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Dark matter - fudge factor?


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I have read read about MOND before. As I understand it It is criticized for being curve fitting measurements rater than a theory. 

I guess the OP needs to take some time to digest the information that is linked to in tis thread 🙂

 

On 25/11/2020 at 18:15, saac said:

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? 

It just sounds like a fudge factor made up to fit nicely in the equations. I'm probably wrong though. 

No I can't tell you where the course of research should be, and if I did nobody would care anyway 😜

 

On 25/11/2020 at 01:27, wulfrun said:

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 this is very true. 

 

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32 minutes ago, Viktiste said:

IIt just sounds like a fudge factor made up to fit nicely in the equations. I'm probably wrong though. 

No I can't tell you where the course of research should be, and if I did nobody would care anyway 😜

 

I think if it is a "fudge factor" then it is a so with a fair amount of weighted and reasoned argument behind it .  In any respect, surely  most of science has progressed  at some point on the basis of a fudge or assumption -  albeit educated ones.   Hasn't it (dark matter ) only really been put forward as a candidate for what may be causing the departure from predictions?  I don't see it as any different from say the work done by Peter Higgs who saw a gap in a pattern and theorized the existence of the Higgs field to fill that gap. The challenge was picked up by the experimentalists to search for it.  It turned out he was right but he may have been wrong and something else would have been found to be the cause of the gap.  Im sure the same process will play out with Dark Matter it will either be confirmed (if within our ability to detect ) or some other candidate will emerge.  Either way our understanding will progress.  If it is found to be a reality I do hope they rename the thing straight away  call it a negative expansion flux , index or accelerant or whatever  - too much woo woo connotations with dark matter and dark energy :) 

 

Other than the MOND posted up by Andrew I'm nowhere near knowledgeable on other current alternative dark matter  candidates but they will no doubt exist.  Would be good to hear what if any are gaining traction. 

 

Jim

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10 hours ago, saac said:

Would be good to hear what if any are gaining traction. 

Jim

The simple answer is no. The difficulty rivals face is that in direct tests of GR both locally of the scale of mm to astronomical distances it does exquisitely well.

This means for rivals that "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." This is a tough challenge and tends to leave them trying to do away with Dark Matter and Dark Energy.

However, "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..." This tends to results in comparing simulations of various cosmological events, galaxy formation or cluster collisions etc and comparing them with what is observed. Such comparisons are rarely definitive.

The quotes are from the review paper I linked to before. Since this review gravitational waves were detected with the form predicted by GR and this ruled out a class of rivals.

Regards Andrew

PS @saac here is the paper again  Modified gravity and cosmology . Just scan it and ignore the maths it is interesting and a lot can be got from it by just reading the intelligible bits!

 

Edited by andrew s
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I think the problem is that people talk in what really amounts to an out of date idea. Basically Gravity.

Einstein in effect said that the material of the universe is not "flat" as required by Newton, that Mass distorted the shape of the universe and movement was not by a force of gravity but because of the change in shape of the universe local to the mass. So No force of Gravity.

Ever attended a talk - RAS? Everything refers to The Force of Gravity. Which in many aspects is out of date by around 100+ years and lets say "innacurate". But it is resolutely stuck to. Using GR explain Star formation without "gravity". Actually fairly easy using a 2D analogy.  Would possibly explain why the rate of star formation is not as predicted and that is likely because predictions are based on a force of gravity.

I would say that it is to an extent the reluctance to "move on" that is the main problem. Newtonian Mechanics is good and in most cases more then adaquate, but it is not 100% accurate. Suppose starting a presentation with: "Right then, Newton got it wrong." Is almost considered blasphemy in many areas

Expect Dark Matter needs input from the partical physics side, which seems somewhat absent. Don't think that from the given fundimental building blocks we have that we could construct an almost inert passive new partical - Dark Matter. Again is this a reluctance to say we were a bit wrong and need to update or modify our old ideas. I did once ask: Do we need an extra Quark? A sort of "Inert Quark". That created a look of horror.

Models of dark matter cause amusement, for most models you need an input of Mass. No one will give you an answer to: What is the approximate Mass? Even "What Mass did you use in this model?".

UCLAN at Astrofest did say Neutrino level, others suggest Super Massive Particals and everything in between.

