arXiv subjects

Bishop Mongwane

Publications and source records attributed to Bishop Mongwane.

10 recordsLinked to original sources

arXiv2026

Hubble tension in k-essence: Evidence for robust tension alleviation

The Hubble tension has come to stay as a major problem in modern cosmology as it continues to plague the standard cosmological model ($Λ$CDM). As one of the viable, self-consistent dark energy theories, k-essence involves nontrivial self-interactions that can modify the background expansion beyond recombination; thereby impacting the sound horizon to last scattering, and hence, the inferred value of the Hubble constant. We examine this tension in two physically motivated k-essence models, dilaton and tachyon, using datasets from Planck and late-Universe probes including Pantheon+SH0ES, cosmic chronometer (CC), Supernova Cosmology Project Union compilation (Union3), Dark Energy Survey Year~5 (DESY5), and Dark Energy Spectroscopic Instrument (DESI) measurements. While $Λ$CDM exhibits inconsistent tension inferences, both k-essence models exhibit a substantial tension alleviation that is robust against the inclusion of the independent late-Universe cosmological datasets, giving consistent tension reduction irrespective of whether the observations are supernovae (Pantheon+SH0ES, Union3, DESY5) alone or in combination with cosmic chronometers (CC) and baryon acoustic oscillation measurements (DESI). The combined late-Universe dataset leads to only $0.14σ$ and $0.69σ$ offsets from the Planck prediction in the dilaton and tachyon models, respectively, compared to $5.89σ$ tension in $Λ$CDM. Both models demonstrate that the inferred tension alleviation is a stable, intrinsic consequence of the underlying k-essence dynamics rather than of model fine tuning: model parameters remain unchanged across datasets. The results establish that the apparent Hubble tension is not an unavoidable feature of late-Universe cosmology but depends critically on the description of dark energy.

astro-ph.CO↗

arXiv2026

Relativistic effects in k-essence

Relativistic effects are sensitive to subtle changes in dark energy. These effects grow on very large scales and at high redshifts, which will be the reach of upcoming surveys. We investigate these effects in both the linear and the angular galaxy power spectra in a late-time universe dominated by cold dark matter and k-essence, focusing on three core models (dilaton, tachyon, and DBI scalar fields) and contrasting their predictions with those of the concordance model. By enforcing identical present-day cosmological parameters, we isolate the imprints of k-essence dynamics and perturbations on very large scales. We found that relativistic corrections dominate on very large scales and grow with redshift, but are largely insensitive to k-essence microphysics in Fourier space, leading to strong degeneracies among the models. However, in the angular power spectrum, where line-of-sight integrals are naturally included, relativistic effects are significantly amplified, yielding better sensitivity to clustering k-essence. In particular, the tachyon exhibits clear deviations across multipoles and redshifts, with distinct imprints in the Doppler and the combined (velocity and gravitational) potentials contributions. Furthermore, our results show that neglecting relativistic corrections can lead to systematic misestimation of deviations of k-essence from the cosmological constant. Our results show the relativistic angular galaxy power spectrum as a more consistent and robust probe of ultra-large-scale physics. These findings underscore the need for full relativistic modelling in next-generation surveys that are targeting horizon-scale modes, where the imprint of non-standard dark energy is most pronounced.

astro-ph.CO↗

arXiv2025

Cosmic magnification in beyond-Horndeski gravity

Cosmic magnification is able to probe the geometry of large-scale structure on cosmological scales, thereby providing another window for probing theories of the late-time cosmic acceleration. It holds the potential to reveal new information on the nature of dark energy and modified gravity. By using the angular power spectrum, we investigated cosmic magnification beyond weak lensing (incorporating all known relativistic corrections) in beyond-Horndeski gravity$-$with both constant phenomenology and dynamic phenomenology, respectively. For both phenomenologies our results show that the total relativistic signal surpasses cosmic variance (considering an SKA2-like sky coverage) in the magnification angular power spectrum at low redshifts ($z\lesssim 0.5$), hence cosmic-variance reduction methods like multi-tracer analysis will not be needed for surveys at the given $z$. For the individual relativistic signals, we found that the Doppler magnification signal also surpasses cosmic variance and remains the dominant signal, at low $z$, for both phenomenologies. However, the integrated-Sachs-Wolfe, the time-delay, and the gravitational (potential) magnification signals, respectively, are subdominant to both the Doppler magnification signal and cosmic variance, at the same $z$; hence multi-tracer analysis will be needed to isolate these signals. At high redshifts ($z\gtrsim 3$), the integrated-Sachs-Wolfe, the time-delay, and the gravitational magnification signals, respectively, appear to surpass cosmic variance and dominate over the Doppler magnification signal for constant phenomenology; whereas for dynamic phenomenology, all these signals diminish significantly and are well below cosmic variance, at all $z$ (consistent with recent quintessence analysis). Suggesting that including time-variation in the parameters will be crucial in identifying the true signature of the beyond-Horndeski gravity.

