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Journal articles on the topic 'Cosmology, Gravity, Alternative theory of gravity, Mimetic gravity'
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1
Sebastiani, Lorenzo, Sunny Vagnozzi, and Ratbay Myrzakulov. "Mimetic Gravity: A Review of Recent Developments and Applications to Cosmology and Astrophysics." Advances in High Energy Physics 2017 (2017): 1–43. http://dx.doi.org/10.1155/2017/3156915.
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Mimetic gravity is a Weyl-symmetric extension of General Relativity, related to the latter by a singular disformal transformation, wherein the appearance of a dust-like perfect fluid can mimic cold dark matter at a cosmological level. Within this framework, it is possible to provide a unified geometrical explanation for dark matter, the late-time acceleration, and inflation, making it a very attractive theory. In this review, we summarize the main aspects of mimetic gravity, as well as extensions of the minimal formulation of the model. We devote particular focus to the reconstruction technique, which allows the realization of any desired expansionary history of the universe by an accurate choice of potential or other functions defined within the theory (as in the case of mimeticf(R)gravity). We briefly discuss cosmological perturbation theory within mimetic gravity. As a case study within which we apply the concepts previously discussed, we study a mimetic Hořava-like theory, of which we explore solutions and cosmological perturbations in detail. Finally, we conclude the review by discussing static spherically symmetric solutions within mimetic gravity and apply our findings to the problem of galactic rotation curves. Our review provides an introduction to mimetic gravity, as well as a concise but self-contained summary of recent findings, progress, open questions, and outlooks on future research directions.
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Sheykhi, Ahmad. "Thermodynamics of apparent horizon in mimetic cosmology." International Journal of Modern Physics D 28, no. 03 (2019): 1950057. http://dx.doi.org/10.1142/s0218271819500573.
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A new perspective toward Einstein’s theory of general relativity, called mimetic gravity, was suggested in [A. H. Chamseddine and V. Mukhanov, J. High Energy Phys. 1311 (2013) 135] by isolating the conformal degree of freedom in a covariant fashion through a re-parametrization of the physical metric in terms of an auxiliary metric and a mimetic field. In this paper, we first derive the Friedmann equations of the Friedmann–Robertson–Walker (FRW) universe with any spatial curvature in mimetic gravity. Then, we disclose that one can always rewrite the Friedmann equations of mimetic cosmology in the form of the first law of thermodynamics, [Formula: see text], on the apparent horizon. We confirm that the entropy associated with the apparent horizon in mimetic cosmology still obeys the area law of entropy which is useful in studying the thermodynamical properties of the black holes in mimetic gravity. We also examine the time evolution of the total entropy in mimetic cosmology and show that, with the local equilibrium assumption, the generalized second law of thermodynamics is fulfilled in a region enclosed by the apparent horizon. Our study further supports the viability of the mimetic gravity from a thermodynamic viewpoint and provides a strong consistency check of this model.
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de Cesare, Marco. "Reconstruction of Mimetic Gravity in a Non-Singular Bouncing Universe from Quantum Gravity." Universe 5, no. 5 (2019): 107. http://dx.doi.org/10.3390/universe5050107.
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We illustrate a general reconstruction procedure for mimetic gravity. Focusing on a bouncing cosmological background, we derive general properties that must be satisfied by the function f(□ϕ) implementing the limiting curvature hypothesis. We show how relevant physical information can be extracted from power-law expansions of f in different regimes, corresponding e.g., to the very early universe or to late times. Our results are then applied to two specific models reproducing the cosmological background dynamics obtained in group field theory and in loop quantum cosmology, and we discuss the possibility of using this framework as providing an effective field theory description of quantum gravity. We study the evolution of anisotropies near the bounce, and discuss instabilities of scalar perturbations. Furthermore, we provide two equivalent formulations of mimetic gravity: one in terms of an effective fluid with exotic properties, the other featuring two distinct time-varying gravitational “constants” in the cosmological equations.
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Momeni, D., P. H. R. S. Moraes, H. Gholizade, and R. Myrzakulov. "Mimetic compact stars." International Journal of Geometric Methods in Modern Physics 15, no. 06 (2018): 1850091. http://dx.doi.org/10.1142/s0219887818500913.
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Modified gravity models have been constantly proposed with the purpose of evading some standard gravity shortcomings. Recently proposed by Chamseddine and Mukhanov, the Mimetic Gravity arises as an optimistic alternative. Our purpose in this work is to derive Tolman–Oppenheimer–Volkoff equations and solutions for such a gravity theory. We solve them numerically for quark star and neutron star cases. The results are carefully discussed.
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Varieschi, Gabriele U. "Relativistic Fractional-Dimension Gravity." Universe 7, no. 10 (2021): 387. http://dx.doi.org/10.3390/universe7100387.
