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Dark matter - fudge factor?


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On a good day I might be able to follow Einstein's special relativity theory. But I don't have the mathematical capabilities to follow Einstein's general relativity theory. If I understand history correctly Einstein didn't manage to do the math of his general relativity theory either and needed help with some of the mathematical  sides of it.

What I find strange is this:

We now have Einstein's general relativity theory accepted as truth theory because it fits most of what we observe. Except galaxy spin speed's. They do not fit .  So we say the theory is still correct, there must be some matter out there we cannot see. Sure there is matter we cannot see, not all matter  emits light, but the amount needed is beyond what we can explain. So we invent a massive fudge factor instead of accepting that our current theory must be incorrect.

 

I don't get it!  I don't get it all!

 

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A "universal law" by its very definition must hold in the entire universe. Whether such laws as we have postulated are universal is another matter. However, the idea that these laws are universal is n

I'd suggest that you're looking at things the wrong way.  The mathematical models we build to describe the universe are exactly that: models.  That means they may not be a perfect fit for every possib

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I'd suggest that you're looking at things the wrong way.  The mathematical models we build to describe the universe are exactly that: models.  That means they may not be a perfect fit for every possible situation, but it doesn't mean they're not useful.  Einstein turned Newtonian physics on its head, but that doesn't mean Newtonian physics was then useless; it was just "incomplete".  But for an awful lot of situations it was also absolutely good enough.  Perhaps it will turn out that Einsteinian physics is the same -- good enough to do even more stuff, but not good enough to explain every single thing we can measure.

Personally I've never really put "truth" and "science" in the same box.  Science is all about building models of the world we perceive that allow us to understand it and do useful stuff with it.  When science models nature so accurately that it isn't possible to tell the difference between the two then perhaps one might consider it "truth", but truth is very much in the eye of the beholder.

So relax, don't worry, have a beer :)  If Newtonian physics works for what you need, use it and be happy.  If Einsteinian physics does the job, dive right in and get on with it.  If neither is a perfect explanation of the universe yet still means your satellite doesn't fall out of the sky then it has achieved something even if it isn't a perfect description of the universe in which we exist.

James

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Not sure that's quite fair. GR is accepted as the best model yet. The "fudge-factor" of dark matter is because they've yet to figure out whether (a) GR is wrong or incomplete or (b) GR is right but dark matter is something real. I think the scientific community has an open mind about what the explanation is. Worth remembering that a theory like GR is still a theory and accepted until there's definitive proof it's wrong. No scientific theory is ever accepted as truth - just something which hasn't yet been proven incorrect.

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We have some very good observational data which backs up GR , gravitational lensing being one of the more compelling I guess and gravitational red shift (1959 Robert Pound and Glen Rebka observed redshift in light in an experimental setup at Harvard Univesity).    So GR appears sound, while no doubt like all theories further layers of refinement may emerge in time, there is nothing really at the moment to suggest that it is significantly flawed.  The  high rotational speed of galaxies may ultimately be accounted for  by what they are calling "dark matter" (I really do wish they had used a different name) or even an error in measurement  of "normal matter"   time may tell. But certainly at the moment there is no basis (no evidence)  for laying  aside GR , it appears to make accurate predictions. 

 

Jim 

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When Newton's laws did not quite explain Mercury's orbit, Einstein came up with another theory. When Einstein's theory does not quite explain galaxies rotation speed, we come up with another theory dark matter. 

?

23 minutes ago, JamesF said:

So relax, don't worry, have a beer :) 

Not relaxing, but have had a beer 😀

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17 minutes ago, wulfrun said:

I think the scientific community has an open mind about what the explanation is

I don't think so. In fact I think you would not be taken seriously in the professional scientific community unless you embrace dark matter.

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7 hours ago, Viktiste said:

I don't think so. In fact I think you would not be taken seriously in the professional scientific community unless you embrace dark matter.

There is a constant flow of achademic papers on modified gravity.  So far they tend to explain one observation e.g. galaxy rotation curves, but not others.

GR has been tested over a very wide spectrum from frame dragging in Earths orbit to neutron star and black hole merges. It is this foundation that make the prediction of dark matter plausible

Just as Netwonian mechanics allowed the prediction of Neptune this can be a good strategy but it can also be wrong as with Vulcan. As more  observations and tests are made we may develop a better theory  of gravity or refine out understanding of dark matter. 

