Effective Newton Constant Research Articles

Why there is something rather than nothing: A proposal for the cosmological constant problem

This work is based on the idea that the observed cosmological constant is preferred for a stable vacuum, where a running gravitational coupling is considered. The most probable value of the cosmological constant is determined by the minimum of the vacuum energy density, and it is just proportional to the matter density. When the cosmological constant is connected with a modified dispersion relation (MDR) via an effective Newton constant, it may impose a constraint on the latter: the exponent of the leading correction should be n = 3.

N -body simulations, halo mass functions, and halo density profile in f(T) gravity

We perform N-body simulations for $f(T)$ gravity using the ME-Gadget code, in order to investigate for the first time the structure formation process in detail. Focusing on the power-law model, and considering the model-parameter to be consistent within 1$\sigma$ with all other cosmological datasets (such as SNIa, BAO, CMB, CC), we show that there are clear observational differences between $\Lambda$CDM cosmology and $f(T)$ gravity, due to the modifications brought about the latter in the Hubble function evolution and the effective $Newton\prime s$ constant. We extract the matter density distribution, matter power spectrum, counts-in-cells, halo mass function and excess surface density (ESD) around low density positions (LDPs) at present time. Concerning the matter power spectrum we find a difference from $\Lambda$CDM scenario, which is attributed to about 2/3 to the different expansion and to about 1/3 to the effective gravitational constant. Additionally, we find a difference in the cells, which is significantly larger than the Poisson error, which may be distinguishable with weak-lensing reconstructed mass maps. Moreover, we show that there are different massive halos with mass $M>10^{14}M_{\odot}/h$, which may be distinguishable with statistical measurements of cluster number counting, and we find that the ESD around LDPs is mildly different. In conclusion, high-lighting possible smoking guns, we show that large scale structure can indeed lead us to distinguish General Relativity and $\Lambda$CDM cosmology from $f(T)$ gravity.

Gravity as a quantum effect on quantum space-time

The 3+1-dimensional Einstein-Hilbert action is obtained from the 1-loop effective action on noncommutative branes in the IIB or IKKT matrix model. The presence of compact fuzzy extra dimensions K as well as maximal supersymmetry of the model is essential. The E-H action can be interpreted as interaction of K with the space-time brane via IIB supergravity, and the effective Newton constant is determined by the Kaluza-Klein scale of K. The classical matrix model defines a pre-gravity action with 2 derivatives less than the induced E-H action, governing the cosmological regime. The perturbative physics is confined to the space-time brane, which for covariant quantum space-times includes all dof of gravity, as well as a tower of higher-spin modes. The vacuum energy of the background is given in terms of the symplectic volume form, and hence does not act as cosmological constant.

Weak-field regime of the generalized hybrid metric-Palatini gravity

In this work we explore the dynamics of the generalized hybrid metric-Palatini theory of gravity in the weak-field, slow-motion regime. We start by introducing the equivalent scalar-tensor representation of the theory, which contains two scalar degrees of freedom, and perform a conformal transformation to the Einstein frame. Linear perturbations of the metric in a Minkowskian background are then studied for the metric and both scalar fields. The effective Newton constant and the PPN parameter $\ensuremath{\gamma}$ of the theory are extracted after transforming back to the (original) Jordan frame. Two particular cases where the general method ceases to be applicable are approached separately. A comparison of these results with observational constraints is then used to impose bounds on the masses and coupling constants of the scalar fields.

Entropy of thermal CFTs on curved backgrounds

We use holography in order to study the entropy of thermal CFTs on (1+1)-dimensional curved backgrounds that contain horizons. Starting from the metric of the BTZ black hole, we perform explicit coordinate transformations that set the boundary metric in de Sitter or black-hole form. For a de Sitter boundary, the dual picture describes a CFT at a temperature different from that of the cosmological horizon. We determine minimal surfaces that allow us to compute the entanglement entropy of a boundary region, as well as the temperature affecting the energy associated with a probe quark on the boundary. For an entangling surface that coincides with the horizon, we study the relation between entanglement and gravitational entropy through an appropriate definition of the effective Newton's constant. We find that the leading contribution to the entropy is proportional to the horizon area, with a coefficient that accounts for the degrees of freedom of a CFT thermalized above the horizon temperature. We demonstrate the universality of our findings by considering the most general metric in a (2+1)-dimensional AdS bulk containing a non-rotating black hole and a static boundary with horizons.

