arXiv subjects

Foteini Skara

Publications and source records attributed to Foteini Skara.

6 recordsLinked to original sources

arXiv2023

On the homogeneity of SnIa absolute magnitude in the Pantheon+ sample

We have analysed the Pantheon+ sample using a new likelihood model that replaces the single SnIa absolute magnitude parameter $M$ used in the standard likelihood model of Brout et. al. with two absolute magnitude parameters $M_<$, $M_>$ and a transition distance $d_{crit}$ that determines the distance at which $M$ changes from $M_<$ to $M_>$. The use of this likelihood dramatically changes the quality of fit to the Pantheon+ sample for a $Λ$CDM background by $Δχ^2=-19.6$. The tension between the $M_<$ and $M_>$ best fit values is at a level more than $3σ$ with a best fit $d_{crit}$ very close to $20Mpc$. The origin of this improvement of fit and $M_<-M_>$ tension is that the new likelihood model, successfully models two signals hidden in the data: 1. The well known systematic effect called 'volumetric redshift scatter bias' which is due to asymmetric peculiar velocity variations at redshifts $z<0.01$ induced by unequal projected volumes at lower and higher distances compared to a given distance and 2. A mild signal for a change of intrinsic SnIa luminosity at about $20Mpc$. This interpretation of the results is confirmed by truncating the $z<0.01$ Hubble diagram data from Pantheon+ where the above systematic is dominant and showing that the $M_<-M_>$ tension decreases from above $3σ$ to a little less than $2σ$. It is also confirmed by a Monte Carlo simulation comparing the SnIa absolute luminosities $M_i$ of SnIa+Cepheid hosts, with the anticipated luminosities in the context of a homogeneous single absolute magnitude $M$. This simulation shows that the maximum significance of the SnIa luminosity transition ($Σ\equiv \frac{|M_>-M_<|}{\sqrt{σ_{M_>}^2+σ_{M_<}^2}}$) in the real data, is larger than the corresponding maximum significance of $94\%$ of the corresponding homogeneous simulated samples.

astro-ph.CO↗

arXiv2022

A reanalysis of the latest SH0ES data for $H_0$: Effects of new degrees of freedom on the Hubble tension

We reanalyze 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 $D_c\simeq 50Mpc$ (about $160Myrs$ ago), the best fit value of the Hubble parameter drops from $H_{0}=73.04\pm1.04\,km\,s^{-1}\,Mpc^{-1}$ to $H_0=67.32\pm 4.64\, km\,s^{-1}\,Mpc^{-1}$ in full consistency with the Planck value. Also, 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 ($Δχ^2 \simeq -15$, $ΔAIC \simeq -13$) according to AIC and BIC model selection criteria.

astro-ph.CO↗

arXiv2022

Challenges for $Λ$CDM: An update

A number of challenges to the standard $Λ$CDM model have been emerging during the past few years as the accuracy of cosmological observations improves. In this review we discuss in a unified manner many existing signals in cosmological and astrophysical data that appear to be in some tension ($2σ$ or larger) with the standard $Λ$CDM model as specified by the Cosmological Principle, General Relativity and the Planck18 parameter values. In addition to the well-studied $5σ$ challenge of $Λ$CDM (the Hubble $H_0$ tension) and other well known tensions (the growth tension, and the lensing amplitude $A_L$ anomaly), we discuss a wide range of other less discussed less-standard signals which appear at a lower statistical significance level than the $H_0$ tension some of them known as 'curiosities' in the data) which may also constitute hints towards new physics. For example such signals include cosmic dipoles (the fine structure constant $α$, velocity and quasar dipoles), CMB asymmetries, BAO Ly$α$ tension, age of the Universe issues, the Lithium problem, small scale curiosities like the core-cusp and missing satellite problems, quasars Hubble diagram, oscillating short range gravity signals etc. The goal of this pedagogical review is to collectively present the current status (2022 update) of these signals and their level of significance, with emphasis on the Hubble tension and refer to recent resources where more details can be found for each signal. We also briefly discuss theoretical approaches that can potentially explain some of these signals.

astro-ph.CO↗

arXiv2022

Gravitational transitions via the explicitly broken symmetron screening mechanism

