Alternative Gravity Research Articles

Black holes in alternative gravity theories

Black holes in alternative gravity theories

Theory of gravity with nonminimal matter-nonmetricity coupling and the de-Sitter swampland conjectures

In this study, we investigate swampland conjectures within the setup of matter and non-metricity nonminimal coupling theories of gravity. We examine how the inflationary solution produced by a single scalar field can be resolved with the swampland criteria in string theory regarding the formation of de Sitter solutions. The new important findings are that the inflationary scenario in our study differs from the one in general relativity because of the presence of a nonminimal coupling term, and that difference gives the correction to general relativity. In addition, we observe that the slow-roll conditions and the swampland conjectures are incompatible with each other for a single scalar field within the framework of nonminimally coupled alternative gravity theories. We predict that these results will hold for a wide range of inflationary scenarios in the context of nonminimal coupling gravitational theories.

The SRG/eROSITA All-Sky Survey. Constraints on f(R) gravity from cluster abundances

The evolution of the cluster mass function traces the growth of the linear density perturbations and can be utilized to constrain the parameters of cosmological and alternative gravity models. In this context, we present new constraints on potential deviations from general relativity by investigating the Hu-Sawicki parametrization of the $f(R)$ gravity with the first Spectrum Roentgen Gamma (SRG)/ All-Sky Survey ( cluster catalog in the western Galactic hemisphere in combination with the overlapping Dark Energy Survey Year-3, KiloDegree Survey, and Hyper Suprime-Cam data for weak lensing mass calibration. For the first time, we present constraints obtained from cluster abundances only. When we consider massless neutrinos, we find a strict upper limit of $ f_ R0 | < -4.31$ at a 95<!PCT!> confidence level. Massive neutrinos suppress structure growth at small scales, and thus have the opposite effect of $f(R)$ gravity. We consequently investigate the joint fit of the mass of the neutrinos with the modified gravity parameter. We obtain $ f_ R0 | < -4.08$ jointly with $ m_ eV $ at a 95<!PCT!> confidence level, which is tighter than the limits in the literature utilizing cluster counts only. At $ f_ R0 |= - 6$, the number of clusters is not significantly changed by the theory. Consequently, we do not find any statistical deviation from general relativity in the study of eRASS1 cluster abundance. Deeper surveys with eROSITA, increasing the number of detected clusters, will further improve constraints on $ |f_ R0 |$ and investigate alternative gravity theories.

Greybody factor of uncharged black hole in symmetric teleparallel gravity

In this study, we investigate the greybody factor of a (3+1)-dimensional black hole solution in the context of alternative f(Q) gravity with minimum scalar field coupling. We obtain the radial equation by applying the Klein–Gordon equation and transforming it into a Schrodinger wave equation using the tortoise coordinate. This transformation enables us to determine the effective potential and its graphical behavior for different values of the coupling constant and relevant physical parameters. We obtain two solutions for the radial equation corresponding to the event and cosmological horizons. To determine the greybody factor and its behavior, we combine these two solutions in the intermediate regime. The greybody factor increases with an increase in radius and coupling constant. This indicates that in alternative gravity, the larger black hole will die sooner as compared to cosmic gravity.

Comparative Analysis of Perturbed $f(R)$ Gravity and Perturbed Rastall Gravity Models in Describing Cosmic Evolution from Early to Late Universe Relative to the $\Lambda$CDM Model

Abstract This study conducts a meticulous examination of the cosmological implications inherent in Rastall gravity and $f(R)$ gravity models, assessing their efficacy across distinct cosmic epochs, from early universe structure formation to late-time acceleration. In the initial stages, both models exhibit commendable compatibility with observed features of structure formation, aligning with the established $\Lambda$CDM model. The derived Jeans' wavenumbers for each model support their viability. However, as the cosmic timeline progresses into the late universe, a discernible disparity surfaces. Utilizing the Markov Chain Monte Carlo method, we reconstruct the deceleration parameter $(q)$ and identify Deceleration - Acceleration redshift transition values. For $f(R)$ gravity, our results align closely with previous studies, emphasizing its superior ability to elucidate the recent cosmic acceleration. In contrast, Rastall gravity exhibits distinct redshift transition values. Our rigorous analysis underscores the prowess of $f(R)$ gravity in capturing the observed cosmic acceleration, positioning it as a compelling alternative to the conventional $\Lambda$CDM model. The discernible shifts observed in the peaks of the CMB power spectrum and evolution of deceleration parameter (q) for both $f(R)$ gravity and Rastall gravity models in the Early and Late universe, in relation to the $\Lambda $CDM model, provide compelling evidence supporting the proposition that these alternative gravity models can account for the anisotropy of the universe without invoking the need for dark energy.