Hawking temperature of black holes using the EUP and EGUP formalisms based on the Einstein-Bohr's photon box

In this paper, by studying the Einstein-Bohr's photon box for weighting a photon, we find that the effective Newton constant can be proposed by the extended uncertainty principle and extended generalized uncertainty principle. We obtain the modified Hawking temperature, mass, specific heat, and entropy by using the modified Schwarzschild metric.

Tension of the EG statistic and redshift space distortion data with the Planck - ΛCDM model and implications for weakening gravity

The $E_G$ statistic is a powerful probe for detecting deviations from GR by combining weak lensing (WL), real-space clustering and redshift space distortion (RSD) measurements thus probing both the lensing and the growth effective Newton constants ($G_L$ and $G_{eff}$). We construct an up to date compilation of $E_G$ statistic data including both redshift and scale dependence ($E_G(R,z)$). We combine this $E_G$ data compilation with an up to date compilation of $f\sigma_8$ data from RSD observations to identify the current level of tension between the Planck/$\Lambda$CDM standard model based on general relativity and a general model independent redshift evolution parametrization of $G_L$ and $G_{eff}$. Each $f\sigma_8$ datapoint considered has been published separately in the context of independent analyses of distinct galaxy samples. However, there are correlations among the datapoints considered due to overlap of the analyzed galaxy samples. Due to these correlations the derived levels of tension of the best fit parameters with Planck/$\Lambda$CDM are somewhat overestimated but this is the price to pay for maximizing the information encoded in the compilation considered. We find that the level of tension increases from about $3.5\sigma$ for the $f\sigma_8$ data compilation alone to about $6\sigma$ when the $E_G$ data are also included in the analysis. The direction of the tension is the same as implied by the $f\sigma_8$ RSD growth data alone (lower $\Omega_m$ and/or weaker effective Newton constant at low redshifts for both the lensing and the growth effective Newton constants ($G_L$ and $G_{eff}$)). These results further amplify the hints for weakening modified gravity discussed in other recent analyses.

J-PAS: forecasts on dark energy and modified gravity theories

ABSTRACT The next generation of galaxy surveys will allow us to test one of the most fundamental assumptions of the standard cosmology, i.e. that gravity is governed by the general theory of relativity (GR). In this paper, we investigate the ability of the Javalambre Physics of the Accelerating Universe Astrophysical Survey (J-PAS) to constrain GR and its extensions. Based on the J-PAS information on clustering and gravitational lensing, we perform a Fisher matrix forecast on the effective Newton constant, μ, and the gravitational slip parameter, η, whose deviations from unity would indicate a breakdown of GR. Similar analysis is also performed for the DESI and Euclid surveys and compared to J-PAS with two configurations providing different areas, namely an initial expectation with 4000 deg2 and the future best case scenario with 8500 deg2. We show that J-PAS will be able to measure the parameters μ and η at a sensitivity of $2\!-\!7{{\ \rm per\ cent}}$, and will provide the best constraints in the interval z = 0.3–0.6, thanks to the large number of ELGs detectable in that redshift range. We also discuss the constraining power of J-PAS for dark energy models with a time-dependent equation-of-state parameter of the type w(a) = w0 + wa(1 − a), obtaining Δw0 = 0.058 and Δwa = 0.24 for the absolute errors of the dark energy parameters.

