Dark matter + gravity is 'right', or our understanding of gravity is wrong?

[cmb]

TL;DR Summary

Is it possible to put into a nutshell why the case for dark matter together with current understanding of gravity is more likely to be right, than simply we don't quite yet understand gravity?

My understanding, possibly my ignorance, is that dark matter is calculated to exist from observations that there's not enough matter to fit observations if the current theory of gravity is right.

Is it possible to put into a nutshell why the case for dark matter together with current understanding of gravity is more likely to be right, than simply we don't quite yet understand gravity?

As a 'for example', If we invent unobservable matter to modify the forces of gravity, why don't we just invent unobservable bubbles of stronger gravity? (note; I don't think this and have no pet theories of this, just an example of 'inventing something else'.)

What is so compelling about "dark matter and current view of gravity" that beats the possibility that we just don't quite understand gravity yet, or that there may be other (non-massive) things that can affect the behaviour of gravity locally or at larger scales that we don't understand yet?

What are the alternatives to the theories of 'dark matter'?

Is there something further to bolster the theory of dark matter, other than effects of gravity that suggests dark matter exists?

 

[.Scott]

The basic evidence are comparisons between the mass of galaxies computed by inventory vs. measuring the effects of galactic gravity.

In most cases, there is a big difference.
But galaxies have been found that show little or no dark matter.

So if its a misunderstanding of gravity - it's a misunderstanding that does not affect all galaxies in the same way.

 

[Bandersnatch]

As a 'for example', If we invent unobservable matter to modify the forces of gravity, why don't we just invent unobservable bubbles of stronger gravity? (note; I don't think this and have no pet theories of this, just an example of 'inventing something else'.)

What is so compelling about "dark matter and current view of gravity" that beats the possibility that we just don't quite understand gravity yet, or that there may be other (non-massive) things that can affect the behaviour of gravity locally or at larger scales that we don't understand yet?

As far as I understand, some general relativistic approaches within the modified gravity territory do just that - include localised sources of gravity. The thing is, localised sources of gravity are what we call matter.
To put in differently, if you modify gravity to have such undetectable bubbles of gravity, you're just adding dark matter.

 

[PeroK]

There is also evidence for dark matter from.the CMBR. See recent responses in another thread by @kimbyd.

 

[cmb]

So if its a misunderstanding of gravity - it's a misunderstanding that does not affect all galaxies in the same way.

Is there a scientifically established reason why gravity should affect galaxies all in the same way?

I mean, yes, our current understanding says it should be the same everywhere.

But is that where it has gone wrong?

If we could 'see' gravity, maybe we would see some galaxies have red gravities and others have green gravities, and we're just colour blind to it?

Or if there is some complex universe-wide vector of gravity as well as local gravity (we can't discriminate locally) that interact?

There is also evidence for dark matter from.the CMBR. See recent responses in another thread by @kimbyd.

Maybe there are both coloured gravities and also a bit of dark matter? But not so much dark matter?I guess my worry is that once we go down the route of relying on, rather than imagining, undetectable things and making predictions from our imaginations, then we're in trouble scientifically.

It's fine and spot on to do that imagining as the means to come up with experiments to detect the stuff, if we find a way to detect and visualise it, top stuff, and several Nobel prizes there for sure. It would clearly be one of the biggest steps ever in science.

But until then, it seems 'unscientific' to claim it exists so as to defend our claim of understanding our theory of gravity when that doesn't actually seem to be working properly?

If my boss says 'why isn't this circuit working yet?' and I say 'well, the only conclusion I can make is there is a bit of dark matter stuck in that capacitor, because I already have theories of how this circuit works from the past and they worked then', then, well, I'd say there are problems with that proposition.

So I guess I am asking what I am not aware of that has been evaluated or discovered that explains why there is so much effort to look for dark matter rather than focus on what we might not understand about gravity?

 

[PeroK]

So I guess I am asking what I am not aware of that has been evaluated or discovered that explains why there is so much effort to look for dark matter rather than focus on what we might not understand about gravity?

The key phrase there is "I'm not aware". If you read Andrew Liddle's Introduction to Modern Cosmology, he lists all current options for dark matter and the modified gravity alternative.

Perhaps, therefore, your consternation at the close-mindedness of the modern cosmologist is simply due to your lack of knowledge of current research?

 

[cmb]

The key phrase there is "I'm not aware". If you read Andrew Liddle's Introduction to Modern Cosmology, he lists all current options for dark matter and the modified gravity alternative.

