secondary branch (Model II) is decisively ruled out. We show that the failure of Model II originates from the interplay between background and perturbation constraints: once late-time distance measurements constrain the expansion history, the model becomes overconstrained, forcing correlated shifts in $Ω_{\rm m}h^2$, $A_s$, $n_s$, and $τ_{\rm reio}$, degrading the fit to the CMB damping tail and driving the optical depth to unphysical values. Our results demonstrate that perturbation observables provide stringent and complementary tests of teleparallel gravity beyond the background expansion history.

astro-ph.CO↗

arXiv2026

Rotating traversable wormholes and particle dynamics in $f(R,T)$ gravity

Traversable wormholes are among the most interesting solutions of gravitational theories, but within General Relativity they generally require exotic matter violating the null energy condition. Modified gravity theories with matter-geometry coupling provide a promising framework in which wormhole geometries may instead be supported by effective gravitational contributions. Motivated by this possibility, we investigate rotating traversable wormholes in $f(R,T)$ gravity, where $R$ is the scalar curvature and $T$ is the trace of the energy-momentum tensor, within the slow-rotation approximation. We construct stationary and axisymmetric wormhole solutions supported by an anisotropic fluid and show that the obtained geometries are regular, asymptotically flat, horizonless, and satisfy the flare-out condition at the throat. A central result is that the matter sector satisfies both the null and strong energy conditions, indicating that traversable rotating wormholes can be supported without exotic matter. We further analyze particle motion, frame dragging, and non-geodesic effects arising from matter-geometry coupling, together with shadow deformation and gravitational lensing signatures induced by rotation. A preliminary stability analysis based on sound-speed conditions indicates the physical viability of the solutions. These results demonstrate that rotating wormholes in $f(R,T)$ gravity constitute physically consistent compact configurations with potentially observable astrophysical signatures.

gr-qc↗

arXiv2025

Buchdahl limit of compact stars in presence of Weyl anomaly

We setup an anisotropic compact star model in presence of Weyl ``trace" anomaly. We derive an exact interior solution which determines the contribution of the vacuum trace anomaly. We introduce a dimentionless parameter, $β$, to characterize this contribution. Applying appropriate matching conditions with the exterior solution, we determine the model parameters in terms of the Weyl anomaly parameter $β$ and the compactness parameter, $C=\frac{2GM}{c^2 \mathcal{R}}$ where $M$ and $\mathcal{R}$ are the mass and the radius of the star. We investigate the parameter space $\{β, C\}$ and the corresponding modifications of Buchdahl limit on the maximum compactness. We use astrophysical observations of mass and radius of the pulsar PSR J0740+6620 to constrain the Weyl anomaly parameter $β$. Also, we investigate the Mass-Radius diagram with other observational constraints from NICER and LIGO/Virgo collaboration.

gr-qc↗

arXiv2025

A preference for dynamical phantom dark energy using one-parameter model with Planck, DESI DR1 BAO and SN data

Baryon Acoustic Oscillation (BAO) provides a powerful tool to measure cosmic expansion and consequently the nature of the Dark Energy (DE). Recent precise BAO measurements by Dark Energy Spectroscopic Instrument data release 1 (DESI DR1), when combined with Cosmic Microwave Background (CMB) data from Planck and Supernovae of Type Ia (SN Ia), favor evolving dark energy over cosmological constant. This result is strongly related to the assumed priors on the Chevallier-Polarski-Linder (CPL) parameterization of DE. We test another parametrization which introduces two free parameters $n$ and $α$, only $n$ is independent. Thus, it reduces the parameter space compared to the CPL model, which derives a more robust preference for evolving DE, if any. The model potentially produces three cosmological scenarios according to the values of its parameters. For $n=3$, the $Λ$CDM model is recovered, quintessence for $n<3$, and phantom for $n>3$. In the present study, we test the model on the background level, and, to our knowledge for the first time, on the linear perturbation level. Bayesian evidence analysis shows a weak preference \textbf{($\mathbf{\ln B \leq 1.8}$)} for dynamical DE in the phantom regime over the cosmological constant DE using Planck, DESI, and PantheonPlus \& SH0ES data, similarly the AIC analysis supports dynamical DE scenario for the same data. The model predicts current phantom DE $w_{de,0} = -1.073 \pm 0.032$ and $H_0=70.9\pm 1.4$ km/s/Mpc when Planck+DESI data is used, which decreases the tension with $H_0$ local measurements to $1.2σ$ level.

