arXiv subjects

Leandros Perivolaropoulos

Publications and source records attributed to Leandros Perivolaropoulos.

At least 37 records · Page 2Linked to original sources

arXiv2025

Dynamical dark energy with AdS-to-dS and dS-to-dS transitions: Implications for the $H_0$ tension

We investigate the dynamics and cosmological implications of dark energy (DE), modeled as a scalar field with a hyperbolic tangent potential that induces a smooth shift in the effective cosmological constant (CC), encompassing transitions such as AdS-dS, 0-dS, and dS-dS, with the mirror AdS-dS as a particular case aligned with the $Λ_{\rm s}$CDM scenario. In our construction, a phantom scalar field with a negative kinetic term drives a bottom-up transition from an AdS-like vacuum at high redshifts to a dS-like vacuum at low redshifts, thereby providing a physical underpinning for the $Λ_{\rm s}$CDM scenario. Despite the negative kinetic term, the step-like form of the potential prevents pathologies such as unbounded energy growth, Big Rip, and violations of the WEC. Our numerical integration of the equations of motion shows that the model is consistent with both CMB data and the SH0ES determination of $H_0$, thereby addressing the $H_0$ tension, with all key kinematical parameters-$H(z)$, $\dot{H}(z)$, and $q(z)$-evolving smoothly. The total energy density of the phantom and matter system remains positive at all times, and the effective EoS stays above -1, ensuring that the WEC is satisfied. While the phantom field's energy density and pressure remain finite throughout, its EoS exhibits a safe singularity as its energy density smoothly crosses zero. We perform analysis of the transition period, demonstrating that the evolution of the DE density from a negative CC-like regime to a positive one does not exactly mirror the behavior of the potential-e.g., it lasts longer-as it also involves the kinetic term. We also show that analogous quintessence models featuring dS-dS transitions predict an $H_0$ value lower than $Λ$CDM, thereby failing to address the $H_0$ tension. Our results establish a robust theoretical foundation for the $Λ_{\rm s}$CDM scenario.

astro-ph.CO↗

arXiv2025

Gravitational Wave Signatures Induced by Dark Fluid Accretion in Binary Systems

We investigate the impact of dark fluid accretion on gravitational waveforms emitted by a compact binary system consisting of a supermassive black hole and a stellar-mass black hole. Using a Lagrangian framework with 1~PN and 2.5~PN corrections, we analyze the effects of the spherically symmetric accretion of a fluid with steady-state flow, including those characterized by an equation of state parameter resembling dark energy, on the binary's dynamics. We validate our approach by comparing it with previous studies in the common region of validity and extend the analysis to include both local effects, such as dynamical friction, and global gravitational interactions with the stellar-mass black hole, focusing on their dependence on the fluid's properties. Our analysis reveals that these interactions induce de-phasing in gravitational waveforms, with the phase shift influenced by the fluid's equation of state and energy density. We also extend the study to sudden cosmological singularities, finding that, although they can deform the binary's orbit from initially circular to elliptical, their effect on de-phasing is negligible for cosmologically relevant energy densities. By incorporating both the local and global gravitational interactions of a fluid on a two-body system into the equations of motion, this preliminary study provides a framework for understanding the interplay between fluid dynamics and gravitational wave emissions in astrophysical systems. It further reinforces the potential for probing the properties of astrophysically relevant fluids through gravitational wave observations.

gr-qc↗

arXiv2024

Hubble Tension or Distance Ladder Crisis?

We present an up-to-date compilation of published Hubble constant ($H_0$) measurements that are independent of the CMB sound horizon scale. This compilation is split in two distinct groups: A. Distance Ladder Measurements sample comprising of 20 recent measurements, primarily from the past four years, utilizing various rung 2 calibrators and rung 3 cosmic distance indicators. B.One-Step Measurements sample including 33 measurements of $H_0$ that are independent of both the CMB sound horizon scale and the distance ladder approach. These 33 measurements are derived from diverse probes such as Cosmic Chronometers, gamma-ray attenuation, strong lensing, megamasers etc. Statistical analysis reveals a significant distinction between the two samples. The distance ladder-based sample yields a best fit $H_0 = 72.8 \pm 0.5$ km s$^{-1}$ Mpc$^{-1}$ with $χ^2/dof=0.51$ indicating some correlations. The one-step measurements result in $H_0 = 69.0 \pm 0.48$ km s$^{-1}$ Mpc$^{-1}$ with $χ^2/dof=1.37$ indicating some internal tension. If two outlier measurements are removed (TDCOSMO.I-2019 known to have systematics and MCP-2020) the best fit of the one step sample reduces to $H_0 = 68.3 \pm 0.5$ km s$^{-1}$ Mpc$^{-1}$ with $χ^2/dof=0.95$, fully self-consistent and consistent with sound horizon based measurements. A Kolmogorov-Smirnov test yields a p-value of 0.0001 suggesting that the two samples are fundamentally distinct, with a probability of less than 0.01\% that they are drawn from the same underlying distribution. These findings suggest that the core of the Hubble tension lies not between early and late-time measurements, but between distance ladder measurements and all other $H_0$ determinations. This discrepancy points to either a systematic effect influencing all distance ladder measurements or a fundamental physics anomaly affecting at least one rung of the distance ladder.