Regards Andrew 

Ps an example from to days new papers https://arxiv.org/abs/1610.07805

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12 hours ago, Viktiste said:

I don't think so. In fact I think you would not be taken seriously in the professional scientific community unless you embrace dark matter.

What's more likely, an error in a theory that has been demonstrated to conform to observations for:

Earth based experiments

Orbit of Mercury precession

Shapiro effect

Deflection of starlight during eclipses

Binary Pulsar orbit decay

Gravitational lensing

Scale of anisotropy of CMBR

 

or 

We can't see everything in a galaxy?

 

 

 

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12 hours ago, Viktiste said:

I don't think so. In fact I think you would not be taken seriously in the professional scientific community unless you embrace dark matter.

Not so. Having been on a couple of science talks at a couple of universities. The dark matter subject was still open to question. 30% ( as a show of hands in the room ) still questioned DM as needing more proof.

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16 hours ago, Viktiste said:

When Newton's laws did not quite explain Mercury's orbit, Einstein came up with another theory. When Einstein's theory does not quite explain galaxies rotation speed, we come up with another theory dark matter. 

?

Not relaxing, but have had a beer 😀

I'm not sure he did.  His rationale for the theory of GR was a little wider than the problem with Mercury's orbit.  Mercury's orbit was used to test the new theory as it presented a measurable effect but I don't think it was the driver.  Look at the end of the day if dark matter is a piece of nonsense then we will find out , if it is not then we will find out.  If it needs a new theory then a new theory will emerge in time ,  that is how science works. Quite exciting either way but it's a long game.   I take it you are not keen on the concept of dark matter as part of the solution , could you explain why and maybe give some insight to where you think the focus of research should be?  I don't really have any strong opinions about it other than I detest the name dark matter and dark energy - it is a gift to pseudo science and lazy media hacks.  It also frightens children :) 

 

Jim 

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It would be best to get a bit more background on dark matter first. Does not need to be very deep - simple reading of Wiki page on Dark matter can help put things into perspective:

https://en.wikipedia.org/wiki/Dark_matter

Here you can find list of major observational evidence in support of existence of dark matter:

image.png.57e7e04a94997e826cfd384d426f1d3a.png

It is not just Galaxy rotation curves - it is much more than that. There is also evidence that goes against modified gravity theories - like discovery of galaxies with little to none of dark matter - those behave in accordance to GR and absence of DM. If DM required modification of GR - then such modification would hold everywhere - or it would need another "strange thing" to explain why there are galaxies that don't behave like modified GR tells they should.

In assessing how successful GR really is - I think it's worth having a read about list of tests used to confirm it:

https://en.wikipedia.org/wiki/Tests_of_general_relativity

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The difficulty with verifying new theories is that they require increasingly precise measurements. Newton's laws could easily be tested on Earth. Testing Einstein's GR required 20-th century technology and measurements within the solar system (and beyond). I read that Eddington's experiment in 1919 reduced to checking angular deflection of almost 2 arcseconds, that is a substantial amount that even amateur equipment nowadays can spot!

Very recently detecting gravitational waves required super precise experiments with light on the edge of what is possible and fighting statistical error.

So new theories need bigger particle accelerators, telescopes, whatever, in short better data. At the moment we just don't have enough to rule out many competing theories involving dark matter. Gradually as the new observations accumulate a theory explaining the paradoxes of dark matter will emerge. It will be the next level of understanding the universe. 

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5 hours ago, vlaiv said:

Here you can find list of major observational evidence in support of existence of dark matter:

image.png.57e7e04a94997e826cfd384d426f1d3a.png

 

These could equally be evidence for an incorrect theory where dark matter doesn't exist

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1 minute ago, philthy said:

These could equally be evidence for an incorrect theory where dark matter doesn't exist

I'm sorry I don't see how that could be could you explain why that could be the case?

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Putting aside dark matter for a moment GR has been tested in a wide range of circumstances and passed. I don't know of any test where it has failed.

If it is not the final answer ( which is quite possible even likely) then any new theory has to make the same predictions as GR. There have been and continue to be new theories of gravity put forward but they have not yet made an real impact.