astro-ph.CO↗

arXiv2025

Stable soliton dark matter wormhole in non-minimally coupled $f({\cal Q},{\cal T})$ gravity

We show that non-minimal coupling between matter and geometry can indeed help in constructing stable, traversable, wormholes (WHs) without requiring exotic matter under certain conditions. In models like $f({\cal Q},{\cal T})={\cal Q}+β{\cal T}$ gravity, where ${\cal Q}$ is the non-metricity scalar, and ${\cal T}$ is the trace of the energy-momentum tensor, the coupling between matter and geometry introduces additional degrees of freedom in terms of the parameter $β$. These can mimic the effects of exotic matter or even replace it entirely under specific parameter choice. The analysis involves deriving WH shape functions based on two dark matter (DM) density profiles: a solitonic core at the center of DM halos, and the outer halo follows the universal Navarro-Frenk-White (NFW) density profile of cold DM (CDM). The wormhole solutions derived in these models satisfy important geometric conditions like: Flaring-out condition (necessary for traversability) and asymptotic flatness condition. For large positive coupling parameter, the null energy condition (NEC) can be satisfied at the wormhole throat, meaning exotic matter is not needed, while the wormhole is no longer Lorentzian and the flaring-out condition is broken. However, for large negative coupling parameter, the NEC can be satisfied, allowing for healthy wormholes without exotic matter, provided the coupling strength remains within certain bounds. In the latter case, the NEC is broken only effectively. We investigate the stability of the obtained wormhole solutions by virtue of a modified version of Tolman-Oppenheimer-Volkoff (TOV) equation, which includes a new force due to matter-geometry non-minimal, showing that these wormholes can be dynamically stable.

gr-qc↗

arXiv2024

Revisiting Flat Rotation Curves in Chern-Simons Modified Gravity

We revisit slow rotating black hole (BH) solutions in Chern-Simons modified gravity (CSMG) by considering perturbative solution about Schwarzschild BH. In particular, the case when nondynamical CSMG with noncanonical CS scalar is considered. We provide a new solution different from the previously obtained one \cite{Konno:2007ze} which we refer to as KMT model. The present solution accounts for frame dragging effect which includes not only radial dependence as in the KMT. Nevertheless, it reduces to KMT as a particular case. We show that the tidal gravitational force (Kretschmann invariant) associated to the present solution contains a term of order $1/r^3$ additional to Schwarzschild but absent from directional divergence, unlike KMT model which diverges along the axis of symmetry. We derive the corresponding circular velocity of a massive test particle in which the KMT velocity is recovered in addition to an extra term $\propto r$. We investigate possible constraints on KMT and the present solutions from the observed rotation curve of UGC11455 galaxy as an example. We show that perturbation solutions cannot physically explain the flattening of galactic rotation curves.

gr-qc↗

arXiv2024

Quadratic Rastall Gravity: from low-mass HESS J1731-347 to high-mass PSR J0952-0607 pulsars