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This paper presents a relativistic version of Newtonian Fractional-Dimension Gravity (NFDG), an alternative gravitational model recently introduced and based on the theory of fractional-dimension spaces. This extended version—Relativistic Fractional-Dimension Gravity (RFDG)—is based on other existing theories in the literature and might be useful for astrophysical and cosmological applications. In particular, in this work, we review the mathematical theory for spaces with non-integer dimensions and its connections with the non-relativistic NFDG. The Euler–Lagrange equations for scalar fields can also be extended to spaces with fractional dimensions, by adding an appropriate weight factor, and then can be used to generalize the Laplacian operator for rectangular, spherical, and cylindrical coordinates. In addition, the same weight factor can be added to the standard Hilbert action in order to obtain the field equations, following methods used for scalar-tensor models of gravity, multi-scale spacetimes, and fractional gravity theories. We then apply the field equations to standard cosmology and to the Friedmann-Lemaître-Robertson-Walker metric. Using a suitable weight vtt, depending on the synchronous time t and on a single time-dimension parameter αt, we extend the Friedmann equations to the RFDG case. This allows for the computation of the scale factor at for different values of the fractional time-dimension αt and the comparison with standard cosmology results. Future additional work on the subject, including studies of the cosmological late-time acceleration, type Ia supernovae data, and related dark energy theory will be needed to establish this model as a relativistic alternative theory of gravity.
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Jones, KRW. "Newtonian Quantum Gravity." Australian Journal of Physics 48, no. 6 (1995): 1055. http://dx.doi.org/10.1071/ph951055.
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We develop a nonlinear quantum theory of Newtonian gravity consistent with an objective interpretation of the wavefunction. Inspired by the ideas of Schrodinger, and Bell, we seek a dimensional reduction procedure to map complex wavefunctions in configuration space onto a family of observable fields in space-time. Consideration of quasi-classical conservation laws selects the reduced one-body quantities as the basis for an explicit quasi-classical coarse-graining. These we interpret as describing the objective reality of the laboratory. Thereafter, we examine what may stand in the role of the usual Copenhagen observer to localise this quantity against macroscopic dispersion. Only a tiny change is needed, via a generically attractive self-potential. A nonlinear treatment of gravitational self-energy is thus advanced. This term sets a scale for all wavepackets. The Newtonian cosmology is thus closed, without need of an external observer. Finally, the concept of quantisation is re-interpreted as a nonlinear eigenvalue problem. To illustrate, we exhibit an elementary family of gravitationally self-bound solitary waves. Contrasting this theory with its canonically quantised analogue, we find that the given interpretation is empirically distinguishable, in principle. This result encourages deeper study of nonlinear field theories as a testable alternative to canonically quantised gravity.
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Ao, Xi-Chen, and Xin-Zhou Li. "de Sitter gauge theory of gravity: an alternative torsion cosmology." Journal of Cosmology and Astroparticle Physics 2011, no. 10 (2011): 039. http://dx.doi.org/10.1088/1475-7516/2011/10/039.
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Bekenstein, Jacob D. "Tensor–vector–scalar-modified gravity: from small scale to cosmology." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369, no. 1957 (2011): 5003–17. http://dx.doi.org/10.1098/rsta.2011.0282.
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The impressive success of the standard cosmological model has suggested to many that its ingredients are all that one needs to explain galaxies and their systems. I summarize a number of known problems with this programme. They might signal the failure of standard gravity theory on galaxy scales. The requisite hints as to the alternative gravity theory may lie with the modified Newtonian dynamics (MOND) paradigm, which has proved to be an effective summary of galaxy phenomenology. A simple nonlinear modified gravity theory does justice to MOND at the non-relativistic level, but cannot be consistently promoted to relativistic status. The obstacles were first side-stepped with the formulation of tensor–vector–scalar theory (T e V e S), a covariant-modified gravity theory. I review its structure, its MOND and Newtonian limits, and its performance in the face of galaxy phenomenology. I also summarize features of T e V e S cosmology and describe the confrontation with data from strong and weak gravitational lensing.
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SCHMIDT, HANS-JÜRGEN. "FOURTH ORDER GRAVITY: EQUATIONS, HISTORY, AND APPLICATIONS TO COSMOLOGY." International Journal of Geometric Methods in Modern Physics 04, no. 02 (2007): 209–48. http://dx.doi.org/10.1142/s0219887807001977.
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The field equations following from a Lagrangian L(R) will be deduced and solved for special cases. If L is a non-linear function of the curvature scalar, then these equations are of fourth order in the metric. In the introduction, we present the history of these equations beginning with the paper of H. Weyl from 1918, who first discussed them as alternative to Einstein's theory. In the third part, we give details about the cosmic no hair theorem, i.e. the details of how within fourth order gravity with L= R + R2, the inflationary phase of cosmic evolution turns out to be a transient attractor. Finally, the Bicknell theorem, i.e. the conformal relation from fourth order gravity to scalar-tensor theory, will be shortly presented.
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Giusti, Andrea. "On the corpuscular theory of gravity." International Journal of Geometric Methods in Modern Physics 16, no. 03 (2019): 1930001. http://dx.doi.org/10.1142/s0219887819300010.
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The aim of this work is to provide a general description of the corpuscular theory of gravity. After reviewing some of the major conceptual issues emerging from the semiclassical and field theoretic approaches to Einstein’s gravity, we present a synthetic overview of two novel (and extremely intertwined) perspectives on quantum mechanical effects in gravity: the horizon quantum mechanics (HQM) formalism and the classicalization scheme. After this preliminary discussion, we then proceed with implementing the latter to several different scenarios, namely self-gravitating systems, the early Universe, and galactic dynamics. Concerning the first scenario, we start by describing the generation of the Newtonian potential as the result of a coherent state of toy (scalar) gravitons. After that we employ this result to study some features of the gravitational collapse and to argue that black holes can be thought of a self-sustained quantum states, at the critical point, made of a large number of soft virtual gravitons. We then refine this simplified analysis by constructing an effective theory for the gravitational potential of a static spherical symmetric system up to the first post-Newtonian correction. Additionally, we employ the HQM formalism to study the causal structure emerging from the corpuscular scenario. Finally, we present a short discussion of corpuscular black holes in lower dimensional spaces. After laying down the basics of corpuscular black holes, we present a generalization of the aforementioned arguments to cosmology. Specifically, we first introduce a corpuscular interpretation of the de Sitter spacetime. Then we use it as the starting point for a corpuscular formulation of the inflationary scenario and to provide an alternative viewpoint on the dark components of the [Formula: see text]CDM model. The key message of this work is that the corpuscular theory of gravity offers a way to unify most of the experimental observations (from astrophysical to galactic and cosmological scales) in a single framework, solely based on gravity and baryonic matter.
