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Zahra Davari

Publications and source records attributed to Zahra Davari.

16 recordsLinked to original sources
arXiv2026

Alleviating Cosmological Tensions with the Hadrosymmetric Twin Higgs

The Hadrosymmetric Twin Higgs (HTH) model provides a natural solution to the little hierarchy problem by incorporating all three generations of quarks in a twin sector. Unlike other Twin Higgs scenarios, such as the Mirror Twin Higgs (MTH), the HTH framework avoids introducing additional light states or radiation and thus remains consistent with stringent bounds on the effective number of relativistic species, $ΔN_{\rm eff}$. Its particle content and interactions also make it difficult to probe at colliders, highlighting the importance of cosmological tests. In this work, we study the cosmological implications of the HTH model, focusing on the persistent tensions in the Hubble constant ($H_0$) and the matter clustering amplitude ($σ_8$). Implementing the HTH sector in a Boltzmann code and confronting it with cosmic microwave background (CMB) data and local $H_0$ measurements, we find that HTH scenario partially reduces the Hubble tension from more than $4σ$ to about $2.5σ$, while also alleviating the $σ_8$ discrepancy. These results demonstrate that the HTH framework not only addresses naturalness in particle physics but also offers a viable route to mitigating current cosmological tensions, thereby strengthening the link between fundamental theory and precision cosmology.

astro-ph.CO↗
arXiv2025

How Complex is Dark Energy? A Bayesian Analysis of CPL Extensions with Recent DESI BAO Measurements

The nature of dark energy is one of the big puzzling issues in cosmology. While $Λ$CDM provides a good fit to the observational data, evolving dark energy scenarios, such as the CPL parametrization, offer a compelling alternative. In this paper, we present a Bayesian model comparison of various dark energy parametrizations using a joint analysis of Cosmic Microwave Background data, DESI Baryon Acoustic Oscillation measurements, and the PantheonPlus (or Union3) Supernovae type Ia sample. We find that while the $Λ$CDM model is initially favored over a constant $w$CDM model, the CPL parametrization is significantly preferred over $w$CDM, reinforcing recent evidence for an evolving dark energy component, consistent with DESI collaboration findings. Crucially, when testing higher-order CPL extensions, the so-called CPL$^+$ and CPL$^{++}$, our Bayesian analysis shows that the observational data do not favor these more complex scenarios compared to the standard CPL. This result indicates that adding excessive complexity to the CPL form is unwarranted by current observations. Interestingly, similar to the CPL parametrization, alternative two-parameter forms, specifically $w_{de}(a) = w_0 + w_b(1-a)^2$ and $w_{de}(a) = w_0 + w_c(1-a)^3$, yield a better fit to observational data than the standard $Λ$CDM cosmology. Our results challenge the necessity for overly complex CPL extensions and confirm that well-chosen two-parameter $w_0w_a$ parametrizations effectively capture DE evolution with current cosmological data, supporting the recent signals for dynamical dark energy by DESI collaboration.

astro-ph.CO↗
arXiv2025

$Λ$CDM model against cosmography: A possible deviation after DESI 2024

In this study, we present an analysis of the standard flat-$Λ$CDM model using a cosmographic approach, incorporating recent DESI BAO observations and Supernovae Type Ia catalogues (SNIa), including the DES-SN5YR and Pantheon+ compilations. We find full consistency between the standard model and the cosmographic approach when considering DESI BAO and SNIa catalogues independently. When combining DESI BAO with SNIa data, we examine the impact of the Planck prior on the sound horizon at the drag epoch, $r_d$, and the Cepheid prior on the absolute magnitude, $M$. Applying the Planck prior on $r_d$ alone yields an $H_0$ value consistent with the Planck measurement, while applying the Cepheid prior on $M$ alone results in an $H_0$ value consistent with the SH0ES measurement. Without any priors, the $H_0$ value obtained has a large error margin, reconciling the Planck and SH0ES measurements. In all cases where individual priors are applied, we observe no significant tension between the flat-$Λ$CDM model and the cosmographic approach. However, when both Planck and Cepheid priors are applied simultaneously, significant tensions arise between the model and cosmography. This tension is even more pronounced when excluding LRG1 and LRG2 from the DESI measurements. These results indicate that the standard model cannot simultaneously reconcile high-redshift Planck CMB observations and local Cepheid measurements. This discrepancy supports the possibility of new physics beyond the standard model or, alternatively, the presence of unrecognized systematic errors in the observational data.

