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.

Filter by
Sorted by
Tagged with
0
votes
1answer
148 views

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 ...
1
vote
1answer
82 views

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 ...
1
vote
1answer
56 views

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+...
1
vote
1answer
35 views

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 ...
1
vote
0answers
52 views

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, ...
0
votes
1answer
79 views

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 ...
0
votes
0answers
19 views

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.
2
votes
1answer
48 views

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}\...
0
votes
0answers
57 views

Modify Newton's gravity to cure the action at a distance

Take a bunch of particles (can be point particles) at ${\bf{x}}_i(t)$ and of mass $m_i>0$. The gravitational field is defined by the Poisson equation $$ \nabla^2 \Phi({\bf{x}},t) = 4 \pi G \rho({\...
0
votes
1answer
105 views

Particular behaviour of 'black hole' horizon in modified gravity

When working in a particular theory of modified gravity, one can see that a solution for a spherically symmetric and static puntual mass is given by \begin{equation} ds^2=-B(r)dt^2+A(r)dr^2+r^2d\theta^...
1
vote
0answers
95 views

What properties of MOND differ from Newtonian gravity?

I know of the following properties that set modified Newtonian dynamics (MOND) apart from Newtonian gravity: There is a $1/r$ dependence instead of a $1/r^2$ dependence, at large distances/low ...
2
votes
1answer
83 views

Higgs non-minimal coupling to gravity: Jordan and Einstein frame

In this paper they consider the Higgs non-minimally coupled to the Ricci scalar. I am trying to recalculate the steps from equation (5) until equation (10). Let's start with (5): $$\int d^4x\sqrt{-g} \...
0
votes
1answer
62 views

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 ...
4
votes
0answers
76 views

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 ...
0
votes
2answers
295 views

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 ...
0
votes
0answers
48 views

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 ...
0
votes
0answers
34 views

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 ...
1
vote
2answers
112 views

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 "...
1
vote
0answers
96 views

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 ...
7
votes
0answers
88 views

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 ...
1
vote
0answers
42 views

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 ...
1
vote
1answer
76 views

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 ...
2
votes
1answer
196 views

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 ...
1
vote
1answer
117 views

$R=0$ 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 ...
1
vote
1answer
133 views

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 ...
1
vote
1answer
86 views

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 ...
2
votes
2answers
150 views

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. ...
3
votes
1answer
140 views

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 ...
7
votes
3answers
133 views

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. ...
1
vote
1answer
51 views

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, ...
3
votes
1answer
117 views

How do you know what is the fate of the universe in a modified theory of gravity?

Usually when you talk about the final stage of the universe you assume that $\Lambda CDM$ is the correct cosmological model and with that you can analyze what could happen for different values of $\...
0
votes
1answer
28 views

Screening tensor-scalar mechanics with equivalence principle

Modified gravity with scalar degrees of freedom generally requires some screening mechanism to pass solar system tests, and that generally breaks down the equivalence principle. Question: is there ...
1
vote
0answers
39 views

What are the key pieces of cosmological evidence if you want to compare the success of dark matter versus quantised inertia?

Quantised Inertia is a controversial fringe theory that is claimed to be an alternative explanation of, for example, galactic motion. The consensus explanation for why the outer stars in galaxies ...
2
votes
0answers
112 views

Dark matter without dark matter - review of suggested theories?

Is there any listing, directory, website or other central resource for suggestions that attempt to explain dark-matter-like behaviour without particulate matter being involved? There's MOND, and ...
1
vote
0answers
31 views

Dark matter as an emergent phenomenon

Dark matter is invoked as a solution to anomalies in galactic rotation curves, galaxy motions in clusters, and gravitational lensing (please mention other evidence for dark matter that is relevant to ...
0
votes
1answer
112 views

Perturbed Ricci scalar in Modified Gravity

When getting the perturbed Ricci scalar in a Modified Gravity theory of the form $\mathcal{L}_{gr}=F\left(\phi,R\right)R$, $\phi$ being a scalar field, it is easy to arrive at an expression of it in ...
1
vote
2answers
161 views