Similar to Rastall gravity we introduce matter-geometry nonminimal coupling which is proportional to the gradient of quadratic curvature invariants. Those are mimicking the conformal trace anomaly when backreaction of the quantum fields to a curved spacetime geometry is considered. We consider a static spherically symmetric stellar structure with anisotropic fluid and Krori-Barua metric potentials model to examine the theory. Confronting the model with NICER+XMM-Newton observational constraints on the pulsar PSR J0740$+$6620 quantifies the amount of the nonminimal coupling via a dimensionless parameter $ε\simeq -0.01$. We verify that the conformal symmetry is broken everywhere inside the pulsar as the trace anomaly $Δ>0$, or equivalently the trace of the stress-energy tensor $\mathfrak{T}<0$, whereas the adiabatic sound speed does not violate the conjecture conformal upper limit $v_r^2/c^2 = 1/3$. The maximum compactness accordingly is $C_\text{max}=0.752$ which is $4\%$ higher than GR. Notably, if the conformal sound speed constraint is hold, observational data excludes $ε\geq 0$ up to $\geq 1.6σ$. The stellar model is consistent with the self-bound structure with soft linear equation of state. Investigating possible connection with MIT bag model of strange quarks sets physical bounds from microscopic physics which confirm the negative value of the parameter $ε$. We estimate a radius $R=13.21 \pm 0.96$ km of the most massive observed compact star PSR J0952$-$0607 with $M=2.35\pm0.17 M_\odot$. Finally, we show that the corresponding mass-radius diagram fits well lowest-mass pulsar HESS J1731$-$347 and highest-mass pulsar PSR J0952$-$0607 ever observed as well as the intermediate mass range as obtained by NICER and LIGO/Virgo observations.

astro-ph.HE↗

arXiv2023

Implications of the Conformal Constraint on Sound Speed on the Radius of PSR J0952-0607 within Rastall Gravity

It has been shown that the nonminimal coupling between geometry and matter can provide models for massive compact stars that are consistent with the conformal bound on the sound speed, $0\leqslant {c}_{s}^{2}\leqslant {c}^{2}/3$, where the core density approaches a few times the nuclear saturation density. We impose the conformal upper bound on the sound speed on Rastall's field equations of gravity, with Krori-Barua potentials in the presence of an anisotropic fluid as a matter source, to estimate the radius of the most massive pulsar ever observed, PSR J0952-0607. For its measured mass $M = 2.35\pm 0.17\, M_\odot$, we obtain a radius $R=14.087 \pm 1.0186$ km as inferred by the model. We investigate a possible connection between Rastall gravity and the MIT bag model with an equation of state, ${p}_{r}(ρ)\approx {c}_{s}^{2}\left(ρ-{ρ}_{\rm{s}}\right)$, in the radial direction, with ${c}_{s}=c/\sqrt{3}$ and a surface density $ρ_\text{s}$ slightly above the nuclear saturation density $ρ_\text{nuc}=2.7\times 10^{14}$ g/cm$^{3}$. The corresponding mass-radius diagram is in agreement with our estimated value of the radius and with astrophysical observations of other pulsars at 68% confidence level.

astro-ph.HE↗

arXiv2023

Constraining Quadratic $f(R)$ Gravity from Astrophysical Observations of the Pulsar J0704+6620

We apply quadratic $f(R)=R+εR^2$ field equations, where $ε$ has a dimension [L$^2$], to static spherical stellar model. We assume the interior configuration is determined by Krori-Barua ansatz and additionally the fluid is anisotropic. Using the astrophysical measurements of the pulsar PSR J0740+6620 as inferred by NICER and XMM observations, we determine $ε\approx \pm 3$ km$^2$. We show that the model can provide a stable configuration of the pulsar PSR J0740+6620 in both geometrical and physical sectors. We show that the Krori-Barua ansatz within $f(R)$ quadratic gravity provides semi-analytical relations between radial, $p_r$, and tangential, $p_t$, pressures and density $ρ$ which can be expressed as $p_r\approx v_r^2 (ρ-ρ_1)$ and $p_r\approx v_t^2 (ρ-ρ_2)$, where $v_r$ ($v_t$) is the sound speed in radial (tangential) direction, $ρ_1=ρ_s$ (surface density) and $ρ_2$ are completely determined in terms of the model parameters. These relations are in agreement with the best-fit equations of state as obtained in the present study. We further put the upper limit on the compactness, which satisfies the $f(R)$ modified Buchdahl limit. Interestingly, the quadratic $f(R)$ gravity with negative $ε$ naturally restricts the maximum compactness to values lower than Buchdahl limit, unlike the GR or $f(R)$ gravity with positive $ε$ where the compactness can arbitrarily approach the black hole limit $C\to 1$. The model predicts a core density a few times the saturation nuclear density $ρ_{\text{nuc}} = 2.7\times 10^{14}$ g/cm$^3$, and a surface density $ρ_s > ρ_{\text{nuc}}$. We provide the mass-radius diagram corresponding to the obtained boundary density which has been shown to be in agreement with other observations.

