Questions tagged [modified-gravity]

A set of theories that attempt to take the basics of general relativity, and extend it in such a way that it solves various problems. This applies to Milgrom's MOND proposal, but also includes such other things as Einstein-Cartan theory, Brans-Dicke theory, and $f(R)$ gravity.

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Why modifying gravity to a fixed distance cant solve dark matter? [closed]

I quote Sabine Hossenfelder: "A modification becoming important at a fixed distance however could never explain the observed rotation velocities for spiral galaxies, whose constant asymptotic ...
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Is MOND equivalent to Modified Gravity?

Usually, we consider two alternative models of dark matter: modified newtonian dynamics (MOND) and modified gravity (MOG). My question is simple: can MOND be made equivalent to MOG or does it stand as ...
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Has MOND been tested or even confirmed for our own galaxy, the Milky Way?

MOND, based on a modifications of Newton's law for small accelerations, describes the rotation curves of stars in most galaxies, especially the outer stars. Has MOND been tested for the stars in our ...
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How to show that our model is close to Einstein's gravity after inflation

We have a modified $f \left( R \right)$ gravity model. How can we show that the proposed model is close to Einstein's gravity after inflation and does not contradict observation data?
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What is the Correction Function in Poisson's Equation for MOND?

I am looking to do a celestial simulation using MOND for the final project of my intermediate mechanics course. From the Wikipedia page, it seems that preserving Newton's Third Law requires deriving a ...
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Differences between the Modified Gravity theories and the Extended Theories of Gravity

I am trying to understand what is the difference between the Modified Gravity theories and the Extended Gravity theories. What is the big difference between the two theories?, and which is giving ...
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Effective field theories in curved spacetime

Loosely speaking, in flat spacetime, one defines the effective Lagrangian by writing down all possible operators compatible with the symmetries and suppressed by some energy scale, and one usually ...
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Physical Situations of Specific Stress Energy Tensor?

I'm having trouble picturing what the physical situation of a non-symmetric stress would be. Say I have a stress tensor $T_{ab} = \begin{pmatrix} T_{00} & T_{01}& 0 & 0 \\ 0&T_{11}&...
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Obtaining the KG equation from Action

After solving the field equation for $$S = \int \sqrt{-g}dx^4[f(\phi)R + h(\phi)g^{\mu \nu}\nabla_{\mu}\phi\nabla_{\nu}\phi - V(\phi)]$$ I have obtained $$2h\square \phi + \frac{\partial h}{\partial \...
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Variation for the Canonical Scalar Field in $f(\phi)R$

I am trying to find the Field equation for $$S = \int \sqrt{-g}dx^4[f(\phi)R + h(\phi)g^{\mu \nu}\nabla_{\mu}\phi\nabla_{\nu}\phi - V(\phi)$$ but I could not take the variation of $$\delta(\sqrt{-g}h(\...
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State of the art on Modified gravity : going beyond the 2nd order differential equations, diffeomorphism invariance breaking, extra degrees of freedom

I am going to do a state of the art on Modified gravity models. I have found a talk that presents the problematic. In particular, it is said the following things : Modifying General Relativity How to ...
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Book/Article recommendation about modified gravity (scalar/tensor/scalar-tensor theories)

Is there a book or article that discusses the modified gravity theories scalar/tensor/scalar-tensor (maybe others as well) theories? (Look at this wiki page for a general overview of the theories that ...
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Problem with field equations in five-dimensional $f(R)$ gravity

I am trying to reproduce the calculations in this paper by Biao Huang, Song Li, and Yongge Ma. The equation of concern is Eq. 9, where the field equation is derived as follows: $$\begin{align} G_{ab}...
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Is Bertrand's Theorem Consistent with GR?

Bertrand's Theorem - Please observe the gif of orbits with different exponents in the denominator. (I couldn't get the image upload feature to work for this) Note, the orbit with a 1.9 exponent ...
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What explains the descrepancy between Asaph Hall's equation for Mercury's precession and GR?

In regard to What is the weight equation through general relativity?, the answer is: $$F=ma=\frac{GMm}{r^2}\frac{1}{\sqrt{1-\frac{2GM}{c^2r}}}.$$ This source provides a different equation: $$f \approx ...
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Does Lorentz invariance imply the impossibility to detect an event horizon from a local experiment?

It appears that it is impossible to detect the event horizon from a local experiment. Is this a consequence that General Relativity always reduces locally to a Lorentz invariant theory? Or in other ...
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What’s wrong with this Nordström-like scalar theory of gravity?