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Bouché, Filippo, Salvatore Capozziello, and Vincenzo Salzano. "Addressing Cosmological Tensions by Non-Local Gravity." Universe 9, no. 1 (2022): 27. http://dx.doi.org/10.3390/universe9010027.
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Alternative cosmological models have been under deep scrutiny in recent years, aiming to address the main shortcomings of the ΛCDM model. Moreover, as the accuracy of cosmological surveys improved, new tensions have risen between the model-dependent analysis of the Cosmic Microwave Background and lower redshift probes. Within this framework, we review two quantum-inspired non-locally extended theories of gravity, whose main cosmological feature is a geometrically driven accelerated expansion. The models are especially investigated in light of the Hubble and growth tension, and promising features emerge for the Deser–Woodard one. On the one hand, the cosmological analysis of the phenomenological formulation of the model shows a lowered growth of structures but an equivalent background with respect to ΛCDM. On the other hand, the study of the lensing features at the galaxy cluster scale of a new formulation of non-local cosmology, based on Noether symmetries, makes room for the possibility of alleviating both the H0 and σ8 tension. However, the urgent need for a screening mechanism arises for this non-local theory of gravity.
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Llinares, Claudio, Robert Hagala, and David F. Mota. "Non-linear phenomenology of disformally coupled quintessence." Monthly Notices of the Royal Astronomical Society 491, no. 2 (2019): 1868–86. http://dx.doi.org/10.1093/mnras/stz2710.
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ABSTRACT The quintessence model is one of the simplest and better known alternatives to Einstein’s theory for gravity. The properties of the solutions have been studied in great detail in the background, linear and non-linear contexts in cosmology. Here we discuss new phenomenology that is induced by adding disformal terms to the interactions. Among other results, we show analytically and using cosmological simulations ran with the code isis that the model possesses a mechanism through which it is possible to obtain repulsive fifth forces, which are opposite to gravity. Although the equations are very complex, we also find that most of the new phenomenology can be explained by studying background quantities. We used our simulation data to test approximate relations that exist between the metric and scalar field perturbations as well as between the fifth force and gravity. Excellent agreement was found between exact and approximated solutions, which opens the way for running disformal gravity cosmological simulations using simply a Newtonian solver. These results could not only help us to find new ways of testing gravity, but also provide new motivations for building alternative models.
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GIDDINGS, STEVEN B. "BLACK HOLES, INFORMATION, AND LOCALITY." Modern Physics Letters A 22, no. 39 (2007): 2949–54. http://dx.doi.org/10.1142/s0217732307025923.
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Three decades of a deepening information paradox suggest the need to revise our basic physical framework. Multiple indicators point toward reassessment of the principle of locality: lack of a precise definition in quantum gravity, behavior of high-energy scattering, hints from strings and AdS/CFT, conundrums of quantum cosmology, and finally lack of good alternative resolutions of the paradox. A plausible conjecture states that the non-perturbative dynamics of gravity is unitary but nonlocal. String theory may directly address these issues but so far important aspects remain elusive. If this viewpoint is correct, critical questions are to understand the "correspondence" limit where nonlocal physics reduces to local quantum field theory, and beyond, to unveil principles of an underlying nonlocal theory.
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MOTOHASHI, HAYATO, ALEXEI A. STAROBINSKY, and JUN'ICHI YOKOYAMA. "f(R) COSMOLOGY AND MASSIVE NEUTRINOS." International Journal of Modern Physics: Conference Series 10 (January 2012): 35–42. http://dx.doi.org/10.1142/s2010194512005739.
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f(R) gravity provides viable cosmology alternative to the ΛCDM model. We discuss the effect of massive neutrinos on matter power spectrum in this theory, to show that the anomalous growth of density fluctuations on small scales due to the scalaron force can be compensated by free streaming of neutrinos. As a result, models which predict observable deviation of the equation-of-state parameter w DE from w DE = -1 may be reconciled with observations of matter clustering if the total neutrino mass is O(0.5 eV ).
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Odintsov, S. D., and V. K. Oikonomou. "Big bounce with finite-time singularity: The F(R) gravity description." International Journal of Modern Physics D 26, no. 08 (2017): 1750085. http://dx.doi.org/10.1142/s0218271817500857.