astro-ph.CO↗
arXiv2024

Modified Compton Effect and CMB Anisotropy

Recent satellite observations have revealed significant anisotropy in the cosmic microwave background (CMB) radiation, a phenomenon that had previously been detected but received limited attention due to its subtlety. With the advent of more precise measurements from satellites, the extent of this anisotropy has become increasingly apparent. This paper examines the CMB radiation by reviewing past research on the causes of CMB anisotropy and presents a new model to explain the observed temperature anisotropy and the anisotropy in the correlation function between temperature and E-mode polarization in the CMB radiation. The proposed model is based on a modified-generalized Compton scattering approach incorporating Loop Quantum Gravity (LQG). We begin by describing the generalized Compton scattering and then discuss the CMB radiation in the context of processes occurring at the last scattering surface. Our findings are derived from the latest observational data from the Planck satellite (2018). In our model, besides the parameters available in the Planck data for the standard model ($Λ$CDM), we introduce two novel parameters: $δ_{L}$, the density of cosmic electrons, and $M^2$, a parameter related to the modified-generalized Compton scattering effects. The results indicate that, based on the 2018 Planck data, small values were obtained for $δ_{L}$ and $M^{2}$, $δ_{L}=1.63\pm0.08(10^{-13})$ and $M^2=2.28\pm0.34(10^{-4})$, showing no significant deviation from the standard model. Moreover, increasing the values of $δ_{L}$ and $M^{2}$ leads to an increase in the range of fluctuations in the CMB temperature anisotropy power spectrum and the correlation function between temperature and E-mode polarization for multipoles $l<500$ until the first peak.

gr-qc↗
arXiv2024

Spherical Collapse Approach for Non-standard Dark Matter Models and Enhanced Early Galaxy Formation in JWST

Using the spherical collapse approach, we investigate the impact of two alternative dark matter models, each characterized by distinct non-zero equations of state, one constant and the other time dependent on the nonlinear regime. Specifically, we compare these models to standard cold dark matter (CDM) by analyzing their influence on the linear density threshold for nonrelativistic component collapse and virial overdensity. Additionally, we explore the number count of collapsed objects, or dark matter halos, analogous to the number count of galaxy clusters. Finally, in light of recent discoveries by the James Webb Space Telescope (JWST), indicating the potential for more efficient early galaxy formation at higher redshifts, we investigate how alternative dark matter assumptions can enhance structure formation efficiency during the early universe.

astro-ph.CO↗
arXiv2023

Dark Matter Cosmology with Varying Viscosity: a Possible Resolution to the $S_8$ Tension

We study varying forms of viscous dark matter and try to address the intriguing tensions of the standard model of cosmology with recent cosmological data, including the Hubble and $S_8$ tensions. We note that by assuming the dark matter viscosity depends on the Hubble parameter, dark matter density, or both, one can improve the statistics. Although the models tend to aggravate the Hubble tension a bit, they tend to reduce the $S_8$ tension, even in comparison with the constant viscosity case. Since similar to viscosity massive neutrinos suppress the power spectrum of matter on small length scales, considering them along with the viscous dark matter, we find that the neutrino mass range is tightened.

astro-ph.CO↗
arXiv2023

$k-$Dependent Dark Matter

With the emersion of precise cosmology and the emergence of cosmic tensions, we are faced with the question of whether the simple model of cold dark matter needs to be extended and whether doing so can alleviate the tensions and improve our understanding of the properties of dark matter. In this study, we investigate one of the generalized models of dark matter so that the behavior of this dark matter changes according to the scale of $k$. In large scales (small $k$'s), the dark matter is cold, while it becomes warm for small scales (large $k$'s). This behavior is modeled phenomenologically for two different scenarios. We show that the $S_8$ tension can be alleviated, but the $H_0$ tension becomes milder while not too much.

hep-ph↗
arXiv2022

Can decaying dark matter scenarios alleviate both $H_0$ and $σ_8$ tensions?

