arXiv subjects

Mahdi Najafi

Publications and source records attributed to Mahdi Najafi.

5 recordsLinked to original sources

arXiv2026

When Dark Energy Turns On: Constraints on a Critical Emergence Model

We investigate a specific emergent dark energy scenario, known as critically emergent dark energy (CEDE), in which dark energy is effectively absent in the early Universe and becomes dynamically relevant only after a critical cosmic epoch through a phase transition. We constrain this model using recent cosmological observations, including cosmic microwave background (CMB) data from \emph{Planck} 2018, baryon acoustic oscillation (BAO) measurements from SDSS and DESI DR2, and two independent Type Ia supernova compilations, PantheonPlus and Union3. Our results show that within the CEDE framework a dark energy phase transition is not ruled out. In particular, CMB-only, CMB+SDSS, and CMB+DESI datasets provide evidence for a nonzero transition scale factor and, according to standard statistical indicators such as $\Delta\chi^2$ and Bayesian evidence, can favor CEDE over the $\Lambda$CDM model. At the same time, we find that CEDE does not fully resolve the Hubble constant tension. Overall, our analysis indicates that dark energy models featuring a phase transition remain a viable and phenomenologically interesting extension of the standard cosmological framework. Upcoming high-precision cosmological surveys will be essential to further assess whether such emergent dark energy scenarios represent a genuine departure from $\Lambda$CDM or an effective description of current data.

astro-ph.CO↗

arXiv2025

A Multi-Probe ISW Study of Dark Energy Models with Negative Energy Density: Galaxy Correlations, Lensing Bispectrum, and Planck ISW-Lensing Likelihood

We investigate the late-time imprints of three dark energy (DE) models, namely, the Chevallier-Polarski-Linder (CPL) parametrization, $\Lambda_{\rm s}$CDM, and an Omnipotent DE model, on cosmological observables sensitive to the time evolution of gravitational potentials. While CPL serves as a reference parameterization, the Omnipotent and $\Lambda_{\rm s}$CDM scenarios were originally proposed as possible solutions to the $H_0$ tension and are selected here because they can yield negative dark energy. These models are examined within a multi-probe framework based on the Integrated Sachs-Wolfe (ISW) effect and the lensing-ISW bispectrum. By analyzing both two- and three-point Cosmic Microwave Background (CMB) correlations, we assess how their late-time dynamics modify the growth and decay of large-scale gravitational potentials compared to the standard $\Lambda$CDM cosmology. Despite producing nearly indistinguishable CMB angular spectra at high multipoles, these models yield distinctive signatures in the low-$\ell$ ISW plateau as well as in higher-order statistics related to ISW, highlighting the power of both large-scale CMB anisotropies and higher-order CMB statistics in testing dark energy physics. Our results demonstrate that combining complementary ISW probes provides an effective way to discriminate between dark energy scenarios and will be crucial to determine whether negative or sign-switching dark energy is ultimately favored or disfavored by forthcoming data.

astro-ph.CO↗

arXiv2024

Robust Preference for Dynamical Dark Energy in DESI BAO and SN Measurements

Recent Baryon Acoustic Oscillation (BAO) measurements released by DESI, when combined with Cosmic Microwave Background (CMB) data from Planck and two different samples of Type Ia supernovae (Pantheon-Plus and DESY5) reveal a preference for Dynamical Dark Energy (DDE) characterized by a present-day quintessence-like equation of state that crossed into the phantom regime in the past. A core ansatz for this result is assuming a linear Chevallier-Polarski-Linder (CPL) parameterization $w(a) = w_0 + w_a (1-a)$ to describe the evolution of the DE equation of state (EoS). In this paper, we test if and to what extent this assumption impacts the results. To prevent broadening uncertainties in cosmological parameter inference and facilitate direct comparison with the baseline CPL case, we focus on 4 alternative well-known models that, just like CPL, consist of only two free parameters: the present-day DE EoS ($w_0$) and a parameter quantifying its dynamical evolution ($w_a$). We demonstrate that the preference for DDE remains robust regardless of the parameterization: $w_0$ consistently remains in the quintessence regime, while $w_a$ consistently indicates a preference for a dynamical evolution towards the phantom regime. This tendency is significantly strengthened by DESY5 SN measurements. By comparing the best-fit $\chi^2$ obtained within each DDE model, we notice that the linear CPL parameterization is not the best-fitting case. Among the models considered, the EoS proposed by Barboza and Alcaniz consistently leads to the most significant improvement.

astro-ph.CO↗

arXiv2024

Dynamical dark energy confronted with multiple CMB missions

The measurements of the cosmic microwave background (CMB) have played a significant role in understanding the nature of dark energy. In this article, we investigate the dynamics of the dark energy equation of state, utilizing high-precision CMB data from multiple experiments. We begin by examining the Chevallier-Polarski-Linder (CPL) parametrization, a commonly used and recognized framework for describing the dark energy equation of state. We then explore the general Exponential parametrization, which incorporates CPL as its first-order approximation, and extensions of this parametrization that incorporate nonlinear terms. We constrain these scenarios using CMB data from various missions, including the Planck 2018 legacy release, the Wilkinson Microwave Anisotropy Probe (WMAP), the Atacama Cosmology Telescope (ACT), and the South Pole Telescope (SPT), as well as combinations with low-redshift cosmological probes such as Baryon Acoustic Oscillations (BAO) and the Pantheon sample. While the $\Lambda$CDM cosmology remains consistent within the 68\% confidence level, we observe that the extensions of the CPL parametrization are indistinguishable for Planck data. However, for ACT and SPT data, the inclusion of additional terms begins to reveal a peak in $w_{\rm a, DE}$ that was previously unconstrained.

astro-ph.CO↗

arXiv2024

Structure Formation in Various Dynamical Dark Energy Scenarios

This research investigates the impact of the nature of Dark Energy (DE) on structure formation, focusing on the matter power spectrum and the Integrated Sachs-Wolfe effect (ISW). By analyzing the matter power spectrum at redshifts $z = 0$ and $z = 5$, as well as the ISW effect on the scale of $\ell = 10-100$, the study provides valuable insights into the influence of DE equations of state (EoS) on structure formation. The findings reveal that dynamical DE models exhibit a stronger matter power spectrum compared to constant DE models, with the JBP model demonstrating the highest amplitude and the CPL model the weakest. Additionally, the study delves into the ISW effect, highlighting the time evolution of the ISW source term $\mathcal{F}(a)$ and its derivative $d\mathcal{F}(a)/da$, and demonstrating that models with constant DE EoS exhibit a stronger amplitude of $\mathcal{F}(a)$ overall, while dynamical models such as CPL exhibit the highest amplitude among the dynamical models, whereas JBP has the lowest. The study also explores the ISW auto-correlation power spectrum and the ISW cross-correlation power spectrum, revealing that dynamical DE models dominate over those with constant DE EoS across various surveys. Moreover, it emphasizes the potential of studying the non-linear matter power spectrum and incorporating datasets from the small scales to further elucidate the dynamical nature of dark energy. This comprehensive analysis underscores the significance of both the matter power spectrum and the ISW signal in discerning the nature of dark energy, paving the way for future research to explore the matter power spectrum at higher redshifts and in the non-linear regime, providing deeper insights into the dynamical nature of dark energy.

astro-ph.CO↗