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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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Solving TOV equations that describes neutron stars in modified f(R, T) gravity

Sorry for the long post, tldr at bottom. I'm trying to use standard RK4 code in C/C++ to solve a coupled system of 2 modified TOV equations in f(R,T) gravity and reproduce some of the results of this ...
hidenori's user avatar
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Second-order equations of motion for higher derivative gravity?

We know that Lovelock gravity is the most general theory of gravity possible for Lagrangians which depend only on the metric tensor and the Riemann tensor \begin{equation*} L = L \left(g_{\alpha\beta},...
Ishan Deo's user avatar
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Why is MOND's standard interpolating function form resemble a Lorentz transformation?

MOND's standard interpolating function $\mu \left(\frac{a}{a_0}\right) = 1/ \sqrt{1+ \left(\frac{a_0}{a}\right)^2}$ with $a_0$ being a constant resembles the form of a Lorentz gamma factor $\gamma = 1/...
Manuel's user avatar
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Variational description of modified Einstein equations

Let us suppose that we have an Einstein equation of the form $$ R_{(\mu \nu)}-\frac{1}{2} g_{\mu \nu} R=8\pi T_{\mu \nu},$$ where $R$ is an affine connection, which differs from the Levi-Civita ...
ProphetX's user avatar
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Torsion and Compatibility with the Metric

Compatibility with a metric, also referred to as metricity, means, I believe, that the covariant derivative of the metric is zero: $$g_{ij;k}=g_{ij,k}-\Gamma^m_{ik}g_{mj}-\Gamma^m_{jk}g_{im}=0$$ This ...
Ric's user avatar
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Is Torsion Observable?

Are there, have there been, or could there be any experiments that might detect any torsion in our corner of the universe? Any results? Or is torsion an unobservable?
Ric's user avatar
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Variation of nonminimal derivative coupling term

all. Can I request you assistance about the following problem? How do I vary this action with respect to metric $\delta g_{ab}$ $$ \int d^4x \sqrt{-g} \Big[\kappa R+ G_{ab}\nabla^a \phi \nabla^b \phi \...
trickymindful's user avatar
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Alternatives to MOND not based on acceleration threshold

The mainstream MOND hypothesis purports different behaviour below some acceleration threshold. Is there some other theory in which gravity would behave differently based on some other condition? I ...
Damian's user avatar
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Ghost-free quadratic gravity

This is an question about how to write an equivalent of "energy-squared" in terms of a gravitational metric. i.e. a spin-2 term that approximates to the spin-0 term $\int (\nabla^\mu \phi \...
bob's user avatar
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Which evidences do we have that general relativity works at large scales?

Recently I've been reading Pedro Ferreira's SciComm book The Perfect Theory: A Century of Geniuses and the Battle over General Relativity. At one of the last chapters, which discusses modified ...
Níckolas Alves's user avatar
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Scale dependent density growth in sub-horizon scales

In standard cosmology, we use \begin{equation} \ddot{\delta} + 2 H \dot{\delta} - 4 \pi G_N \rho \delta=0 \tag{1} \end{equation} to study the structure growth in sub-horizon scales. However, at the ...
t-rex's user avatar
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Conformal transformation to Einstein frame for a Non-minimally coupled Ricci and Maxwell term

I am currently working on a modified gravity theory which has non-minimal coupling between Ricci scalar and Maxwell term. The precise action is $$\int d^4x\sqrt{-g} \left(R + \alpha R^2 + (1 + \beta R)...
Suriyah R K's user avatar
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3 answers
164 views

Why are dark matter and dark energy favoured over changes to our physical models? [closed]

I am instinctively skeptical of the existence of "dark matter" and "dark energy". Together, they strike me as being analogous to luminiferous aether -- something that was invented ...
spraff's user avatar
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New models for extreme scales? [duplicate]

Physicists struggling to explain too-fast spinning galaxies with standard models of gravity is weird to me. If we can accept that normal physics don't apply on a quantum level why shouldn't the same ...
Fayjakin's user avatar
1 vote
1 answer
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Strong Equivalence Principle and Milgrom's Law

I am reading this paper by Milgrom(1983) that suggests a modification to Newton's Second Law in order to do away with the requirement of dark matter on astrophysical scales. My question is regarding a ...
Ambica Govind's user avatar
2 votes
1 answer
131 views

What is the MOND elliptical orbit speed equation?

