Search Results (5)

We reanalyze in a simple and comprehensive manner the recently released SH0ES data for the determination of $H_0$. We focus on testing the homogeneity of the Cepheid+SnIa sample and the robustness of the results in the presence of new degrees of freedom in the modeling of Cepheids and SnIa. We thus focus on the four modeling parameters of the analysis: the fiducial luminosity of SnIa $M_B$ and Cepheids $M_W$ and the two parameters ($b_W$ and $Z_W$) standardizing Cepheid luminosities with period and metallicity. After reproducing the SH0ES baseline model results, we allow for a transition of the value of any one of these parameters at a given distance $D_c$ or cosmic time $t_c$, thus adding a single degree of freedom in the analysis. When the SnIa absolute magnitude $M_B$ is allowed to have a transition at $D_c\simeq 50$ Mpc (about 160 Myrs ago), the best-fit value of the Hubble parameter drops from $H_0=73.04\pm 1.04$ km s${}^{-1}$ Mpc${}^{-1}$ to $H_0=67.32\pm 4.64$ km s${}^{-1}$ Mpc${}^{-1}$ in full consistency with the Planck value. Additionally, the best-fit SnIa absolute magnitude $M_B^{>}$ for $D>D_c$ drops to the Planck inverse distance ladder value $M_B^{>}=-19.43\pm 0.15$, while the low distance best fit $M_B^{<}$ parameter remains close to the original distance ladder calibrated value $M_B^{<}=-19.25\pm 0.03$. Similar hints for a transition behavior is found for the other three main parameters of the analysis ($b_W$, $M_W$ and $Z_W$) at the same critical distance $D_c\simeq 50$ Mpc, even though in that case, the best-fit value of $H_0$ is not significantly affected. When the inverse distance ladder constraint on $M_B^{>}$ is included in the analysis, the uncertainties for $H_0$ reduce dramatically ($H_0=68.2\pm 0.8$ km s${}^{-1}$ Mpc${}^{-1}$), and the $M_B$ transition model is strongly preferred over the baseline SH0ES model ($\Delta {\chi }^2\simeq -15$, $\Delta AIC\simeq -13$) according to the AIC and BIC model selection criteria. Full article

Ultra Long Period Cepheids are becoming a very interesting and important topic thanks to the contribution that they can give to understanding the current tension existing between the early-universe and local Hubble constant measurements. These bright pulsating variables are observable up to cosmological distances (larger than 100 Mpc) allowing us, in principle, to measure the Hubble constant without the need for secondary indicators, thus reducing the possible systematic errors in the calibration of the extragalactic distance scale. The Ultra Long Period Cepheids also represent a useful tool for obtaining information on the star formation history of the host galaxy and a challenge for the evolutionary and pulsational models, particularly in the very metal poor regime. In this paper, the largest known ULP sample, consisting of 72 objects, including 10 new candidates, is analyzed to give an observational and theoretical overview of their role as distance indicators and of their evolutionary properties. Full article

It has recently been suggested that a gravitational transition of the effective Newton’s constant $G_{\mathrm{eff}}$ by about 10%, 50–150 Myrs ago could lead to the resolution of both the Hubble crisis and the growth tension of the standard $\Lambda$CDM model. Hints for such an abrupt transition with weaker gravity at times before the transition, have recently been identified in Tully–Fisher galactic mass-velocity data, and also in Cepheid SnIa calibrator data. Here we use Monte-Carlo simulations to show that such a transition could significantly increase (by a factor of 3 or more) the number of long period comets (LPCs) impacting the solar system from the Oort cloud (semi-major axis of orbits ≳${10}^4\mathrm{AU}$). This increase is consistent with observational evidence from the terrestrial and lunar cratering rates, indicating that the impact flux of kilometer sized objects increased by at least a factor of 2 over that last 100 Myrs compared to the long term average. This increase may also be connected with the Chicxulub impactor event that produced the Cretaceous–Tertiary (K-T) extinction of 75% of life on Earth (including dinosaurs) about 66 Myrs ago. We use Monte-Carlo simulations to show that for isotropic Oort cloud comet distribution with initially circular orbits, random velocity perturbations (induced e.g., by passing stars and/or galactic tidal effects), lead to a deformation of the orbits that increases significantly when $G_{\mathrm{eff}}$ increases. A 10% increase in $G_{\mathrm{eff}}$ leads to an increase in the probability of the comets to enter the loss cone and reach the planetary region (pericenter of less than 10 AU) by a factor that ranges from 5% (for velocity perturbation much smaller than the comet initial velocity) to more than 300% (for total velocity perturbations comparable with the initial comet velocity). Full article

Globular clusters are both primary fossils of galactic evolution and formation and are ideal laboratories for constraining the evolution of low-mass and metal-poor stars. RR Lyrae and type II Cepheid variables are low-mass, radially pulsating stars that trace old-age stellar populations. These stellar standard candles in globular clusters are crucial for measuring their precise distances and, in turn, absolute ages, and for the calibration of the extragalactic distance scale. Herein, the evolutionary stages of RR Lyrae and type II Cepheids are discussed, and their pulsation properties, including the light curves, color–magnitude and period–amplitude diagrams, and period–luminosity relations in globular clusters at optical and infrared wavelengths are presented. The RR Lyrae visual magnitude–metallicity relation and the multiband period–luminosity–metallicity relations in globular clusters covering a wide metallicity range are also discussed in detail for their application to the RR Lyrae-based distance scale. Full article

We present a detailed and pedagogical analysis of recent cosmological data, including CMB, BAO, SnIa and the recent local measurement of $H_0$. We thus obtain constraints on the parameters of these standard dark energy parameterizations, including $\Lambda CDM$, and $H\left(z\right)$ deformation models such as $wCDM$ (constant equation of state w of dark energy), and the CPL model (corresponding to the evolving dark energy equation-of-state parameter $w\left(z\right)=w_0+w_a\frac{z}{1+z}$). The fitted parameters include the dark matter density ${\mathrm{\Omega }}_{0m}$, the SnIa absolute magnitude M, the Hubble constant $H_0$ and the dark energy parameters (e.g., w for $wCDM$). All models considered lead to a best-fit value of M that is inconsistent with the locally determined value obtained by Cepheid calibrators (M tension). We then use the best-fit dark energy parameters to reconstruct the quintessence Lagrangian that would be able to reproduce these best-fit parameterizations. Due to the derived late phantom behavior of the best-fit dark energy equation-of-state parameter $w\left(z\right)$, the reconstructed quintessence models have a negative kinetic term and are therefore plagued with instabilities. Full article