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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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 ...
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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 \...
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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. ...
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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 ...
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$\rho-3P$ 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 ...
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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 ...
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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 ...
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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-...
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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 ...
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Einstein-Cartan Theory with modern Cartan structural equations?

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 ...
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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 ...
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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. ...
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Is there any evidence for dark matter besides gravitational effects?

Is there any evidence for dark matter besides just its gravitational effects? What I’m getting at is...why are we so quick to assume it’s any kind of “matter” at all rather than just some unexplained ...
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Square-root matrix in Bimetric Gravity

The Hasan-Rosen formulation of bimetric gravity can schematically be written as[1]: \begin{equation} \mathcal{L}_{bi} = \mathcal{L}_g + \mathcal{L}_f + \mathcal{L}_{int} \end{equation} where $\...
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Metric vs coframe energy-momentum tensor in metric-affine gravity

Conventions Latin indices represent components in the anholonomic frame and greek ones are for coordinate components. I will call $R_{\mu \nu} := R_{\mu \rho \nu}{}^{\rho}$ (Ricci tensor) and $\bar{R}...
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Stuckelberg Formalism for Spin-2 - Metric Signature and Ghost Fields

Caution: This question may be trivial to experts, since I am looking at the consequence of metric conventions on the nature of fields in the calculation. My aim is to spot an error in either my ...
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Alternative to the explanation of large scales phenomena [duplicate]

I've been reading a lot trough different articles about dark-matter and dark-energy and how they could explain observations such as the acceleration of the expansion of the universe or the ...
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Will modified gravity helps find new technologies or improve existing technologies in our life on Earth? [closed]

If we were able to find a modified theory of gravity or a theory that better explains gravity than general relativity and fixes the problems of general relativity in places were general relativity ...
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Can Dark Matter be…(massive) gravitons?

The dark matter particle has NOT been found. Despite the quantity of models, I wonder if dark matter could be some (exotic) type of (interacting) gravitons. Is that possible?
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Classical mechanics gravitational waves

Would there be gravitational waves even if general relativity was wrong? For example imagine there was a theory of gravity that was consistent with special relativity. How different could ...
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The role of the Hopf Fibration in Kaluza-Klein theory

I have been learning recently about the Hopf Fibration and its relation to physics. My professor has told me that it is one of the simplest methods of dimensional reduction in Kaluza-Klein theory. ...
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What's the point of the Hoyle–Narlikar theory of gravity?

Many extensions of general relativity have been proposed, to solve various problems or expand it's range as a theorem. The one of these that caught my eye most recently is Hoyle–Narlikar theory, a ...
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Gravitons with negative mass?

I have been reading several papers on massive gravity. All of them have equations that involve the square of the graviton mass, rather than graviton mass itself. See for example, equations 43 and 44 ...
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Violation of the strong equivalence principle in scalar-tensor theories

As I understand it the strong equivalence principle is the statement that within a infinitesimal neighbourhood of any spacetime point all physical interactions (gravitational and non-gravitational) ...
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Implications of the LIGO detections on the 'Modified Gravity' Program

We all know that GR needs modification at the microscopic scale but there are some attempts to modify GR in the classical regime as well, known by the name of "Modified Gravity". As far as I ...
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Corrections to precession of the perihelion of Mercury

Consider one of the classical observables of General Relativity, the precession of the perihelion of an elliptical orbit: $$ \delta\varphi=\frac{6\pi G^2M^2m^2}{c^2L^2} $$ I am interested in known ...
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Einstein's General Theory of Relativity [closed]

How a Total Solar Eclipse Helped Prove Einstein Right About Relativity That historic experiment was carried out on May 29, 1919. British astronomer Sir Arthur Eddington was paying attention to ...
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Dark matter and simulations of Modified Newtonian Dynamics

It is well known that dark matter was introduced to explain the orbits of stars in a galaxy. My main question is this: Were these calculations done using Newton or Einstein? People I ask tell me ...
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Derivatives of distributions in general relativity

I am having some trouble when trying to reproduce some calculations involving the description of distributions (mostly used in spacetime junction conditions). I am trying to reproduce the ...
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$f(R)$ spherically symmetric field equations derivation mistake?

Abstract: An f(R) gravitation for galactic environments. We propose an action-based f(R) modification of Einstein's gravity which admits of a modified Schwarzschild-deSitter metric. In the ...
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Why is Brans-Dicke Theory considered as a failed attempt to incorporate Mach’s principle in a relativistic theory of gravity?