astro-ph.CO↗

arXiv2024

Effects of a local physics change on the SH0ES determination of $H_0$

The Hubble tension, a significant discrepancy between the Hubble constant ($H_0$) values derived from early-time (Cosmic Microwave Background and Baryon Acoustic Oscillations) and late-time (Cepheid-calibrated Type Ia Supernovae) measurements, remains a major challenge in cosmology. Traditional attempts to resolve this tension have struggled to maintain consistency with dynamical and geometrical probes at redshifts $0.01 < z \lesssim 2.5$. We explore a novel model introducing new degrees of freedom in local physical laws affecting calibrators like Cepheids and Type Ia Supernovae within a distance of $d \lesssim 50$ Mpc ($z \lesssim 0.01$). Specifically, we incorporate a gravitational transition causing a change in the gravitational constant ($G$) at a specific distance, affecting the Cepheid Period-Luminosity Relation (PLR) and the absolute magnitude of SNe Ia. We verify the inverse scaling of SN luminosity $L$ with Chandrasekhar Mass $M_C$ in a changed $G$ scenario as predicted using a semi-analytical model in a recent theoretical study \cite{Wright2018}. Fixing $ΔG/G \approx 0.04$, our model naturally resolves the Hubble tension, yielding a best-fit $H_0$ value consistent with the Planck measurement, even without using Planck data. This approach suggests a potential resolution to the Hubble tension by aligning $H_0$ with high-redshift CMB measurements.

astro-ph.CO↗

arXiv2024

Transition dynamics in the $Λ_{\rm s}$CDM model: Implications for bound cosmic structures

We explore the predictions of $Λ_{\rm s}$CDM, a novel framework suggesting a rapid anti-de Sitter (AdS) to de Sitter (dS) vacua transition in the late Universe, on bound cosmic structures. In its simplest version, the cosmological constant, $Λ_{\rm s}$, abruptly switches sign from negative to positive, attaining its present-day value at a redshift of ${z_\dagger\sim 2}$. The $Λ_{\rm s}$CDM model emerges as a promising solution to major cosmological tensions, particularly the $H_0$ and $S_8$ tensions, as well as other less definite tensions. A key aspect of our investigation is examining the impact of the abrupt $Λ_{\rm s}$CDM model on the formation and evolution of bound cosmic structures. We identify three primary influences: (i) the negative cosmological constant (AdS) phase for $z > z_\dagger$, (ii) the abrupt transition marked by a type II (sudden) singularity, leading to an abrupt increase in the universe's expansion rate at $z=z_\dagger$, and (iii) an increased expansion rate in the late universe under a positive cosmological constant for $z < z_\dagger$, compared to $Λ$CDM. Utilizing the spherical collapse model, we investigate the non-linear evolution of bound cosmic structures within the $Λ_{\rm s}$CDM framework. We find that the virialization process of cosmic structures, and consequently their matter overdensity, varies depending on whether the AdS-dS transition precedes or follows the turnaround. Specifically, structures virialize with either increased or reduced matter overdensity compared to the Planck-$Λ$CDM model, contingent on the timing of the transition. Additionally, our results demonstrate that the sudden singularity does not result in the dissociation of bound systems. Despite its profound nature, the singularity exerts only relatively weak effects on such systems, thereby reinforcing the model's viability in this context.