So the best theory we have requires dark matter to explain a range of observations. As far as I know there is no competing theory that can pass the tests GR has passed (where dark matter is not involved e.g frame dragging and gravity waves ) and explains the "dark matter" effects but without the dark matter.

If anyone knows of one then please give me a reference as I would live to follow it up.

Regards Andrew 

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It might be worth considering that proposing new "stuff" is quite common. Sometimes it works out and the theory holds up. Examples would be neutrinos,  anti matter, Neptune, and Higgs boson .

Sometimes it doesn't work out. Examples being the ether, celestial spheres and Vulcan. Then new theories are needed.

This is just how science works. 

Regards Andrew 

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Here the synopsis of a as not yet peer reviewed paper on Modified Newtonian Dynamics. Posted on ArXiv.org where per published papers are collected.

The global stability of M33 in MOND

Indranil Banik (Bonn), Ingo Thies (Bonn), Graeme Candlish (Valparaiso), Benoit Famaey (Strasbourg), Rodrigo Ibata (Strasbourg), Pavel Kroupa (Bonn, Prague)

The dynamical stability of disk galaxies is sensitive to whether their anomalous rotation curves are caused by dark matter halos or Milgromian Dynamics (MOND). We investigate this by setting up a MOND model of M33. We first simulate it in isolation for 6 Gyr, starting from an initial good match to the rotation curve (RC). Too large a bar and bulge form when the gas is too hot, but this is avoided by reducing the gas temperature. A strong bar still forms in 1 Gyr, but rapidly weakens and becomes consistent with the observed weak bar. Previous work showed this to be challenging in Newtonian models with a live dark matter halo, which developed strong bars. The bar pattern speed implies a realistic corotation radius of 3 kpc. However, the RC still rises too steeply, and the central line of sight velocity dispersion (LOSVD) is too high. We then add a constant external acceleration field of 8.4×10−12 m/s2 at 30∘ to the disk as a first order estimate for the gravity exerted by M31. This suppresses buildup of material at the centre, causing the RC to rise more slowly and reducing the central LOSVD. Overall, this simulation bears good resemblance to several global properties of M33, and highlights the importance of including even a weak external field on the stability and evolution of disk galaxies. Further simulations with a time-varying external field, modeling the full orbit of M33, will be needed to confirm its resemblance to observations.%We finally discuss a novel observational test using the warp induced by the external field in nearly edge-on galaxies.

It shows science at work. MOND models do well at galaxy rotation curves.

Regards Andrew 

 

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Not sure if any one is interested but I have had a look at the literature on GR v alternative theories of gravity this is from a 2012 review:

"From a theoretical perspective, model building is an important part of understanding and explaining existing data, as well as making predictions for the future. Tight constraints already available on solar system and astrophysical scales, however, mean that model builders are presented with a choice: They can either study minimal deviations away from General Relativity, or must otherwise look for mechanisms that hide modifications to gravity on the scales probed by experiment. The former of these has value for understanding the special nature of General Relativity, and the consequences of moving away from it, while the latter provides an exciting opportunity to try and solve some of the cosmological puzzles that have arisen with the discovery of dark matter and energy.

Modified gravity necessarily involves additional fields, extra dimensions, or broken symmetries, since we know that GR is the unique diffeomorphism invariant theory of a single rank-2 tensor that can be constructed from the metric variation of an action in four dimensions. An important consideration is then whether or not these deviations manifest themselves at the level of the background cosmology, or merely at the level of perturbations. Of course, if we wish to account for dark energy, or solve the cosmological constant problem using modified gravity, these deviations must be manifest in the solutions of the Friedmann equations. We must also require, however, that they do not spoil the successful predictions of the standard cosmology, such as the abundance of light elements, the peak positions of the CMB acoustic spectrum, or the predictions for baryon acoustic oscillations. This requires the background FLRW cosmology of the modified theories to closely mimic the standard evolution of ΛCDM from nucleosynthesis through to matter domination."

I have underlined two key section of the discussion. Removing Dark energy and matter come at a cost. There is no free lunch to be had.

 

The extremely long and technical review is available here Modified gravity and cosmology  I admit I just scan read it and don't understand it all. The most recent review I found is 2014.

