articles by Waleed El Hanafy
Papers by Waleed El Hanafy
The Astrophysical Journal
It has been shown that the nonminimal coupling between geometry and matter can provide models for... more 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 ≤ c s 2 ≤ 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 ± 0.17 M ⊙, we obtain a radius R = 14.087 ± 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 ) ≈ c s 2 ρ − ρ s , in the radial direction, with c s = c / 3 and a surface density ρ s slightly above the nuclear saturation density ρ nuc = 2.7 × 1014 g cm–3. The corresponding mass–radius diagram is in agreement wit...
The European Physical Journal C
We study various forms of diagonal tetrads that accommodate Black Hole solutions in f(T) gravity ... more 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 ...
Astrophysics and Space Science, 2016
We derive an exact f (T) gravity in the absence of ordinary matter in Friedmann-Robertson-Walker ... more We derive an exact f (T) gravity in the absence of ordinary matter in Friedmann-Robertson-Walker (FRW) universe, where T is the teleparallel torsion scalar. We show that vanishing of the energy-momentum tensor T µν of matter does not imply vanishing of the teleparallel torsion scalar, in contrast to general relativity, where the Ricci scalar vanishes. The theory provides an exponential (inflationary) scale factor independent of the choice of the sectional curvature. In addition, the obtained f (T) acts just like cosmological constant in the flat space model. Nevertheless, it is dynamical in non-flat models. In particular, the open universe provides a decaying pattern of the f (T) contributing directly to solve the fine-tuning problem of the cosmological constant. The equation of state (EoS) of the torsion vacuum fluid has been studied in positive and negative Hubble regimes. We study the case when the torsion is made of a scalar field (tlaplon) which acts as torsion potential. This treatment enables to induce a tlaplon field sensitive to the symmetry of the spacetime in addition to the reconstruction of its effective potential from the f (T) theory. The theory provides six different versions of inflationary models. The real solutions are mainly quadratic, the complex solutions, remarkably, provide Higgs-like potential.
The Astrophysical Journal
Millisecond pulsars are perfect laboratories to test possible matter–geometry coupling and its ph... more 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 Newton X-ray-observational constraints on the mass and radius of the pulsar PSR J0740+6620, we determine the 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 ≤ c 2 / 3 unlike the general relativistic treatment. We note that no equations of state are assumed; the model how...
Late time cosmic acceleration may be achieved by modifying gravity on large scales. This should a... more 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 ΛCDM. Using Planck 2018 CMB (TT+TE+EE+lensing) alone, best fits predict those parameters to be almost identical to ΛCDM, with slightly smaller χ^2_min. The resulting H_0=72.24± 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 c...
arXiv: Cosmology and Nongalactic Astrophysics, 2020
Assuming the universe to be spatially flat with homogeneous and isotropic background, we study th... more Assuming the universe to be spatially flat with homogeneous and isotropic background, we study the cosmological viability of the infrared corrected $f(T)=T e^{\beta T_0/T}$ teleparallel gravity in terms of its background expansion history, including its effect on the linear growth of matter perturbations. As the dimensionless parameter $\beta$ is completely constrained by the current density parameter, this theory does not introduce extra free parameters. This novel feature renders the theory statistically comparable on equal footing with $\Lambda$CDM, which is not common with modified gravity based cosmological models. 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 -- the joint likelihood analysis of the $f(T)$ gravity shows very good agreement with data to within 1$\sigma$. It...
In the present work, we discusses the two-body problem in the context of General Relativity(GR) t... more In the present work, we discusses the two-body problem in the context of General Relativity(GR) theory. Since binary pulsars represent a natural laboratory for testing relativistic field theories, it was interesting to study the problem of two bodies, in a strong field regime. Hence, this thesis is not only a complete review of the work done on binary pulsars but also, it includes an important step towards a more general solution to the problem of the strong field of the two bodies; which is still far beyond the reach of researchers. This is done by modifying Curzon solution to describe the field of binaries. A generalized formula for red shift and Shapiro time delay were derived including additional terms. Also, energy localization and the possibility of gravitational waves emission have been discussed. This thesis provides a necessary background for the relativists, astronomers and astrophysicists who work in the binary pulsars.
The Astrophysical Journal, 2019
We propose an f (T ) teleparallel gravity theory including a torsional infrared (IR) correction. ... more 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 ≈ 0.058 at reionization redshift z re ∼ 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 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 ΛCDM predictions and current data for z 2.
Journal of Cosmology and Astroparticle Physics, 2018
We use dynamical system methods to explore the general behaviour of f (T) cosmology. In contrast ... more We use dynamical system methods to explore the general behaviour of f (T) cosmology. In contrast to the standard applications of dynamical analysis, we present a way to transform the equations into a one-dimensional autonomous system, taking advantage of the crucial property that the torsion scalar in flat FRW geometry is just a function of the Hubble function, thus the field equations include only up to first derivatives of it, and therefore in a general f (T) cosmological scenario every quantity is expressed only in terms of the Hubble function. The great advantage is that for one-dimensional systems it is easy to construct the phase space portraits, and thus extract information and explore in detail the features and possible behaviours of f (T) cosmology. We utilize the phase space portraits and we show that f (T) cosmology can describe the universe evolution in agreement with observations, namely starting from a Big Bang singularity, evolving into the subsequent thermal history and the matter domination, entering into a late-time accelerated expansion, and resulting to the de Sitter phase in the far future. Nevertheless, f (T) cosmology can present a rich class of more exotic behaviours, such as the cosmological bounce and turnaround, the phantom-divide crossing, the Big Brake and the Big Crunch, and it may exhibit various singularities, including the non-harmful ones of type II and type IV. We study the phase space of three specific viable f (T) models offering a complete picture. Moreover, we present a new model of f (T) gravity that can lead to a universe in agreement with observations, free of perturbative instabilities, and applying the Om(z) diagnostic test we confirm that it is in agreement with the combination of SNIa, BAO and CMB data at 1σ confidence level.
Astrophysics and Space Science, 2016
In a recent work, a particular class of f (T ) gravity, where T is the teleparallel torsion scala... more In a recent work, a particular class of f (T ) gravity, where T is the teleparallel torsion scalar, has been derived. This class has been identified by flat-like universe (FLU) assumptions . The model is consistent with the early cosmic inflation epoch. A quintessence potential has been constructed from the FLU f (T )-gravity. We show that the first order potential of the induced quintessence is a quasi inverse power law inflation with an additional constant providing an end of the inflation with no need to an extra mechanism. At e-folds N * = 55 before the end of the inflation, this type of potential can perform both E and B modes of the cosmic microwave background (CMB) polarization pattern.
The European Physical Journal C, 2017
A non-diagonal vielbein ansatz is applied to the N -dimension field equations of f (T ) gravity. ... more A non-diagonal vielbein ansatz is applied to the N -dimension field equations of f (T ) gravity. An analytical vacuum solution is derived for the quadratic polynomial f (T ) = T + T 2 and an inverse relation between the coupling constant and the cosmological constant . Since the induced metric has off-diagonal components, it cannot be removed by a mere coordinate transformation, the solution has a rotating parameter. The curvature and torsion scalars invariants are calculated to study the singularities and horizons of the solution. In contrast to general relativity, the Cauchy horizon differs from the horizon which shows the effect of the higher order torsion. The general expression of the energy-momentum vector of f (T ) gravity is used to calculate the energy of the system. Finally, we have shown that this kind of solution satisfies the first law of thermodynamics in the framework of f (T ) gravitational theories.
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articles by Waleed El Hanafy
Papers by Waleed El Hanafy