arXiv subjects

Mariam Bouhmadi-López

Publications and source records attributed to Mariam Bouhmadi-López.

At least 19 recordsLinked to original sources

arXiv2026

Reconstructing sign-switching dark energy histories: Scalar-field regularity, conditional potential comparison, and representative dynamics

Phenomenologically similar sign-switching dark-energy histories need not have comparable scalar-field realizations. We reconstruct minimally coupled scalars with fixed kinetic sign for three prescribed histories: the error-function model (ECDM), the compact smooth-step model (SSCDM), and the ladder-like model (L$Λ$CDM). Continuous negative-to-positive density crossings select the phantom branch. ECDM yields a smooth on-shell potential at every finite redshift; its equation-of-state pole at the density zero is only a ratio singularity. Exact SSCDM has a regular trajectory but a $C^1$, non-$C^2$ endpoint potential, $V-V_e\propto|ϕ-ϕ_e|^{4/3}$. Its non-Lipschitz force permits delayed departures from frozen plateaus, so the reconstructed potential and plateau data do not uniquely generate the prescribed history. The exact Ladder requires distributional kinetic stress and has no ordinary classical realization in the adopted one-field action. In conditional synthetic comparisons, the sigmoid--Gaussian family ranks highest for ECDM and the generalized axion-like family for SSCDM, although the fitted axion exponents $n<1/2$ imply a divergent endpoint force. Representative regular forward solutions exhibit negative-to-positive scalar-density crossings, with the potential zero preceding the density zero. In a closure test, unretuned evolution of the top-ranked ECDM template tracks the target approximately, $\max|Δ\widetildeΩ_ϕ|\simeq0.16$. Closure fails structurally for compact SSCDM: no potential with a locally Lipschitz force can reproduce its exact finite-duration frozen plateaus from exactly frozen initial data. Field-map existence, invertibility, and endpoint regularity must be assessed before interpreting a phenomenological history as scalar dynamics. The phantom action is used only as a homogeneous effective proxy.

gr-qc↗

arXiv2026

Three-form dark energy: constraints and multi-probe comparison with $Λ$CDM

Three-form fields provide a theoretically well-motivated framework for dark energy, arising in higher-dimensional theories and exhibiting a rich cosmological phenomenology. We investigate a minimally coupled three-form dark energy model with a Gaussian potential and constrain it using current cosmological observations, including CMB shift parameters, DESI DR2 baryon acoustic oscillation measurements, Pantheon+ supernovae with and without SH0ES calibration, cosmic chronometers, and gamma-ray bursts. Parameter estimation is performed within a Bayesian Markov-chain Monte Carlo framework, while model comparison relies on several information criteria and the Bayesian evidence, as well as tension statistics. We find that the three-form model provides a viable and competitive description of the expansion history of the Universe. It is mildly preferred over $Λ$CDM for the combination of early and late-time datasets that are heavily tensioned (CMB+BAO and Pantheon+SH0ES). This preference decreases to neutrality for the other, less tensioned combination of early and late-time data, while for individual early-time or late-time datasets analysed separately, the information criteria are neutral or favour $Λ$CDM. This suggests that the additional degrees of freedom of the three-form field may help accommodate cosmological observations of different origins within a common framework. The reconstructed dark energy dynamics exhibit a characteristic phantom phase at intermediate redshifts while approaching a cosmological-constant-like behaviour at early and late times, providing a distinctive observational signature. Although the model does not significantly alleviate the Hubble tension despite allowing higher values of $H_0$, it remains consistent with current observations and offers a well-motivated alternative to $Λ$CDM whose predictions can be tested by future cosmological surveys.

astro-ph.CO↗

arXiv2026

Alleviating the Hubble Tension with Smooth Sign-Switching Dark Energy: Full CMB Constraints with DESI and PantheonPlus

Sign-switching dark energy has recently been proposed as a minimal modification of the late-time expansion history aimed at alleviating tensions within the standard cosmological model. In this work, we investigate ECDM, a smooth realisation of this scenario, with the dark energy density gradually transitioning from a negative to a positive value. We develop a consistent formulation of the perturbation equations that remains well behaved even when the dark energy equation-of-state parameter diverges during the transition. We confront the model with a comprehensive set of cosmological observations, including cosmic microwave background measurements from Planck 2018, ACT DR6 and SPT-3G, baryon acoustic oscillation measurements from DESI DR2, Type Ia supernova distances from Pantheon+, and local Hubble constant measurement of SH0ES. The inclusion of perturbations allows us to assess the impact of the model on structure growth and CMB anisotropies, providing a more thorough test of sign-switching dark energy. Our results show that this class of models is fully compatible with current precision cosmological observations while alleviating the Hubble tension and providing a compelling modification of the late-time dynamics of the Universe.