Perhaps, therefore, your consternation at the close-mindedness of the modern cosmologist is simply due to your lack of knowledge of current research?

Well, yes and no. There is no balanced view on this in scientific discussions I can see. It's 'there IS dark matter, and modified gravity is somewhat questionable' sort of view. That's the general trend of views that I see. Am I mislead by my lack of awareness of discussions not so exposed in the public eye?

Why is the discussion in public domain physics biased in that way, when there are alternative theories?

 

[PeroK]

Why the popular scientific press reports what it does is not something I can answer.

That said, dark matter is the leading candidate perhaps for the simple reason those wanting to win the Nobel prizes that see you dangling before them choose the horse that they feel has the best chance!

And, your more esoteric theories might simply lead to a lifetime of wasted study. That's, of course, not something of concern to the armchair theorist, who is not obliged to spend years of research in pursuit of his theories.

 

[weirdoguy]

Why is the discussion in public domain physics biased in that way, when there are alternative theories?

Maybe because most people who knows technical details of both dark matter models and alternative models think that dark matter models are better and simpler? As to why it is so - just learn those technical details. Not every model needs to be treated equally. I don't see why should I waste my whole career on something I don't think is worth it. General relativity is unimaginably consistent with all observations, so it's simpler to invoke some new form of matter (which is not that weird, just look at the history of particle physics) than to modify model of gravity. Every alternative model trying to modify gravity has big problems with reproducing every single thing that GR predicts and that is confirmed by observations. And that is a big issue - you can't just throw away something as succesfull as GR having almost nothing to replace it. If you knew some technical details you would see that dark matter IS the simplest way to solve the problem.

 

[cmb]

Maybe because most people who knows technical details of both dark matter models and alternative models think that dark matter models are better and simpler? As to why it is so - just learn those technical details. Not every model needs to be treated equally. I don't see why should I waste my whole career on something I don't think is worth it. General relativity is unimaginably consistent with all observations, so it's simpler to invoke some new form of matter (which is not that weird, just look at the history of particle physics) than to modify model of gravity. Every alternative model trying to modify gravity has big problems with reproducing every single thing that GR predicts and that is confirmed by observations. And that is a big issue - you can't just throw away something as succesfull as GR having almost nothing to replace it. If you knew some technical details you would see that dark matter IS the simplest way to solve the problem.

Ah, I see, so the 'in a nutshell' argument I'm after is that if one seeks to modify gravity one has to also modify GR?

Well, that is a bit of a bigger hurdle then. I can buy that.

I also appreciate the argument that dark matter is more likely to win prizes than giving some old theory a good kicking and a work-over. [I have paraphrased responses there, but that's my 'take-home' on that point!]

OK, thanks.

 

[weirdoguy]

if one seeks to modify gravity one has to also modify GR?

Not necessarily, one can throw away GR and use this new model. But this new model has to be consistent with everything that GR predicted and has been confirmed observationaly/experimentaly. Well, so in a way it has to have most of the GR built in it, one way or another. Thus far modified gravity models have problem with that.

 

[Scott Scott]

I think perhaps that the focus on gravity, however misguided or controversial, must be joined by the idea that zero physical evidence exists of what creates dark matter and dark energy. Something is happening because we can see the effects. But like the philosophers who spent their lives looking for that elusive Chinese elixir of immortality or the Philosopher's stone and not finding anything, cosmologists seem to keep banging their heads fruitlessly against the wall of dark stuff that nobody can find rather than looking at things that they know exist or to follow other paths of inquiry around the obstacle. This doesn't seem rational. Some people use the problem of dark stuff to suggest that humans don't have the brain power to solve all problems in physics, but they may only not yet have found the correct path to solving this particular problem. One things for sure, the expensive experiments to find the elusive particles making up dark matter don't seem to be working.

 

[weirdoguy]

cosmologists seem to keep banging their heads fruitlessly against the wall of dark stuff that nobody can find rather than looking at things that they know exist or to follow other paths of inquiry around the obstacle.

Sigh... If you would spent enough time reading about the dark matter problem (or just read the very thread you are responding to) you would know that cosmologists are trying to follow other paths. It's just those paths are not as fruitfull as dark matter path.

One things for sure, the expensive experiments to find the elusive particles making up dark matter don't seem to be working.

What experiments?

pontificating from the comfort of your armchair

"Weird" thing that most of those who pontificate are not physicists and don't even know the technical details of things they say are not "rational"

 

[PeroK]

I think we can all agree that pontificating from the comfort of your armchair is a lot easier than finding solutions to the unanswered questions in physics.

