Mond beats Dark Matter

[QuantumT]

Quote

We can model the motions of planets in the Solar System quite accurately using Newton’s laws of physics. But in the early 1970s, scientists noticed that this didn’t work for disc galaxies – stars at their outer edges, far from the gravitational force of all the matter at their centre – were moving much faster than Newton’s theory predicted.

This made physicists propose that an invisible substance called “dark matter” was providing extra gravitational pull, causing the stars to speed up – a theory that’s become hugely popular. However, in a recent review my colleagues and I suggest that observations across a vast range of scales are much better explained in an alternative theory of gravity proposed by Israeli physicist Mordehai Milgrom in 1982 called Milgromian dynamics or Mond – requiring no invisible matter.

Mond’s main postulate is that when gravity becomes very weak, as occurs at the edge of galaxies, it starts behaving differently from Newtonian physics. In this way, it is possible to explain why stars, planets and gas in the outskirts of over 150 galaxies rotate faster than expected based on just their visible mass. But Mond doesn’t merely explain such rotation curves, in many cases, it predicts them.

https://theconversation.com/dark-matter-our-review-suggests-its-time-to-ditch-it-in-favour-of-a-new-theory-of-gravity-186344

 

So, the question is: Is Mond good enough to gain consensus favor?

[MigL]

MOND can be modelled in two ways ...
As a modification of Newton's second, where the a in F=Ma is modified.
Or the inverse square relation ( 1/r² ) is modified in Newonian gravity.

See here ...

Modified Newtonian dynamics - Wikipedia

The Wiki article, should you choose to read it, explains some insurmountable problems with MOND and TeVeS, its relativistic formulation, such as the problem with the observations of the Bullet Cluster collision of 2006 which suggests that Modified Newtonian Dynamics is not enough to account for those observations, and that some form of 'dark' matter is still required.

[swansont]

!

Moderator Note

Moved out of science news because this is not news

 

 

Put another way: you can't just look at galaxy rotation curves, i.e. MOND fails when you look at a wider range of phenomena. It can only be part of an answer if you get rid of dark matter. This is discussed in the link MigL provided

 

[QuantumT]

I'm just the messenger, and wanted the opinions of you fine gentlemen.

Sorry for putting this in the wrong section. It sounded like news to me.

Edited July 12, 2022 by QuantumT

[swansont]

Ethan Siegel has written a number of blog posts on the topic. There are more than just these two

https://medium.com/starts-with-a-bang/the-death-of-dark-matters-1-competitor-98edff3a066f

https://medium.com/starts-with-a-bang/could-dark-matter-not-exist-at-all-b0e69fd41848

[QuantumT]

Thank you very much for your replies and the links.

But I get the feeling that you didn't bother to read the OP article, and might be unaware of the extensive research put into it. It's not a new attempt based on old data, but a comparison between the two models, based on recently discovered shortcomings (of CDM) and (fixed) computational mistakes.

Here's a few key excerpts.

Comparing CDM and MOND after error correction:

Spoiler

We introduced a concept known as “theoretical flexibility” to capture the underlying idea of Occam’s razor that a theory with more free parameters is consistent with a wider range of data – making it more complex. In our review, we used this concept when testing the standard cosmological model and Mond against various astronomical observations, such as the rotation of galaxies and the motions within galaxy clusters.

Each time, we gave a theoretical flexibility score between –2 and +2. A score of –2 indicates that a model makes a clear, precise prediction without peeking at the data. Conversely, +2 implies “anything goes” – theorists would have been able to fit almost any plausible observational result (because there are so many free parameters). We also rated how well each model matches the observations, with +2 indicating excellent agreement and –2 reserved for observations that clearly show the theory is wrong. We then subtract the theoretical flexibility score from that for the agreement with observations, since matching the data well is good – but being able to fit anything is bad.

A good theory would make clear predictions which are later confirmed, ideally getting a combined score of +4 in many different tests (+2 -(-2) = +4). A bad theory would get a score between 0 and -4 (-2 -(+2)= -4). Precise predictions would fail in this case – these are unlikely to work with the wrong physics.

We found an average score for the standard cosmological model of –0.25 across 32 tests, while Mond achieved an average of +1.69 across 29 tests. The scores for each theory in many different tests are shown in figures 1 and 2 below for the standard cosmological model and Mond, respectively.

Diagrams:

Spoiler

Conclusion:

Spoiler

There are many other failures of the standard cosmological model that we investigated in our review, with Mond often able to naturally explain the observations. The reason the standard cosmological model is nevertheless so popular could be down to computational mistakes or limited knowledge about its failures, some of which were discovered quite recently. It could also be due to people’s reluctance to tweak a gravity theory that has been so successful in many other areas of physics.