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An alternative to the Big Bang cosmologies is obtained by the Big Bounce cosmologies. In this paper, we study a bounce cosmology with a Type IV singularity occurring at the bouncing point in the context of [Formula: see text] modified gravity. We investigate the evolution of the Hubble radius and we examine the issue of primordial cosmological perturbations in detail. As we demonstrate, for the singular bounce, the primordial perturbations originating from the cosmological era near the bounce do not produce a scale-invariant spectrum and also the short wavelength modes after these exit the horizon, do not freeze, but grow linearly with time. After presenting the cosmological perturbations study, we discuss the viability of the singular bounce model, and our results indicate that the singular bounce must be combined with another cosmological scenario, or should be modified appropriately, in order that it leads to a viable cosmology. The study of the slow-roll parameters leads to the same result indicating that the singular bounce theory is unstable at the singularity point for certain values of the parameters. We also conformally transform the Jordan frame singular bounce, and as we demonstrate, the Einstein frame metric leads to a Big Rip singularity. Therefore, the Type IV singularity in the Jordan frame becomes a Big Rip singularity in the Einstein frame. Finally, we briefly study a generalized singular cosmological model, which contains two Type IV singularities, with quite appealing features.
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Starkman, Glenn D. "Modifying gravity: you cannot always get what you want." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369, no. 1957 (2011): 5018–41. http://dx.doi.org/10.1098/rsta.2011.0292.
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The combination of general relativity (GR) and the Standard Model of particle physics disagrees with numerous observations on scales from our Solar System up. In the canonical concordance model of Lambda cold dark matter ( Λ CDM) cosmology, many of these contradictions between theory and data are removed or alleviated by the introduction of three completely independent new components of stress energy—the inflaton, dark matter and dark energy. Each of these in its turn is meant to have dominated (or to currently dominate) the dynamics of the Universe. There is, until now, no non-gravitational evidence for any of these dark sectors, nor is there evidence (though there may be motivation) for the required extension of the Standard Model. An alternative is to imagine that it is GR that must be modified to account for some or all of these disagreements. Certain coincidences of scale even suggest that one might expect not to make independent modifications of the theory to replace each of the three dark sectors. Because they must address the most different types of data, attempts to replace dark matter with modified gravity are the most controversial. A phenomenological model (or family of models), modified Newtonian dynamics, has, over the last few years, seen several covariant realizations. We discuss a number of challenges that any model that seeks to replace dark matter with modified gravity must face: the loss of Birkhoff's theorem, and the calculational simplifications it implies; the failure to explain clusters, whether static or interacting, and the consequent need to introduce dark matter of some form, whether hot dark matter neutrinos or dark fields that arise in new sectors of the modified gravity theory; the intrusion of cosmological expansion into the modified force law, which arises precisely because of the coincidence in scale between the centripetal acceleration at which Newtonian gravity fails in galaxies and the cosmic acceleration. We conclude with the observation that, although modified gravity may indeed manage to replace dark matter, it is likely to do so by becoming or at least incorporating a dark matter theory itself.
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Solanki, Raja, Simran Arora, Pradyumn Kumar Sahoo, and Pedro H. R. S. Moraes. "Bulk Viscous Fluid in Symmetric Teleparallel Cosmology: Theory versus Experiment." Universe 9, no. 1 (2022): 12. http://dx.doi.org/10.3390/universe9010012.
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The standard formulation of General Relativity Theory, in the absence of a cosmological constant, is unable to explain the responsible mechanism for the observed late-time cosmic acceleration. On the other hand, by inserting the cosmological constant in Einstein’s field equations, it is possible to describe the cosmic acceleration, but the cosmological constant suffers from an unprecedented fine-tuning problem. This motivates one to modify Einstein’s spacetime geometry of General Relativity. The f(Q) modified theory of gravity is an alternative theory to General Relativity, where the non-metricity scalar Q is the responsible candidate for gravitational interactions. In the present work, we consider a Friedmann–Lemâitre–Robertson–Walker cosmological model dominated by bulk viscous cosmic fluid in f(Q) gravity with the functional form f(Q)=αQn, where α and n are free parameters of the model. We constrain our model with the Pantheon supernovae dataset of 1048 data points, the Hubble dataset of 31 data points, and the baryon acoustic oscillations dataset consisting of 6 data points. We find that our f(Q) cosmological model efficiently describes the observational data. We present the evolution of our deceleration parameter with redshift, and it properly predicts a transition from decelerated to accelerated phases of the universe’s expansion. Furthermore, we present the evolution of density, bulk viscous pressure, and the effective equation of state parameter with redshift. Those show that bulk viscosity in a cosmic fluid is a valid candidate to acquire the negative pressure to drive the cosmic expansion efficiently. We also examine the behavior of different energy conditions to test the viability of our cosmological f(Q) model. Furthermore, the statefinder diagnostics are also investigated in order to distinguish among different dark energy models.
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Alam, Shadab, Christian Arnold, Alejandro Aviles, et al. "Towards testing the theory of gravity with DESI: summary statistics, model predictions and future simulation requirements." Journal of Cosmology and Astroparticle Physics 2021, no. 11 (2021): 050. http://dx.doi.org/10.1088/1475-7516/2021/11/050.
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Abstract Shortly after its discovery, General Relativity (GR) was applied to predict the behavior of our Universe on the largest scales, and later became the foundation of modern cosmology. Its validity has been verified on a range of scales and environments from the Solar system to merging black holes. However, experimental confirmations of GR on cosmological scales have so far lacked the accuracy one would hope for — its applications on those scales being largely based on extrapolation and its validity there sometimes questioned in the shadow of the discovery of the unexpected cosmic acceleration. Future astronomical instruments surveying the distribution and evolution of galaxies over substantial portions of the observable Universe, such as the Dark Energy Spectroscopic Instrument (DESI), will be able to measure the fingerprints of gravity and their statistical power will allow strong constraints on alternatives to GR. In this paper, based on a set of N-body simulations and mock galaxy catalogs, we study the predictions of a number of traditional and novel summary statistics beyond linear redshift distortions in two well-studied modified gravity models — chameleon f(R) gravity and a braneworld model — and the potential of testing these deviations from GR using DESI. These summary statistics employ a wide array of statistical properties of the galaxy and the underlying dark matter field, including two-point and higher-order statistics, environmental dependence, redshift space distortions and weak lensing. We find that they hold promising power for testing GR to unprecedented precision. The major future challenge is to make realistic, simulation-based mock galaxy catalogs for both GR and alternative models to fully exploit the statistic power of the DESI survey (by matching the volumes and galaxy number densities of the mocks to those in the real survey) and to better understand the impact of key systematic effects. Using these, we identify future simulation and analysis needs for gravity tests using DESI.