astro-ph.CO↗

arXiv2026

Observational constraints on 3-forms dark energy

3-forms are natural candidates for describing the late-time accelerated expansion of the Universe, as they can inherently reproduce a positive cosmological constant when lacking an evolving potential. When such a potential is present, a 3-form field may exhibit either quintessence-like or phantom-like behaviour. In this paper, we consider a late-time effective dark energy model described by a 3-form with a Gaussian potential, stable during the dark-energy-dominated era. We constrain this model observationally by performing a Markov Chain Monte Carlo (MCMC) analysis employing a comprehensive cosmological dataset, including Planck PR4 cosmic microwave background (CMB) data, DESI DR1 baryon acoustic oscillation (BAO) measurements, Pantheon+ Type Ia supernovae data, low-$z$ Cepheid calibrators, and DES Y1 large-scale structure observations. We demonstrate that the 3-form model successfully increases the predicted Hubble parameter of CMB and BAO data from $67.89\pm0.36{\rm km/s/Mpc}$ of $Λ$CDM model to $68.29^{+0.56}_{-0.61}{\rm km/s/Mpc}$ by approaching the potential peak at the right time, thus mildly reducing the tension with the late-time observation. Overall, the 3-form field serves as a promising candidate of phantom-like dark energy from both theoretical and observational points of view.

astro-ph.CO↗

arXiv2026

Sign-Switching Dark Energy: Smooth Transitions with Recent DESI DR2 Observations

Sign-switching dark energy provides a novel mechanism for modifying the late-time expansion history of the Universe without invoking additional fields or finely tuned initial conditions. In this work, we investigate a class of background--level cosmological models in which the dark energy contribution changes sign at a transition redshift $z_\dagger$, producing a sharp deviation from standard $Λ$CDM dynamics. We confront these models with a comprehensive set of cosmological observations, including Planck 18 cosmic microwave background (CMB) measurements, DESI DR2 Baryonic Acoustic Oscillation (BAO) data and the Pantheon+ $\&$ SH0ES Type Ia supernova sample (SN). Using a full Markov Chain Monte Carlo (MCMC) analysis, we find that the sign-switching scenario significantly alleviates the Hubble tension while obtaining better results when statistically comparing with $Λ$CDM, as quantified by the Akaike and Bayesian information Criteria. Although the model is explored only at the background level, the improvement in the inferred Hubble constant demonstrates that sign-switching dark energy offers a promising and physically economical pathway toward resolving late-universe discrepancies.

astro-ph.CO↗

arXiv2026

Dark energy driven by an oscillating generalised axion-like quintessence field

Generalised axion-like scalar fields provide a well-motivated framework for describing the late-time acceleration of the Universe. As the field evolves, it rolls down its potential and, depending on its mass and initial conditions, it may either still be approaching the minimum or already oscillating around it. These two dynamical regimes require distinct treatments of cosmological perturbations. In this work, we perform a detailed analysis of linear cosmological perturbations in the regime where the dark-energy scalar field undergoes coherent oscillations about the minimum of its potential. We show that the standard effective fluid description breaks down in this phase and develop a consistent field-based perturbation framework, which we use to assess the impact of oscillatory dark energy on the growth of cosmic structures.

astro-ph.CO↗

arXiv2026

Quadratic energy-momentum squared gravity: constraints from big bang nucleosynthesis