We generalize the symmetron screening mechanism by allowing for an explicit symmetry breaking of the symmetron $ϕ^4$ potential. A coupling to matter of the form $A(ϕ)=1+\frac{ϕ^2}{M^2}$ leads to an explicitly broken symmetry with effective potential $V_{eff}(ϕ)=-μ^2 (1-\fracρ{μ^2 M^2})ϕ^2 +\fracλ{2}ϕ^4 + 2 \varepsilon ϕ^3+\fracλ{2}η^4$. Due to the explicit symmetry breaking induced by the cubic term we call this field the 'asymmetron'. For large matter density $ρ>ρ_*\equiv μ^2M^2+\frac{9}{4}\varepsilonηM^2$ the effective potential has a single minimum at $ϕ=0$ leading to restoration of General Relativity as in the usual symmetron screening mechanism. For low matter density however, there is a false vacuum and a single true vacuum due to the explicit symmetry breaking. This is expected to lead to an unstable network of domain walls with slightly different value of the gravitational constant $G$ on each side of the wall. This network would be in constant interaction with matter overdensities and would lead to interesting observational signatures which could be detected as gravitational and expansion rate transitions in redshift space. Such a gravitational transition has been recently proposed for the resolution of the Hubble tension.

astro-ph.CO↗

arXiv2021

Hubble tension or a transition of the Cepheid SnIa calibrator parameters?

We re-analyze the Cepheid data used to infer the value of $H_0$ by calibrating SnIa. We do not enforce a universal value of the empirical Cepheid calibration parameters $R_W$ (Cepheid Wesenheit color-luminosity parameter) and $M_H^{W}$ (Cepheid Wesenheit H-band absolute magnitude). Instead, we allow for variation of either of these parameters for each individual galaxy. We also consider the case where these parameters have two universal values: one for low galactic distances $D D_c$ where $D_c$ is a critical transition distance. We find hints for a $3σ$ level mismatch between the low and high galactic distance parameter values. We then use AIC and BIC criteria to compare and rank the following types of models: Base models: Universal values for $R_W$ and $M_H^{W}$ (no parameter variation), I Individual fitted galactic $R_W$ with a universal fitted $M_H^{W}$, II Universal fixed $R_W$ with individual fitted galactic $M_H^{W}$, III Universal fitted $R_W$ with individual fitted galactic $M_H^{W}$, IV Two universal fitted $R_W$ (near and far) with one universal fitted $M_H^{W}$, V Universal fitted $R_W$ with two universal fitted $M_H^{W}$ (near and far), VI Two universal fitted $R_W$ with two universal fitted $M_H^{W}$ (near and far). We find that the AIC and BIC criteria consistently favor model IV instead of the commonly used Base model where no variation is allowed for the Cepheid empirical parameters. The best fit value of the SnIa absolute magnitude $M_B$ and of $H_0$ implied by the favored model IV is consistent with the inverse distance ladder calibration based on the CMB sound horizon $H_0=67.4\pm 0.5\,km\,s^{-1}\,Mpc^{-1}$. Thus in the context of the favored model IV the Hubble crisis is not present. This model may imply the presence of a fundamental physics transition taking place at a time more recent than $100\,Myrs$ ago.

astro-ph.CO↗

arXiv2020

Weak gravity on a $Λ$CDM background

We consider Horndeski modified gravity models obeying stability, velocity of gravitational waves $c_T$ equals $c$ and quasistatic approximation (QSA) on subhorizon scales. We assume further a $Λ$CDM background expansion and a monotonic evolution on the cosmic background of the $α$ functions as $α_i= α_{i0}~a^s$ where $i=M,B$, $a$ is the scale factor and $α_{i0}$ ($α_{M0}, α_{B0}$), $s$ are arbitrary parameters. We show that the growth and lensing reduced (dimensionless) gravitational couplings $μ\equiv G_{\rm growth}/G$, $Σ\equiv G_{\rm lensing}/G$ exhibit the following generic properties today: $Σ_0 < 1$ for all viable parameters, $μ_0<1$ (weak gravity today) is favored for small $s$ while $μ_0>1$ is favored for large $s$. We establish also the relation $μ\geq Σ$ at all times. Taking into account the $fσ_8$ and $E_G$ data constrains the parameter $s$ to satisfy $s\lesssim 2$. Hence these data select essentially the weak gravity regime today ($μ_0<1$) when $s<2$, while $μ_0>1$ subsists only marginally for $s\approx 2$. At least the interval $0.5\lesssim s \lesssim 2$ would be ruled out in the absence of screening. We consider further the growth index $γ(z)$ and identify the $(α_{M0},α_{B0},s)$ parameter region that corresponds to specific signs of the differences $γ_0-γ_0^{ΛCDM}$, and $γ_1-γ_1^{ΛCDM}$, where $γ_0\equiv γ\bigl|_{z=0}$ and $γ_1\equiv \frac{{\rm d}γ}{\rm d z}\bigl|_{z=0}$. In this way important information is gained on the past evolution of $μ$. We obtain in particular the signature $γ_0>γ_0^{ΛCDM}$ for $s<2$ in the selected weak gravity region.

gr-qc↗