astro-ph.CO↗

arXiv2024

Hubble tension tomography: BAO vs SnIa distance tension

We investigate the redshift dependence of the Hubble tension by comparing the luminosity distances obtained using an up-to-date BAO dataset (including the latest DESI data) calibrated with the CMB-inferred sound horizon, and the Pantheon+ SnIa distances calibrated with Cepheids. Using a redshift tomography method, we find: 1) The BAO-inferred distances are discrepant with the Pantheon+ SnIa distances across all redshift bins considered, with the discrepancy level varying with redshift. 2) The distance discrepancy is more pronounced at lower redshifts ($z \in [0.1,0.8]$) compared to higher redshifts ($z\in [0.8,2.3]$). The consistency of $Λ$CDM best fit parameters obtained in high and low redshift bins of both BAO and SnIa samples is investigated and we confirm that the tension reduces at high redshifts. Also a mild tension between the redshift bins is identified at higher redshifts for both the BAO and Pantheon+ data with respect to the best fit value of $H_0$ in agreement with previous studies which find hints for an 'evolution' of $H_0$ in the context of $Λ$CDM. These results confirm that the low redshift BAO and SnIa distances can only become consistent through a re-evaluation of the distance calibration methods. An $H(z)$ expansion rate deformation alone is insufficient to resolve the tension. Our findings also hint at a possible deviation of the expansion rate from the Planck18/$Λ$CDM model at high redshifts $z\gtrsim 2$. We show that such a deformation is well described by a high redshift transition of $H(z)$ like the one expressed by $Λ_s$CDM even though this alone cannot fully resolve the Hubble tension due to its tension with intermediate/low $z$ BAO data.

astro-ph.CO↗

arXiv2024

Special Issue on Modified Gravity Approaches to the Tensions of $Λ$CDM: Goals and Highlights

The Special Issue on "Modified Gravity Approaches to the Tensions of $Λ$CDM"} in the Universe journal tackles significant challenges faced by the $Λ$CDM model, including discrepancies in the Hubble constant, growth rate of structures, and cosmological anisotropies. These issues suggest foundational cracks in the model, raising questions about the validity of General Relativity, dark energy, and cosmological principles at large scales. This collection brings together leading researchers to delve into Modified Gravity theories as potential solutions. Covering approaches from Scalar-Tensor theories to $f(R,T)$ gravity and beyond, each contribution presents innovative research aimed at addressing the limitations of the $Λ$CDM model. This Special Issue not only highlights the theoretical and empirical strengths of Modified Gravity models but also opens avenues for future investigations, emphasizing the synergy between theoretical advancements and observational evidence to deepen our cosmological understanding.

gr-qc↗

arXiv2023

The Laser Interferometer Space Antenna mission in Greece White Paper

The Laser Interferometer Space Antenna (LISA) mission, scheduled for launch in the mid-2030s, is a gravitational wave observatory in space designed to detect sources emitting in the millihertz band. LISA is an ESA flagship mission, currently entering the Phase B development phase. It is expected to help us improve our understanding about our Universe by measuring gravitational wave sources of different types, with some of the sources being at very high redshifts $z\sim 20$. On the 23rd of February 2022 we organized the 1$^\mathrm{st}$ {\it LISA in Greece Workshop}. This workshop aimed to inform the Greek scientific and tech industry community about the possibilities of participating in LISA science and LISA mission, with the support of the Hellenic Space Center (HSC). In this white paper, we summarize the outcome of the workshop, the most important aspect of it being the inclusion of $15$ Greek researchers to the LISA Consortium, raising our total number to $22$. At the same time, we present a road-map with the future steps and actions of the Greek Gravitational Wave community with respect to the future LISA mission.

gr-qc↗

arXiv2023

The density of virialized clusters as a probe of dark energy

We use the spherical collapse model to demonstrate that the observable average density of virialized clusters depends on the properties of dark energy along with the properties of gravity on cluster scales and can therefore be used as a probe of these properties. As an application of this approach we derive the predicted virialized densities and radii of cluster mass structures for a wide range of values of the cosmological constant (including negative values) as a function of the turnaround redshift. For the value of $Ω_{Λ,0}=-0.7$ (with $Ω_{m,0}=0.3$) preferred by $Λ$ sign-switching models ($Λ_s\text{CDM}$) proposed for the resolution of the Hubble and $S_8$ tensions, we find an amplification of the density of virialized clusters which can be as large as $80\%$ compared to \plcdm for a turnaround redshift $z_{\text{max}} \gtrsim 2$. Such an amplification may lead to more efficient early galaxy formation in this class of models in accordance with the recent findings of JWST.

astro-ph.CO↗

arXiv2023

Effects of a Late Gravitational Transition on Gravitational Waves and Anticipated Constraints

We investigate the evolution of gravitational waves through discontinuous evolution (transition) of the Hubble expansion rate $H(z)$ at a sudden cosmological singularity, which may be due to a transition of the value of the gravitational constant. We find the evolution of the scale factor and the gravitational wave waveform through the singularity by imposing the proper boundary conditions. We also use existing cosmological data and mock data of future gravitational wave experiments (the ET) to impose current and anticipated constraints on the magnitude of such a transition. We show that mock data of the Einstein Telescope can reduce the uncertainties by up to a factor of three depending on the cosmological parameter considered.