Current tensions in cosmology, including $H_0$ and $σ_8$, provide one of the strong reasons to suspect the existence of physics beyond the standard model of cosmology ($Λ$CDM). In this paper, we investigate if there is a relation between these tensions and beyond cold dark matter scenarios. To model non-CDM, we assume a decaying dark matter (DDM) which is unstable and may decay into two daughter particles, a combination of cold dark matter, warm dark matter, and dark radiation, to explore a vast era of possibilities. We checked our model against CMB data and could show that decaying dark matter seems not a promising candidate to address the cosmological tensions.

astro-ph.CO↗
arXiv2021

MOG cosmology without dark matter and the cosmological constant

In this work, we investigate the MOdified Gravity (MOG) theory for dynamics of the universe and compare the results with the $Λ$CDM cosmology. We study the background cosmological properties of the MOG model and structure formation at the linear perturbation level. We compare the two models with the currently available cosmological data by using statistical Bayesian analyses. After obtaining updated constraints on the free parameters, we use some methods of model selection to assist in choosing the more consistent model such as the reduced chi-squared ($χ^2_{\rm red}$) and a number of the basic information criteria such as the Akaike Information Criterion (AIC), the Bayes factor or Bayesian Information Criterion (BIC), and Deviance Information Criterion (DIC). MOG model appears to be consistent with the $Λ$CDM model by the results of $χ^2_{\rm red}$ and DIC for an overall statistical analysis using the background data and the linear growth of structure formation.

astro-ph.CO↗
arXiv2020

Testing MOdified Gravity (MOG) theory and dark matter model in Milky Way using the local observables

In this paper, we have investigated one of the alternative theories to dark matter named MOdified Gravity (MOG) by testing its ability to describe the local dynamics of the Milky Way in vertical and transverse directions with the baryonic matter. MOG is designed to interpret the dynamics of galaxies and cluster of galaxies without the need for dark matter. We use local observational data such as the vertical dispersion, rotation curve, surface density and number density of stars in the Milky Way to obtained the parameters of MOG and the baryonic component of MW by implementing a Bayesian approach to the parameter estimation based on a Markov Chain Monte Carlo method. We compare our results with the dark matter model of MW. The two models of MOG and CDM are able to describe equally well the rotation curve and the vertical dynamics of stars in the local MW. The best values for the free parameters of MOG in this analysis is obtained as $α= 8.99 \pm 0.02 $ and $μ=0.054\pm 0.005$ kpc$^{-1}$. Also, we obtain the parameters of the generalized gNFW model in the dark matter model. Our best value of bulge mass from MOG is $(1.06 \pm 0.26)\times10^{10}\rm M_{\odot}$ which is consistent with the estimations form the microlensing observations.

astro-ph.GA↗
arXiv2020

Reply to: Overconfidence in Bayesian analyses of galaxy rotation curves

Cameron {\it et al.}~2019 (hereafter C19) recommends a more cautious and rigorous approach to statistical analysis in astronomy. We welcome this particular side of their communication as it helps stimulating the effort towards the adoption of better statistical methods in galaxy rotation curves, an effort to which we contributed with Rodrigues {\it et al.}~2018 (hereafter R18). Indeed, R18 was the first work that, in order to conclude on the universality of the acceleration scale $a_0$, studied the posterior distributions on $a_0$ of a large set of galaxies. As C19 agrees, the credible intervals were found within the Bayesian framework, that is, the marginalized posteriors on $a_0$ were found using Bayes' theorem to update the priors in light of the observational data; this process was done without introducing any approximation. Considering R18, C19 also remarks that: i) better methods to select the nuisance parameters and the corresponding priors could be used; ii) a quality cut based on $χ^2$ values should not be used, and iii) the compatibility of the posteriors should be assessed in a more robust way. In the following, after first clarifying the context of our work, we address these criticisms.

astro-ph.GA↗
arXiv2019

Cosmological constrains on minimally and non-minimally coupled scalar field models

