# 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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### 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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### Is there any $N$-Body software available that allows you to change the code to try different laws of gravity?

I am looking for some software (free or for a charge) that allows the user to modify the code to try different laws of gravity. This will be used to model galaxies, dark matter, galaxy clusters, etc.
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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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### 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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### No-ghost conditions and Laplacian instability of cosmological perturbations

I have been reading about cosmological perturbations for my degree work and I have to develop a stability analysis in the cosmological model that I study. However, in many articles about modified ...
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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 ...
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### Black Holes in Higher Derivative Gravity

It is well known that Einstein Theory of General Relativity has pathological solutions, called black holes, where the theory fails at some point and gives us infinite values. In order to solve this ...
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### Squaring the Einstein-Hilbert action - how much damage can a simple squaring do?

I am interested in understanding the consequences of squaring the Einstein Hilbert action: $$S=\int \sqrt{\left( \left(R\sqrt{|\det g_{\mu\nu}|} \right)^2 + L_m \right)}d^4x$$ The field equations ...
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### If space is expanding, what is the effect on $1/r^2$?

Space is expanding, and this effect can be only detected at large scales, on the scale of galaxies and larger. If space is expanding, the expression $1/r^2$ for the law of gravity will be modified. ...
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### Unimodular gravity and Lovelock theorem

I was reading about Unimodular gravity, this is a modified theory of gravity that postulates that the gravity is only invariant under volume preserving diffeomorphism. So it breaks the full ...
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### Where is the wiggle room in current gravity theories?

As far as I know, General Relativity has long since been proved experimentally to every qualified person's entire satisfaction, and modern technology such as GPS relies on its accurate predictions. ...
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### Is there any free utility/software to generates and numerically solve the field equation for modified gravity $f(R)$

I'm looking for a free software (preferred to run under Windows) that can generate the gravity field equations for a given $f(R)$ model in modified gravity. I used Maxima to calculate a few terms, ...
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### Metric vs coframe energy-momentum tensor in metric-affine gravity

Conventions Latin indices represent components in the anholonomic frame and greek ones are for coordinate components. I will call $R_{\mu \nu} := R_{\mu \rho \nu}{}^{\rho}$ (Ricci tensor) and \$\bar{R}...
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### Stuckelberg Formalism for Spin-2 - Metric Signature and Ghost Fields

Caution: This question may be trivial to experts, since I am looking at the consequence of metric conventions on the nature of fields in the calculation. My aim is to spot an error in either my ...