I got very perplexed while reading a few papers on the old Nordström theory of relativistic scalar gravity. I would like to know what's wrong with the following, which isn't exactly the same as ...
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Which geodesics light travels on: metric or affine in Einstein-Cartan theory?

In a generic Lorentzian spacetime solving the Einstein equations since both the metric and affine geodesic coincide this question doesn't arise. But in the Einstein–Cartan theory it is not very ...
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Is a vacuum-energy smaller than zero forbidden? Why?

Einstein's Field Equations allow for the derivation of Newton's law and this, together with the velocity profile of the stars within the galaxies and the galaxies within the galaxy clusters, leads to ...
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Does Higher spinor field bend Space-time?

Consider the free massless spin-$n/2$ field in a general curved space-time ($M$, $g$): \begin{equation} \nabla^{AA'}\phi_{\underbrace{AB\cdots L}_n} =0\end{equation}If $\phi_{AB\cdots L}$ has charge $...
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Asymptotic behaviour of a non-linear differential equation in Gravitation

I have been recently working on modifications to General Relativity, by adding new curvature terms in the Lagrangian density of the theory. In one of these theories (Einsteinian Cubic Gravity), the ...
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Newtonian quantum gravity

Can someone give me reference about Newtonian (non-relativistic) quantum gravity like unifying Newtonian gravity with quantum mechanics?
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Newton's Law of Gravitation and dark matter [closed]

Before the days of Einstein the observation that light incident to the Earth's surface appears to be constant no matter whether the transmitter is travelling away from the earth or towards the earth ...
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Could quantum vacuum polarization increase GR frame dragging beyond the predicted values and therefore replace DM explanation of galactic rotation? [duplicate]

image source credits:David Butler This anomalous speed rotation distribution of galaxies is today mainly contributed to Dark Matter. However, since a definitive experimental measurement and ...
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Graviton propagator in Horndeski theory

Let $\phi$ be a scalar field and $g_{\mu \nu} = \eta_{\mu \nu}+h_{\mu \nu}/M_p$ where $M_p$ is the Planck mass (so we assume we deal with perturbations). Let $\Lambda_2,\Lambda_3$ be energy scales ...
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Feynman rules Horndeski theory

Let $\phi$ be a scalar field and $g_{\mu \nu} = \eta_{\mu \nu}+h_{\mu \nu}/M_p$ where $M_p$ is the Planck mass (so we assume we deal with perturbations). Let $\Lambda_2,\Lambda_3$ be energy scales ...
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Switch from $AdS$ to $dS$ in quadratic gravity using $f(R)$ trick: problem

I have some difficulties with effective quadratic gravity involving a cosmological constant with the "wrong sign". The following is the setup of my question. Let's assume one has the ...
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Contradictions in articles about renormalizability of Einstein-conformal gravity

Background: In the calculations I've done, I've found an action of the following form: \begin{equation} S=\int d^4x\,\sqrt{-g}\left( \xi^2R+\frac{1}{120}\xi^2C_{\alpha \beta \mu \nu}C^{\alpha \beta \...
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Can Integrating out Dark Matter give Modified Gravity?

I'm sure I misunderstood something obvious, but reading this question, I wondered what philosophically is the difference between modified gravity (like TeVeS, f(R), etc.) and dark matter, if/since we ...
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If MOND were true, what would that imply for the geometrical description of gravity as curved spacetime?

As I understand it, Modified Newtonian Dynamics, or MOND (Milgrom M., 1983, ApJ, 270, 365), slightly alters Newton's Law of Gravity by introducing a low acceleration limit below which (for an object ...
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Equations of motion of the metric in Classical Dilaton Gravity

In these lecture notes by Strominger section 3.3 we derive the equations of motion of the Classical Dilaton Gravity action $$ S = \frac{1}{2\pi}\int{d^2x}{\sqrt{-g}e^{-2\phi}\left(R +4 (\nabla\phi)^2+...
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Why are wide binary systems assumed to contain negligible dark matter?

I'm doing a project that simulates wide binaries to do gravity tests, specifically a dark matter vs MOND test. I've come across similar papers, and their justification for using wide binaries for this ...
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Is the Palatini formulation of a gravity theory a metric one?

As far as i know, the Einstein equivalence principle (EEP) is the starting point to explain gravity as a geometric phenomenom. It allows you to link gravity with two geometrical objects: a metric, ...
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If MOND requires fixes at LOW accelerations, why don't we see it in table experiments?

