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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Equivalence principle(s) and geodesic equation(s)
The equivalence principle (EP) is often stated in many different forms, sometimes leading to different interpretations. To make things easier for beginners to understand, I find it useful to give a ...
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Is shape dynamics capable of explaining dark matter?
I recently got introduced to the incredibly fascinating subject of Shape Dynamics: for example see https://arxiv.org/abs/1409.0105
Shape Dynamics uses conformal three-dimensional geometry to build up ...
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Can dark matter be isolated from baryonic matter?
The above is an image to test Verlinde's emergent gravity theory (2016, https://arxiv.org/abs/1611.02269). The research team observered galaxies and masses beyond, used gravitational lensing (y-axis) ...
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Cosmology, MOND and TeVeS
I have read the Wiki page about TeVeS. It says that MOND (or rather, AQUAL) is its nonrelativistic limit.
I wrote a thesis on cosmology eons ago (no, seriously, more than 50 years ago!) and though I ...
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Right hand side of Einstein field equation
Why can't the RHS of the Einstein field equations take a form like $T_{\mu \nu}$ plus some coefficient multiplied with $g_{\mu \nu} T$?
It should also be covariantly conserved, I suppose? For example, ...
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Where does the motivation for discovering a 5th force come from? Has there been any evidence found yet? [duplicate]
Electromagnetism, nuclear strong, nuclear weak, and the weird notably weaker force of gravity. Now a force is something fundamental to reality, gravity described as the bending of spacetime causing ...
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Is this already an established functional relationship or have I created hodgepodge?
Last winter I started toying with the galaxy gravitational rotation curve graphs. I started modifying the exponent of $r$ that in effect change the $1/r^2$ law and therefore correct the mismatch, ...
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Dark matter, MOND or flattened gravitational fields? [closed]
Could there not be a third variant to explain why e.g. long-distance multistar systems rotate faster than Newton's law of gravity suggests?
In addition to the Dark matter hypothesis and MOND then, ...
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Does MOND reduce the discrepancy in a quantifiable way? [closed]
Which of the following statements accurately describes the impact of Modified Newtonian Dynamics (MOND) on the observed "missing baryonic mass" discrepancy in galaxy clusters?
MOND is a ...
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Does MOND respect linear superposition of gravitational field intensities?
Does Milgrom's MOND respect linear superposition of gravitational field intensities as Newtonian gravity does?
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Brans-Dicke formalism: Validity for curve of scale factor vs cosmic time
Within the framework of the Brans-Dicke formalism, after having run a MCMC sampler and, once the best-fits have been found, I inject them into an ODE system resulting from the modified Friedmann ...
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Modified Gravity against Dark Matter
I am looking for examples of Modified Gravity theories that have been developed with the aim of dispensing dark matter, besides Milgrom's MOND, Bekenstein's TeVeS, and the recent relativistic version ...
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Gravitational field intensity in mass disk
To calculate the gravitational field intensity or acceleration in a mass disk (like a galaxy), should I do a(r)=G×Mt/r^2 or a(r)=G×M(r)/r^2 with Mt being the total mass of the disk/galaxy and M(r) the ...
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How to derive the general Horndeski's action?
I have seen that Horndeski's theory can be written in terms of an action as :
$$
S\left[g_{\mu \nu}, \phi\right]=\int \mathrm{d}^4 x \sqrt{-g}\left[\sum_{i=2}^5 \frac{1}{8 \pi G_{\mathrm{N}}} \mathcal{...
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Seeking Empirical Data Sources Relevant for an Alternative Gravity Theory [closed]
I am currently developing an alternative theory of gravity and am in need of empirical data sources that could assist in evaluating the potential validity and implications of this theory. This model, ...
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Lapse Function?
I have this 5-dimensional metric
\begin{equation}
ds^2= N^2(z)dz^{2} + a^{2}(z) q _{\mu\nu}(x) dx^{\mu} dx^{\nu}
\end{equation}
I'm trying to understand what is meant by a lapse function. Would $N(z)...
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How to canonicalize a coupled scalar kinetic term?
I am working with a classical action in curved space-time that looks something like:
\begin{equation}
S = \int d^4x \frac{1}{16G\pi}\sqrt{-g} \left[R - \frac{K_\Phi}{\Phi^2} \partial_\mu \Phi \partial^...
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Second-order perturbation in Brans-Dicke gravity
Let be $g_{\mu \nu} = \eta_{\mu\nu}+h_{\mu\nu}$ the perturbation of the metric and $\phi=\phi_0 + \varphi$ the perturbation of a field.
The lagrangian of a scalar-tensor theory of gravity is:
\...
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Does a particle which crosses the galaxy straight (not orbiting) suffer MOND's force in MOND theory?
Lets suppose a particle coming from intergalatic space crosses a galaxy. The particle is not rotating the galaxy, so it has no angular velocity or acceleration. The particle is attracted to the galaxy ...
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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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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 $$\delta(\sqrt{-g}h(\...
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State of the art on Modified gravity : going beyond the 2nd order differential equations, diffeomorphism invariance breaking, extra degrees of freedom
I am going to do a state of the art on Modified gravity models. I have found a talk that presents the problematic. In particular, it is said the following things :
Modifying General Relativity
How to ...
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Book/Article recommendation about modified gravity (scalar/tensor/scalar-tensor theories)
Is there a book or article that discusses the modified gravity theories scalar/tensor/scalar-tensor (maybe others as well) theories? (Look at this wiki page for a general overview of the theories that ...
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Is Bertrand's Theorem Consistent with GR?
Bertrand's Theorem - Please observe the gif of orbits with different exponents in the denominator. (I couldn't get the image upload feature to work for this)
Note, the orbit with a 1.9 exponent ...
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What explains the descrepancy between Asaph Hall's equation for Mercury's precession and GR?
In regard to What is the weight equation through general relativity?, the answer is:
$$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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