 

[DennisN]

But like the philosophers who spent their lives looking for that elusive Chinese elixir of immortality or the Philosopher's stone and not finding anything, cosmologists seem to keep banging their heads fruitlessly against the wall of dark stuff that nobody can find rather than looking at things that they know exist or to follow other paths of inquiry around the obstacle. This doesn't seem rational. (my bolding)

I have a completely different view on this. What you call "obstacle" I call scientific indications/evidence of something we do not yet understand. Which is one of the most exciting things there are in science!

 

[MikeeMiracle]

The Science Asylum has just done a good video on the evidence for Dark matter

 

[ohwilleke]

Is it possible to put into a nutshell why the case for dark matter together with current understanding of gravity is more likely to be right, than simply we don't quite yet understand gravity?

The answer is that dark matter is not more likely to be right.

This is something of a hit and go answer that I may expand on later, but the key point is that the dark matter v. gravity debate is more open than it has ever been before.

The list of problems with the LambdaCDM (for cold dark matter with a cosmological constant) model are growing, and several gravity oriented approaches (of which there are many in addition to "toy-model" MOND) are viable. Every dark matter particle theory has serious problems. There is no longer a definitive preference for one or the other from observation. I'll restate observations I've made elsewhere (basically self-plagiarizing with minor editting):

Modified gravity theories are credible. There are deep, potentially intractable problems with a dark matter particle approach. Also, the weight of the evidence as shifted as astronomers, particle physicists and theorists have provided us with more relevant evidence and with more ideas about how to solve the problem, even in the last few years in this very active area of ongoing research.

This is as it should be because dark matter phenomena constitute the most striking case in existence today where the combination of general relativity and the Standard Model of Particle Physics simply cannot explain the empirical evidence without some kind of new physics (or a new understanding of how to apply existing physics) of some kind.

1. Any viable dark matter theory has to be able to explain why the distribution of luminous matter in a galaxy predicts observed dark matter phenomena so tightly and with so little scatter in multiple respects such as rotation curves and bulge sizes. These relationships persist even in cases that in a non-gravitational theory should not naturally hold. For example, planetary nebulae distantly rotating ellipical galaxies show the same dynamics of stars at the fringe of spiral galaxies do. Similarly, these relationships persist in gas rich galaxies and dwarf galaxies (which have as predicted about 0.2% ordinary matter if GR is correct in a universe that is overall 17% dark matter) despite that they are beyond the scope of the data used to formulate the theories.

One of the more successful recent efforts to reproduce the baryonic Tully-Fischer relation with CDM models is L.V. Sales, et al., "The low-mass end of the baryonic Tully-Fisher relation" (February 5, 2016). It explains:

[T]he literature is littered with failed attempts to reproduce the Tully-Fisher relation in a cold dark matter-dominated universe. Direct galaxy formation simulations,for example, have for many years consistently produced galaxies so massive and compact that their rotation curves were steeply declining and, generally, a poor match to observation. Even semi-analytic models, where galaxy masses and sizes can be adjusted to match observation, have had difficulty reproducing the Tully-Fisher relation, typically predicting velocities at given mass that are significantly higher than observed unless somewhat arbitrary adjustments are made to the response of the dark halo.

The paper manages to simulate the Tully-Fisher relation only with a model that has sixteen parameters carefully "calibrated to match the observed galaxy stellar mass function and the sizes of galaxies at z = 0" and "chosen to resemble the surroundings of the Local Group of Galaxies", however, and still struggles to reproduce the one parameter fits of the MOND toy-model from three decades ago. Any data set can be described by almost any model so long as it has enough adjustable parameters.

Much of the improvement over prior models has come from efforts to incorporate feedback between baryonic and dark matter into the models, but this has generally been done in a manner than is more ad hoc than it is firmly rooted in rigorous theory or empirical observations of the feedback processes in action.

One of the more intractable problems with simulations based upon a dark matter particle model that has been pointed out, for example, in Alyson M. Brooks, Charlotte R. Christensen, "Bulge Formation via Mergers in Cosmological Simulations" (12 Nov 2015) is that their galaxy and mass assembly model dramatically understates the proportion of spiral galaxies in the real world which are bulgeless, which is an inherent difficulty with the process by which dark matter and baryonic matter proportions are correlated in dark matter particle models which are not a problem for modified gravity models. They note that:

[W]e also demonstrate that it is very difficult for current stellar feedback models to reproduce the small bulges observed in more massive disk galaxies like the Milky Way. We argue that feedback models need to be improved, or an additional source of feedback such as AGN is necessary to generate the required outflows.

