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.
Questions tagged [modified-gravity]
182 questions
56
votes
4 answers
Are modified theories of gravity credible?
I'm a statistician with a little training in physics and would just like to know the general consensus on a few things.
I'm reading a book by John Moffat which basically tries to state how GR makes failed predictions in certain situations. I know GR…
dcl
- 673
17
votes
1 answer
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 calculations in this paper. In the appendix, I can't derive…
DisStudent
- 171
14
votes
4 answers
Does MOND make good predictions?
Well, it does according to this preprint for certain scales.
What would be a simple way to explain MOND to a layman?
Does it ignore mainstream physics? How much?
Mark
- 345
12
votes
0 answers
What are Galileons good for?
Lately I've seen many papers (for example "The galileon as a local modification of gravity"; 292 total hits on the arXiv) on types of field theories known as Galileons, and I'm wondering what the motivation for studying them is. I often hear of them…
Surgical Commander
- 4,057
11
votes
1 answer
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 states that they put constraints on the number of…
enumaris
- 4,154
11
votes
1 answer
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) Bimetric gravity was shown to be inconsistent with the…
user106422
10
votes
4 answers
What current alternatives are there to the standard concordance model of cosmology?
The current "standard model" or concordance model of cosmology is Lambda CDM which includes late time acceleration due to a cosmological constant, cold dark matter as the missing matter component and an inflationary period at very early times.
What…
ihuston
- 231
10
votes
2 answers
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 just work different on larger scales? After years of…
Noon36
- 178
- 10
9
votes
1 answer
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 understand, these attempts can be possibly justified in…
user87745
8
votes
2 answers
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. Teleparallel gravity is equivalent to general relativity…
tikaresty
- 131
8
votes
2 answers
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 fluke of gravity?
Ashish
- 183
8
votes
1 answer
Does dark matter have to be matter?
There is a very similar question to this one: Is dark matter really matter?. But the particular aspect I'm asking about seems not to be mentioned there. So, here we go:
Does dark matter have to exist as matter? Or, is it possible that space-time is…
user1264
- 513
8
votes
2 answers
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 understand the appeal of these theories. Are they studied…
Frotaur
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8
votes
1 answer
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
$$ \Pi^{\beta}{}_{\mu\nu}= h^{\beta}_{i} \partial_{\nu}h^{i}_{\mu} - \Gamma^{\beta}{}_{\mu\nu} \, $$
where $ \Gamma^{\beta}{}_{\mu\nu} $ is the…
7
votes
0 answers
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 gauge fixing term $\mathcal{L}_{G F}=…
Akoben
- 2,549