We study the minimally and non-minimally coupled scalar field models as possible alternatives for dark energy, the mysterious energy component that is driving the accelerated expansion of the universe. After discussing the dynamics at both the background and perturbation level, we confront the two models with the latest cosmological data. After obtaining updated constraints on their parameters we perform model selection using the basic information criteria. We found that the $Λ$CDM model is strongly favored when the local determination of the Hubble constant is not considered and that this statement is weakened once local $H_0$ is included in the analysis. We calculate the parameter combination $S_8=σ_8\sqrt{Ω_{m}/0.3}$ and show the decrement of the tension with respect to the Planck results in the case of minimally and non-minimally coupled scalar field models. Finally, for the coupling constant between DE and gravity, we obtain the constraint $ξ\simeq -0.06^{+0.19}_{-0.19}$, approaching the one from solar system tests $|ξ| \lesssim 10^{-2}$ and comparable to the conformal value $ξ=1/6$ at $1σ$ uncertainty.

gr-qc↗
arXiv2018

Absence of a fundamental acceleration scale in galaxies

The Radial Acceleration Relation confirms that a nontrivial acceleration scale can be found in the average internal dynamics of galaxies. The existence of such a scale is not obvious as far as the standard cosmological model is concerned, and it has been interpreted as a possible sign of modified gravity. The implications could be profound: it could in principle explain galactic dynamics without large amounts of yet-undetected dark matter and address issues that the standard cosmological model faces at galactic scales. Here, we consider 193 disk galaxies from the SPARC and THINGS databases and, using Bayesian inference, we show that the probability of existence of a fundamental acceleration that is common to all the galaxies is essentially zero: the $p$-value is smaller than $10^{-20}$ or, equivalently, the null hypothesis is rejected at more than 10$σ$. We conclude that the acceleration scale unveiled by the Radial Acceleration Relation is of emergent nature, possibly caused by a complex interplay between baryons and dark matter. In particular, the MOND theory, or any other theory that behaves like it at galactic scales, is ruled out as a fundamental theory for galaxies at more than 10$σ$.

astro-ph.GA↗
arXiv2018

New parameterization for unified dark matter and dark energy

In this paper we investigate a new phenomenological parameterization for unified dark matter and dark energy based on the polynomial expansion of the barotropic equation of state parameter $w$. Our parameterization provides well-behaving evolution of $w$ for both small and big redshifts as well as in the far future. The dark fluid described by our parameterization behaves for big redshifts like a dark matter. Therefore one can parameterize dark energy and dark matter using a single dark fluid, like in the case of the Chaplygin gas. Within this parameterization we consider 2 models: one with DE barotropic parameter fixed to be $-1$ and the second one, where $w \neq -1$ is chosen to match the best fit to the data. We study main cosmological properties of these models at the expansion and perturbation levels. Based on Markov chain Monte Carlo method with currently available cosmic observational data sets, we constrain these models to determine the cosmological parameters at the level of background and clustering of matter. We consider the interaction between DM and DE which directly affects the evolution of matter and its clustering. Our model appears to be perfectly consistent with the $Λ$CDM model, while providing unification of DE and DM.

gr-qc↗
arXiv2015

Growth of matter perturbations in clustered holographic dark energy cosmologies

We investigate the growth of matter fluctuations in holographic dark energy cosmologies. First we use an overall statistical analysis involving the latest observational data in order to place constraints on the cosmological parameters. Then we test the range of validity of the holographic dark energy models at the perturbation level and its variants from the concordance $Λ$ cosmology. Specifically, we provide a new analytical approach in order to derive, for the first time, the growth index of matter perturbations. Considering a homogeneous holographic dark energy we find that the growth index is $γ\approx \frac{4}{7}$ which is somewhat larger ($\sim 4.8\%$) than that of the usual $Λ$ cosmology, $γ^{(Λ)}\approx \frac{6}{11}$. Finally, if we allow clustering in the holographic dark energy models then the asymptotic value of the growth index is given in terms of the effective sound speed $c_{\rm eff}^2$, namely $γ\approx \frac{3(1-c_{\rm eff}^2)}{7}$.

astro-ph.CO↗