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The next time I drop my lump hammer on my foot I shall console myself that the blistering pain  ringing through my very core is not the result of a force rather my foot preventing the hammer continuing on it' geodesic line of travel :)  I await the "happiest thoughts of my life"  moment as Einstein commented as the pain slowly subsides - it never comes  -  gravity remain a stubborn force!

It's a hard idea to shake , it's embedded in our experience, deeply rooted from childhood onward. Described in one of the books I'm reading, The Ascent Of Gravity by Marcus Chown,  " our only real and tangible everyday interaction with the fundamental forces" - electromagnetic competing equally one could argue but point taken . Only until the very senior stage of school physics at say A level, Adv Higher and the Int baccalaureate does GR feature in any meaningful way (still only introductory and very much qualitative ). Only then does the geometric nature of gravity reveal itself. So to the majority of school leavers and general public gravity remains the familiar force of the lump hammer dropped on a foot!  In the professional realm  engineers (my own background) have no need or care to see gravity other than the agent of force (weight)  as they strive to keep aircraft in the air and bridges standing.   The thing is, the force nature of gravity is not redundant and although not a complete description, depending upon the application at hand it is perfectly fitting. I'm not one hundred percent certain of this but I would be surprised if in any of the orbital mechanics mathematics of the Apollo programme reflected anything but Newtonian mechanics, where gravity prevailed as a force of attraction.  I guess much as we talk about the duality of light (particle and wave nature) we could play fast and loose and think about the duality of gravity - the model chosen to fit the application.   I bet the the cosmologists working on dark matter theories unlike us armchair warriors know exactly when it is appropriate to consider either nature of gravity though.  What I will say is that this forum  is a fantastic resource; I've certainly benefited from reading many of the really informed posts from contributors like Andrew S, Vlaiv, and Georges Jones to name only a few.  I often find I benefit from pre reading on here before trying to make sense of the more academic publications where the maths can be impenetrable.  This really is a great community in that respect. 

 

Regarding need for another quark - there was news of a new penta quark  last year, don't think it was necessarily hinting anything towards gravity/dark matter though though.   Now there is a thing - all school books need to be revised!

https://home.cern/news/news/physics/lhcb-experiment-discovers-new-pentaquark

 

Jim 

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8 minutes ago, saac said:

The next time I drop my lump hammer on my foot I shall console myself that the blistering pain  ringing through my very core is not the result of a force rather my foot preventing the hammer continuing on it' geodesic line of travel :)  I await the "happiest thoughts of my life"  moment as Einstein commented as the pain slowly subsides - it never comes  -  gravity remain a stubborn force!

Next time you drop a hammer on your foot just be grateful that the "Force of Gravity" is so weak 😂

Dave

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1 minute ago, Davey-T said:

Next time you drop a hammer on your foot just be grateful that the "Force of Gravity" is so weak 😂

Dave

Don't know why everyone keeps blaming gravity for this, when it clearly states that both hammer and feather fall with same acceleration.

Remember that next time you drop feather on your foot :D

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2 minutes ago, vlaiv said:

Don't know why everyone keeps blaming gravity for this, when it clearly states that both hammer and feather fall with same acceleration.

Remember that next time you drop feather on your foot :D

I knew I was doing something wrong  vlaiv :)   Alexa - set reminder - "remember to replace lump hammers with ostrich feathers ":) 

Jim 

 

 

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Is equivalence of gravitational and inertial mass one of those things that any substitute theory of gravity needs to explain?

As far as I understand, equivalence stems from the fact that there are no two different things - there is just inertial mass. Other part is down to energy / mass equivalence (one that bends the space - it is energy that bends the space).

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21 minutes ago, vlaiv said:

Is equivalence of gravitational and inertial mass one of those things that any substitute theory of gravity needs to explain?

As far as I understand, equivalence stems from the fact that there are no two different things - there is just inertial mass. Other part is down to energy / mass equivalence (one that bends the space - it is energy that bends the space).

There are 3 levels of equivalence principle:

Weak Equivalence Principle (WEP): All uncharged, freely falling test particles follow the same trajectories, once an initial position and velocity have been prescribed.

