Thanks to the release of the extraordinary EHT image of shadow attributed to the M87* supermassive black hole (SMBH), we have a novel window to assess the validity of fundamental physics in the strong-field regime. Motivated by this, we consider Joha
nnsen & Psaltis metric parameterized by mass, spin, and an additional dimensionless hair parameter $epsilon$. This parametric framework in the high rotation regimes provides a well-behaved bed to the strong-gravity test of the no-hair theorem (NHT) using the EHT data. Incorporating the $epsilon$ into the standard Kerr spacetime enrich it in the sense that, depending on setting the positive and negative values for that, we deal with alternative compact objects: deformed Kerr naked singularity and Kerr BH solutions, respectively. Shadows associated with these two possible solutions indicate that the deformation parameter $epsilon$ affects the geometry shape of standard shadow such that it becomes more oblate and prolate with $epsilon<0$ and $epsilon>0$, respectively. By scanning the window associated with three shadow observables oblateness, deviation from circularity, and shadow diameter, we perform a numerical analysis within the range $a_*=0.9mp0.1$ of the dimensionless rotation parameter, to find the constraints on the hair parameter $epsilon$ in both possible solutions. For both possible signs of $epsilon$, we extract a variety of upper bounds that are in interplay with $a_*$. Although by approaching the rotation parameters to the extreme limit, the allowable range of both hair parameters becomes narrower, the hairy Kerr BH solution is a more promising candidate to play the role of the alternative compact object instead of the standard Kerr BH. The lack of tension between hairy Kerr BH with the current observation of the EHT shadow of the M87* SMBH carries this message that there is the possibility of NHT violation.
We determine the relationship between the turnaround radius, $R_{rm t}$, and mass, $M_{rm t}$, in $Lambda$CDM, and in dark energy scenarios, using an extended spherical collapse model taking into account the effects of shear and vorticity. We find a
more general formula than that usually described in literature, showing a dependence of $R_{rm t}$ from shear, and vorticity. The $R_{rm t}-M_{rm t}$ relation differs from that obtained not taking into account shear, and rotation, especially at galactic scales, differing $simeq 30%$ from the result given in literature. This has effects on the constraint of the $w$ parameter of the equation of state. We compare the $R_{rm t}-M_{rm t}$ relationship obtained for the $Lambda$CDM, and different dark energy models to that obtained in the $f(R)$ modified gravity (MG) scenario. The $R_{rm t}-M_{rm t}$ relationship in $Lambda$CDM, and dark energy scenarios are tantamount to the prediction of the $f(R)$ theories. Then, the $R_{rm t}-M_{rm t}$ relationship is not a good probe to test gravity theories beyond Einsteins general relativity.
We investigate the efficiency of screening mechanisms in the hybrid metric-Palatini gravity. The value of the field is computed around spherical bodies embedded in a background of constant density. We find a thin shell condition for the field dependi
ng on the background field value. In order to quantify how the thin shell effect is relevant, we analyze how it behaves in the neighborhood of different astrophysical objects (planets, moons or stars). We find that the condition is very well satisfied except only for some peculiar objects. Furthermore we establish bounds on the model using data from solar system experiments such as the spectral deviation measured by the Cassini mission and the stability of the Earth-Moon system, which gives the best constraint to date on $f(R)$ theories. These bounds contribute to fix the range of viable hybrid gravity models.
In this work we will test an alternative model of gravity belonging to the large family of galileon models. It is characterized by an intrinsic breaking of the Vainshtein mechanism inside large astrophysical objects, thus having possibly detectable o
bservational signatures. We will compare theoretical predictions from this model with the observed total mass profile for a sample of clusters of galaxies. The profiles are derived using two complementary tools: X-ray hot intra-cluster gas dynamics, and strong and weak gravitational lensing. We find that a dependence with the dynamical internal status of each cluster is possible; for those clusters which are very close to be relaxed, and thus less perturbed by possible astrophysical local processes, the galileon model gives a quite good fit to both X-ray and lensing observations. Both masses and concentrations for the dark matter halos are consistent with earlier results found in numerical simulations and in the literature, and no compelling statistical evidence for a deviation from general relativity is detectable from the present observational state. Actually, the characteristic galileon parameter $Upsilon$ is always consistent with zero, and only an upper limit ($lesssim0.086$ at $1sigma$, $lesssim0.16$ at $2sigma$, and $lesssim0.23$ at $3sigma$) can be established. Some interesting distinctive deviations might be operative, but the statistical validity of the results is far from strong, and better data would be needed in order to either confirm or reject a potential tension with general relativity.