Exploring the physical properties of anisotropic compact objects and constraining their mass–radius relations beyond standard limit in [formula omitted]-gravity

The progress in theoretical modeling of compact high-mass stars in the context of alternative gravity has become important in minimizing challenges associated with neutron star (NS) radius measurements. On this basis, we have presented a rigorous study of compact objects beyond the standard limit in gravity F(Q) in particular F(Q)=b1Q+b2 where b1 and b2 are constants. After formulating the basic equations and finding their relevant solutions by assuming a well-behaved ansatz for the metric potential as well as for anisotropy monitored by the parameters μ1,μ2,b1, along with imposing the boundary conditions on the system under treatment, we have developed a stellar model for anisotropic stars. Specifically, we explored the physical properties of these anisotropic stars and provided novel mass–radius (M−R) relations with models falling in the mass gap of the events GW190814 and GW200210, as well as the effects of slow rotation and moment of inertia on these findings. Interestingly, the behavior of the M−R curves represents a polytropic-type equation of state (EOS) for a negative Λ, while a positive Λ corresponds to a quark matter EOS. However, the analysis reveals that the predicted radii of the compact object observed in the GW190814 event, with a mass of 2.5–2.67 M⊙, is approximately 11.4 km for the de Sitter (dS) case and 11.8 km for the anti-de Sitter (AdS) case, assuming a specific value of the parameter b1=1.1. Furthermore, for b1>1.1, the model predicts the existence of more compact stars with a maximum mass of approximately 3M⊙, which is in good agreement with the R2 model of NSs maintaining the Sly EOS. From the I−M curves, it can be observed that higher values of b1 and μ1 in F(Q) gravity can sustain high moments of inertia (I) of stars with high masses. Interestingly, we obtained the range of I for the dS space as [1.92,3.92]×1045gcm2 and for the AdS space as [2.01,4.03]×1045gcm2 for 1.0≤b1≤1.2.

Testing MOND using the dynamics of nearby stellar streams

The stellar halo of the Milky Way is built up at least in part from debris from past mergers. The stars from these merger events define substructures in phase space, for example in the form of streams, which are groups of stars that move on similar trajectories. The nearby Helmi streams discovered more than two decades ago are a well-known example. Using 6D phase-space information from the Gaia space mission, recent work showed that the Helmi streams are split into two clumps in angular momentum space. This substructure can be explained and sustained in time if the dark matter halo of the Milky Way takes a prolate shape in the region probed by the orbits of the stars in the streams. Here, we explore the behaviour of the two clumps identified in the Helmi streams in a modified Newtonian dynamics (MOND) framework to test this alternative model of gravity. We performed orbit integrations of Helmi streams member stars in a simplified MOND model of the Milky Way and using the more sophisticated phantom of RAMSES simulation framework. We find with both approaches that the two Helmi streams clumps do not retain their identity and dissolve after merely Myr. This extremely short timescale would render the detection of two separate clumps very unlikely in MONDian gravity. The observational constraints provided by the streams, which MOND fails to reproduce in its current formulation, could potentially also be used to test other alternative gravity models.

Stability analysis of wormhole solutions in [formula omitted] gravity utilizing Karmarkar condition with radial dependent redshift function

In this work, we examine the properties of wormhole (WH) solutions in the context of modified f(Q) gravity. The shape function (ξS(r)) and redshift function (ζ(r)) are crucial in wormhole modeling since they determine the metric of a wormhole and define its attributes. The investigation provides the interaction between f(Q) gravity features and gravitational effects, guaranteeing insights into potential wormhole configurations. Subsequently, we have looked at the behavior of energy conditions and the fundamental properties of WHs. Additionally, the volume integral quantifier (V IQ) is addressed, providing insight into the quantity of exotic matter needed in the vicinity of the wormhole throat. Furthermore, the stability of the WH solutions in both cases is examined using the Tolman–Oppenheimer–Volkoff (TOV) equation where it is evident that both WH solutions meet the equilibrium requirements. Both cases ensures the real-world acceptability of WH solutions due to the rising nature of active gravitational mass (Mactive). Our findings contribute to the current discussion about alternative gravity theories and exotic spacetime geometries.