Einstein Equivalence Principle (EEP): The WEP is valid, and furthermore in all freely falling frames one recovers (locally, and up to tidal gravitational forces) the same laws of special relativistic physics, independent of position or velocity.

Strong Equivalence Principle (SEP): The WEP is valid for massive gravitating objects as well as test particles, and in all freely falling frames one recovers (locally, and up to tidal gravitational forces) the same special relativistic physics, independent of position or velocity.

but yes it needs to explain the equivalence.

From the paper linked to above.

36 minutes ago, Davey-T said:

Next time you drop a hammer on your foot just be grateful that the "Force of Gravity" is so weak 😂

Dave

Stop blaming the hammer it's your foot that is applying a proper acceleration (deceleration ) force to the hammer.

Regards Andrew

Edited by andrew s
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40 minutes ago, saac said:

....  The thing is, the force nature of gravity is not redundant and although not a complete description, depending upon the application at hand it is perfectly fitting. I'm not one hundred percent certain of this but I would be surprised if in any of the orbital mechanics mathematics of the Apollo programme reflected anything but Newtonian mechanics, where gravity prevailed as a force of attraction. ...

 

Jim 

I think this is correct, there is no need to do anything more complex than Newtonian physics for the majority of space exploration. GR needs to be accounted for in things like SatNav and comms satellites.

I think gravity persists simply because it's convenient to think of it that way and it's adequate for a great deal of processing, predictions and so on. That's also true for other areas of science, it's simpler to think of things in certain ways even when we know it isn't actually true. Atomic orbitals anyone? Electricity a bit like water in plumbing?

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26 minutes ago, andrew s said:

 

Stop blaming the hammer it's your foot that is applying a proper acceleration (deceleration ) force to the hammer.

Regards Andrew

So many ways to think about what caused the pain - and there's me , just thinking about the pain. I'm missing out on so much of the experience 😜

Jim

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Today, at 1 pm local (9 pm BT), I will attend the following Zoom talk. Normally, the talk would be in-person, and I and other folks would get to talk with the speaker over a meal. Because of COVID, the talk is virtual. 🙁

 

"Title:  Searching for Dark Forces in the Cosmos

 Abstract:  New fundamental forces can play a crucial role in models of dark matter and they are predicted by many theories of particle physics beyond the Standard Model. Such new forces are said to be "dark" if they interact only very weakly with ordinary matter. They could take many forms, with specific examples being similar to the electromagnetic or strong forces, and in some cases they can even be the source of dark matter themselves. In this talk I will describe how such forces might arise and what they can do in the early universe. In particular, I will show how detailed measurements of primordial element abundances and the cosmic microwave background radiation can be used to search for them."

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3 hours ago, andrew s said:

@George Jones please give us an update on the talk and your take on it.

Interested to here the  speakers view on why the LHC has not found any evidence for particles beyond the standard model . Other than the standard " they are beyond its energy range".

Regards Andrew 

The speaker is one of the authors of the recent paper

https://arxiv.org/abs/2003.02273

so I suspect the talk will be an upper-level undergrad version of this. He might not mention the LHC.

 

 

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I meant to get to this earlier, but I have been doing a COVID-related overload, and things have been somewhat hectic.

The talk included theoretical results of the paper to which I linked in my previous post. The authors of the paper take the view that gravity is okay, and that new unseen and transparent dark matter is needed to account for the motions of stars in galaxies, and galaxies in clusters of galaxies. Observations of the relative abundances of primordial elements, and of the Cosmic Microwave Background (CMB), indicate that dark matter is not made of the particles that account for normal matter's mass, protons and neutrons.

To date, we only have evidence that dark matter interacts via gravity, but many physicist think/hope that dark matter interacts via other (quantum) forces. They think that dark matter has quantum interactions, because this would give scenarios for dark matter production in the early universe similar to the production of normal matter, e.g., matter antimatter annihilation/creation that continues until the universe expands enough to stop these processes. Physicists hope that dark matter interacts with normal matter, as this gives ways for experimentally seeing signatures of dark matter.

If there are other interactions (besides gravity) involving dark matter interactions, they could be between: 1) dark matter and dark matter (just as there are interactions between normal matter and normal matter), and/or 2) dark matter and normal matter.