The huge lead of Mond over the standard cosmological model in our study led us to conclude that Mond is strongly favoured by the available observations. While we do not claim that Mond is perfect, we still think it gets the big picture correct – galaxies really do lack dark matter.

The full study:

https://www.mdpi.com/2073-8994/14/7/1331/htm

 

Note: Being from a not-English-speaking country I might have chosen some wrong words to describe certain things. Sorry about that.

[swansont]

Quote

We found an average score for the standard cosmological model of –0.25 across 32 tests, while Mond achieved an average of +1.69 across 29 tests

I have to wonder, what was the score on the three omitted tests? Assuming the other 29 are the same.

They claim “ no major problems were identified for Mond“ but that’s not what the other articles say. How might one reconcile all this?

[mistermack]

It's an interesting concept, if it is indeed more predictive than Dark Matter. But I'm naturally sceptical of new models that are tailored to fit a known phenomenon. The tailoring can be pretty creative, and you don't always get told what wiggling has been done, to get to a destination that you already know. 

Even if Mond IS a better predictor of the motion of stars, it doesn't mean that it's based on a principle that's right. 

Comparing the Dark Matter idea, and Mond, they are both hypothesising something unknown, causing an observed phenomenon. But Dark Matter is more logical. Mond is talking about gravity not behaving in the way in which it has been observed and measured time and time again, just because the field is low strength. I haven't seen any suggestion of a mechanism why that should be so. 

DM speculates on the presence of an unknown form of matter. That's not exactly without precedent. But it also needs to be "tailored" to each galaxy to fit. 

Having a guess, I'd say that DM is a more likely scenario. I can't conjure up a hypothetical reason for gravity to go haywire, just because the field is low strength, but I can imagine a previously unknown form of matter being identified. Of course, the fact that I can't picture it doesn't make it any less viable. 

[Markus Hanke]

15 hours ago, QuantumT said:

But I get the feeling that you didn't bother to read the OP article, and might be unaware of the extensive research put into it.

The trouble is that MOND is a non-relativistic theory, so comparing it to standard cosmology is kind of useless. At a minimum you’d have to use one of its relativistic generalisations - TeVeS being the most common and popular. And here’s where the issues start, because TeVeS has some serious problems, both so far as observational data is concerned, and in terms of mathematical consistency. 

And even if you could get it to work properly, you end up with various extra vector and scalar fields that are needed in the model - for which of course there’s no experimental evidence whatsoever. So in the end you just replace Dark Matter and Dark Energy with a bunch of extra unknown fields. It really doesn’t solve anything, on a conceptual level.

[QuantumT]

On 7/14/2022 at 10:02 AM, Markus Hanke said:

The trouble is that MOND is a non-relativistic theory, so comparing it to standard cosmology is kind of useless. At a minimum you’d have to use one of its relativistic generalisations - TeVeS being the most common and popular. And here’s where the issues start, because TeVeS has some serious problems, both so far as observational data is concerned, and in terms of mathematical consistency. 

And even if you could get it to work properly, you end up with various extra vector and scalar fields that are needed in the model - for which of course there’s no experimental evidence whatsoever. So in the end you just replace Dark Matter and Dark Energy with a bunch of extra unknown fields. It really doesn’t solve anything, on a conceptual level.

Thanks for letting me down easy. Again.

So you read the full study?

[Markus Hanke]

9 hours ago, QuantumT said:

Thanks for letting me down easy. Again.

So you read the full study?

I have followed the debates about the nature of the ‘dark sector’ for many years now, and have looked at the mathematical formalisms of all the various candidate models and ideas, some of them in detail. So I’m drawing from a diverse range of sources, not just a single paper or author. 

If you look at the bigger picture, you’ll find that many of the alternative models may be better at explaining specific phenomena - but at the cost of failing miserably with other observational data. Furthermore, very many of these alternatives require extra fields or extra dimensions, or make ad-hoc assumptions that aren’t based on any known physics - so they try to explain one unknown by proposing other unknowns, which is kind of useless. For example, the paper you quote assumes the existence of sterile neutrinos below a certain critical mass limit in order to match observations. Other known problems with MOND are never addressed at all.

On a meta level, taking into account all available observational data at this point in time, standard GR still provides the best fit. Im aware of the problems in standard cosmology of course, but I don’t think any of the currently existing alternatives provides a good enough solution. That includes MOND and its relativistic generalisations.

 

[LaurieAG]

I read this article by Pavel Kroupa today, he takes a different approach. He's a professor of astrophysics at University of Bonn, in Germany, where he heads the Stellar Populations and Dynamics research group.

https://iai.tv/articles/dark-matter-doesnt-exist-auid-2180

[Markus Hanke]

4 hours ago, LaurieAG said:

I read this article by Pavel Kroupa today, he takes a different approach. He's a professor of astrophysics at University of Bonn, in Germany, where he heads the Stellar Populations and Dynamics research group.

https://iai.tv/articles/dark-matter-doesnt-exist-auid-2180

I also do not think that Dark Matter exists in the way it is generally conceived of, ie as a particulate substance made from hitherto undiscovered particles.