Modified gravity theories with a screening mechanism have acquired much interest recently in the quest for a viable alternative to General Relativity on cosmological scales, given their intrinsic property of being able to pass Solar System scale test
s and, at the same time, to possibly drive universe acceleration on much larger scales. Here, we explore the possibility that the same screening mechanism, or its breaking at a certain astrophysical scale, might be responsible of those gravitational effects which, in the context of general relativity, are generally attributed to Dark Matter. We consider a recently proposed extension of covariant Galileon models in the so-called beyond Horndeski scenario, where a breaking of the Vainshtein mechanism is possible and, thus, some peculiar observational signatures should be detectable and make it distinguishable from general relativity. We apply this model to a sample of clusters of galaxies observed under the textit{CLASH} survey, using both new data from gravitational lensing events and archival data from X-ray intra-cluster hot gas observations. In particular, we use the latter to model the gas density, and then use it as the only ingredient in the matter clusters budget to calculate the expected lensing convergence map. Results show that, in the context of this extended Galileon, the assumption of having only gas and no Dark Matter at all in the clusters is able to match observations. We also obtain narrow and very interesting bounds on the parameters which characterize this model. In particular, we find that, at least for one of them, the general relativity limit is excluded at $2sigma$ confidence level, thus making this model clearly statistically different and competitive with respect to general relativity.
We present measurements of the spatial clustering statistics in redshift space of various scalar field modified gravity simulations. We utilise the two-point and the three-point correlation functions to quantify the spatial distribution of dark matte
r halos within these simulations and thus discern between the models. We compare $Lambda$CDM simulations to various modified gravity scenarios and find consistency with previous work in terms of 2-point statistics in real and redshift-space. However using higher order statistics such as the three-point correlation function in redshift space we find significant deviations from $Lambda$CDM hinting that higher order statistics may prove to be a useful tool in the hunt for deviations from General Relativity.
The field equations in FRW background for the so called C-theories are presented and investigated. In these theories the usual Ricci scalar is substituted with $f(mathcal{R})$ where $mathcal{R}$ is a Ricci scalar related to a conformally scaled metri
c $hat{g}_{mu u} = mathcal{C}(mathcal{R})g_{mu u}$, where the conformal factor itself depends on $mathcal{R}$. It is shown that homogeneous perturbations of this Ricci scalar around general relativity FRW background of a large class of these theories are either inconsistent or unstable.
It is shown that extensions to General Relativity, which introduce a strongly coupled scalar field, can be viable if the interaction has a non-conformal form. Such disformal coupling depends upon the gradients of the scalar field. Thus, if the field
is locally static and smooth, the coupling becomes invisible in the solar system: this is the disformal screening mechanism. A cosmological model is considered where the disformal coupling triggers the onset of accelerated expansion after a scaling matter era, giving a good fit to a wide range of observational data. Moreover, the interaction leaves signatures in the formation of large-scale structure that can be used to probe such couplings.
The scalar-tensor Dirac-Born-Infeld (DBI) inflation scenario provides a simple mechanism to reduce the large values of the boost factor associated with single field models with DBI action, whilst still being able to drive 60 efolds of inflation. Usin
g a slow-roll approach, we obtain an analytical expression for the spectral index of the perturbations and, moreover, determine numerically the regions of the parameter space of the model capable of giving rise to a power spectrum with amplitude and spectral index within the observed bounds. We find that regions that exhibit significant DBI effects throughout the inflationary period can be discarded by virtue of a blue-tilted spectral index, however, there are a number of viable cases --- associated with a more red-tilted spectral index --- for which the boost factor is initially suppressed by the effect of the coupling between the fields, but increases later to moderate values.
The Dirac-Born-Infeld (DBI) action has been widely studied as an interesting example of a model of k-inflation in which the sound speed of the cosmological perturbations differs from unity. In this article we consider a scalar-tensor theory in which
the matter component is a field with a DBI action. Transforming to the Einstein frame, we explore the effect of the resulting coupling on the background dynamics of the fields and the first-order perturbations. We find that the coupling forces the scalar field into the minimum of its effective potential. While the additional scalar field contributes significantly to the energy density during inflation, the dynamics are determined by the DBI field, which has the interesting effect of increasing the number of efolds of inflation and decreasing the boost factor of the DBI field. Focusing on this case, we show, with the benefit of numerical examples, that the power spectrum of the primordial perturbations is determined by the behaviour of the perturbations of the modified DBI field.