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Hamber, Herbert W., Lu Heng Sunny Yu, and Hasitha E. Pituwala Kankanamge. "Gravitational Fluctuations as an Alternative to Inflation III. Numerical Results." Universe 6, no. 7 (2020): 92. http://dx.doi.org/10.3390/universe6070092.
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Power spectra play an important role in the theory of inflation, and their ability to reproduce current observational data to high accuracy is often considered a triumph of inflation, largely because of a lack of credible alternatives. In previous work we introduced an alternative picture for the cosmological power spectra based on the nonperturbative features of the quantum version of Einstein’s gravity, instead of currently popular inflation models based on scalar fields. The key ingredients in this new picture are the appearance of a nontrivial gravitational vacuum condensate (directly related to the observed cosmological constant), and a calculable renormalization group running of Newton’s G on cosmological scales. More importantly, one notes the absence of any fundamental scalar fields in this approach. Results obtained previously were largely based on a semi-analytical treatment, and thus, while generally transparent in their implementation, often suffered from the limitations of various approximations and simplifying assumptions. In this work, we extend and refine our previous calculations by laying out an updated and extended analysis, which now utilizes a set of suitably modified state-of-the-art numerical programs (ISiTGR, MGCAMB and MGCLASS) developed for observational cosmology. As a result, we are able to remove some of the approximations employed in our previous studies, leading to a number of novel and detailed physical predictions. These should help in potentially distinguishing the vacuum condensate picture of quantum gravity from that of other models such as scalar field inflation. Here, besides the matter power spectrum P m ( k ) , we work out, in detail, predictions for what are referred to as the TT, TE, EE, BB angular spectra, as well as their closely related lensing spectra. However, the current limited precision of observational data today (especially on large angular scales) does not allow us yet to clearly prove or disprove either set of ideas. Nevertheless, by exploring in more details the relationship between gravity and cosmological matter and radiation both analytically and numerically, together with an expected future influx of increasingly accurate observational data, one can hope that the new quantum gravitational picture can be subjected to further stringent tests in the near future.
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Baumgärtel, Christof, and Simon Maher. "Foundations of Electromagnetism: A Review of Wilhelm Weber’s Electrodynamic Force Law." Foundations 2, no. 4 (2022): 949–80. http://dx.doi.org/10.3390/foundations2040065.
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This article reviews the electrodynamic force law of Wilhelm Weber and its importance in electromagnetic theory. An introduction is given to Weber’s force and it is shown how it has been utilised in the literature to explain electromagnetism as well as phenomena in other disciplines of physics, where the force law has connections to the nuclear force, gravity, cosmology, inertia and quantum mechanics. Further, criticism of Weber’s force is reviewed and common misconceptions addressed and rectified. It is found that, while the theory is not without criticism and has much room for improvement, within the limitations of its validity, it is equally as successful as Maxwell’s theory in predicting certain phenomena. Moreover, it is discussed how Weber offers a valid alternative explanation of electromagnetic phenomena which can enrich and complement the field perspective of electromagnetism through a particle based approach.
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Wang, Ji-Yao, Chao-Jun Feng, Xiang-Hua Zhai, and Xin-Zhou Li. "Solar system tests of a new class of f(z) theory." International Journal of Modern Physics D 29, no. 08 (2020): 2050060. http://dx.doi.org/10.1142/s0218271820500601.
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Recently, a new kind of [Formula: see text] theory is proposed to provide a different perspective for the development of reliable alternative models of gravity in which the [Formula: see text] Lagrangian terms are reformulated as polynomial parametrizations [Formula: see text]. In the previous study, the parameters in the [Formula: see text] models have been constrained by using cosmological data. In this paper, these models will be tested by the observations in the solar system. After solving the Ricci scalar as a function of the redshift, one could obtain [Formula: see text] that could be used to calculate the standard Parametrized-Post-Newtonian (PPN) parameters. First, we fit the parametric models with the latest cosmological observational data. Then, the tests are performed by solar system observations. And last we combine the constraints of solar system and cosmology together and reconstruct the [Formula: see text] actions of the [Formula: see text] parametric models.
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Serpa, Nilo. "The Structure of Space-time and the Emergence of Complex Life." CALIBRE - Revista Brasiliense de Engenharia e Física Aplicada 7 (March 3, 2022): 1. http://dx.doi.org/10.17648/calibre.v7.2188.