General relativity doesn't naturally supply such a feedback mechanism.

2. The fact that it is possible to explain pretty much all galactic rotation curves with a single parameter implies that any dark matter theory also can't be too complex, because otherwise it would take more parameters to fit the data. The relationships that modified gravity theories show exist are real, whether or not the proposed mechanism giving rise to those relationships is real or not. A dark matter theory shouldn't have more degrees of freedom than a toy model theory that can explain the same data. The number of degrees of freedom it takes to explain a data set is insensitive to the particular underlying nature of the correct theory to explain that data.

Also, while I don't have references to them easily at hand at the moment, early dark matter simulations quickly revealed that models with one primary kind of dark matter fit the data much better than those with multiple kinds of dark matter that significantly contribute to these phenomena.

This simplicity requirement greatly narrows the class of dark matter candidates that need to be considered, and hence, the number of viable dark matter particle theories that a modified gravity theory must compete with in a credibility contest.

3. There are fairly tight constraints from astronomy observations on the parameter space of dark matter. Alyson Brooks, "Re-Examining Astrophysical Constraints on the Dark Matter Model" (July 28, 2014). These rule out pretty much all cold dark matter models except "warm dark matter" (WDM) (at a keV scale mass that is at the bottom of the range permitted by the lamdaCDM model) and "self-interacting dark matter" (SIDM) (which escapes problems that otherwise plague cold dark matter models with a fifth force that only acts between dark matter particles requiring at least a beyond the Standard Model fermion and a beyond the Standard Model force carried by a new massive boson with a mass on the order of 1-100 MeV).

4. Direct detection experiments (especially LUX) rule out any dark matter candidates that interact via any of the three Standard Model forces (including the weak force) at masses down to 1 GeV (also here).

5. Another blow is the non-detection of annihilation and decay signatures. Promising data from the Fermi satellite's observation of the galactic center have now been largely ruled out as dark matter signatures in Samuel K. Lee, Mariangela Lisanti, Benjamin R. Safdi, Tracy R. Slatyer, and Wei Xue. "Evidence for unresolved gamma-ray point sources in the Inner Galaxy." Phys. Rev. Lett. (February 3, 2016). And, signs of what looked like a signal of warm dark matter annihilation have likewise proved to be a false alarm.

6. The CMS experiment at the LHC rules out a significant class of low mass WIMP dark matter candidates, while other LHC results exclude essentially all possible supersymmetric candidates for dark matter. If SUSY particles exist, they would be both too heavy to constitute warm dark matter (almost all types of SUSY particles are excluded up to about 40 GeV by the LHC which is too heavy) and they would also lack the right kind of self-interactions force within a SUSY context to be a SIDM candidate. This has particularly broad implications because SUSY is the low energy effective theory of almost all popular GUT theories and viable string theory vacua.

7. While MOND requires dark matter in galactic clusters, including the particularly challenging case of the bullet cluster, this defect is not shared by all modified gravity theories (see, e.g., here and here). Many of the theories that can successfully explain the bullet cluster are able to do so mostly because the collision can be decomposed into gas and galaxy components that have independent effects from each other under the theories in question. The bullet cluster is also one of the main constraints on SIDM parameter space (which itself basically does modify gravity but just does so in the dark sector, limiting those modifications to dark matter particles only), and is tough to square with manner dark matter particle theories.

8. It is possible in a modified gravity theory but very challenging in a dark matter particle theory, to explain why the mass to luminosity ratio of ellipical galaxies varies by a factor of four, systemically based upon the degree to which they are spherical or not.

9. Many of the modified gravity proposals mature enough to receive attention to their fit to cosmological data can meet that test as well. See, e.g., here.

10. In short, while a dark matter hypothesis alone can explain the apparently missing matter in any given situation, in order to get a descriptive theory, you need to be able to describe the highly specific manner in which it is distributed in the universe relative to the baryonic matter in the universe, ideally in a manner that predicts new phenomena, rather than merely post-dicting already observed results that went into the formulation of the model.

Modified gravity theories have repeatedly been predictive, while dark matter theories have still not figured out how to distribute it properly throughout the universe without "cheating" in how the models testing them are set up, and have failed to make any correct predictions of new phenomena below the cosmic microwave background radiation scale of cosmology.

To be clear, I am not asserting that modified gravity is indeed to correct explanation of all or any of the phenomena attributed to dark matter, nor am I asserting that any of the modified gravity theories currently in wide circulation are actually correct descriptions of Nature.