In this work, we extend the standard cosmological model within the quadratic energy-momentum squared gravity (qEMSG) framework, introducing a nonminimal interaction between the usual material field ($T_{μν}$) and its accompanying partner field (qEMSF, $T_{μν}^{\rm qEMSF}$), defined by $f(\mathbf{T}^2)=α\mathbf{T}^2$ with $\mathbf{T^2}=T_{μν}T^{μν}$. Adopting an analytical approach within the qEMSG framework, we present a comprehensive exploration of Big Bang Nucleosynthesis (BBN) dynamics. Our analysis selects the radiation-dominated universe solution compatible with the standard cosmological model limit as $α\rightarrow0$ and reveals that qEMSF interaction model can modify the radiation energy density's evolution, potentially altering neutron-proton interconversion rates and consequently affecting $^4$He abundance in various ways. By explicitly defining modifications to the predicted primordial $^4$He mass fraction, $Y_{\rm p}$, we establish the most stringent cosmological constraints on the parameter $α$ based on recent measurements of $Y_{\rm p}$: $(-8.81\leqα\leq8.14)\times10^{-27}\,\mathrm{eV}^{-4}$ (68% CL) from Aver et al.'s primordial $^4$He abundance measurements, aligning with $α=0$. Additionally, $(3.48\leqα\leq4.43)\,\times 10^{-27}\rm{eV}^{-4}$ (68% CL) from Fields et al.'s estimates, utilizing the Planck-CMB estimated baryon density within the standard cosmological model framework, diverges from $α=0$, thereby lending support to the qEMSF interaction model. The study also highlights the bidirectional nature of energy-momentum/entropy transfer in qEMSF interaction model, depending on the sign of $α$. The implications of qEMSF in the presence of additional relativistic relics are also explored, showcasing the model's potential to accommodate deviations from standard cosmology and the Standard Model of particle physics.

astro-ph.CO↗

arXiv2026

Modified Teleparallel $f(T)$ Gravity, DESI BAO and the $H_0$ Tension

We investigate whether late-time modifications of gravity in the teleparallel framework can impact the current tension in the Hubble constant $H_0$, focusing on $f(T)$ cosmology as a minimal and well-controlled extension of General Relativity. We consider three representative $f(T)$ parametrisations that recover the teleparallel equivalent of General Relativity at early times and deviate from it only at late epochs. The models are confronted with unanchored Pantheon+ Type~Ia supernovae, DESI DR2 baryon acoustic oscillations, compressed Planck cosmic microwave background distance priors, and redshift-space distortion data, allowing us to jointly probe the background expansion and the growth of cosmic structures. Two of the three models partially shift the inferred value of $H_0$ towards local measurements, while the third worsens the discrepancy. This behaviour is directly linked to the effective torsional dynamics, with phantom-like regimes favouring higher $H_0$ and quintessence-like regimes producing the opposite effect. A global statistical comparison shows that the minimal $f(T)$ extensions considered here are not favoured over $Λ$CDM by the combined data. Nevertheless, our results demonstrate that late-time torsional modifications can non-trivially redistribute current cosmological tensions among the background and growth sectors.

gr-qc↗

arXiv2026

The Sign-Switching of the Cosmological Constant

We propose and investigate a class of dynamical dark energy models in which the cosmological constant evolves from negative values in the early Universe to a positive value at low redshifts. This framework includes a generalised ladder-step evolution, as well as smooth-transition scenarios, providing a unified description of sign-changing cosmological constants. We analyse the theoretical construction and background dynamics of these models using cosmographic diagnostics. Extending this study to the linear perturbation regime, we solve the perturbation equations from the radiation-dominated era with adiabatic initial conditions. We examine the evolution of the matter density contrast, gravitational potential, and the $fσ_8$ observable. Our results are compared against the standard $Λ$CDM model and confronted with current observational data, illustrating the phenomenological viability of sign-changing dark energy models and revealing distinctive imprints on cosmic structure formation arising from the transition of the cosmological constant.

astro-ph.CO↗

arXiv2025

Insights in $f(Q)$ cosmology: the relevance of the connection

We explore the role of the affine connection in $f(Q)$ gravity, a modified theory where gravity is governed by non-metricity within the symmetric teleparallel framework. Although the connection is constrained to be flat and torsionless, it is not uniquely determined by the metric, allowing for multiple physically distinct formulations. We analyze three such connections compatible with a homogeneous and isotropic universe to show that they yield markedly different cosmological dynamics, even under the same functional form of $f(Q)$. Using both analytical and numerical methods, including a Born-Infeld type model of $f(Q)$, we demonstrate that specific connections can resolve cosmological singularities like the Big Bang and Big Rip, replacing them with smooth de Sitter phases. Others retain singularities but with notable modifications in their behavior. These findings highlight the physical relevance of connection choice in $f(Q)$ gravity and its potential to address fundamental cosmological questions.