Non-linear matter power spectrum without screening dynamics modelling in f(R) gravity

ABSTRACT Halo model is a physically intuitive method for modelling the non-linear power spectrum, especially for the alternatives to the standard ΛCDM models. In this paper, we examine the Sheth–Tormen barrier formula adopted in the previous CHAM method. As an example, we model the ellipsoidal collapse of top-hat dark matter haloes in f(R) gravity. A good agreement between Sheth–Tormen formula and our result is achieved. The relative difference in the ellipsoidal collapse barrier is less than or equal to $1.6{{\ \rm per\ cent}}$. Furthermore, we verify that, for F4 and F5 cases of Hu–Sawicki f(R) gravity, the screening mechanism does not play a crucial role in the non-linear power spectrum modelling up to k ∼ 1 h Mpc−1. We compare two versions of modified gravity modelling, namely with/without screening. We find that by treating the effective Newton constant as constant number, Geff = 4/3GN is acceptable. The scale dependence of the gravitational coupling is subrelevant. The resulting spectra in F4 and F5, are in $0.1{{\ \rm per\ cent}}$ agreement with the previous CHAM results. The published code is accelerated significantly. Finally, we compare our halo model prediction with N-body simulation. We find that the general spectrum profile agrees, qualitatively. However, via the halo model approach, there exists a systematic underestimation of the matter power spectrum in the comoving wavenumber range between 0.3 and 3 h Mpc−1. These scales are overlapping with the transition scales from two-halo term dominated regimes to those of one-halo term dominated regimes.

Limitations on Standard Sirens tests of gravity from screening

Modified gravity theories with an effective Newton constant that varies over cosmological timescales generally predict a different gravitational wave luminosity distance than General Relativity. While this holds for a uniform variation, we show that if locally screened at the source and at the observer as required to pass stringent astrophysical tests of gravity, the General Relativistic distance is restored. In the absence of such a screening, the same effect must modify electromagnetic luminosity distances inferred from supernovae Type Ia, to the extent that the effects can cancel in the comparison. Hence, either the modifications considered employ screening, which leaves no signature in Standard Sirens of a cosmological modification of gravity, or screening does not operate, in which case there can be a signal that is however well below the forseeable sensitivity of the probe when astrophysical bounds are employed. We recover these results both in the Jordan and Einstein frames, paying acute attention to pecularities of each frame such as the notion of redshift or geodesic motions. We emphasise that despite these limitations, Standard Sirens provide valuable independent tests of gravity that differ fundamentally from other probes, a circumstance that is generally important for the wider scope of gravitational modifications and related scenarios. Finally, we use our results to show that the gravitational wave propagation is not affected by dark sector interactions, which restores a dark degeneracy between conformal and disformal couplings that enables observationally viable cosmic self-acceleration to emenate from those.

Induced gravity and quantum cosmology

We study the Wheeler-DeWitt equation for a class of induced gravity models in the minisuperspace approximation. In such models a scalar field nonminimally coupled to gravity determines the effective Newton's constant. For simplicity our analysis is limited to power-law potentials for the scalar field which have exact classical solutions. We show that these models have exact solutions also when quantised. Finally the Einstein Frame form of these solutions is obtained and a classical-quantum correspondence is found. Realistic induced gravity models also must include a symmetry breaking term which is needed in order to obtain a gravitational constant, successful inflation and a subsequent standard cosmological evolution. Nonetheless the potentials considered are important as they may describe the inflationary phase when the symmetry breaking part of the potential is negligible.

The Effective Field Theory of Dark Energy Diagnostic of Linear Horndeski Theories After GW170817 and GRB170817A

We summarize the effective field theory of dark energy construction to explore observable predictions of linear Horndeski theories. We review the diagnostic of these theories on the correlation of the large-scale structure phenomenological functions: the effective Newton constant, the light deflection parameter, and the growth function of matter perturbations. We take this opportunity to discuss the evolution of the bounds the propagation speed of gravitational waves has undergone and use the most restrictive one to update the diagnostic.

Hints of modified gravity in cosmos and in the lab?