But, the examples of modified gravity theories that we do have are sufficient to make clear that some kind of modified gravity theory is a credible possible solution to the problem of dark matter phenomena.

It is also a more credible solution than it used to be because the case for the most popular dark matter particle theories has grown steadily less compelling as various kinds of dark matter candidates have been ruled out and as more data has narrowed the parameter space available for the dark matter candidates. The "WIMP miracle" that motivated a lot of early dark matter proposals is dead.

While this comment doesn't comprehensively review all possible dark matter candidates and affirmatively rule them out, it does make clear that none of the easy solutions that had been widely expected to work out in the 20th century have survived the test of time into 2016. Over the past decade or so, only a few viable dark matter particle theories have survived, while myriad new modified gravity theories have been developed and not been ruled out.

Other post-2016 issues include (some overlapping):

* the too-dense-to-be-satellites problem. Mohammadtaher Safarzadeh, Abraham Loeb "A New Challenge for Dark Matter Models" arXiv:2017.03478 (July 7, 2021).

* the gravitational lensing of subhalos in galactic clusters recently observed to be much more compact and less "puffy" than LambdaCDM would predict.

* a KIDS telescope observation of very large scale structure which shows it to be 8.3% smoother (i.e. less clumpy) than predicted by LambdaCDM.

* the Hubble tension that shows that Hubble's constant, which is a measure of the expansion rate of the universe, is about 10% smaller when measured via cosmic microwave background radiation (with a small margin of error) than when measured by a wide variety of measures at times much more removed from the Big Bang that the time at which the cosmic microwave background came into being.

* The inferred dark matter halo shapes are usually wrong (too cuspy and not in the NFW distribution predicted by the theory).

* The correspondence between the distribution of ordinary matter and inferred dark matter in galaxies is too tight; truly collisionless dark matter should have less of a tight fit in its distribution to ordinary matter distributions than is observed. This is also the case in galaxy clusters.

* It doesn't explain systemic variation in the amount of apparent dark matter in elliptical galaxies, or why spiral galaxies have smaller proportions of ordinary matter than elliptical galaxies in same sized inferred dark matter halos, or why thick spiral galaxies have more inferred dark matter than thin ones.

* It doesn't explain why satellite galaxies are consistently located in a two dimensional plane relative to the core galaxy.

* Not as many satellite galaxies are observed as predicted, or why the number of satellite galaxies is related to budge mass in spiral galaxies.

* The aggregate statistical distribution of galaxy types and shapes, called the "halo mass function" is wrong.

* Galaxies are observed sooner after the Big Bang than expected.

* The temperature of the universe measured by 21cm background radio signals is consistent with no dark matter and inconsistent with sufficient dark matter for LambdaCDM to work.

* It doesn't explain strong statistical evidence of an external field effect that violates the strong equivalence principle.

* It doesn't do a good job of explaining the rare dwarf galaxies (that are usually dark matter dominated) that seem to have no dark matter either

* It doesn't explain deficits of X-ray emissions in low surface brightness galaxies.

* It predicts too few galaxy clusters.

* It gets globular cluster formation wrong.

* It doesn't explain evidence of stronger than expected gravitational effects in wide binary stars.

* There are too many galaxy clusters colliding at speeds that are too high relative to each other.

* It doesn't explain the "cosmic coincidence" problem (that the amount of ordinary matter, dark matter and dark energy are of the same order of magnitude at this moment in the history of the Universe since the Big Bang).

* Every measure of detecting it directly has come up empty (including not just dedicated direct detection experiments but particle collider searches, searches for cosmic ray signals of dark matter annihilation, and indirect searches combined with direct searches and also here). But it requires particles and forces of types not present in the Standard Model or general relativity to fit what is observed.

* It has made very few ex ante predictions and those it has made have often been wrong, while MOND has a much better track record despite being far simpler (which should matter).

* There are alternative modified gravity theories to toy model MOND that explain pretty much everything that dark matter particle theories do (including, e.g., the cosmic coincidence problem, clusters, the Bullet Cluster, galaxy formation, the cosmic background radiation pattern observed), with fewer problems and anomalies.

 

[ohwilleke]

The basic evidence are comparisons between the mass of galaxies computed by inventory vs. measuring the effects of galactic gravity.

In most cases, there is a big difference.
But galaxies have been found that show little or no dark matter.

So if its a misunderstanding of gravity - it's a misunderstanding that does not affect all galaxies in the same way.