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<p><strong>Abstract</strong>. This article is a continuation of the work started in 2016 by Serpa and Steiner on the structure of space-time. The original theory proposes an alternative to the idea of direct quantization of gravity through a semi-classical evolutionary model, now associated with an entropic principle applied to the accelerated expansion of the universe. This principle relates the emergence of intelligent life to an expectation value of the expansion rate of space-time. The theory is consistent with general relativity and observational cosmology at its current stage of technological development, making a test program plausible in the relatively near future.</p><p><strong>Key words</strong>: space-time, gravity, entropy, general relativity.</p><p>=====================================================================</p><p><strong>Resumo</strong>. Este artigo é uma continuação do trabalho iniciado por Serpa e Steiner em 2016 sobre a estrutura do espaço-tempo. A teoria original propõe uma alternativa à ideia de quantização direta da gravidade por meio de um modelo evolutivo semiclássico, agora associado a um princípio entrópico aplicado à expansão acelerada do universo. Este princípio relaciona o surgimento de vida inteligente a um valor de espera da taxa de expansão do espaço-tempo. A teoria é consistente com a relatividade geral e com a cosmologia observacional em seu estágio atual de desenvolvimento tecnológico, tornando plausível um programa de teste em futuro relativamente próximo.</p><p><strong>Palavras-chave</strong>: espaço-tempo, gravidade, entropia, relatividade geral.</p>
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Shestakova, Tatyana P. "On the meaning of the wave function of the Universe." International Journal of Modern Physics D 28, no. 13 (2019): 1941009. http://dx.doi.org/10.1142/s0218271819410098.
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The meaning of the wave function of the Universe was actively discussed in 1980s. In most works on quantum cosmology, it is accepted that the wave function is a probability amplitude for the Universe to have some space geometry, or to be found in some point of the Wheeler superspace. It seems that the wave function gives maximally objective description compatible with quantum theory. However, the probability distribution does not depend on time and does not take into account the existing of our macroscopic evolving Universe. What we wish to know is how quantum processes in the Early Universe determined the state of the present Universe in which we are able to observe macroscopic consequences of these quantum processes. As an alternative to the Wheeler–DeWitt quantum geometrodynamics, we consider the picture that can be obtained in the extended phase space approach to quantization of gravity. The wave function in this approach describes different states of the Universe which correspond to different stages of its evolution.
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EL-NABULSI, R. A. "MAXWELL BRANE COSMOLOGY WITH HIGHER-ORDER STRING CURVATURE CORRECTIONS, A NONMINIMALLY COUPLED SCALAR FIELD, DARK MATTER–DARK ENERGY INTERACTION AND A VARYING SPEED OF LIGHT." International Journal of Modern Physics D 18, no. 02 (2009): 289–318. http://dx.doi.org/10.1142/s0218271809014431.
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We investigate the cosmological effects of an alternative theory of gravity on the four-dimensional Randall–Sundrum braneworld of type II with a higher-order string curvature term added to the action. We discuss the possibility of a varying speed of light, which has recently attracted considerable attention, in the presence a Maxwell field and of a dynamically evolving bulk scalar field nonminimally coupled to scalar curvature in a quadratic form, together with a dark matter–dark energy interaction term. After deriving the modified Friedmann equation on the brane, as well as the scalar field equations, we then analyze the dynamical equations obtained so far. Special attention is paid to scaling solutions which could be important building blocks in constructing the models of dark energy. The constructed model exhibits several features of cosmological and astrophysical interest for both the early and the late universe, consistent with recent observations, in particular the ones concerned with celerity of light, four and five gravitational constants, black hole masses and entropies.
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Safo, Ramsès Bounkeu. "Disruptive Gravity: A Quantizable Alternative to General Relativity." Global Journal of Science Frontier Research, July 22, 2020, 43–57. http://dx.doi.org/10.34257/gjsfravol20is8pg43.
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Gravity is the most problematic interaction of modern science. Questioning the very foundations of gravity might be the key to understanding it better since its description changed over time. Newton described it as a force, Einstein described it as a spacetime curvature and this paper shows how gravity can be described as a force able to bend spacetime instead. Applied to cosmology, gravity as a spacetime bending force doesn't require Dark Energy. Described as a spacetime bending force, gravity becomes quantizable as a force in curved spacetime which is compatible with the Standard Model of particle physics. Therefore, one could associate the Standard Model to this theory and achieve Quantum Gravity.
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Venikoudis, S. A., K. V. Fasoulakos та F. P. Fronimos. "Late-time Cosmology of scalar field assisted f(𝒢) gravity". International Journal of Modern Physics D 31, № 05 (2022). http://dx.doi.org/10.1142/s0218271822500389.
Full textAbstract:
In this work, we present the late-time behaviour of the universe in the context of Einstein–Gauss–Bonnet gravitational theory. The theory involves a scalar field, which represents low-effective quantum corrections, assisted by a function [Formula: see text] solely depending from the Gauss–Bonnet topological invariant [Formula: see text]. It is considered that the dark energy serves as the impact of all geometric terms, which are included in the gravitational action and the density of dark energy acts as a time-dependent cosmological constant evolving with an infinitesimal rate and driving the universe into an accelerating expansion. We examine two cosmological models of interest. The first involves a canonical scalar field in the presence of a scalar potential while the second, involves a scalar field which belongs to a generalized class of theories [Formula: see text] namely the k-essence scalar field in the absence of scalar potential. As it is proved, the aforementioned models are in consistency with the latest Planck data and in relatively good agreement with the [Formula: see text]CDM standard cosmological model. The absence of dark energy oscillations at the early stages of matter-dominated era, which appear in alternative scenarios of cosmological dynamics in the context of modified [Formula: see text] gravitational theories, indicates an advantage of the theory for the interpretation of late-time phenomenology.