astro-ph.CO↗

arXiv2025

Investigating the imprint of quintessence in cosmic magnification

We study cosmic magnification beyond lensing in a late-time universe dominated by quintessence and cold dark matter. The cosmic magnification angular power spectrum, especially going beyond the well-known lensing effect, provides an independent avenue for investigating the properties of quintessence, and hence, dark energy. By analysing the magnification power spectrum at different redshifts, it is possible to extract new information about the large-scale imprint of dark energy, including whether we are able to disentangle different models from one another. Using three well-known quintessence models, we analyse the cosmic magnification angular power spectrum while taking relativistic corrections into account. We found that it will be difficult to distinguish between quintessence models, and quintessence from the cosmological constant, in lensing magnification angular power spectrum on large scales, at redshifts $z \,{\leq}\, 1$; whereas, when relativistic corrections are incorporated, the total magnification angular power spectrum holds the potential to distinguish between the models, at the given $z$. At $z \,{\geq}\, 3$, the lensing magnification angular power spectrum can be a reasonable approximation of the total magnification angular power spectrum. We also found that both the total relativistic and the Doppler magnification signals, respectively, surpass cosmic variance at $z \,{\leq}\, 0.5$: hence the effect may be detectable at the given $z$. On the other hand, the ISW and the time-delay magnification signals, respectively, are surpassed by cosmic variance on all scales, at epochs up to $z \,{=}\, 4.5$, with the gravitational-potential magnification signal being zero.

astro-ph.CO↗

arXiv2025

Quasinormal modes of a Schwarzschild black hole within the Bondi-Sachs framework

Studies of quasinormal modes (QNMs) of black holes have a long and well established history. Predominantly, much research in this area has customarily focused on the equations given by Regge, Wheeler and Zerilli. In this work we study linearized perturbations of a Schwarzschild black hole using the Characteristic formulation of numerical relativity, with an emphasis on the computation of QNMs. Within this formalism, the master equation describing gravitational perturbations is known to satisfy a fourth order differential equation. We analyse the singular points of this master equation, and obtain series solutions whose coefficients are given by three term recurrence relations, from which Leaver's continued fraction method can be applied. Using this technique, we recover the standard Schwarzschild quasinormal modes. In addition, we find that imposing purely outgoing boundary conditions, a natural feature of the Bondi-Sachs framework, leads to the recovery of the algebraically special mode.

gr-qc↗

arXiv2019

Probing beyond-Horndeski gravity on ultra-large scales

The beyond-Horndeski gravity has recently been reformulated in the dark energy paradigm - which has been dubbed, Unified Dark Energy (UDE). The evolution equations for the given UDE appear to correspond to a non-conservative dark energy scenario, in which the total energy-momentum tensor is not conserved. We investigate both the background cosmology and, the large-scale imprint of the UDE by probing the angular power spectrum of galaxy number counts, on ultra-large scales; taking care to include the full relativistic corrections in the observed overdensity. The background evolution shows that only an effective mass smaller than the Planck mass is needed in the early universe in order for predictions in the given theory to match current observational constraints. We found that the effective mass-evolution-rate parameter, which drives the evolution of the UDE, acts to enhance the observed power spectrum and, hence, relativistic effects (on ultra-large scales) by enlarging the UDE sound horizon. Conversely, both the (beyond) Horndeski parameter and the kineticity act to diminish the observed power spectrum, by decreasing the UDE sound horizon. Our results show that, in a universe with UDE, a multi-tracer analysis will be needed to detect the relativistic effects in the large-scale structure. In the light of a multi-tracer analysis, the various relativistic effects hold the potential to distinguish different gravity models. Moreover, while the Doppler effect will remain significant at all epochs and, thus can not be ignored, the integrated Sachs-Wolfe, the time-delay and the potential (difference) effects, respectively, will only become significant at epochs near z=3 and beyond, and may be neglected at late epochs. In the same vein, the Doppler effect alone can serve as an effective cosmological probe for the large-scale structure or gravity models, in the angular power spectrum - at all z.