And there are theories modifying gravity back to at least 1981 in which not all galaxies are affected in the same way, for example, in MOND, due to the external field effect. See also: https://arxiv.org/abs/2009.11525 and

[Submitted on 3 May 2018 (v1), last revised 19 Sep 2018 (this version, v3)]

NGC 1052-DF2 And Modified Gravity (MOG) Without Dark Matter​

J. W. Moffat, V. T. Toth

We model the velocity dispersion of the ultra-diffuse galaxy NGC 1052-DF2 using Newtonian gravity and modified gravity (MOG). The velocity dispersion predicted by MOG is higher than the Newtonian gravity prediction, but it is fully consistent with the observed velocity dispersion that is obtained from the motion of 10 globular clusters.

Comments:	4 pages, version accepted for publication in MNRAS Letters
Subjects:	General Relativity and Quantum Cosmology (gr-qc); Astrophysics of Galaxies (astro-ph.GA)
Journal reference:	Mon.Not.Roy.Astron.Soc. Letters 482 (2019) L1-L3
DOI:	10.1093/mnrasl/sly176
Cite as:	arXiv:1805.01117 [gr-qc]
	(or arXiv:1805.01117v3 [gr-qc] for this version)

 

[ohwilleke]

Not necessarily, one can throw away GR and use this new model. But this new model has to be consistent with everything that GR predicted and has been confirmed observationaly/experimentaly. Well, so in a way it has to have most of the GR built in it, one way or another. Thus far modified gravity models have problem with that.

There are relativistic approaches dealing with dark matter and dark energy as gravitational phenomena.

 

[ohwilleke]

Maybe because most people who knows technical details of both dark matter models and alternative models think that dark matter models are better and simpler?

There are very few such people and many (probably most) of them prefer alternatives to dark matter particle models.

 

[ohwilleke]

There is also evidence for dark matter from.the CMBR. See recent responses in another thread by @kimbyd.

But see, e.g., Constantinos Skordis, Tom Złosnik, "A new relativistic theory for Modified Newtonian Dynamics" arXiv (June 30, 2020):

We propose a relativistic gravitational theory leading to Modified Newtonian Dynamics, a paradigm that explains the observed universal acceleration and associated phenomenology in galaxies. We discuss phenomenological requirements leading to its construction and demonstrate its agreement with the observed Cosmic Microwave Background and matter power spectra on linear cosmological scales. We show that its action expanded to 2nd order is free of ghost instabilities and discuss its possible embedding in a more fundamental theory.

and also
[Submitted on 2 Sep 2014]

Structure Growth and the CMB in Modified Gravity (MOG)​

J. W. Moffat

An important piece of evidence for dark matter is the need to explain the growth of structure from the time of horizon entry and radiation-matter equality to the formation of stars and galaxies. This cannot be explained by using general relativity without dark matter. So far, dark matter particles have not been detected in laboratory measurements or at the LHC. We demonstrate that enhanced structure growth can happen in a modified gravity theory (MOG). The vector field and particle introduced in the theory to explain galaxy and cluster dynamics plays an important role in generating the required structure growth. The particle called the phion (a light hidden photon) is neutral and is a dominant, pressureless component in the MOG Friedmann equations, before the time of decoupling. The dominant energy density of the phion particle in the early universe, generates an explanation for the growth of density perturbations. The angular acoustical power spectrum due to baryon-photon pressure waves is in agreement with the Planck 2013 data. As the universe expands and large scale structures are formed, the density of baryons dominates and the rotation curves of galaxies and the dynamics of clusters are explained in MOG, when the phion particle in the present universe is ultra-light. The matter power spectrum determined by the theory is in agreement with current galaxy redshift surveys.

Comments:	10 pages, 3 figures
Subjects:	Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc)
Cite as:	arXiv:1409.0853 [astro-ph.CO]
	(or arXiv:1409.0853v1 [astro-ph.CO] for this version)

 

[John Jordano]

Summary:: Is it possible to put into a nutshell why the case for dark matter together with current understanding of gravity is more likely to be right, than simply we don't quite yet understand gravity?

My understanding, possibly my ignorance, is that dark matter is calculated to exist from observations that there's not enough matter to fit observations if the current theory of gravity is right.

Is it possible to put into a nutshell why the case for dark matter together with current understanding of gravity is more likely to be right, than simply we don't quite yet understand gravity?

As a 'for example', If we invent unobservable matter to modify the forces of gravity, why don't we just invent unobservable bubbles of stronger gravity? (note; I don't think this and have no pet theories of this, just an example of 'inventing something else'.)

