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.
179 questions
5
votes
1
answer
94
views
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 ...
- 51
1
vote
0
answers
32
views
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 ...
- 45
1
vote
1
answer
37
views
References on Starobinsky inflation and quadratic gravity in cosmology
I'm learning about inflation and would like to read more about Starobinsky inflation and the predictions of quadratic gravity (i.e., Starobinsky plus Weyl squared) to inflationary cosmology. An ideal ...
0
votes
1
answer
161
views
Why is the deep MOND regime not the Newtonian acceleration plus a constant of acceleration?
In the standard deep MOND regime,
$$g= \sqrt{(GMa_{0}/r^2)}.$$
Why is the deep MOND regime not simply the sum of Newtonian acceleration and the acceleration scale constant?
$$g= GM/r^2 + a_{0}.$$
...
0
votes
0
answers
77
views
Non-metric gravity calculations
According to "Gravity and Strings" by T. Ortin (2015), the non-metricity tensor is calculated as
$$
Q_{\rho\mu\nu}\equiv\nabla_\rho g_{\mu\nu}=\partial_\rho g_{\mu\nu}-\Gamma^\beta_{\rho\mu}...
- 7
3
votes
0
answers
63
views
References on classical quadratic gravity
Quadratic gravity is an interesting model of modified gravity because it leads to a perturbatively renormalizable theory of quantum gravity (Stelle, K. S. Phys. Rev. D 16 (1977), 953). I know of a few ...
1
vote
0
answers
24
views
What are the differences and similarities between inflaton, dilaton, scaleron, and other similar hypothetical particles?
Lately I've been reading about modified gravity and I'm a bit lost on the names of hypothetical particles. Let me give an example. I was reading about Starobinsky inflation, which prescribes a ...
- 23k
2
votes
0
answers
51
views
Second-order equations of motion for higher derivative gravity?
We know that Lovelock gravity is the most general theory of gravity possible for Lagrangians which depend only on the metric tensor and the Riemann tensor
\begin{equation*}
L = L \left(g_{\alpha\beta},...
- 1,857
0
votes
0
answers
39
views
Why is MOND's standard interpolating function form resemble a Lorentz transformation?
MOND's standard interpolating function
$\mu \left(\frac{a}{a_0}\right) = 1/ \sqrt{1+ \left(\frac{a_0}{a}\right)^2}$
with $a_0$ being a constant
resembles the form of a Lorentz gamma factor
$\gamma = 1/...
2
votes
0
answers
78
views
Variational description of modified Einstein equations
Let us suppose that we have an Einstein equation of the form
$$ R_{(\mu \nu)}-\frac{1}{2} g_{\mu \nu} R=8\pi T_{\mu \nu},$$
where $R$ is an affine connection, which differs from the Levi-Civita ...
- 731
4
votes
1
answer
356
views
Torsion and Compatibility with the Metric
Compatibility with a metric, also referred to as metricity, means, I believe, that the covariant derivative of the metric is zero:
$$g_{ij;k}=g_{ij,k}-\Gamma^m_{ik}g_{mj}-\Gamma^m_{jk}g_{im}=0$$
This ...
- 133
3
votes
0
answers
131
views
Is Torsion Observable?
Are there, have there been, or could there be any experiments that might detect any torsion in our corner of the universe? Any results?
Or is torsion an unobservable?
- 133
0
votes
0
answers
25
views
Variation of nonminimal derivative coupling term
all. Can I request you assistance about the following problem?
How do I vary this action with respect to metric $\delta g_{ab}$
$$
\int d^4x \sqrt{-g} \Big[\kappa R+ G_{ab}\nabla^a \phi \nabla^b \phi \...
1
vote
1
answer
63
views
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 ...
- 13
0
votes
0
answers
92
views
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 \...
- 287
2
votes
1
answer
153
views
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 ...
- 23k
0
votes
0
answers
25
views
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 ...
- 11
1
vote
0
answers
84
views
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)...
- 23
0
votes
3
answers
180
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 ...
- 5,188
0
votes
0
answers
30
views
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 ...
- 21
1
vote
1
answer
72
views
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 ...
- 1,637
2
votes
1
answer
152
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 ...
- 85
0
votes
1
answer
33
views
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 ...
- 6,727
0
votes
0
answers
24
views
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 ...
1
vote
0
answers
33
views
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 ...
0
votes
1
answer
111
views
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_\...
- 33
3
votes
0
answers
178
views
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 ...
- 3,313
3
votes
0
answers
122
views
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 ...
- 93
2
votes
0
answers
66
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 ...
- 4,448
2
votes
1
answer
355
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, ...
- 41
2
votes
1
answer
52
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, ...
- 21
1
vote
1
answer
129
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, ...
- 531
0
votes
1
answer
102
views
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 ...
2
votes
1
answer
58
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?
1
vote
0
answers
66
views
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 ...
- 45
0
votes
0
answers
38
views
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 ...
0
votes
1
answer
51
views
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
86
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)...
- 11
1
vote
0
answers
93
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^...
1
vote
0
answers
105
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:
\...
- 179
0
votes
0
answers
34
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 ...
-1
votes
1
answer
91
views
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 ...
2
votes
1
answer
170
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 ...
- 6,727
2
votes
2
answers
222
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 ...
- 805
1
vote
0
answers
49
views
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?
- 75
0
votes
0
answers
66
views
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 ...
- 31
4
votes
0
answers
120
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 ...
- 305
4
votes
0
answers
111
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}&...
0
votes
1
answer
62
views
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
$$2h\square \phi + \frac{\partial h}{\partial \...
- 3,160
0
votes
2
answers
201
views
Variation for the Canonical Scalar Field in $f(\phi)R$
I am trying to find the Field equation for
$$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(\...
- 3,160