arXiv subjects

Mahmoud Hashim

Publications and source records attributed to Mahmoud Hashim.

10 recordsLinked to original sources

arXiv2026

A Six-Parameter Teleparallel Cosmology Beyond $\Lambda$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 $\Lambda$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 $\Lambda$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 $\Lambda$CDM in terms of the values of $\chi^2$ 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 $\Lambda$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 $\Lambda$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 $\Omega_{\rm m}h^2$, $A_s$, $n_s$, and $\tau_{\rm reio}$, 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↗

arXiv2025

Clustered unified dark sector cosmology: Background evolution and linear perturbations in light of observations

We consider unified dark sector models in which the fluid can collapse and cluster into halos, allowing for hierarchical structure formation to proceed as in standard cosmology. We show that both background evolution and linear perturbations tend towards those in $\LCDM$ as the clustered fraction $f \rightarrow 1$. We confront such models with various observational datasets, with emphasis on the relatively well motivated standard Chaplygin gas. We show that the strongest constraints come from secondary anisotropies in the CMB spectrum, which prefer models with $f \rightarrow 1$. However, as a larger Hubble constant is allowed for smaller $f$, values of $f \simeq 0.99$ (rather than tending to exact unity) are favored when late universe expansion data is included, with $f \simeq 0.97$ and $H_0 \simeq 70 {\rm km/s/Mpc}$ allowed at the 2-$\sigma$ level. Such values of $f$ imply extremely efficient clustering into nonlinear structures. They may nevertheless be compatible with clustered fractions in warm dark matter based cosmologies, which have similar minimal halo mass scales as the models considered here. Tight CMB constraints on $f$ also apply to the generalized Chaplygin gas, except for models that are already quite close to $\LCDM$, in which case all values of $0 \le f \le 1$ are allowed. In contrast to the CMB, large scale structure data, which were initially used to rule out unclustered unified dark matter models, are far less constraining. Indeed, late universe data, including the large scale galaxy distribution, prefer models that are far from $\LCDM$. But these are in tension with the CMB data.

astro-ph.CO↗

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↗

arXiv2023

Building stellar bulges and halo cores from massive clumps observed in the DYNAMO-HST sample

We present N-body simulations of the process of bulge formation in disc galaxies due to inward migration of massive stellar clumps. The process is accompanied by dark halo heating, with a quasi-isothermal core replacing the initial central density cusp, transforming an initially dark matter dominated central region into a baryon dominated one. The characteristics of the clumps are chosen to be compatible with low redshift observations of stellar clumps in DYNAMO-HST galaxies, which may be relatively long lived in terms of being robust against internal starburst-instigated disruption. We thus test for disruption due to tidal stripping using different clump internal radial profiles; Plummer, Hernquist and Jaffe, in ascending order of steeper central density profile. Our calculations predict that in order for clump migration to be effective in building galactic bulges and dark halo cores, steeply increasing central clump profiles, or a less massive or less concentrated haloes, are preferred. The dependence on such factors may contribute to the diversity in observed total mass distributions and resulting rotation curves in galaxies. When the process is most efficient, a 'bulge-halo conspiracy', with a singular isothermal total density akin to that observed bright galaxies, results.

astro-ph.GA↗

arXiv2022

Halo heating from fluctuating gas in a model dwarf

The cold dark matter (CDM) structure formation scenario faces challenges on (sub)galactic scales, central among them being the `cusp-core' problem. A known remedy, driving CDM out of galactic centres, invokes interactions with baryons, through fluctuations in the gravitational potential arising from feedback or orbiting clumps of gas or stars. Here we interpret core formation in a hydrodynamic simulation in terms of a theoretical formulation, which may be considered a generalisation of Chandrasekhar's theory of two body relaxation to the case when the density fluctuations do not arise from white noise; it presents a simple characterisation of the effects of complex hydrodynamics and `subgrid physics'. The power spectrum of gaseous fluctuations is found to follow a power law over a range of scales, appropriate for a fully turbulent compressible medium. The potential fluctuations leading to core formation are nearly normally distributed, which allows for the energy transfer leading to core formation to be described as a standard diffusion process, initially increasing the velocity dispersion of test particles as in Chandrasekhar's theory. We calculate the energy transfer from the fluctuating gas to the halo and find it consistent with theoretical expectations. We also examine how the initial kinetic energy input to halo particles is redistributed to form a core. The temporal mass decrease inside the forming core may be fit by an exponential form; a simple prescription based on our model associates the characteristic timescale with an energy relaxation time. We compare the resulting theoretical density distribution with that in the simulation.

astro-ph.GA↗

arXiv2021

Dissipative Unified Dark Fluid: Observational Constraints

We adopt a standard FRW cosmology with a unified scenario, where the usual dark matter and dark energy sectors are replaced by a single dissipative unified dark fluid (DUDF). The equation of state of such fluid can asymptote between two power laws. As a result, it enables fluid to have a smooth transition from dust at early times to dark energy at late times. The dissipation is represented by a bulk viscosity with a constant coefficient, whereas shear viscosity is excluded due to the isotropy of the universe. We performed a likelihood analysis using recent observational datasets from local $H_0$ measurements, SNe Ia, observational Hubble data, BAO, and CMB to put cosmological constraints on the model. The special case of the non-dissipative unified dark fluid (UDF) is also studied, while a similar analysis is performed on the $\Lambda$CDM model for comparison. We got an $H_0$ value of $70.02$ $km$ $s^{-1} Mpc^{-1}$ for DUDF and $70.25$ $km$ $s^{-1} Mpc^{-1}$ for the UDF model. Our analyses revealed that the DUDF model has the lower $\chi_{\rm min}^2$-value. Based on model selection statistics in the form of the Akaike Information Criterion (AIC), we compare different models to select the favored one due to the observational data used. Our results revealed that the UDF model has the minimum AIC, with the conclusion that it is the most favorable model for the data. $\Delta AIC$ value of other models are then measured to this model. This difference indicated that the DUDF model is a substantial model on the level of empirical support. Examining the evolution of the deceleration parameter, the effective equation of state parameter, and the density parameter indicated that our unified fluid doesn't deviate from the standard $\Lambda$CDM model at early times, with the ability to play the role of the cosmological constant by accelerating the universe at the late times.

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 $\Lambda$CDM. Using Planck 2018 CMB (TT+TE+EE+lensing) alone, best fits predict those parameters to be almost identical to $\Lambda$CDM, with slightly smaller $\chi^2_{min}$. 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 $\Lambda$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 $\Lambda$CDM, with slight deviation at low-multipole $\ell < 30$, where cosmic variance is large. The matter power spectrum is also quite close to $\Lambda$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↗

arXiv2020

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 $\Lambda$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 $\Lambda$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↗

arXiv2014

Degeneracy between primordial non-Gaussianity and interaction in the dark sector

If dark energy and dark matter interact via exchange of energy and momentum, then this may affect the galaxy power spectrum on large scales. When this happens, it may be degenerate with the signal from primordial non-Gaussianity via scale-dependent bias. We consider a class of interacting dark energy models and show that the matter overdensity is scale-dependent on large scales. We estimate the effective non-Gaussianity arising from the large-scale effects of interaction in the dark sector. The signal of dark sector interaction can be disentangled from a primordial non-Gaussian signal by measuring the power at two redshifts.

astro-ph.CO↗