What is so compelling about "dark matter and current view of gravity" that beats the possibility that we just don't quite understand gravity yet, or that there may be other (non-massive) things that can affect the behaviour of gravity locally or at larger scales that we don't understand yet?

What are the alternatives to the theories of 'dark matter'?

Is there something further to bolster the theory of dark matter, other than effects of gravity that suggests dark matter exists?

All models of dark matter + gravity that I'm aware of do their calculations using Newtonian Gravity. The reason is that it is exceedingly difficult to do the calculations using General Relativity. And so scientists operate under the belief and implicit assumption that GR doesn't affect galaxy dynamics, and NM is fine. But a set of conjectures about 10 years ago and more recent papers in the last year or so have shown rather strongly that his assumption is false. Galaxy shape and mass distribution has a subtle but significant effect on the GR effects.

Our "understanding of gravity" isn't wrong, we're just using the wrong theory of gravity. GR has been around for about 100 years. The math is really hard to apply to gravity size, mass and shape objects. But that's not a good excuse for not doing the math.

 

[cmb]

All models of dark matter + gravity that I'm aware of do their calculations using Newtonian Gravity. The reason is that it is exceedingly difficult to do the calculations using General Relativity. And so scientists operate under the belief and implicit assumption that GR doesn't affect galaxy dynamics, and NM is fine. But a set of conjectures about 10 years ago and more recent papers in the last year or so have shown rather strongly that his assumption is false. Galaxy shape and mass distribution has a subtle but significant effect on the GR effects.

Our "understanding of gravity" isn't wrong, we're just using the wrong theory of gravity. GR has been around for about 100 years. The math is really hard to apply to gravity size, mass and shape objects. But that's not a good excuse for not doing the math.

I'm unconvinced. Why would people stick with "Newtonian Gravity" if there was something better to use?

Surely GR is not 'sooooo' complicated that it is impossible to model it (instead of "Newtonian gravity"). Is that what you are trying to say?

Thanks for your contribution but I can't follow your point. If you have some relevant reference confirming your assertion, that would be helpful.

 

[wiganshale]

I once read that there was an imbalance in the observed matter and anti-matter in the Universe i.e. not enough anti-matter. Is it possible that galaxies may have a corona of anti-matter, in some form, which we may not be able to see but which "pushes" matter into its well?

 

[cmb]

I once read that there was an imbalance in the observed matter and anti-matter in the Universe i.e. not enough anti-matter. Is it possible that galaxies may have a corona of anti-matter, in some form, which we may not be able to see but which "pushes" matter into its well?

What makes you say that anti-matter causes anti-gravity? I think if something is massive then the gravity is one direction towards it, anti- or not.

 

[wiganshale]

What makes you say that anti-matter causes anti-gravity? I think if something is massive then the gravity is one direction towards it, anti- or not.

F= - G m1m2/ r*r

 

[cmb]

F= - G m1m2/ r*r

Yes, but anti-matter is not 'negative matter'.

E=mc^2. An anti-proton does not have 'negative energy'. That'd mean if a proton and an anti-proton met, they'd annihilate to zero net energy, and you know that's not right, don't you?

 

[weirdoguy]

E=mc^2. An anti-proton does not have 'negative energy'.

E=mc^2 is only a rest energy, not total energy, so I don't know why you are invoking that The key property is that anti-proton has positive mass.

 

[cmb]

E=mc^2 is only a rest energy, not total energy, so I don't know why you are invoking that

Oh, don't get me going. Maybe when it is traveling at an imaginary velocity it can have even more negative energy.

... hey, maybe that's it, 'dark space' where particles moving in that have a negative kinetic energy ...

 

[wiganshale]

Yes, but anti-matter is not 'negative matter'.

E=mc^2. An anti-proton does not have 'negative energy'. That'd mean if a proton and an anti-proton met, they'd annihilate to zero net energy, and you know that's not right, don't you?

Anti-matter has a negative charge, so will repel matter. I imagine an egg, where the yolk is matter and the Albumen is anti-matter. the yolk coheres and the albumen coheres. The albumen acts like a corset pulling itself tight around the yolk and compressing it. When galaxies collide it would be something like two soap bubbles reorganising themselves without any noticeable annihilations. Maybe there'd be extra gamma emitted by colliding galaxies. Why the yolk would always be matter, I don't know. How you would tell if a galaxy had an anti-matter yolk, I wouldn't know. I've never formally studied Astronomy, black matter etc. and I don't have more than a pair of binoculars, so I can't really join any serious discussion. However I think the idea of galaxies acting like rigid bodies because of an encasing anti-matter shell, is slightly more appealing than some sort of plum duff with no discernible properties other than having mass, which doesn't condense.