Gravity as a repulsive force

As we know that universe is expanding, why can't we think the gravitational force as repulsive in nature in the realms of dark matter and dark energy?
0
votes
1answer
154 views

Riemann Dual Tensor and Scalar Field Theory

I'm trying to find the component equation of motion for the action in a paper. The action for the system is, $$S=\frac{m_P^2}{8\pi}\int d^4x\sqrt{-g}\bigg(\frac{R}{2}-\frac{1}{2}\partial_\mu\phi\...
0
votes
0answers
51 views

Effective exapsnion of Brans-Dicke like gravity

I'm working on the effects of higher derivative terms in gravitational theories and I have a question based on the effective expansion of a Brans-Dicke-like theory. Essentially my question is, in my ...
5
votes
1answer
673 views

Is Hilbert-Einstein action just the leading order of some kind of series?

Introducing the action for the gravitational field my GR professor stated that, in principle, one could write it as $$S = k\int d^4x\sqrt{g}(\sum_n\sum_m a_{nm} R_n^m - 2\Lambda), \space \space \...
1
vote
0answers
54 views

A doubt in Modified gravity

I am new to dark matter and modified gravity so excuse and inform me if I am wrong. If changes are made in the Friedman equations then there wouldn't always be an underlying action action principle. ...
2
votes
1answer
120 views

Is the Palatini-Lovelock action of order $k$ topological in $2k$ dimensions?

I am interested in Lovelock actions in the metric-affine (or Palatini) formalism. It is well-known that the metric version (starting from the Levi-Civita curvature) of the Lovelock lagrangian of order ...
2
votes
1answer
57 views

$\rho-3P$ for ultra-relativistic regime in early universe

I am trying to read this paper, in which they try to get dark energy by modifying the EFEs to its unimodular form. As interesting as it may be for someone here, I'm struggling to understand the ...
1
vote
0answers
60 views

Have fractional order differential models been explored as an alternative to standard gravitational field theory?

Since Einstein introduced his field equations and general theory of relativity, experimental evidence, at least on the cosmic scale has repeatedly supported the theory. Nevertheless, many seeking to ...
10
votes
1answer
763 views

Why does only gravity leak into extra non-compact dimensions?

This article K. Pardo, et. al., Limits on the number of spacetime dimensions from GW170817, Journal of Cosmology and Astroparticle Physics, Vol. 2018, 2018. which was published recently in JCAP ...
2
votes
1answer
105 views

What does it mean for a graviton to have mass?

On wikipedia we can read: Astronomical observations of the kinematics of galaxies, especially the galaxy rotation problem and modified Newtonian dynamics, might point toward gravitons having non-...
-1
votes
1answer
125 views

What kind of modified gravity could still explain all dark matter claims if we never more directly detect dark matter? [closed]

I'm curious about this. I saw this: https://www.forbes.com/sites/brucedorminey/2018/07/18/is-time-running-out-on-dark-matter/ and they're suggesting that as direct searches for dark matter more and ...
2
votes
0answers
177 views

Einstein-Cartan Theory with modern Cartan structural equations? [duplicate]

I cannot find real derivations and analysis of the Einstein-Cartan Theory. This can probably be a neat up-climb to a Cartan structural summit, or very close. I am not asking this because I want it ...
1
vote
1answer
89 views

Which metric is the main one in a metric-affine theory? $g^{\mu\nu}$ or its conformal counterpart?

For a metric-affine theory, I have found a conformal metric as $$h_{\mu\nu}=f(R)~g_{\mu\nu}.$$ My question is which metric is more important? For example in finding the weak field limit of such a ...
4
votes
2answers
349 views

Why do we study teleparallel gravity if it is equivalent to general relativity?

We know there are many modified theories of gravity, like $f(R)$ gravity, $f(T)$ gravity, and alternative theory of gravity, that is teleparallel gravity. Currently I am studying this theory. ...