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Kaczmarek, Adam Z., and Dominik Szczȩśniak. "Cosmology in the mimetic higher-curvature $$f(R,R_{\mu \nu }R^{\mu \nu })$$ gravity." Scientific Reports 11, no. 1 (2021). http://dx.doi.org/10.1038/s41598-021-97907-y.
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AbstractIn the framework of the mimetic approach, we study the $$f(R,R_{\mu \nu }R^{\mu \nu })$$ f ( R , R μ ν R μ ν ) gravity with the Lagrange multiplier constraint and the scalar potential. We introduce field equations for the discussed theory and overview their properties. By using the general reconstruction scheme we obtain the power law cosmology model for the $$f(R,R_{\mu \nu }R^{\mu \nu })=R+d(R_{\mu \nu }R^{\mu \nu })^p$$ f ( R , R μ ν R μ ν ) = R + d ( R μ ν R μ ν ) p case as well as the model that describes symmetric bounce. Moreover, we reconstruct model, unifying both matter dominated and accelerated phases, where ordinary matter is neglected. Using inverted reconstruction scheme we recover specific $$f(R,R_{\mu \nu }R^{\mu \nu })$$ f ( R , R μ ν R μ ν ) function which give rise to the de-Sitter evolution. Finally, by employing the perfect fluid approach, we demonstrate that this model can realize inflation consistent with the bounds coming from the BICEP2/Keck array and the Planck data. We also discuss the holographic dark energy density in terms of the presented $$f(R,R_{\mu \nu }R^{\mu \nu })$$ f ( R , R μ ν R μ ν ) theory. Thus, it is suggested that the introduced extension of the mimetic regime may describe any given cosmological model.
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López-Corredoira, Martín, and Louis Marmet. "Alternative ideas in cosmology." International Journal of Modern Physics D, March 18, 2022. http://dx.doi.org/10.1142/s0218271822300142.
Full textAbstract:
Some remarkable examples of alternative cosmological theories are reviewed here, ranging from a compilation of variations on the Standard Model through the more distant quasi-steady-state cosmology, plasma cosmology, or universe models as a hypersphere, to the most exotic cases including static models. The present-day Standard Model of cosmology, [Formula: see text]CDM, gives us a representation of a cosmos whose dynamics is dominated by gravity (Friedmann equations derived from general relativity) with a finite lifetime, large scale homogeneity, expansion and a hot initial state, together with other elements necessary to avoid certain inconsistencies with observations (inflation, nonbaryonic dark matter, dark energy, etc.). There are however some models with characteristics that are close to those of the Standard Model but differing in some minor aspects; we call these “variations on the Standard Model”. Many of these models are indeed investigated by some mainstream cosmologists: different considerations on CP violation, inflation, number of neutrino species, quark-hadron phase transition, baryonic or nonbaryonic dark-matter, dark energy, nucleosynthesis scenarios, large-scale structure formation scenarios; or major variations like a inhomogeneous universe, Cold Big Bang, varying physical constants or gravity law, zero-active mass (also called “[Formula: see text]”), Milne, and cyclical models. At the most extreme distance from the Standard Model, the static models, a noncosmological redshift includes “tired-light” hypotheses, which assume that the photon loses energy owing to an intrinsic property or an interaction with matter or light as it travels some distance, or other nonstandard ideas. Our impression is that none of the alternative models has acquired the same level of development as [Formula: see text]CDM in offering explanations of available cosmological observations. One should not, however, judge any theory in terms of the number of observations that it can successfully explain (ad hoc in many cases) given the much lower level of development of the alternative ones, but by the plausibility of its principles and its potential to fit data with future improvements of the theories. A pluralist approach to cosmology is a reasonable option when the preferred theory is still under discussion.
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Bessa, P., M. Campista, and A. Bernui. "Observational constraints on Starobinsky f(R) cosmology from cosmic expansion and structure growth data." European Physical Journal C 82, no. 6 (2022). http://dx.doi.org/10.1140/epjc/s10052-022-10457-z.
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AbstractThe unknown physical nature of the Dark Energy motivates in cosmology the study of modifications of the gravity theory at large distances. One of these types of modifications is to consider gravity theories, generally termed as f(R). In this paper we use observational data to both constrain and test the Starobinsky f(R) model (Starobinsky in JETP Lett 86(3):157–163, 2007), using updated measurements from the dynamics of the expansion of the universe, H(z); and the growth rate of cosmic structures, $$[f\sigma _8](z)$$ [ f σ 8 ] ( z ) , where the distinction between the concordance $$\varLambda $$ Λ CDM model and modified gravity models f(R) becomes clearer. We use MCMC likelihood analyses to explore the parameters space of the f(R) model using H(z) and $$[f\sigma _8](z)$$ [ f σ 8 ] ( z ) data, both individually and jointly, and further, examine which of the models best fits the joint data. To further test the Starobinsky model, we use a method proposed by Linder (Astropart Phys 86:41–45, 2017), where the data from the observables is jointly binned in redshift space. This allows one to further explore the model’s parameter that better fits the data in comparison to the $$\varLambda $$ Λ CDM model. The joint analysis of H(z) and $$[f\sigma _8](z)$$ [ f σ 8 ] ( z ) show that the $$n=2$$ n = 2 –Starobinsky f(R) model fits well the observational data. In the end, we confirm that this joint analysis is able to break the degenerescence between modified gravity models as proposed in the original work (Starobinsky 2007). Our results indicate that the f(R) Starobinsky model provides a good fit to the currently available data for a set of values of its parameters, being, therefore, a possible alternative to the $$\varLambda $$ Λ CDM model.