General Relativity (GR) is consistent with a wide range of experiments/observations from millimeter scales up to galactic scales and beyond. However, there are reasons to believe that GR may need to be modified because it includes singularities (it is an incomplete theory) and also it requires fine-tuning to explain the accelerating expansion of the universe through the cosmological constant. Therefore, it is important to check various experiments and observations beyond the above range of scales for possible hints of deviations from the predictions of GR. If such hints are found it is important to understand which classes of modified gravity theories are consistent with them. The goal of this review is to summarize recent progress on these issues. On sub-millimeter scales, we show an analysis of the data of the Washington experiment [D. J. Kapner, T. S. Cook, E. G. Adelberger, J. H. Gundlach, B. R. Heckel, C. D. Hoyle and H. E. Swanson, Phys. Rev. Lett. 98 (2007) 021101, arXiv:hep-ph/0611184 [hep-ph]] searching for modifications of Newton’s Law on sub-millimeter scales and demonstrate that a spatially oscillating signal is hidden in this dataset. We demonstrate that even though this signal cannot be explained in the context of standard modified theories (viable scalar tensor and [Formula: see text] theories), it is a rather generic prediction of nonlocal gravity theories. On cosmological scales we review recent analyses of Redshift Space Distortion (RSD) data which measure the growth rate of cosmological perturbations at various redshifts and show that these data are in some tension with the [Formula: see text]CDM parameter values indicated by Planck/2015 CMB data at about [Formula: see text] level. This tension can be reduced by allowing for an evolution of the effective Newton constant that determines the growth rate of cosmological perturbations. We conclude that even though this tension between the data and the predictions of GR could be due to systematic/statistical uncertainties of the data, it could also constitute early hints pointing towards a new gravitational theory.

Constraining power of cosmological observables: Blind redshift spots and optimal ranges

A cosmological observable measured in a range of redshifts can be used as a probe of a set of cosmological parameters. Given the cosmological observable and the cosmological parameter, there is an optimum range of redshifts where the observable can constrain the parameter in the most effective manner. For other redshift ranges the observable values may be degenerate with respect to the cosmological parameter values and thus inefficient in constraining the given parameter. These are blind redshift ranges. We determine the optimum and blind redshift ranges of basic cosmological observables with respect to three cosmological parameters: the matter density parameter ${\mathrm{\ensuremath{\Omega}}}_{m}$, the equation-of-state parameter $w$ (assumed constant), and a modified gravity parameter ${g}_{a}$ which parametrizes a possible evolution of the effective Newton's constant as ${G}_{\mathrm{eff}}(z)={G}_{N}(1+{g}_{a}(1\ensuremath{-}a{)}^{2}\ensuremath{-}{g}_{a}(1\ensuremath{-}a{)}^{4})$ (where $a=\frac{1}{1+z}$ is the scale factor and ${G}_{N}$ is Newton's constant of general relativity). We consider the following observables: the growth rate of matter density perturbations expressed through $f(z)$ and $f{\ensuremath{\sigma}}_{8}(z)$, the distance modulus $\ensuremath{\mu}(z)$, baryon acoustic oscillation observables ${D}_{V}(z)\ifmmode\times\else\texttimes\fi{}\frac{{r}_{s}^{\mathrm{fid}}}{{r}_{s}}$, $H\ifmmode\times\else\texttimes\fi{}\frac{{r}_{s}}{{r}_{s}^{\mathrm{fid}}}$ and ${D}_{A}\ifmmode\times\else\texttimes\fi{}\frac{{r}_{s}^{\mathrm{fid}}}{{r}_{s}}$, $H(z)$ measurements, and the gravitational wave luminosity distance. We introduce a new statistic ${S}_{P}^{O}(z)\ensuremath{\equiv}\frac{\mathrm{\ensuremath{\Delta}}O}{\mathrm{\ensuremath{\Delta}}P}(z)\ifmmode\cdot\else\textperiodcentered\fi{}{V}_{\mathrm{eff}}^{1/2}$, including the effective survey volume ${V}_{\mathrm{eff}}$, as a measure of the constraining power of a given observable $O$ with respect to a cosmological parameter $P$ as a function of redshift $z$. We find blind redshift spots ${z}_{b}$ (${S}_{P}^{O}({z}_{b})\ensuremath{\simeq}0$) and optimal redshift spots ${z}_{s}$ (${S}_{P}^{O}({z}_{s})\ensuremath{\simeq}\mathrm{max}$) for the above observables with respect to the parameters ${\mathrm{\ensuremath{\Omega}}}_{m}$, $w$, and ${g}_{a}$. For example, for $O=f{\ensuremath{\sigma}}_{8}$ and $P=({\mathrm{\ensuremath{\Omega}}}_{m},w,{g}_{a})$ we find blind spots at ${z}_{b}\ensuremath{\simeq}(1,2,2.7)$, respectively, and optimal (sweet) spots at ${z}_{s}=(0.5,0.8,1.2)$. Thus, probing higher redshifts may in some cases be less effective than probing lower redshifts with higher accuracy. These results may be helpful in the proper design of upcoming missions aimed at measuring cosmological observables in specific redshift ranges.

