Phichayoot Baisri | Searcharxiv

arXiv2026

$δ$-CDM: A Minimal Deformation of $Λ$CDM with Scalar Field Reconstruction

Recent DESI BAO observations provide intriguing hints that dark energy may be dynamical in nature. To investigate deviations of the dark energy equation of state (EoS) from $w = -1$, we introduce the $δ$-CDM framework, a controlled deformation of $Λ$CDM in which deviations from a cosmological constant are parametrized by a redshift-dependent function $δ(z)$, defined through $w_{\rm de}(z) = -1 + δ(z)$. As an illustrative example, we reconstruct $δ(z)$ using effective scalar field dynamics of thawing type, encompassing both quintessence and phantom regimes within a unified description. Notably, the reconstructed $δ(z)$ is independent of the specific scalar field realization, ensuring theoretical robustness. Using Planck CMB-SPA data, DESI DR2 BAO measurements, and the Pantheon+ supernova sample within a Bayesian Markov Chain Monte Carlo analysis, we find that the $\tilde{w}_0\tilde{w}_a$ parametrization is preferred over this thawing-type realization of deviations from $w = -1$. Overall, the $δ$-CDM framework provides a minimal yet flexible extension of $Λ$CDM, capable of capturing late-time dynamical features of dark energy.

astro-ph.CO↗

arXiv2023

Cosmology in holographic non-minimal derivative coupling theory: constraints from inflation and variation of gravitational constant

We consider a cosmological model of non-minimal derivative coupling (NMDC) to gravity with holographic effect from Bekenstein-Hawking entropy using Hubble horizon IR cutoff. Holographic parameter $c$ is constant in a range, $0 \leq c < 1$. NMDC effect allows gravitational constant to be time-varying. Definition of holographic density include time-varying part of the gravitational constant. NMDC part reduces strength of gravitational constant for $\k > 0$ and opposite for $\k < 0$. The holographic part enhances gravitational strength. We use spectral index and tensor-to-scalar ratio to test the model against CMB constraint. Number of e-folding is chosen to be $N \geq 60$. Potentials, $V = V_0 ϕ^n $ with $n = 2, 4$, and $V = V_0 \exp{(-βϕ)}$ are considered. Combined parametric plots of $\k$ and $ϕ$ show that the allowed regions of the power spectrum index and of the tensor-to-scalar ratio are not overlapping. NMDC inflation is ruled out and the holographic NMDC inflation is also ruled out for $0 < c < 1$. NMDC significantly changes major anatomy of the dynamics, i.e. it gives new late-time attractor trajectories in acceleration regions. The holographic part clearly affects pattern of trajectories. However, for the holographic part to affect shape of the acceleration region, the NMDC field must be in presence. To constrain the model at late time, variation of gravitational constant is considered. Gravitational-wave standard sirens and supernovae data give a constraint, $\dot{G}/G|_{t_0} \lesssim 3\times10^{-12} \, \text{year}^{-1}$ \cite{Zhao:2018gwk} which, for this model, results in $ 10^{-12} \, \text{year}^{-1} \, \gtrsim \, {- κ} \dotϕ\ddotϕ/{M^2_{\p}}\,. $ Positive $\k$ is favored and greater $c^2$ results in lifting up lower bound of $\k$.

gr-qc↗