In Generalized Brans-Dicke theory: A dynamical systems analysis by Nandan Roy and Narayan Banerjee, Brans-Dicke theory is described as a failed attempt to incorporate Mach’s principle in a ...
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Emergent Gravity theory by Verlinde

Erik Verlinde has proposed a emergent structure of Gravity in a recent paper Emergent Gravity and the Dark Universe Abstract from paper cited above Recent theoretical progress indicates that ...
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Dark Matter vs Modified Gravity

Why do cosmologists and astrophysicists assume that the reason for the higher velocities of outer stars in galaxies is due to matter at all? The name dark matter seems misleading. Couldn't gravity ...
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f(R) action and GYH term

I have the following question. I have done integration by parts and I'm now applying the Gauss-Stokes theorem to get the boundary terms. I'm not interested in ignoring the terms, rather keeping them ...
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Auto-parallel Transport or Principle of Extremum Action?

In an affinely connected spacetime with a metric compatible connection, the equation of the curve in which the tangent vector at each point is the result of the parallel transport of every tangent ...
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The inverse of disformal metric?

From this paper on equation (62), they show the disformal transformation of metric $$ \tilde{g}_{\mu\nu}=A(\phi,X)g_{\mu\nu}+B(\phi,X)\nabla_\mu\phi\nabla_\nu\phi,~~~\tilde{\phi}=\phi. $$ Then the ...
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Shouldn't Torsion be eliminated on the basis of Equivalence Principle?

An affinely connected spacetime with a metric compatible connection can, in principle, have a non-vanishing anti-symmetric part; where the definition of the connection is given by defining parallel ...
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What is the aim of modified GR theories?

Having finished an introductory course in GR, I started reading a bit about the modified general relativity theories, especially f(R) GR and scalar-tensor theories. However, I am unable to ...
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Does dark matter indicate a problem with general relativity?

In light of the failure of the LUX experiment to find dark matter I was wondering if the whole idea of dark matter possibly indicates some problem with GR rather than simply being as yet undiscovered. ...
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Have there been any physics experiments or searches to try to disprove the hypothesised existence of dark matter?

Have there been any experiments or searches in which scientists have sought to disprove the hypothesis of the existence of dark matter? If so, what were the results? For example, a non-gravitational ...
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Is there any problem with the idea that gravity is repulsive at long distances?

If gravity is repulsive at long distances, would this suffice in explaining "dark energy"? Was there any reason why a new concept (dark energy) was introduced rather than just assuming that gravity ...
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Does Dark Matter Exert The Same Gravitational Force As Ordinary Matter?

I have read through this Wikipedia article on Dark Matter and it contains references to the methods used to demonstrate the existence of dark matter. A galaxy rotation curve is a plot of the ...
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Deriving the field equations for a dark energy / modified gravity effective field theory

Question I'm trying to derive the modified gravity EFT field equations and, from their 00 component, this Friedmann equation: \begin{equation} H^{2}+H\frac{\dot{\Omega}}{\Omega}=\frac{\kappa \rho_{m}+...
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Is the Weitzenböck connection the only connection with torsion, but without curvature?

In teleparallel gravity, the (local) connection coefficients of the Weitzenböck connection are given by $$ \Pi^{\beta}{}_{\mu\nu}= h^{\beta}_{i} \partial_{\nu}h^{i}_{\mu} - \Gamma^{\beta}{}_{\mu\nu}...
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What is basic tensor algebra in teleparallel equivalent of general relativity?

Teleparallel gravity represents a viable alternative to general relativity where gravitation comes from torsion rather that curvature. The theory is based on a new modified connection, and the ...
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How does Modified Newtonian Dynamics (MOND) violate Newton's laws?

I have read some articles on MOND which indicate that a modification of gravity violate Newton's third law, but I don't know how exactly that works. I do know that if Newtonian acceleration is smaller ...
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$f^{\prime}(R)=0$ in $f(R)$ gravity

Suppose in a certain $f(R)$ gravity theory, $f^{\prime}(R)=0$ for some finite value of $R$. (e.g. let $f(R)=R+\alpha R^2$ with $\alpha<0$. $f^{\prime}(R)=0$ at $R=-\frac{1}{2\alpha}$.) Also ...
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Charged gravity: what magnitudes, if any, would be consistent with observations so far? [duplicate]

(Disclaimer: the following might fit better on Worldbuilding - on the one hand, I'm not looking to write a story, but on the other, I don't know enough physics to know whether this is a trivial "no, ...
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Could there be a “massive gravity” theory?

If we talk about a "quantum theory" of General Relativity, we know that the particle that mediates the gravitational force would be the so called Graviton, a massless particle with spin $2$. I ...
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Gravity with more than one metric tensor

As weird as it sounds, yes, there are gravity theories with more than one metric tensor. This is called bimetric gravity. My question to those who have encountered bimetric gravity before: a) ...