Generalized uncertainty principles, effective Newton constant and the regular black hole

In this paper, the quantum spacetime with a running gravitational coupling is explored. Analyzing the gravity-induced quantum interference pattern and the Gedanken for weighting a photon, we find that the running Newton constant can be inspired by the generalized uncertainty principles. A characteristic momentum associated with the tidal effect is suggested, which incorporates the quantum effect with the geometric nature of gravity. When the simplest generalized uncertainty principle is considered, the minimal model of the regular black holes is reproduced by the effective Newton constant. The black hole’s tunneling probability, accurate to the second order correction, is carefully analyzed. We find that the tunneling probability is regularized by the size of the black hole remnant. Moreover, for a given initial black hole, the remnant is the final state of a transition process that the probability is minimal. We also suggest a theory of modified gravity, by substituting the effective Newton constant into the Hilbert–Einstein action.

Self-acceleration in scalar-bimetric theories

We describe scalar-bimetric theories where the dynamics of the Universe are governed by two separate metrics, each with an Einstein-Hilbert term. In this setting, the baryonic and dark matter components of the Universe couple to metrics which are constructed as functions of these two gravitational metrics. The scalar field, contrary to dark energy models, does not have a potential whose role is to mimic a late-time cosmological constant. The late-time acceleration of the expansion of the Universe can be easily obtained at the background level in these models by appropriately choosing the coupling functions appearing in the decomposition of the vierbeins for the baryonic and dark matter metrics. We explicitly show how the concordance model can be retrieved with negligible scalar kinetic energy. This requires the scalar coupling functions to show variations of order unity during the accelerated expansion era. This leads in turn to deviations of order unity for the effective Newton constants and a fifth force that is of the same order as Newtonian gravity, with peculiar features. The baryonic and dark matter self-gravities are amplified although the gravitational force between baryons and dark matter is reduced and even becomes repulsive at low redshift. This slows down the growth of baryonic density perturbations on cosmological scales, while dark matter perturbations are enhanced. In our local environment, the upper bound on the time evolution of Newton's constant requires an efficient screening mechanism that both damps the fifth force on small scales and decouples the local value of Newton constant from its cosmological value. This cannot be achieved by a quasi-static chameleon mechanism, and requires going beyond the quasi-static regime and probably using derivative screenings, such as Kmouflage or Vainshtein screening, on small scales.

Note on the initial conditions within the effective field theory approach of cosmic acceleration

By using the effective field theory approach, we investigate the role of initial condition for the dark energy or modified gravity models. In details, we consider the constant and linear parametrization of the effective Newton constant models. Firstly, under the adiabatic assumption, the correction from the extra scalar degree of freedom in the beyond $\Lambda$CDM model is found to be negligible. The dominant ingredient in this setup is the primordial curvature perturbation originated from inflation mechanism, and the energy budget of the matter components is not very crucial. Secondly, the iso-curvature perturbation sourced by the extra scalar field is studied. For the constant and linear model of the effective Newton constant, there is no such kind of scalar mode exist. For the quadratic model, there is a non-trivial one. However, the amplitude of the scalar field is damped away very fast on all scales. Consequently, it could not support a reasonable structure formation. Finally, we study the importance of the setup of the scalar field starting time. By setting different turn-on time, namely $a=10^{-2} $ and $a=10^{-7} $, we compare the cosmic microwave background radiation temperature, lensing deflection angle auto-correlation function as well as the matter power spectrum in the constant and linear model. We find there is an order of $\mathcal{O}(1\%)$ difference in the observable spectra for constant model, while for the linear model, it is smaller than $\mathcal{O}(0.1\%)$.