MOND is controlled by $\mu=\mu(\frac{a}{a_0})$ and $\mu\rightarrow1$ when $a\rightarrow0$ But on my table $a=0$ so shouldn't I see ...
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Dimension of operators in modified gravity

I was reading a paper arXiv:1903.06784 where the following action is considered: \begin{equation} S\sim \int d^4x \sqrt{-g}\bigg[M^2_{Pl}R-\frac{1}{2}(\nabla\phi)^2 -\frac{1}{2}\mu^2\phi^2-\frac{1}{2}\...
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Modify Newton's gravity to cure the action at a distance

Take a bunch of particles (can be point particles) at ${\bf{x}}_i(t)$ and of mass $m_i>0$. The gravitational field is defined by the Poisson equation $$ \nabla^2 \Phi({\bf{x}},t) = 4 \pi G \rho({\...
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Particular behaviour of 'black hole' horizon in modified gravity

When working in a particular theory of modified gravity, one can see that a solution for a spherically symmetric and static puntual mass is given by \begin{equation} ds^2=-B(r)dt^2+A(r)dr^2+r^2d\theta^...
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What properties of MOND differ from Newtonian gravity?

I know of the following properties that set modified Newtonian dynamics (MOND) apart from Newtonian gravity: There is a $1/r$ dependence instead of a $1/r^2$ dependence, at large distances/low ...
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Higgs non-minimal coupling to gravity: Jordan and Einstein frame

In this paper they consider the Higgs non-minimally coupled to the Ricci scalar. I am trying to recalculate the steps from equation (5) until equation (10). Let's start with (5): $$\int d^4x\sqrt{-g} \...
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A rapid question on $f(R)$ gravity

Is it possible to construct a $f(R)$ where, $$f(R) = \alpha R \tag{1} ?$$ I'm asking this for two reasons: $1)$ I'm quite a freshman on modified theories of gravity and $2)$ Most of the times I see ...
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How robust is this 2020 measurement of the external field effect (EFE) in MOND?

There's a paper from a few months ago that claims to have measured this unique Modified Newtonian Dynamics (MOND) effect at high significance. The result seems very interesting: The effect is unique ...
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How is the External Field Effect in MOND conceptually distinct from Newtonian gravity and GR?

I’m trying to understand the External Field Effect (EFE) in Modified Newtonian Dynamics (MOND) and how it is conceptually distinct from GR and Newtonian gravity. More specifically, the descriptions I ...
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Books about the development/history of gravitational theory

I am looking for a book about the history of gravitational theory. It should obviously include discussions of Newton, Einstein, and their theories, and hopefully it would include the work of other ...
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Why can't Dark Matter be curvature without a proximal mass source? [closed]

I don't understand why Dark Matter or MOND/"gravity works different on bigger scales" are the only options to explain the observational data. Why can't it just be curvature without a "...
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Obtaining General Relativity

Considering the action given in the following post:Induced gravitation action under what conditions does GR originate from this induced gravitation model? Also, what can we say about the cosmological ...
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Propagator in Brans-Dicke Gravity

Consider an action of the form $$ S = -\frac{2}{\kappa^2}\int d^4x\sqrt{-g}~\left(\phi R + \phi\mathcal{L}_{matter}\right). $$ Expanding this to second order in $h_{\mu\nu}$ and including a harmonic ...
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Decoupling higher order actions

I’m dealing with non-minimally coupling Quadratic gravity in the weak field limit, and as a result of the perturbation $g_{\mu\nu}= \eta_{\mu\nu} +h_{\mu\nu}$ I get some kinetic term mixing. On doing ...
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Induced gravitation action

Considering the action for induced gravity: $$S=\int d^4x\sqrt{-g}\left(\epsilon\phi^2 R-\frac{1}{2}(\partial\phi)^2+V(\phi)\right).$$ I was trying to get the metric field equations by doing the usual ...
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Metric field equations for the Jordan-Brans-Dicke action

Considering the Jordan-Brans-Dicke action: $$S=\int d^4x\sqrt{-g}\left(\phi R+\frac\omega\phi(\partial\phi)^2+\mathfrak{L_{m}}(\psi)\right).$$ I was trying to get the metric field equations by varying ...
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$R=0$ solution in field equations

I am dealing with some General Relativity extensions and I am not sure about my knowledge in basic GR since I am having some weird troubles with what I think are basic concepts. As far as I know, if ...