What is Modified gravity: Definition and 18 Discussions

Alternatives to general relativity are physical theories that attempt to describe the phenomenon of gravitation in competition to Einstein's theory of general relativity. There have been many different attempts at constructing an ideal theory of gravity.These attempts can be split into four broad categories based on their scope. In this article, straightforward alternatives to general relativity are discussed, which do not involve quantum mechanics or force unification. Other theories which do attempt to construct a theory using the principles of quantum mechanics are known as theories of quantized gravity. Thirdly, there are theories which attempt to explain gravity and other forces at the same time; these are known as classical unified field theories. Finally, the most ambitious theories attempt to both put gravity in quantum mechanical terms and unify forces; these are called theories of everything.
None of these alternatives to general relativity have gained wide acceptance. General relativity has withstood many tests, remaining consistent with all observations so far. In contrast, many of the early alternatives have been definitively disproven. However, some of the alternative theories of gravity are supported by a minority of physicists, and the topic remains the subject of intense study in theoretical physics.

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  1. F

    A Modified Gravity against Dark Matter

    I am looking for examples of Modified Gravity theories that have been developed with the aim of dispensing dark matter, besides Milgrom's [MOND][1], Bekenstein's [TeVeS][2], and the recent relativistic version of MOND ([RMOND][3]) by Skordis and Zlosnik. Are there any other examples of similar...
  2. D

    A Graviton propagator in Horndeski theory

    Let ##\phi## be a scalar field and ##g_{\mu \nu} = \eta_{\mu \nu}+h_{\mu \nu}/M_p## where ##M_p## is the Planck mass (so we assume we deal with perturbations). Let ##\Lambda_2,\Lambda_3## be energy scales such that ##\Lambda_2 \gg \Lambda_3##. These are defined by ##\Lambda^2_2 = M_p H_0## and...
  3. W

    Reading for dark matter versus modified gravity

    Does anyone have any good laypersons reads to get a good summary the dark matter versus modified gravity? Thanks
  4. R

    PhD Inquiry, Modified Gravity or String Theory

    Hello everyone, I am currently finishing my Masters in Theoretical Physics and looking forward to continue in a PhD. I always thought I would love String Theory, and now that i am actually doing it I see that my feeling was correct. I do not know why but trying to understand its concepts...
  5. ohwilleke

    I Are there issues with Conformal Gravity as a DM solution?

    One approach that has been proposed to replicate the description of dark matter phenomena in galaxies with a gravity modification that is set forth in a simplified manner in the phenomenological toy model theory MOND is to do so through a variation on Einstein's General Relativity known as...
  6. W

    I List of modified gravity theories

    There is a an encyclopaedia of all the different inflationary models. But I am wondering is there is a similar list of modified gravity models ?
  7. D

    I Investigating a Possible Derivation Error in f(R) Gravity Field Equation

    In this paper (https://arxiv.org/abs/astro-ph/0603302) the authors derive the field equations for f(R) gravity considering a spherically symmetric and static metric. Now the Ricci scalar only depends on r so you could write f(R(r)) = g(r) for some g. However what it seems the authors have done...
  8. wolram

    Are modified gravity models viable

    Different things keep popping up in the literature that pose Dark matter and Dark energy are not needed to explain the universe, the latest MOG i have come across is this: http://arxiv.org/pdf/1510.07037.pdf What are the chances that a modified gravity theory will replace the standard model?
  9. ohwilleke

    An interesting new paper on f(X) gravity

    http://arxiv.org/abs/1508.04641 The body text of the paper ends up calling the "Hybrid metric-Palatini gravity theory" f(X) gravity. From the body text (citations and internal references omitted without notation): The paper closes by identifying many issues for further study and arguing...
  10. C

    Dark matter -- particles or modified gravity?

    What is the evidence for dark matter being new particles as opposed to some modified form of gravity?
  11. M

    Deur's modified gravity: a type of MOND inspired by QCD

    I found a few curious arxiv papers by Alexandre Deur (via Twitter). Deur works in QCD. He explains QCD strings (flux line between quark and antiquark) as due to attraction between gluon field lines, ultimately due to gluon-gluon interactions. Phenomenologically this adds a linear term to the...
  12. W

    Dark Matter or Modified Gravity ?

    What are your personal opinions ??, Do you guys think gravity is just not fully understood that is why we can't explain certain events or DM is the probable answer to these discrepancies ? I personally don't think we need to create a somewhat imaginary matter (dark matter and dark energy) to...
  13. alemsalem

    Dark matter vs. modified gravity?

    what's the status of that? also why not just look for two systems that have the same visible distribution but with a different dark matter content?
  14. E

    How Does Modified Gravity Theory Address Black Holes and the Universe's Origins?

    Friends: I have been studying John Moffats Modified Gravity theory. I find it very interesting and I agree with him in that I think Dark Matter and Dark Energy are merely mediums to make our current equations work so we can predict the movements of the cosmos. However I am unclear on what...
  15. wolram

    Modified Gravity with Baryon Oscillations: SDSS to WFMOS

    http://arxiv.org/abs/astro-ph/0605278 Searching for modified gravity with baryon oscillations: from SDSS to WFMOS Authors: Kazuhiro Yamamoto, Bruce A. Bassett, Robert C. Nichol, Yasushi Suto, Kazuhiro Yahata Comments: 16 pages, submitted to PRD We discuss how the baryon acoustic...
  16. wolram

    Cosmological Expansion & Uniqueness of Gravity: Multamaki and Vilja

    http://arxiv.org/abs/astro-ph/0506692 Title: Cosmological expansion and the uniqueness of gravitational action Authors: T. Multamaki, I. Vilja Comments: 4 pages Modified theories of gravity have recently been studied by several authors as possibly viable alternatives to the cosmological...