 

Regards Andrew

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13 hours ago, andrew s said:

Here the synopsis of a as yet peer reviewed paper on Modified Newtonian Dynamics. Posted on ArXiv.org where per published papers are collected.

The global stability of M33 in MOND

Indranil Banik (Bonn), Ingo Thies (Bonn), Graeme Candlish (Valparaiso), Benoit Famaey (Strasbourg), Rodrigo Ibata (Strasbourg), Pavel Kroupa (Bonn, Prague)

The dynamical stability of disk galaxies is sensitive to whether their anomalous rotation curves are caused by dark matter halos or Milgromian Dynamics (MOND). We investigate this by setting up a MOND model of M33. We first simulate it in isolation for 6 Gyr, starting from an initial good match to the rotation curve (RC). Too large a bar and bulge form when the gas is too hot, but this is avoided by reducing the gas temperature. A strong bar still forms in 1 Gyr, but rapidly weakens and becomes consistent with the observed weak bar. Previous work showed this to be challenging in Newtonian models with a live dark matter halo, which developed strong bars. The bar pattern speed implies a realistic corotation radius of 3 kpc. However, the RC still rises too steeply, and the central line of sight velocity dispersion (LOSVD) is too high. We then add a constant external acceleration field of 8.4×10−12 m/s2 at 30∘ to the disk as a first order estimate for the gravity exerted by M31. This suppresses buildup of material at the centre, causing the RC to rise more slowly and reducing the central LOSVD. Overall, this simulation bears good resemblance to several global properties of M33, and highlights the importance of including even a weak external field on the stability and evolution of disk galaxies. Further simulations with a time-varying external field, modeling the full orbit of M33, will be needed to confirm its resemblance to observations.%We finally discuss a novel observational test using the warp induced by the external field in nearly edge-on galaxies.

It shows science at work. MOND models do well at galaxy rotation curves.

Regards Andrew 

 

I wasn't aware of MOND and I found that account really interesting - thanks for posting that Andrew.  As as you say a good example of science at work .    There is useful background introduction  to MOND on wikiipedia for anybody interested.  https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

Jim 

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11 minutes ago, saac said:

I wasn't aware of MOND and I found that account really interesting - thanks for posting that Andrew.  As as you say a good example of science at work .    There is useful background introduction  to MOND on wikiipedia for anybody interested.  https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

Jim 

While it does well with galaxy rotation (that's what it way aimed at) it does less well with clusters of galaxies and as the Wikipedia article notes it still needs dark matter to help it out in such cases. 

Regards Andrew 

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12 minutes ago, andrew s said:

While it does well with galaxy rotation (that's what it way aimed at) it does less well with clusters of galaxies and as the Wikipedia article notes it still needs dark matter to help it out in such cases. 

Regards Andrew 

It's intriguing in that respect. I can't think of any parallels of the top of my head but there must be other models in other areas of physics which only take us so far before breaking down (classical and quantum must be replete with examples).  I think it is  intuitive that the reach of a model must surely  be limited by the boundary conditions in which it was framed.  We run into problems then as our capability  allows for more accurate or rather greater resolution in our measurements then the more susceptible our models become to falling to the effects of unknowns.   I hope that makes sense - I think I know what Im talking about but I often confuse myself :) 

Jim 

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17 minutes ago, saac said:

I think it is  intuitive that the reach of a model must surely  be limited by the boundary conditions in which it was framed.

I'm sure this is not limited merely to physics.  But I bet it's quite often what has happened when you hear a scientist say: "Oh!  That's odd." :D

James

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5 minutes ago, JamesF said:

I'm sure this is not limited merely to physics.  But I bet it's quite often what has happened when you hear a scientist say: "Oh!  That's odd." :D

James

May be apocryphal but the story goes the discovery of the strange quark was exactly that   " that's strange " :) 

jim 

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Two example would be:

Galileo's dynamics subsumed as a low speed case of Special relativity.

Newton's gravity being a limiting case of General Relativity.

There was also the ideas Plank introduced to avoid the problem of black body radiation curve but I can't  recall the name of the original theory

 

That's strange or odd s the life blood of science @JamesF.

Regards Andrew 

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