However, neither do I believe that any of the currently existing alternatives provide a better solution than standard cosmology does.

Furthermore, some of the assertions made in this article are concerning, eg the claim that (paraphrase) “all predictions made by MOND have been verified”. This is quite simply wrong (some of its predictions are in fact in direct contradiction to observation), and I am very surprised that a qualified astrophysicist would say something like this. 

[swansont]

6 hours ago, LaurieAG said:

I read this article by Pavel Kroupa today, he takes a different approach. He's a professor of astrophysics at University of Bonn, in Germany, where he heads the Stellar Populations and Dynamics research group.

https://iai.tv/articles/dark-matter-doesnt-exist-auid-2180

"Dark matter is wrong" does not equate to "MOND is correct" as is suggested by the article. If DM is wrong, then it's wrong. That's as far as it goes.

And also, as Markus points out, the author is simply ignoring issues with MOND. Such argument is trash, from a scientific standpoint.

The objection that "The theory of dark matter makes no predictions as to what the particles ought to be and what to look for" reminds me of the search for the neutrino/antineutrino. A particle that was proposed because the behavior shown in experiments didn't match up with theory - energy was missing, you had a continuous spectrum of electron energy, and angular momentum wasn't being conserved. There was no theory that said a neutrino should exist but the observation said something had to be there, and also said that any such particle wouldn't be interacting electromagnetically. (it was "dark" even though this is not what we call dark matter). Lo and behold, it was eventually detected, more than a decade after being proposed. The theory to explain it also came after, and the original proposal had to be refined over the course of time. It took decades to work all of this out.  

 

[kba]

How it is possible to discuss what "gravity theory" is better while them all don't explain the essense of gravity?

Edited July 22, 2022 by kba

[Markus Hanke]

1 hour ago, kba said:

How it is possible to discuss what "gravity theory" is better while them all don't explain the essense of gravity?

What do you mean by “essence of gravity”?

[kba]

On 7/14/2022 at 1:32 AM, mistermack said:

Even if Mond IS a better predictor of the motion of stars, it doesn't mean that it's based on a principle that's right. 

Comparing the Dark Matter idea, and Mond, they are both hypothesising something unknown, causing an observed phenomenon.

I'm so agree with these.

I have an explanation for the essense of gravity, as a dynamical force, which based on particles' model.

And this explanation predicts very weak acceleration (caused by gravity of far stars, accordingly with Mach's principle) for any bodies and particles for their inertial traveling.

Such weak acceleration, in the scales of galaxy, provides the high speed for peripheral stars and rotating curve which differs with newtonian one. And no DM is needed.

Also it provides very high speed and energy for particles in the cosmic rays. And no supernovas are needed.

Probably, it could be connected with some kinds of bursters, with ultraspeed extragalaxy stars, etc., anything that depend on accelerated traveling.

17 minutes ago, Markus Hanke said:

What do you mean by “essence of gravity”?

Its nature, its origin, its source.

Edited July 23, 2022 by kba

[Markus Hanke]

7 hours ago, kba said:

I have an explanation for the essense of gravity, as a dynamical force, which based on particles' model.

And this explanation predicts very weak acceleration (caused by gravity of far stars, accordingly with Mach's principle) for any bodies and particles for their inertial traveling.

A test particle in free fall under the influence of gravity does not experience any forces - which is to say that an accelerometer comoving with such a test particle reads exactly zero at all times. This needs to be true in all potential models of gravity, since this is what we observe in the real world. If that weren’t so, the motion wouldn’t be inertial, and thus the test particle wouldn’t trace out a geodesic.

[kba]

9 hours ago, Markus Hanke said:

A test particle in free fall under the influence of gravity does not experience any forces - which is to say that an accelerometer comoving with such a test particle reads exactly zero at all times

This is correct because a test particle and an accelerometer (actually, particles it consists) already [in free fall] under the same gravity acceleration.

Edited July 23, 2022 by kba

[Markus Hanke]

2 hours ago, kba said:

under the same gravity acceleration.

As I said, there is no proper acceleration for free fall motion, so no forces are acting on the test particle. There is only coordinate acceleration as calculated by any specific outside observer using his own set of coordinates, but this does not correspond to any physical force, since no accelerometer exists that reads this quantity. It’s merely a frame-dependent accounting device.

Either way, if you wish to present and discuss your idea, it will be best to open your own thread in “Speculations”. This here is not the right place for it.