gr-qc↗

arXiv2025

Addressing $H_0$ and $S_8$ tensions within $f(Q)$ cosmology

We investigate the viability of $f(Q)$ gravity as an alternative framework to address the $H_0$ and $S_8$ tensions in cosmology. Focusing on three representative $f(Q)$ models, we perform a comprehensive Bayesian analysis using a combination of cosmological observations, including cosmic chronometers, Type Ia supernovae, gamma-ray bursts, baryon acoustic oscillations, and CMB distance priors. Our results demonstrate that most of these models can yield higher values of $H_0$ than those predicted by $Λ$CDM, offering a partial alleviation of the tension. In addition, one model satisfies the condition $G_{\mathrm{eff}} < G$, making it a promising candidate for addressing the $S_8$ tension. However, these improvements are accompanied by mild internal inconsistencies between different subsets of data, which limit the overall statistical preference relative to $Λ$CDM. Despite this, $f(Q)$ gravity remains a promising and flexible framework for late-time cosmology, and our results motivate further exploration of extended or hybrid models that may reconcile all observational constraints.

astro-ph.CO↗

arXiv2025

Cosmological perturbations for smooth sign-switching dark energy models

In this work, we carry out a comprehensive perturbative analysis of four cosmological models featuring a sign-switching cosmological constant. Among these, we include the well-known $Λ_{\rm s}$CDM model, characterised by an abrupt transition from a negative to a positive cosmological constant. We also consider the L$Λ$CDM model, which exhibits a generalised ladder-step evolution, as well as the SSCDM and ECDM models, both of which undergo a smooth sign change at comparable redshifts. We solve the linear cosmological perturbation equations from the radiation-dominated era, imposing initial adiabatic conditions for matter and radiation, for modes well outside the Hubble radius in the early Universe. We analyse the behaviour of the matter density contrast, the gravitational potential, the linear growth rate, the matter power spectrum, and the $fσ_8$ evolution . These results are contrasted with predictions from the standard $Λ$CDM model and are confronted with observational data.

gr-qc↗

arXiv2025

Cosmographic analysis of sign-switching dark energy

Inspired by the well-studied $Λ_{\rm s}$CDM model, we propose and investigate a class of dynamical dark energy models that evolve from a negative cosmological constant, transitioning to a positive value at low redshifts. Specifically, we introduce a dark energy framework based on a generalised ladder-step function for the cosmological constant. Furthermore, we present two additional models in which the cosmological constant undergoes a smooth sign change at a specified redshift. We provide a detailed discussion of the construction and theoretical properties of these models, analysing their background cosmological evolution using a cosmographic approach and the statefinder hierarchy parameters. Our results are compared with those obtained for the standard $Λ$CDM and $Λ_{\rm s}$CDM models. We also perform a careful analysis of the types of singularities that may arise in these models due to a sign change in the cosmological constant. Notably, we show that a continuous sign change removes the sudden singularity present in the $Λ_{\rm s}$CDM model, replacing it with a milder $w$-singularity.

gr-qc↗

arXiv2025

Observational constraints on generalised axion-like potentials for the late Universe

Axions have emerged as compelling candidates for describing the dark sector of the Universe. In this work, we explore quintessence models inspired by axion-like potentials as a dynamical alternative to the cosmological constant. These models naturally exhibit a tracking behaviour, reducing the need for fine-tuned initial conditions. We perform a Markov chain Monte Carlo (MCMC) analysis on a complete cosmological dataset of Planck PR4 cosmic microwave background (CMB), DESI DR1 baryon acoustic oscillation (BAO), Pantheon+ type Ia supernovae, low-$z$ Cepheid anchors, and DES Y1 large scale structure measurement. Unfortunately, neither the Hubble tension nor the $S_8$ tension is eased. The model does reach parity with $Λ$CDM model statistically according to DIC, WAIC and Bayesian ratio, suggesting that a quintessence model may still help with the cosmic tension by further extending the model we have investigated.

astro-ph.CO↗

arXiv2025

Speeding up the Universe with a generalised axion-like potential

Understanding the late-time acceleration of the Universe is one of the major challenges in cosmology today. In this paper, we present a new scalar field model corresponding to a generalised axion-like potential. In fact, this model can be framed as a quintessence model based on physically motivated considerations. This potential is capable of alleviating the coincidence problem through a tracking regime. We will as well prove that this potential allows for a late-time acceleration period induced by an effective cosmological constant, which is reached without fine-tuning the initial conditions of the scalar field. In our model, the generalised axion field fuels the late-time acceleration of the Universe rather than fuelling an early dark energy era. Additionally, we will show how the late-time transition to dark energy dominance could be favoured in this model, since the density parameter of the scalar field will rapidly grow in the late phase of the tracking regime.