astro-ph.CO↗

arXiv2017

Characteristic Formulation for Metric $f(R)$ Gravity

In recent years, the Characteristic formulation of numerical relativity has found increasing use in the extraction of gravitational radiation from numerically generated spacetimes. In this paper, we formulate the Characteristic initial value problem for $f(R)$ gravity. We consider, in particular, the vacuum field equations of Metric $f(R)$ gravity in the Jordan frame, without utilising the dynamical equivalence with scalar-tensor theories. We present the full hierarchy of non-linear hypersurface and evolution equations necessary for numerical implementation in both tensorial and eth forms. Furthermore, we specialise the resulting equations to situations where the spacetime is almost Minkowski and almost Schwarszchild using standard linearization techniques. We obtain analytic solutions for the dominant $\ell=2$ mode and show that they satisfy the concomitant constraints. These results are ideally suited as testbed solutions for numerical codes. Finally, we point out that the Characteristic formulation can be used as a complementary analytic tool to the $1+1+2$ semi-tetrad formulation.

gr-qc↗

arXiv2016

On the Hyperbolicity and Stability of $3+1$ Formulations of Metric $f(R)$ Gravity

$3+1$ formulations of the Einstein field equations have become an invaluable tool in Numerical relativity, having been used successfully in modeling spacetimes of black hole collisions, stellar collapse and other complex systems. It is plausible that similar considerations could prove fruitful for modified gravity theories. In this article, we pursue from a numerical relativistic viewpoint the $3+1$ formulation of metric $f(R)$ gravity as it arises from the fourth order equations of motion, without invoking the dynamical equivalence with Brans-Dicke theories. We present the resulting system of evolution and constraint equations for a generic function $f(R)$, subject to the usual viability conditions. We confirm that the time propagation of the $f(R)$ Hamiltonian and Momentum constraints take the same Mathematical form as in general relativity, irrespective of the $f(R)$ model. We further recast the 3+1 system in a form akin to the BSSNOK formulation of numerical relativity. Without assuming any specific model, we show that the ADM version of $f(R)$ is weakly hyperbolic and is plagued by similar zero speed modes as in the general relativity case. On the other hand the BSSNOK version is strongly hyperbolic and hence a promising formulation for numerical simulations in metric $f(R)$ theories.

gr-qc↗

arXiv2015

Toward a Consistent Framework for High Order Mesh Refinement Schemes in Numerical Relativity

It has now become customary in the field of numerical relativity to couple high order finite difference schemes to mesh refinement algorithms. To this end, different modifications to the standard Berger-Oliger adaptive mesh refinement algorithm have been proposed. In this work we present a fourth order stable mesh refinement scheme with sub-cycling in time for numerical relativity. We do not use buffer zones to deal with refinement boundaries but explicitly specify boundary data for refined grids. We argue that the incompatibility of the standard mesh refinement algorithm with higher order Runge Kutta methods is a manifestation of order reduction phenomena, caused by inconsistent application of boundary data in the refined grids. Our scheme also addresses the problem of spurious reflections that are generated when propagating waves cross mesh refinement boundaries. We introduce a transition zone on refined levels within which the phase velocity of propagating modes is allowed to decelerate in order to smoothly match the phase velocity of coarser grids. We apply the method to test problems involving propagating waves and show a significant reduction in spurious reflections.

gr-qc↗

arXiv2012

Cosmic Electromagnetic Fields due to Perturbations in the Gravitational Field

We use non-linear gauge-invariant perturbation theory to study the interaction of an inflation produced seed magnetic field with density and gravitational wave perturbations in an almost Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime. We compare the effects of this coupling under the assumptions of poor conductivity, infinite conductivity and the case where the electric field is sourced via the coupling of velocity perturbations to the seed field in the ideal magnetohydrodynamic (MHD) regime, thus generalizing, improving on and correcting previous results. We solve our equations for long wavelength limits and numerically integrate the resulting equations to generate power spectra for the electromagnetic field variables, showing where the modes cross the horizon. We find that the rotation of the electric field dominates the power spectrum on small scales, in agreement with previous arguments.

gr-qc↗