Papers by Waleed El Hanafy
Toward a concordance teleparallel Cosmology II: Linear perturbation
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...
Advances in Mathematical Physics, 2016
The importance of Einstein’s geometrization philosophy, as an alternative to the least action pri... more The importance of Einstein’s geometrization philosophy, as an alternative to the least action principle, in constructing general relativity (GR), is illuminated. The role of differential identities in this philosophy is clarified. The use of Bianchi identity to write the field equations of GR is shown. Another similar identity in the absolute parallelism geometry is given. A more general differential identity in the parameterized absolute parallelism geometry is derived. Comparison and interrelationships between the above mentioned identities and their role in constructing field theories are discussed.
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 scalar... 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 [1]. 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. An... 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.
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.
Physical Review D, 2016
We investigate a bounce inflation model with a graceful exit into the Friedmann-Robertson-Walker ... more We investigate a bounce inflation model with a graceful exit into the Friedmann-Robertson-Walker (FRW) decelerated Universe within f (T) gravity framework, where T is the torsion scalar in the teleparallelism. We study the cosmic thermal evolution, the model predicts a supercold Universe during the precontraction phase, which is consistent with the requirements of the slow-roll models, while it performs a reheating period by the end of the contraction with a maximum temperature just below the grand unified theory (GUT) temperature. However, it matches the radiation temperature of the hot big bang at later stages. The equation-of-state due to the effective gravitational sector suggests that our Universe is self-accelerated by teleparallel gravity. We assume the matter component to be a canonical scalar field. We obtain the scalar field potential that is induced by the f (T) theory. The power spectrum of the model is nearly scale invariant. In addition, we show that the model unifies inflaton and quintessence fields in a single model. Also, we revisited the primordial fluctuations in f (T) bounce cosmology, to study the fluctuations that are produced at the precontraction phase.
Astrophysics and Space Science, 2016
An exact solution of two singularities in the teleparallel equivalent to general relativity theor... more An exact solution of two singularities in the teleparallel equivalent to general relativity theory has been obtained. A holographic visualization of the binary black holes (BBHs) space-time, due to the non vanishing torsion scalar field, has been given. The acceleration tensor of BBHs space-time has been calculated. The results identify the repulsive gravity zones of the BBHs field. The total conserved quantities of the BBHs has been evaluated. Possible gravitational radiation emission by the system has been calculated without assuming a weak field initial data.
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Papers by Waleed El Hanafy