Inspired by the recent conjecture originated from graduated dark energy that the Universe has recently transitioned from anti-de Sitter vacua to de Sitter vacua, we extend the $Λ$CDM model by a cosmological constant ($Λ_{\rm s}$) that switches sign at a certain redshift $z_\dagger$, and we call this model $Λ_{\rm s}$CDM. We discuss the construction and theoretical features of this model and find out that, when the consistency of $Λ_{\rm s}$CDM with the CMB data is ensured, (i) $z_\dagger\gtrsim1.1$ is implied by the condition that the Universe monotonically expands, (ii) $H_0$ and $M_B$ (type Ia supernovae absolute magnitude) values are inversely correlated with $z_\dagger$ and reach $H_0\approx74.5~{\rm km\, s^{-1}\, Mpc^{-1}}$ and $M_B\approx-19.2\,{\rm mag}$ for $z_\dagger=1.5$, in agreement with the SH0ES measurements, and (iii) $H(z)$ presents an excellent fit to the Ly-$α$ measurements provided that $z_\dagger\lesssim 2.34$. We further investigate the model constraints by using the full Planck CMB data set, with and without BAO data. We find that the CMB data alone does not constrain $z_\dagger$, but the CMB+BAO data set favors the sign switch of $Λ_{\rm s}$ providing the constraint: $z_\dagger=2.44\pm0.29$ (68% C.L.). Our analysis reveals that the lower and upper limits of $z_\dagger$ are controlled by the Galaxy and Ly-$α$ BAO measurements, respectively, and the larger $z_{\dagger}$ values imposed by the Galaxy BAO data prevent the model from achieving the highest local $H_0$ measurements. In general, $Λ_{\rm s}$CDM (i) relaxes the $H_0$ tension while being fully consistent with the TRGB measurements, (ii) relaxes the $M_B$ tension, (iii) removes the discrepancy with the Ly-$α$ measurements, (iv) relaxes the $S_8$ tension, and (v) finds a better agreement with the BBN constraints on the physical baryon density. [Abridged]
It has recently been shown that the class of running vacuum models (RVMs) has the capacity to fit the overall cosmological observations better than the concordance $Λ$CDM model, therefore supporting the possibility of dynamical dark energy (DE). Apart from the cosmic microwave background (CMB) anisotropies, the most crucial datasets involved are: i) baryonic acoustic oscillations (BAO), and ii) direct large scale structure (LSS) formation data. Analyses mainly focusing on CMB and with insufficient BAO+LSS input generally fail to capture the dynamical DE signature, whereas the few existing stud…
Alleviating the Hubble Tension with Smooth Sign-Switching Dark Energy: Full CMB Constraints with DESI and PantheonPlus
IKERBASQUE, Basque Foundation for Science（伊卡巴斯基克巴斯克科学基金会）；Department of Physics, University of the Basque Country UPV/EHU（巴斯克大学物理系）；EHU Quantum Center, University of the Basque Country UPV/EHU（巴斯克大学量子中心）；Department of Physics, Southern University of Science and Technology（南方科技大学物理系）
Combining pre- and post-recombination new physics to address cosmological tensions: case study with varying electron mass and sign-switching cosmological constant
The possibility that the $H_0$ tension is a sign of a physics beyond the $Λ$CDM model is one of the most exciting possibilities in modern cosmology. The challenge of solving this problem is complicated by several factors, including the worsening of the tension on $σ_8$ parameter when that on $H_0$ is raised. Furthermore, the perspective from which the problem is viewed should not be underestimated, since the tension on $H_0$ can also be interpreted as a tension on the size of the acoustic horizon, $r_s$, which deserves proper discussion. The common approach in the literature consists in propos…
Barrow proposed that the area law of the entropy associated with a horizon might receive a "fractal correction" due to quantum gravitational effects -- in place of $S\propto A$, we have instead $S\propto A^{1+δ/2}$, where $0\leqslant δ\leqslant 1$ measures the deviation from the standard area law ($δ=0$). Based on black hole thermodynamics, we argue that the Barrow entropy should run (i.e., energy scale dependent), which is reasonable given that quantum gravitational corrections are expected to be important only in high energy regime. When applied to the Friedmann equation, we demonstrate the …
At present, cosmological observations set the most stringent bound on the neutrino mass scale. Within the standard cosmological model ($Λ$CDM), the Planck collaboration reports $\sum m_ν< 0.12\,\text{eV}$ at 95% CL. This bound, taken at face value, excludes many neutrino mass models. However, unstable neutrinos, with lifetimes shorter than the age of the universe $τ_ν\lesssim t_U$, represent a particle physics avenue to relax this constraint. Motivated by this fact, we present a taxonomy of neutrino decay modes, categorizing them in terms of particle content and final decay products. Taking in…
A Friendly Phantom: Late-time AdS-to-dS transition and cosmological tensions
Istanbul Technical University（伊斯坦布尔理工大学）；University of Ioannina（约阿尼纳大学）；Deutsches Zentrum für Astrophysik（德国天体物理中心）；Odessa National University（敖德萨国立大学）
BAO miscalibration cannot rescue late-time solutions to the Hubble tension
University of Trento（特伦托大学）；Trento Institute for Fundamental Physics and Applications (TIFPA)-INFN（特伦托基础物理与应用研究所（TIFPA）-意大利国家核物理研究所）；Istanbul Technical University（伊斯坦布尔技术大学）；University of Sheffield（谢菲尔德大学）；Universidade Federal do Espírito Santo（圣埃斯皮里图联邦大学）；INAF – Osservatorio Astronomico di Trieste（意大利国家天体物理研究所的里雅斯特天文台）；IFPU – Institute for Fundamental Physics of the Universe（宇宙基础物理研究所）；University of Ioannina（约阿尼纳大学）
Impostor Among $ν$s: Dark Radiation Masquerading as Self-Interacting Neutrinos
Saha Institute of Nuclear Physics（萨哈核物理研究所）；Homi Bhabha National Institute（霍米·巴巴国立学院）；Washington University（华盛顿大学）；Johannes Gutenberg-Universität Mainz（美因茨约翰内斯古腾堡大学）；Southern University of Science and Technology（南方科技大学）；The University of Texas at Austin（德克萨斯大学奥斯汀分校）