Reconstruction of cosmological matter perturbations in modified gravity

The analysis of perturbative quantities is a powerful tool to distinguish between different dark energy models and gravity theories degenerated at the background level. In this work, we generalize the integral solution of the matter density contrast for general relativity gravity [V. Sahni and A. Starobinsky, Int. J. Mod. Phys. D 15, 2105 (2006)., U. Alam, V. Sahni, and A. A. Starobinsky, Astrophys. J. 704, 1086 (2009).] to a wide class of modified gravity (MG) theories. To calculate this solution, it is necessary to have prior knowledge of the Hubble rate, the density parameter at the present epoch (${\mathrm{\ensuremath{\Omega}}}_{m0}$), and the functional form of the effective Newton's constant that characterizes the gravity theory. We estimate in a model-independent way the Hubble expansion rate by applying a nonparametric reconstruction method to model-independent cosmic chronometer data and high-$z$ quasar data. In order to compare our generalized solution of the matter density contrast, using the nonparametric reconstruction of $H(z)$ from observational data, with a purely theoretical one, we choose a parametrization of the screened modified gravity and the ${\mathrm{\ensuremath{\Omega}}}_{m0}$ from WMAP-9 Collaborations. Finally, we calculate the growth index for the analyzed cases, finding very good agreement between theoretical values and the obtained ones using the approach presented in this work.

Tension and constraints on modified gravity parametrizations of Geff(z) from growth rate and Planck data

We construct an updated and extended compilation of growth rate data consisting of 34 points and including corrections for model dependence. In order to maximize the independence of the datapoints we also construct a subsample of this compilation (`Gold' growth dataset) which consists of 18 datapoints. We test the consistency of this dataset with the best fit Planck15/$\Lambda$CDM parameters in the context of General Relativity (GR) using the evolution equation for the growth factor $\delta(a)$ with a $w$CDM background. We find tension at the $\sim 3 \sigma$ level between the best fit parameters $w$ (the dark energy equation of state), $\Omega_{0m}$ (the matter density parameter) and $\sigma_8$ (the matter power spectrum normalization on scales $8h^{-1}$Mpc) and the corresponding Planck15/$\Lambda$CDM parameters. We show that the tension disappears if we allow for evolution of the effective Newton's constant, parametrized as $G_{eff}(a)/G_N = 1 + g_a(1-a)^n-g_a(1-a)^{2n}$ with $n\ge2$ where $g_a$, $n$ are parameters of the model, $a$ is the scale factor and $z = 1/a-1$ is the redshift. This parametrization satisfies three criteria: a. $G_{eff} > 0$, b. Consistency with Big Bang Nucleosynthesis ($G_{eff}(a\ll 1)/G_N=1$), c. Consistency with solar system tests ($G_{eff}(a=1)/G_N=1$ and $G_{eff}'(a=1)/G_N=0$). We show that the best fit form of $G_{eff}(z)$ obtained from the growth data corresponds to weakening gravity at recent redshifts (decreasing function of $z$) and we demonstrate that this behavior is not consistent with any scalar-tensor Lagrangian with a real scalar field. Finally, we use MGCAMB to find the best fit $G_{eff}(z)$ obtained from the Planck CMB power spectrum on large angular scales and show that it is a mildly increasing function of $z$, in $3\sigma$ tension with the corresponding decreasing best fit $G_{eff}(z)$ obtained from the growth data.

Cosmic growth signatures of modified gravitational strength

Cosmic growth of large scale structure probes the entire history of cosmic expansion and gravitational coupling. To get a clear picture of the effects of modification of gravity we consider a deviation in the coupling strength (effective Newton's constant) at different redshifts, with different durations and amplitudes. We derive, analytically and numerically, the impact on the growth rate and growth amplitude. Galaxy redshift surveys can measure a product of these through redshift space distortions and we connect the modified gravity to the observable in a way that may provide a useful parametrization of the ability of future surveys to test gravity. In particular, modifications during the matter dominated era can be treated by a single parameter, the ``area'' of the modification, to an accuracy of ∼0.3% in the observables. We project constraints on both early and late time gravity for the Dark Energy Spectroscopic Instrument and discuss what is needed for tightening tests of gravity to better than 5% uncertainty.