According to MOND, beyond $~a_0~$, the circular orbit speed is derived with $~v = (GMa_0)^{1/4}~$. That gives a constant orbit speed regardless of radius, as in flat rotation curves. But accelerations ...
user141183's user avatar
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Can axion fields make the gravitational constant to oscillate in harmonic way?

Axions (and more generally, I think scalar fields) can make fundamental constants to vary. Does it include the gravitational constant? And if so, can it be make to oscillate as a simple harmonic ...
riemannium's user avatar
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What modifications/corrections are made by $f(R)$ gravity model on Morris-Thorne wormhole metric?

What modifications/corrections are made by the $f(R)$ gravity model on the Morris-Thorne wormhole metric? Specifically, how does the physics associated with the metric change in $f(R)$ gravity compare ...
PARTHA PRATIM NATH's user avatar
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Tachyonic instabilities [closed]

How to find tachyonic instabilities for some parameter ranges in the $f(R)$ gravity model? I have tried to find the parameter ranges for exponential $f(R)$ model, Starobinsky $f(R)$ model and ...
PARTHA PRATIM NATH's user avatar
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1 answer
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Confusion about 'minimally coupled' and 'massive' Scalar Tensor Theory

So if I write down a general action of Scalar Tensor Theory in a Jordan frame as $$ S = \frac{1}{16\pi G_0}\int \left( f_1(\varphi) R - f_2(\varphi) g^{\alpha \beta} \partial_\alpha \varphi \partial_\...
JoeGlas's user avatar
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3 votes
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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 ...
Eletie's user avatar
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3 votes
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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 ...
MartyMcFly's user avatar
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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) ...
Koen de Jong's user avatar
2 votes
0 answers
54 views

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 ...
Alfred's user avatar
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2 votes
1 answer
346 views

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, ...
Yuan Liu's user avatar
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16 views

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 ...
Troy Dube''s user avatar
2 votes
1 answer
50 views

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, ...
Sandman's user avatar
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1 vote
1 answer
121 views

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, ...
Lehs's user avatar
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1 answer
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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 ...
anonymous's user avatar
2 votes
1 answer
55 views

Does MOND respect linear superposition of gravitational field intensities?

Does Milgrom's MOND respect linear superposition of gravitational field intensities as Newtonian gravity does?
Manuel's user avatar
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1 vote
0 answers
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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 ...
guizmo133's user avatar
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0 answers
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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 ...
Manuel's user avatar
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0 votes
1 answer
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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, ...
1 vote
0 answers
70 views

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)...
mattdwyer's user avatar
1 vote
0 answers
77 views

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^...
Relatively General's user avatar
1 vote
0 answers
101 views

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: \...
gravitone123's user avatar
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0 answers
33 views

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 ...
Manuel's user avatar
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-1 votes
1 answer
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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 ...
Manuel's user avatar
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2 votes
1 answer
142 views

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 ...
riemannium's user avatar
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2 votes
2 answers
214 views

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 ...
KlausK's user avatar
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1 vote
0 answers
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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?
Saber's user avatar
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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 ...
Aryan Prasad's user avatar
4 votes
0 answers
110 views

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 ...
Joel's user avatar
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4 votes
0 answers
107 views

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}&...
PhysicsStudent101's user avatar
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1 answer
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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 \...
seVenVo1d's user avatar
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0 votes
2 answers
188 views

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(\...
seVenVo1d's user avatar
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0 votes
1 answer
143 views

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 ...
user avatar
1 vote
1 answer
87 views

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 ...
1 vote
1 answer
154 views

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 ...
HardlyCurious's user avatar
2 votes
1 answer
131 views

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 ...
HardlyCurious's user avatar