Preliminary sensitivity study for a gravitational redshift measurement with China’s Lunar exploration project

General relativity (GR) is a highly successful theory that describes gravity as a geometric phenomenon. The gravitational redshift, a classic test of GR, can potentially be violated in alternative gravity theories, and experimental tests on this effect are crucial for our understanding of gravity. In this paper, considering the space-ground clock comparisons with free-space links, we discuss a high-precision Doppler cancellation-based measurement model for testing gravitational redshift. This model can effectively reduce various sources of error and noise, reducing the influences of the first-order Doppler effect, atmospheric delay, Shapiro delay, etc. China’s Lunar Exploration Project (CLEP) is proposed to equip the deep-space H maser with a daily stability of 2×10−15 , which provides an approach for testing gravitational redshift. Based on the simulation, we analyze the space-ground clock comparison experiments of the CLEP experiment, and simulation analysis demonstrates that under ideal condition of high-precision measurement of the onboard H-maser frequency offset and drift, the CLEP experiment may reach the uncertainty of 3.7×10−6 after a measurement session of 60 days. Our results demonstrate that if the issue of frequency offset and drift is solved, CLEP missions have a potential of testing the gravitational redshift with high accuracy.

Testing the cosmological Poisson equation in a model-independent way

We show how one can test the cosmological Poisson equation by requiring only the validity of three main assumptions: the energy-momentum conservation equations of matter, the equivalence principle, and the cosmological principle. We first point out that one can only measure the combination M≡Ωm(0)μ, where μ quantifies the deviation of the Poisson equation from the standard one and Ωm(0) is the fraction of matter density at present. Then we employ a recent model-independent forecast for the growth rate f(z) and the expansion rate E(z) to obtain constraints on M for a survey that approximates a combination of the Dark Energy Spectroscopic Instrument (DESI) and Euclid. We conclude that a constant M can be measured with a relative error σM=4.5%, while if M is arbitrarily varying in redshift, it can be measured only to within 13.4% (1 σ c.l.) at redshift z=0.9, and 15-22% up to z=1.5. We also project our constraints on some parametrizations of M proposed in literature, while still maintaining model-independence for the background expansion, the power spectrum shape, and the non-linear corrections. Generally speaking, as expected, we find much weaker model-independent constraints than found so far for such models. This means that the cosmological Poisson equation remains quite open to various alternative gravity and dark energy models.

Measuring the Lense–Thirring Orbital Precession and the Neutron Star Moment of Inertia with Pulsars

Neutron stars (NSs) are compact objects that host the densest forms of matter in the observable universe, providing unique opportunities to study the behaviour of matter at extreme densities. While precision measurements of NS masses through pulsar timing have imposed effective constraints on the equation of state (EoS) of dense matter, accurately determining the radius or moment of inertia (MoI) of an NS remains a major challenge. This article presents a detailed review on measuring the Lense–Thirring (LT) precession effect in the orbit of binary pulsars, which would give access to the MoI of NSs and offer further constraints on the EoS. We discuss the suitability of certain classes of binary pulsars for measuring the LT precession from the perspective of binary star evolution and highlight five pulsars that exhibit properties promising to realise these goals in the near future. Finally, discoveries of compact binaries with shorter orbital periods hold the potential to greatly enhance measurements of the MoI of NSs. The MoI measurements of binary pulsars are pivotal to advancing our understanding of matter at supranuclear densities, as well as improving the precision of gravity tests, such as the orbital decay due to gravitational wave emission, and of tests of alternative gravity theories.

Perfect Fluid with Heat Flow in f(T) Theory of Gravity

Bianchi Type-I cosmological models have been a subject of extensive research in cosmology due to their simplicity and relevance in understanding the dynamics of the early Universe. In this study, we investigate the dynamics of such models within the framework of f(T) gravity, an alternative theory of gravity that extends teleparallel gravity by introducing a general function of the torsion scalar, T. We focus on the presence of a perfect fluid with heat flow in the cosmic medium. By solving the field equations of f(T) gravity, we obtain exact solutions for the Bianchi Type-I cosmological models. These solutions provide valuable insights into the evolution of the Universe and how it is influenced by the modified gravity theory. Furthermore, we derive cosmological parameters in terms of redshift, offering a convenient way to interpret observational data and connect theoretical predictions to empirical measurements. Our findings not only contribute to a deeper understanding of the dynamics of Bianchi Type-I cosmological models but also provide a foundation for comparing f(T) gravity with standard general relativity in the context of observational cosmology. This research paves the way for further exploration of alternative gravity theories and their implications for the early Universe’s evolution and structure.