The very interesting talk was about theoretical models that have both 1) and 2), where 1) is used to generate the masses of dark matter particles, and 2) is used to predict observable effects of the models.

Two scenarios were considered for generation dark matter mass. One was a possible a dark electromagnetic-like interaction between dark matter and dark matter. The analogy is not exact, as, unlike normal photons, the dark photons have mass that they acquire from a proposed dark Higgs-like particle. Another possibility is that there is a dark colour-like force. In the normal colour nucleon force, gluons carry the colour force, and thus gluons can interact with gluons. In the proposed dark colour-like interactions, dark gluons interact to form massive dark glueballls.

In both types of models, it is proposed that there is a "feeble" interaction between dark matter and normal matter. This feeble interaction can inject energy into the normal universe. If his injection is early enough, it can affect the relative primordial abundances of elements. If this energy injection is later, it can affect the the thermal spectrum of the CMB. If this injection is later still, it can affect the anisotropy spectrum of the CMB. High precision cosmological measurements could reveal these effects.

Possible results for the LHC and for high precision cosmological measurements probe complementary interaction strengths. The cosmological results are for weaker interactions.

 

 

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I agree with the original poster. There are things we may not understand for another 1000 years.  I also think it’s quite an arrogant assumption to say that the same universal laws apply everywhere, despite current observations. Maybe some do and others don’t. We may find eventually that this is incorrect.

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27 minutes ago, kirkster501 said:

I agree with the original poster. There are things we may not understand for another 1000 years.  I also think it’s quite an arrogant assumption to say that the same universal laws apply everywhere, despite current observations. Maybe some do and others don’t. We may find eventually that this is incorrect.

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 not just an assumption, it has been tested quite extensively. By observing patterns of stellar evolution through spectroscopy, and study of supernovae, we can quite confidently state that the laws of nature hold over tens, even hundreds of millions of lightyears. Stellar evolution depends on all four forces, and any small deviation in any of them would have profound implications for stellar evolution. The tiniest changes would changes things like spectral lines of the elements, the energy yield of fusion, the gravitational pressure in the centre of stars as a function of stellar mass, or the mass at which white dwarfs get fed up and explode.

As for me, as of November 1, I am joining the hunt for particles that might explain dark matter, by helping in the development of new algorithms for detection of gamma ray events using the Cherenkov Telescope Array (under construction).

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1 hour ago, kirkster501 said:

I agree with the original poster. There are things we may not understand for another 1000 years.  I also think it’s quite an arrogant assumption to say that the same universal laws apply everywhere, despite current observations. Maybe some do and others don’t. We may find eventually that this is incorrect.

I don't think scientists assume the universality of laws. They may postulate the idea but they then test it by observation.  I am not sure where it's up to but an experiment to see if the fine structure constant varies with time is planned / underway. 

As we get new instruments they do new test e.g. the arrival time of gravitational  and electromagnetic waves to test the universality of the speed of "light".

Regards Andrew 

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21 hours ago, andrew s said:

we desperately need more data to constrain the speculation.

I agree., and over the next 10 years or so, we will accumulate a lot more data from the LHC, from experiments that attempt direct detection of dark matter, and from other observations. If, at the end of 10 years, we are still in a dark matter muddle, I think we will be in crisis mode. Other physicist would disagree, and would echo SuperTramp. Crisis? What Crisis?

Experimental data has already killed off substantial portions of "theory space" for dark matter. Theory space, however, is an expanding universe.

Edited by George Jones
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45 minutes ago, George Jones said:

I agree., and over the next 10 years or so, we will accumulate a lot more data from the LHC, from experiments that attempt direct detection of dark matter, and from other observations. If, at the end of 10 years, we are still in a dark matter muddle, I think we will be in crisis mode. Other physicist would disagree, and would echo SuperTramp. Crisis? What Crisis?

Experimental data has already killed off substantial portions of "theory space" for dark matter. Theory space, however, is an expanding universe.

Just hope I am still here to see it reach a conclusion. Regards Andrew

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Me too!

Even if it does reach a conclusion, we might not like the conclusion. Paraphrasing  Planck: "Science advances one funeral at a time."

Actual version: "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it."

Edited by George Jones
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