Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions · HackerPapers discussions

Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Thu, 11 Aug 2022 14:33:01 GMT

Famaey & McGaugh's review <https://arxiv.org/abs/1112.3960> (Stacey McGaugh is <https://tritonstation.com/>, a frequent critic of particle dark matter) §7 is all about adding whole new fields ("parameters") to standard General Relativity in order to capture higher-order terms in post-Newtonian expansions (in 1/c^n or in the metric tensor) of theories that preserve the central characteristics of Milgrom's MOND. Those characteristics are: no "non-luminous" or "hidden" matter sources -- the glowing, refracting, and obscuring dusts are the sole sources of gravitation -- and in circumstances where…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Tue, 19 Oct 2021 21:15:05 GMT

> There is something fundamental in the universe that we don't understand. Good! I hope there's lots of fundamental things we don't understand well yet. We can then enjoy improving our understanding. "The paper" below is https://arxiv.org/abs/2007.00082 == https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.12... which is the topic of the fine article linked at the top. > If this result holds, it could be an amazing leap I'm not sure if one can use the term result here. In the paper there is just a write-down of a theory that is deliberately designed to match an extra couple lines of…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Tue, 19 Oct 2021 19:11:57 GMT

This is a surprisingly good write-up. The Skordis & Zlosnik paper can be found at https://arxiv.org/abs/2007.00082 I'll start with some comments for non-experts, and then escalate about ten paragraphs further down. tl;dr: there are lots of things that hang on General Relativity being correct practically everywhere in our universe, the only "hiding places" that don't break astrophysical objects (or even laboratory experiments, including in robotic laboratories we have sent to other places in our solar system) is very close to the big bang, deep inside black holes, and in large masses brought…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Fri, 28 May 2021 01:05:36 GMT

No, it is unfair to call MOND (and at least much of "the like") as a dark matter theory, because it manifestly generates a solution (or solutions) in the absence of any matter at all. MOND is characterized by an interpolating function between its fundamental constant a_0 and acceleration, such that in the "deep-MOND" regime, F = m \frac{a^2}{a_0} rather than Newton's F = ma. Both MOND and Newton work in everywhere-vacuum, and in everywhere-vacuum perturbed by a point mass. Neither the vacuum case nor the point mass case qualifies as a "dark matter theory" any more than the vacuum…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Sun, 24 Nov 2019 19:33:17 GMT

"The relativistic versions of MOND" are just adding fields to General Relativity. Thus making them relativistic theories. Famaey and McGaugh §7. https://arxiv.org/abs/1112.3960 Consequently the rest of your last paragraph is incorrect. "we need dark matter to explain any theory of gravity" No. MOND is a theory of gravitation, but it's not relativistic, and it does not work at scales larger than that of galaxies (it is notably wrong with respect to the peculiar motions of galaxies in massive clusters, and a residual mass term must be added ibid. §6.6.4 : essentially, MOND still needs dark…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Sat, 11 May 2019 11:48:39 GMT

> What does Dark matter explain that MOND can't MOND describes, it doesn't explain. By adding a simple function to Newton's law of universal gravitation, it does a very good job of describing the momentum-deficit in the bulk motion of large optical and radio sources in a variety of galaxies compared to the predictions from Newton's unmodified theory. That it does so, and with such a simple modification, is interesting. Milgrom's function in MOND is almost certainly the simplest post-Newtonian gravitational term extending Newton's theory consistent with observations. (There are other…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Thu, 06 Dec 2018 04:41:58 GMT

> Dark matter can be placed arbitrarily Yes. > so it can match pretty much anything At the time you do the initial placement. But that's not the only relevant time. You can scatter any type of DM on a spacelike hypersurface any way you want, but then you have to figure out the successor (and predecessor) spacelike hypersurface, and there you have to bite a bullet: you can constrain the DM or throw away the well-posedness of the initial-value problem. Your constraints are dominated by the hyperbolic-elliptic PDEs. The hyperbolic portion lets you lay down initial values for your dark matter on…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Wed, 11 Apr 2018 01:09:54 GMT

> As a non-scientist Unfortunately you're missing the thrust of Hossenfelder's developing argument: in upgrading MOND to a full relativistic theory, you necessarily seriously blur the difference between "modified gravity" and "particle dark matter". There's some meat on this in the comments on that blog page, notably the point raised (and her subsequent reference to it) by "M_Malenfant". Essentially her point is that when you relativize some theories of MOND (detailed examples in Famaey & McGauch https://arxiv.org/abs/1112.3960 section 7) you are inevitably adding a field to the existing…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Thu, 05 Apr 2018 18:33:31 GMT

This is a couple of days old, and was discussed at length here: https://news.ycombinator.com/item?id=16701248 (~ 280 comments). Since then, however, McGauch (of modified gravity) put out a blog entry (dated yesterday): https://tritonstation.wordpress.com/2018/04/04/the-dwarf-gal... I'd reword his second last sentence by adding a couple of words in two places: "MOND is one example of a class of bets that what we call dark matter is really the manifestation of gravitational physics beyond General Relativity"; he himself has catalogued several non-General-Relativity theories that resemble MOND…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Tue, 22 Aug 2017 19:29:40 GMT

Second thing first: > ... much the same way physicists dislike the standard model in all of its' ugliness - even though it works Personally I think the U(1)xSU(2)xSU(3) group theory of the Standard Model of Particle Physics is quite pretty. AFAIK the problems with the Standard Model are rarely put as issues of "ugliness" but rather that it's known to be incomplete -- in particular the gravitational sector is not dealt with at all -- and is probably slightly incorrect (depending, for example, on how you deal with neutrino masses). It has some practical problems too, namely that there are at…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Wed, 01 Mar 2017 14:13:27 GMT

> I've seen people create variants of MOND that try to address the Bullet Cluster problem, but you start having to have some sort of really weird gravity memory effect in space or something, where space itself "remembers" that it used to have a lot of matter in it and still has a small amount of gravitation in it despite no longer containing the corresponding mass Which variants are these? cf. section 8.3, https://arxiv.org/abs/1112.3960 Several of the relativstic MOND theories (see chapter 7 of the same review) are detailed, and I don't see how any of the 8.3 set matches the quote above.…

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Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions — mentioned by raattgift

Wed, 01 Mar 2017 12:41:53 GMT

The problem is that your "lens" statement doesn't convey much information; if it goes deep, unfortunately it's not obvious. For example, it doesn't tell anyone whether you think that CDM is sparticles or a quantization of a function on the Ricci scalar or of a second metric or whatever. The latter two are awfully hard to distinguish from relativistic approaches to MOND; indeed you could quantize most of the approaches in Chapter 7 of https://arxiv.org/abs/1112.3960 . More generally, you can put a scalar or other degree of freedom on either side of the usual write-down of the Einstein Field…

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