gr-qc↗

arXiv2022

Impact of Rastall gravity on mass, radius and sound speed of the pulsar PSR J0740+6620

Millisecond pulsars are perfect laboratories to test possible matter-geometry coupling and its physical implications in light of recent Neutron Star Interior Composition Explorer (NICER) observations. We apply Rastall field equations of gravity, where matter and geometry are nonminimally coupled, to Krori-Barua interior spacetime whereas the matter source is assumed to be anisotropic fluid. We show that all physical quantities inside the star can be expressed in terms of Rastall, $ε$, and compactness, $C=2GM/Rc^2$, parameters. Using NICER and X-ray Multi-Mirror X-ray observational constraints on the mass and radius of the pulsar PSR J0740+6620 we determine Rastall parameter to be at most $ε=0.041$ in the positive range. The obtained solution provides a stable compact object; in addition the squared sound speed does not violate the conjectured sound speed $c_s^2\leq c^2/3$ unlike the general relativistic treatment. We note that no equations of state are assumed; the model however fits well with linear patterns with bag constants. In general, for $ε>0$, the theory predicts a slightly larger size star in comparison to general relativity for the same mass. This has been explained as an additional force, due to matter-geometry coupling, in the hydrodynamic equilibrium equation, which contributes to partially diminish the gravitational force effect. Consequently, we calculate the maximal compactness as allowed by the strong energy condition to be $C = 0.735$ which is $\sim 2\%$ higher than general relativity prediction. Moreover, for the surface density at saturation nuclear density $ρ_{\text{nuc}} = 2.7\times 10^{14}$ g/cm$^3$ we estimate the maximum mass $M=4 M_\odot$ at radius $R=16$ km.

astro-ph.HE↗

arXiv2022

Revisiting diagonal tetrads: New Black Hole solutions in $f(T)$ gravity

We study various forms of diagonal tetrads that accommodate Black Hole solutions in $f(T)$ gravity with certain symmetries. As is well-known, vacuum spherically symmetric diagonal tetrads lead to rather boring cases of constant torsion scalars. We extend this statement to other possible horizon topologies, namely, spherical, hyperbolic and planar horizons. All such cases are forced to have constant torsion scalars to satisfy the anti-symmetric part of the field equations. We give a full classification of possible vacuum static solutions of this sort. Furthermore, we discuss addition of time-dependence in all the above cases. We also show that if all the components of a diagonal tetrad depend only on one coordinate, then the anti-symmetric part of the field equations is automatically satisfied. This result applies to the flat horizon case with Cartesian coordinates. For solutions with a planar symmetry (or a flat horizon), one can naturally use Cartesian coordinates on the horizon. In this case, we show that the presence of matter is required for existence of non-trivial solutions. This is a novel and very interesting feature of these constructions. We present two new exact solutions, the first is a magnetic Black Hole which is the magnetic dual of a known electrically charged Black Hole in literature. The second is a dyonic Black Hole with electric and magnetic charges. We present some features of these Black holes, namely, extremality conditions, mass, behavior of torsion and curvature scalars near the singularity.