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"Preface." Journal of Physics: Conference Series 2197, no. 1 (2022): 011001. http://dx.doi.org/10.1088/1742-6596/2197/1/011001.
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The 12th international symposium, The Vigier Centenary - 3rd Regime Natural Science Towards a Physics of The Observer: Honoring Noted Mathematical Physicist Jean-Pierre Vigier, 20-23 September 2021, was postponed three times due to travel restrictions imposed by the Coronavirus pandemic. The conference was originally to be held in historic Liege, Belgium, but ended up with a Virtual Meeting venue at the Noetic Advanced Studies Institute, Escalante Desert Research Facility, Beryl, Utah, USA. This symposium – the Centenary of J-P Vigier’s birth (January 16, 1920 Paris, France to May 4, 2004, Paris, France) marks the final conference in the series running for 27-years. Vigier was often called ‘Le Heretique du Physique’ in the French media. Most of his work – over 400 papers, chapters and books relates to cutting-edge extensions or alternative interpretations to ‘mainstream physics’ such as photon mass, or the de Broglie-Bohm-Vigier Causal-Stochastic Interpretation of Quantum Mechanics for example. Einstein asked Vigier to become his assistant at Princeton’s Advanced Studies Institute; but since Vigier was ‘politically involved’ with the French in Vietnam the US government would not allow him entry to the USA at that time. Thus, he became Nobelist Louis de Broglie’s assistant instead. The Vigier conference began in 1995, with the first two conferences held at York University in Toronto, Canada. It was one of a handful of such Avant-garde series ‘allowing’ physicists to color outside-the-box; including CASYS, PIRT and ANPA, which emerged in the last decades of the 20th-century to cover areas of fundamental physics which at that time had no obvious outlet for discussion. These are vital areas for discussion now, but at the time, particle physics was thought solved by string theory and supersymmetry, and gravity by various approaches to quantizing general relativity. A surprise is likely to occur with the looming experimental awakening of the imminent paradigm shift to 3rd regime Natural Science; current thinking, for all practical purposes, insists that gravity must be quantized. This arises from the belief that the quantum mechanical stochastic foam is an impenetrable basement of reality. String theory (now a 12D M-theory), an evolution of 5D Kaluza-Klein theory, has been developed as the putative theory of quantum gravity; but there is no a priori reason that gravity must be quantized. The Standard Model (SM) of particle physics and cosmology, while known to be incomplete, still myopically governs ‘allowed’ research avenues.
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Momeni, Davood. "Position-dependent mass quantum systems and ADM formalism." SciPost Physics Proceedings, no. 4 (August 13, 2021). http://dx.doi.org/10.21468/scipostphysproc.4.009.
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The classical Einstein-Hilbert (EH) action for general relativity (GR) is shown to be formally analogous to the classical system with position-dependent mass (PDM) models. The analogy is developed and used to build the covariant classical Hamiltonian as well as defining an alternative phase portrait for GR. The set of associated Hamilton’s equations in the phase space is presented as a first-order system dual to the Einstein field equations. Following the principles of quantum mechanics, I build a canonical theory for the classical general. A fully consistent quantum Hamiltonian for GR is constructed based on adopting a high dimensional phase space. It is observed that the functional wave equation is timeless. As a direct application, I present an alternative wave equation for quantum cosmology. In comparison to the standard Arnowitt-Deser-Misner(ADM) decomposition and quantum gravity proposals, I extended my analysis beyond the covariant regime when the metric is decomposed into the 3+13+1 dimensional ADM decomposition. I showed that an equal dimensional phase space can be obtained if one applies ADM decomposed metric.
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Springel, Volker, Rüdiger Pakmor, Oliver Zier, and Martin Reinecke. "Simulating cosmic structure formation with the GADGET-4 code." Monthly Notices of the Royal Astronomical Society, July 1, 2021. http://dx.doi.org/10.1093/mnras/stab1855.
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Abstract Numerical methods have become a powerful tool for research in astrophysics, but their utility depends critically on the availability of suitable simulation codes. This calls for continuous efforts in code development, which is necessitated also by the rapidly evolving technology underlying today’s computing hardware. Here we discuss recent methodological progress in the GADGET code, which has been widely applied in cosmic structure formation over the past two decades. The new version offers improvements in force accuracy, in time-stepping, in adaptivity to a large dynamic range in timescales, in computational efficiency, and in parallel scalability through a special MPI/shared-memory parallelization and communication strategy, and a more-sophisticated domain decomposition algorithm. A manifestly momentum conserving fast multipole method (FMM) can be employed as an alternative to the one-sided TreePM gravity solver introduced in earlier versions. Two different flavours of smoothed particle hydrodynamics, a classic entropy-conserving formulation and a pressure-based approach, are supported for dealing with gaseous flows. The code is able to cope with very large problem sizes, thus allowing accurate predictions for cosmic structure formation in support of future precision tests of cosmology, and at the same time is well adapted to high dynamic range zoom-calculations with extreme variability of the particle number density in the simulated volume. The GADGET-4 code is publicly released to the community and contains infrastructure for on-the-fly group and substructure finding and tracking, as well as merger tree building, a simple model for radiative cooling and star formation, a high dynamic range power spectrum estimator, and an initial conditions generator based on second-order Lagrangian perturbation theory.