 

[Ibix]

Anti-matter has a negative charge, so will repel matter.

No. This is not correct. Antimatter has an opposite electric charge (which may be positive or negative and could form neutral atoms) but it has positive mass so is gravitationally attractive the same as normal matter.

I've never formally studied Astronomy,

Then you don't even understand the details of the problem. Please note that PF is not for discussion of personal theories, as set out in the terms you agreed to when you joined.

 

[DaveC426913]

Anti-matter has a negative charge.

No. Antimatter simply has opposite charge: anti-electrons have a positive charge, whereas antiprotons have a negative charge. Anti hydrogen is neutral.

Antimatter produces the same positive gravity as matter.

 

[cmb]

Anti-matter has a negative charge

Positively not positrons, which I am positive are positive.

Anyway, galactic matter is net neutral, as would be galactic anti-matter.

 

[ohwilleke]

I once read that there was an imbalance in the observed matter and anti-matter in the Universe i.e. not enough anti-matter. Is it possible that galaxies may have a corona of anti-matter, in some form, which we may not be able to see but which "pushes" matter into its well?

No. This isn't possible. Anti-matter interacts vigorously with ordinary matter. If there were anti-matter haloes around galaxies we'd see massive numbers of diffuse flashes everywhere.

 

[ohwilleke]

I'm unconvinced. Why would people stick with "Newtonian Gravity" if there was something better to use?

Surely GR is not 'sooooo' complicated that it is impossible to model it (instead of "Newtonian gravity"). Is that what you are trying to say?

Thanks for your contribution but I can't follow your point. If you have some relevant reference confirming your assertion, that would be helpful.

The basic problem is that there isn't a literature out there of really rigorous analysis of GR minus Newtonian gravity in galaxy scale systems. There is back of napkin rough estimates making lots of spherical cow type assumptions, but there isn't much literature quantifying the magnitude of the non-linear effects of GR in non-spherical systems of galaxy scale. They are clearly not zero and haven't been rigorously bounded.

This isn't entirely a coincidence.

There is a statement in the Misner, Thorne and Archibald Gravitation textbook which implies that no observable effects arise from self-interactions of gravitational fields, which is almost surely an overstatement, and leaves people with an overstated impression of what is being said.

There have also been a number of papers looking at different non-linear GR effect (like gravitomagnetism) that don't work and have been rejected.

There is also the look for you keys under the lamp post effect. It is much, much easier to do Newtonian approximations, and it is much much easier to model GR effects in the spherically symmetric case in which the effect the Deur is interested in cancels out due to symmetry considerations.

Doing hard work to find something with a low Baysean probability of working out isn't the most attractive course of action.

I'd also note that another possibility is that Deur is indeed actually using equations that for subtle reasons are not actually standard Einstein equation GR (as he has asserted that they are). But suppose that is the case.

The equations still reproduce dark matter and dark energy and early structure phenomena. They reproduce MOND in disk galaxies and solve the insufficiency of dark matter phenomena in galactic clusters. They make novel observational predictions that have been confirmed that aren't found in other modified gravity theories. And, they manifestly reduce to the weak field Newtonian approximation (in a manner naturally generalized to be relativistic) on their face below a threshold that corresponds to the MOND constant. These equations do all of these things with a single field, not the scalar-tensor or scalar-vector-tensor fields that many other relativistic modified gravity theories require. These equations do all of these things without particle dark matter and without a cosmological constant of a substance to provide dark energy. This is still a very good day's work.

So, even if he is mistaken and he is not actually doing exactly Einstein's GR, if he has a versatile set of equations that can reproduce the phenomenology of both dark matter and dark energy, not just dark matter as MOND does, over a much wide range of applicability than MOND, in a quite simple and elegant way. Thus, whatever he has done to subtly modify Einstein's GR may be an accurate description of nature, and Einstein's GR may not be quite the right description of how gravity really acts in the infrared in systems that aren't spherically symmetrical. Whether or not it is truly faithful to Einstein's GR is something of an academic question if it works better to describe the universe.

After all, it's a dirty little secret but GR theorists actually have several different slight variations of GR on offer already that differ in subtle ways that aren't currently experimentally or observationally testable, such as teleparallel gravity, Einstein-Gauss-Bonnet, Einstein-Cartan theory, metric-affine gravity, etc. So, it really wouldn't be all that remarkable if the correct theory was a subtle tweak from the original 1917 version.