astro-ph.CO↗

arXiv2023

On the isotropy of SnIa absolute magnitudes in the Pantheon+ and SH0ES samples

We use the hemisphere comparison method to test the isotropy of the SnIa absolute magnitudes of the Pantheon+ and SH0ES samples in various redshift/distance bins. We compare the identified levels of anisotropy in each bin with Monte-Carlo simulations of corresponding isotropised data to estimate the frequency of such levels of anisotropy in the context of an underlying isotropic cosmological. We find that the identified levels of anisotropy in all bins are consistent with the Monte-Carlo isotropic simulated samples. However, in the real samples for both the Pantheon+ and the SH0ES cases we find sharp changes of the level of anisotropy occuring at distances less than $40Mpc$. For the Pantheon+ sample we find that the redshift bin $[0.005,0.01]$ is significantly more anisotropic than the other 5 redshift bins considered. For the SH0ES sample we find a sharp drop of the anisotropy level at distances larger than about $30Mpc$. These anisotropy transitions are relatively rare in the Monte-Carlo isotropic simulated data and occur in $2\%$ of the SH0ES simulated data and at about $7\%$ of the Pantheon+ isotropic simulated samples. This effect is consistent with the experience of an off center observer in a $30Mpc$ bubble of distinct physics or systematics.

astro-ph.CO↗

arXiv2023

Modified Gravity and Cosmology: An Update by the CANTATA Network

General Relativity and the $Λ$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. This work is a Review of the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization was performed in the framework of the COST European Action ``Cosmology and Astrophysics Network for Theoretical Advances and Training Actions''.

gr-qc↗

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

Late-transition vs smooth $H(z)$ deformation models for the resolution of the Hubble crisis

Gravitational transitions at low redshifts ($z_t<0.1$) have been recently proposed as a solution to the Hubble and growth tensions. Such transitions would naturally lead to a transition in the absolute magnitude $M$ of type Ia supernovae (SnIa) at $z_t$ (Late $M$ Transitions - $LMT$) and possibly in the dark energy equation of state parameter $w$ (Late $w-M$ Transitions - $LwMT$). Here, we compare the quality of fit to cosmological data of this class of models, with the corresponding quality of fit of the cosmological constant model ($Λ$CDM) and some of the best smooth $H(z)$ deformation models ($w$CDM, CPL, PEDE). We also perform model selection via the Akaike Information Criterion and the Bayes factor. We use the full CMB temperature anisotropy spectrum data, the baryon acoustic oscillations (BAO) data, the Pantheon SnIa data, the SnIa absolute magnitude $M$ as determined by Cepheid calibrators and the value of the Hubble constant $H_0$ as determined by local SnIa calibrated using Cepheids. We find that smooth $H(z)$ deformation models perform worse than transition models for the following reasons: 1) They have a worse fit to low-$z$ geometric probes (BAO and SnIa data); 2) They favor values of the SnIa absolute magnitude $M$ that are lower as compared to the value $M_c$ obtained with local Cepheid calibrators at $z<0.01$; 3) They tend to worsen the $Ω_\mathrm{m,0}-σ_\mathrm{8,0}$ growth tension. We also find that the $w-M$ transition model ($LwMT$) does not provide a better quality of fit to cosmological data than a pure $M$ transition model ($LMT$) where $w$ is fixed to the \lcdm value $w=-1$ at all redshifts. We conclude that the $LMT$ model has significant statistical advantages over smooth late-time $H(z)$ deformation models in addressing the Hubble crisis.

astro-ph.CO↗

arXiv2022

Observational constraints on the deceleration parameter in a tilted universe

We study a parametrization of the deceleration parameter in a tilted universe, namely a cosmological model equipped with two families of observers. The first family follows the smooth Hubble flow, while the second are the real observers residing in a typical galaxy inside a bulk flow and moving relative to the smooth Hubble expansion with finite peculiar velocity. We use the compilation of Type Ia Supernovae (SnIa) data, as described in the Pantheon dataset, to find the quality of fit to the data and study the redshift evolution of the deceleration parameter. In so doing, we consider two alternative scenarios, assuming that the bulk-flow observers live in the $Λ$CDM and in the Einstein-de Sitter universe. We show that a tilted Einstein-de Sitter model can reproduce the recent acceleration history of the universe, without the need of a cosmological constant or dark energy, by simply taking into account linear effects of peculiar motions. By means of a Markov Chain Monte Carlo (MCMC) method, we also constrain the magnitude and the uncertainties of the parameters of the two models. From our statistical analysis, we find that the tilted Einstein-de Sitter model, equipped with one or two additional parameters that describe the assumed large-scale velocity flows, performs similar to the standard $Λ$CDM paradigm in the context of model selection criteria (Akaike Information Criterion and Bayesian Information Criterion).

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↗