gr-qc↗

arXiv2021

Toward a concordance teleparallel Cosmology I: Background Dynamics

Assuming a spatially flat universe, we study the cosmological viability of an infrared corrected teleparallel gravity model, which accounts for late acceleration by weakening gravity at later times on cosmological distances. The theory does not introduce any additional free parameters into the cosmological model, as is commonly the case with modified gravity based cosmologies. This feature renders the cosmological model statistically comparable, on equal footing, with $Λ$CDM. In this context, using recent cosmological observations -- Pantheon supernova Type Ia, Hubble constant $H_0$, Baryon acoustic oscillation, redshift space distortions, Big Bang nucleosynthesis and the cosmic microwave background constraint on the decoupling acoustic scale -- we show that, although the exponential infrared-corrected gravity and $Λ$CDM are physically different, they are phenomenologically and statistically equivalent. However, the former is more adept at fitting accurately determined observational constraints while decreasing the $H_0$ tension without worsening the $S_8$ tension. This calls for full examination of the empirical viability of the theory at the linear perturbation level, which is the subject of paper II.

astro-ph.CO↗

arXiv2021

Toward a concordance teleparallel Cosmology II: Linear perturbation

Late time cosmic acceleration may be achieved by modifying gravity on large scales. This should also have consequences on the evolution of perturbations. We thus extend our study of exponential infrared $f(T)$ teleparallel gravity to examine the viability of the theory at the linear perturbation level, evaluating the full CMB and matter power spectra. As the theory does not introduce extra free parameters, it fits within the minimal six parameter space of standard $Λ$CDM. Using Planck 2018 CMB (TT+TE+EE+lensing) alone, best fits predict those parameters to be almost identical to $Λ$CDM, with slightly smaller $χ^2_{min}$. The resulting $H_0=72.24\pm 0.64$ km/s/Mpc, which "practically" alleviates the tension with local measurements, due to late time phantom behaviour. Inclusion of BAO data however reduces $H_0$, reflecting furthermore systematic deviations from data that are also present in supernova distances and the growth rate of structure (increasing the apparent tension in the latter case). As the theory, unlike other viable $f(T)$ models, does not reduce to $Λ$CDM through extra free parameters, those conclusions are generic; applying to any modified gravity or dynamical dark energy with phantom behaviour. With best fit parameters, the present scenario produces a CMB spectrum almost identical to $Λ$CDM, with slight deviation at low-multipole $\ell < 30$, where cosmic variance is large. The matter power spectrum is also quite close to $Λ$CDM; with percent level scale free modifications affecting modes significantly smaller than the horizon, arising primarily from modified background evolution. More significant deviations appear on larger scales, and may in principle distinguish modified gravity scenarios of the type studied here from dynamical dark energy.

astro-ph.CO↗

arXiv2018

$H_0$ Tension and the Phantom Regime: A Case Study In Terms of an Infrared $f(T)$ Gravity

We propose an $f(T)$ teleparallel gravity theory including a torsional infrared (IR) correction. We show that the governing Friedmann's equations of a spatially flat universe include a phantom-like effective dark energy term sourced by the torsion IR correction. As has been suggested, this phantom phase does indeed act as to reconcile the tension between local and global measurements of the current Hubble value $H_0$. The resulting cosmological model predicts an electron scattering optical depth $τ_e\thickapprox 0.058$ at reionization redshift $z_{re} \sim 8.1$, in agreement with observations. The predictions are however in contradiction with baryon acoustic oscillations (BAO) measurements, particularly the distance indicators. We argue that this is the case with any model with a phantom dark energy model that has effects significant enough at redshifts $z \lesssim 2$ as to be currently observable. The reason being that such a scenario introduces systematic differences in terms of distance estimates in relation to the standard model; e.g., if the angular diameter distance to the recombination era is to be kept constant while $H_0$ is increased in the context of a phantom scenario, the distances there are systematically overestimated to all objects at redshifts smaller than recombination. But no such discrepancies exist between $Λ$CDM predictions and current data for $z \lesssim 2$.

gr-qc↗