Stable gravastar with large surface redshift in Einstein's gravity with two scalar fields

We propose a class of models, in which stable gravastar with large surface redshift becomes a solution. In recent decades, gravastars have become a plausible substitute for black holes. Researchers have explored stable gravastar models in various alternative gravity theories, in addition to the conventional framework of general relativity. In this paper, we present a stellar model within the framework of Einstein's gravity with two scalar fields, in accordance with the conjecture proposed by Mazur and Mottola [Proc. Nat. Acad. Sci. 101 (2004), 9545-9550]. In the model, the two scalar fields do not propagate by imposing constraints in order to avoid ghosts. The gravastar comprises two distinct regions, namely: (a) the interior region and (b) the exterior region. We assume the interior region consists of the de Sitter spacetime, and the exterior region is the Schwarzschild one. The two regions are connected with each other by the shell region. On the shell, we assume that the metric is given by a polynomial function of the radial coordinate r. The function has six constants. These constants are fixed by the smooth junction conditions, i.e., the interior region with the interior layer of the shell and the exterior region with the exterior layer of the shell. From these boundary conditions, we are able to write the coefficients of the scalar fields in terms of the interior radius and exterior radius. To clarify the philosophy of this study, we also give two examples of spacetimes that asymptote as the de Sitter spacetime for small r and as the Schwarzschild spacetime for large r. Exploration is focused on the physical attribute of the shell region, specifically, its proper length. The gravastar model's stability has frequently been examined by analyzing the relationship between surface redshift and shell thickness, a comparison we also undertake with previous models. Especially, we show that there exists a stable gravastar with a large surface redshift prohibited by the instability in the previous works. Furthermore, by checking the effective equation of state parameters, we show that the gravastar geometry realized in this paper by using two scalar fields could be difficult to generate with ordinary fluid.

A possible quantum gravity hint in binary black hole merger

We present a semi-rigorous justification of Bekenstein's Generalized Second Law of Thermodynamics applicable to a universe with black holes present, based on a generic quantum gravity formulation of a black hole spacetime, where the bulk Hamiltonian constraint plays a central role. Specializing to Loop Quantum Gravity, and considering the inspiral and post-ringdown stages of binary black hole merger into a remnant black hole, we show that the Generalized Second Law implies a lower bound on the non-perturbative LQG correction to the Bekenstein-Hawking area law for black hole entropy. This lower bound itself is expressed as a function of the Bekenstein-Hawking area formula for entropy. Results of the analyses of LIGO-VIRGO-KAGRA data recently performed to verify the Hawking Area Theorem for binary black hole merger are shown to be entirely consistent with this Loop Quantum Gravity-induced inequality. However, the consistency is independent of the magnitude of the LQG corrections to black hole entropy, depending only on the negative algebraic sign of the quantum correction. We argue that results of alternative quantum gravity computations of quantum black hole entropy, where the quantum entropy exceeds the Bekenstein-Hawking value, may not share this consistency.

Reciprocity Relation between Alternative Gravity Formulas

Reciprocity Relation between Alternative Gravity Formulas

Compact stars in f(T)=T+ξTβ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(T) = T +\\xi T^\\beta $$\\end{document} gravity

The Teleparallel Theory is equivalent to General Relativity, but whereas in the latter gravity has to do with curvature, in the former gravity is described by torsion. As is well known, there is in the literature a host of alternative theories of gravity, among them the so called extended theories, in which additional terms are added to the action, such as for example in the f(R) and f(T) gravities, where R is the Ricci scalar and T is the scalar torsion, respectively. One of the ways to probe alternative gravity is via compact objects. In fact, there is in the literature a series of papers on compact objects in f(R) and f(T) gravity. In particular, there are several papers that consider f(T)=T+ξT2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f(T) = T + \\xi T^2$$\\end{document}, where ξ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\xi $$\\end{document} is a real constant. In this paper, we generalise such extension considering compact stars in f(T)=T+ξTβ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$f (T ) = T + \\xi T^\\beta $$\\end{document} gravity, where ξ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\xi $$\\end{document} and β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta $$\\end{document} are real constants and looking out for the implications in their maximum masses and compactness in comparison to the General Relativity. Also, we are led to constrain the β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta $$\\end{document} parameter to positive integers which is a restriction not imposed by cosmology.

