All Questions
Tagged with mond or modified-gravity
186 questions
-7
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88
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How the universe could work without dark matter? [closed]
I have a 7 page document that outlines the current 4 major anomalies. Universal expansion (dark matter), voids, early star formation, and local galactic environment) and a possible radical explanation ...
2
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0
answers
101
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Semolina porridge rotation vs. galactic rotation
Please excuse if this question sounds naïve — my aim is simply to understand the different kinds of gravity models.
In Wikipedia there is this animation about galaxy rotation curves: without dark ...
- 87
1
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90
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How to derive the field equations for cubic gravity in arXiv:1704.01590?
For the cubic gravity with Lagrangian
$$
\sqrt{-g} \left(R+\frac{1}{\Lambda^4}R_{\mu \nu}{ }^{\alpha \beta} R_{\alpha \beta}{ }^{\gamma \sigma} R_{\gamma \sigma}{ }^{\mu \nu}\right),
$$
where $\Lambda$...
- 11
0
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0
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91
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Does MOND work the same for small masses?
To explain observed “flat” rotation curves of galaxies, “Milgrom’s Law” Modified Newtonian Dynamics (MOND) “kicks in” at gravitational accelerations below $a_0 \sim 10^{-10}~\mathrm{m/s^2}$. But ...
- 19
1
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1
answer
110
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Mass function of compact stars in $f(R)$ gravity
(re-typing this question because screenshots aren't welcome)
Hello, I am currently studying Compact Stars in the $R + \alpha R^2$ model of gravity. In this model, our field equations take the ...
- 13
0
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0
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138
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Scale-invariant Brans-Dicke Einstein frame?
The Brans-Dicke theory is a scalar-tensor theory of gravity that introduces a scalar field $\phi$ to account for a varying gravitational constant. It is a generalization of general relativity.
In the ...
1
vote
1
answer
86
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Paper for Modified Brans-Dicke field equations
Can anyone point me to a paper where they give the field equations for the following modified Brans-Dicke action with non-minimal coupling to matter
$$S = \frac{1}{16 \pi}\int d^4x\sqrt{-g}
\left(\phi ...
0
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0
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99
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Should the Brans-Dicke scalar field couple to the matter Lagrangian?
The Brans-Dicke action is generally given by:
$$S = \frac{1}{16 \pi}\int d^4x\sqrt{-g}
\left(\phi R - \frac{\omega}{\phi}\partial_a\phi\partial^a\phi\right) + \int d^4 x \sqrt{-g} \,\mathcal{L}_\...
0
votes
1
answer
120
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How would change in the model of gravity affect orbital precession/regression?
If we model the fall-off of gravity as $1/r^e$, the newtonian model has $e=2$, giving the well know $1/r^2$. This yields a steady state elliptical orbit.
How would the orbit change for $e<2$ (which ...
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1
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0
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90
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Error in Barrow's paper? [closed]
I'm currently going through John D. Barrow and S. Cotsakis' 1988 paper "INFLATION AND THE CONFORMAL STRUCTURE OF HIGHER-ORDER GRAVITY THEORIES" (https://doi.org/10.1016/0370-2693(88)90110-4)....
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2
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57
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What mass density regulates screening mechanisms of modified gravity theories?
I attended a seminar today about modified gravity theories - broadly things like $f(R)$ gravity. The speaker talked through screening mechanisms and how they could depend on the local density and, in ...
- 19k
0
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0
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87
views
Does any combination of quadratic Riemann invariants approximate to the Klein-Gordon action squared?
I am considering the action (inspired by MOND) given by $$\mathcal{L} = f( (\nabla^\mu \phi \nabla_\mu \phi)^2 )$$
where $f$ is an arbitrary function.
But I want to get it from a Newtonian ...
- 397
1
vote
1
answer
128
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Why Modified Newtonian dynamics is not tested with modern gravimeters?
According to Wikipedia, Standard Newtonian gravity might not hold for smaller than $10^{-10}\ \mathrm{m/s^2}$ accelerations and Modified Newtonian Dynamics might explain the differences. If I ...
- 71
6
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1
answer
215
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How does MOND explain ‘low dark matter’ galaxies?
MOND attempts to solve the rotation curve problem of galaxies by modifying the law of gravitation. However, there are galaxies (notably NGC 1277) where the rotation curves do more-or-less agree with ...
- 61
1
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0
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100
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Interpretation of an anticorrelation between $H_0$ and $\log_{10}(\omega_{BD})$ with Brans-Dicke's theory in a triplot diagram
I get below the following contours of a MCMC run with the main cosmological parameters for Brans-Dickce's theory without introducing a cosmological constant ($\Lambda=0$) and considering only baryonic ...
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