astro-ph.CO↗

arXiv2025

Cosmological perturbations in a generalised axion-like dark energy model

We analyse the cosmological evolution of a generalised axion-like field that drives the late-time acceleration of the Universe. This model can exhibit tracking behaviour, which alleviates the coincidence problem. The cosmological perturbations are carried within a multi-fluid approach where the scalar field is described by a non-adiabatic fluid, i.e., one whose speed of sound in the rest frame differs from the adiabatic one. The cosmological perturbations are solved since the radiation-dominated epoch, imposing initial adiabatic conditions for matter, radiation and the dark energy component, for modes well outside the Hubble horizon in the past. We analyse the homogeneous curvature perturbation, gravitational potential and dark energy perturbations in this model, as well as the matter power spectrum and fσ8. We discuss which parameters of the model are more favoured observationally.

astro-ph.CO↗

arXiv2024

Dark energy with a shift-symmetric scalar field: obstacles, loophole hunting and dead ends

We discuss the possibility of a scalar field being the fundamental description of dark energy. We focus on shift-symmetric scalar-tensor theories since this symmetry potentially avoids some fine-tuning problems. We also restrict attention to theories satisfying that the propagation speed of gravitational waves is equal to the speed of light. These considerations lead us to investigate shift-symmetric Kinetic Gravity Braiding theories. Analysing the stability of scalar linear perturbations, we discuss the conditions that seems to be necessary to describe (super) accelerated cosmic expansion without introducing instabilities. However, it has been previously established that the linearised analysis does not guarantee the stability of this non-canonical scalar theory, as potentially dangerous interactions between dark energy fluctuations and tensor perturbations (essentially gravitational waves) appear at a higher order in perturbation theory. Indeed, although we shall point out that the standard proof of absence of dark energy stable braiding models due to this interaction has a possible way-out, we find general arguments suggesting that there are no dark energy stable solutions that can exploit this loophole. Thus, we discuss future research directions for finding viable fundamental descriptions of dark energy. We also provide a dictionary between the covariant version of the scalar field theory and that of the Effective Field Theory approach, explicitly computing the parameters in the latter formalism in terms of the functions appearing in the covariant version, and its derivatives. To the best of our knowledge, this is the first time these expressions are explicitly obtained up-to arbitrary order in perturbation theory.

gr-qc↗

arXiv2024

Equivalence of matter-type modified gravity theories to general relativity with nonminimal matter interaction

In this study, we first establish that gravity models incorporating matter-related terms, such as $f(\mathcal{L}_{\rm m})$, $f(g_{μν} T^{μν})$, and $f(T_{μν} T^{μν})$, into the usual matter Lagrangian density $\mathcal{L}_{\rm m}$, are equivalent to general relativity with nonminimal matter interactions. Through the redefinition $\mathcal{L}_{\rm m}+f \rightarrow \mathcal{L}_{\rm m}^{\rm tot}$, these models are exactly GR, yet the usual material field $T_{μν}$ and its accompanying partner, the modification field $T_{μν}^{\rm mod}$, engage in nonminimal interactions. Specifically, $\nabla^μT_{μν}=-Q_ν=-\nabla^μT_{μν}^{\rm mod}$, where $Q_ν$ is the interaction kernel that governs the rate of energy transfer. Our focus narrows on the specific model of $f(T_{μν} T^{μν})$, known as Energy-Momentum Squared Gravity, where the usual material field $T_{μν}$ is accompanied by an \textit{energy-momentum squared field} (EMSF), $T_{μν}^{\rm emsf}$, along with a sui generis nonminimal interaction between them. We demonstrate that a particular $T_{μν}^{\rm emsf}$ can be introduced by \textit{removing} $\frac{\partial^2 \mathcal{L}_{\rm m}}{\partial g^{μν} \partial g^{σε}}$ (the new term emerging in models that incorporate scalars formed from $T_{μν}$), thanks to the freedom in determining the interaction kernel, but this approach compromises the Lagrangian formulation of EMSG. Additionally, we address the ambiguities regarding the perfect fluid stemming from this new term. We show the proper way of calculating this term for a perfect fluid, revealing that it is indeed non-zero, contrary to common assumption in the literature. Finally, we re-examine cosmological models within the realm of EMSG, offering new insights into the applicability and interpretation of our findings in EMSG and similar theoretical frameworks.

gr-qc↗