The other way around: from alternative gravity to entropy

Since the seminal work of Verlinde, the idea that gravity may be an emergent force of entropic origin has gained widespread attention. Many generalizations of this key idea have been considered in the literature, starting from well-known and well-motivated generalized entropies to derive generalized gravity theories. Here, we approach the problem from the opposite direction. We ask whether phenomenologically motivated generalized gravitational theories, yet lacking a strong theoretical justification, may find their origin in an entropic scenario. We examine a set of seven proposals of modified gravity, which have been introduced either (i) as large-scale corrections to Newtonian gravity, aimed at reproducing astrophysical observations in the far field, or (ii) as small-scale corrections, in order to regularize the singularity in the near field. For each proposal, we construct the underlying entropy, producing the desired dynamics in an entropic scenario. This reveals previously unnoticed connections between various proposals. The class of entropies introduced by Sheykhi and Hendi (2011 Phys. Rev. D 84 044023), exhibiting power-law corrections to the area law, appears to cover a number of useful phenomenological proposals, while the concept of fractional gravity is shown to arise from the recently introduced Barrow entropy. Other entropic forms, involving different type of corrections, also emerge from this procedure. We discuss their implications and their connections with entropies previously introduced in the literature. To broaden our analysis, we extend our discussion to the cosmological context, and examine the effect of these entropies on Friedmann equations.

Correcting the gravitational dipole direction for a partial sky survey

. The gravitational dipole approach is suitable for measuring β, the velocity parameter scale and, consequently, fσ 8, the product of the cosmic growth rate, f, and the matter density fluctuation on scales of 8 Mpc/h, σ 8. In cosmology, measurements of fσ 8 are important because they are a powerful tool for constraining alternative gravity models. However, in gravitational dipole analyses, if one uses a survey with incomplete sky coverage, this will certainly bias the measurement of the direction and magnitude of the dipole, which must be corrected accordingly. In this work we follow a new approach developed in ref. [1] to correct gravitational dipole measurements for partial sky data. Specifically, we study the gravitational dipole direction, important information to obtain an unbiased value of β. Our results show that, when comparing the velocity of the Local Group with the gravitational dipole, the correction procedure can actually recover the dipole direction with great precision, ensuring a robust result for β.

Exploring new subclass of k-inflation: Tachyon inflation in [formula omitted] gravity model

It is explained that any scalar field in f(R,T) gravity model could present a new subclass of the k-essence model for inflation. While the case of the quintessence has been studied, there is an empty gap required to be filled by investigating more types of field models. Here, we are aiming to partially fill the gap and concentrate on the tachyon field. In this study, we explore the phenomenon of tachyon inflation in the context of the alternative gravity theory f(R,T)=R+ηT. By applying slow-roll approximations, we examine the model in detail for three different potentials: power-law, generalized T-mode, and inverse hyperbolic cosh. Using observational data and Python coding, we determine a range of values for the model’s free parameters that allow it to fit the data perfectly. In essence, this study provides a comprehensive analysis of tachyon inflation in the R+ηT gravity theory, offering new insights into this alternative framework and its potential to explain tachyon inflation.

Turkey in-between the EU and China: from Europeanization to cooperation with China

ABSTRACT Turkey has been on the path of EU membership since the 2000s, and the democratization process was well underway during the initial years of its candidacy. However, this trend was reversed substantially, with Turkey growing increasingly authoritarian during the 2010s. This substantial democratic backsliding has led to increasing authoritarian cooperation with the authoritarian powers on the rise, one of which is China, whose increasing engagement and cooperation with Turkey marked an alternative gravity centre for Turkey to be pulled by. This article argues that Turkey, in line with the worsening domestic authoritarianism, has been engaging with the authoritarian powers for survival rather than engaging with the EU, which provided legitimacy to the rule of the AKP during the initial years of its rule.