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Author Topic: Why is it called dark matter instead of dark gravity?  (Read 1499 times)

Offline IAMREALITY

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Hi all!

As is the case with many lovers of Physics, dark matter and dark energy fascinate me. I think about them often, trying to wrap my head around them.

But I'm curious about something with dark matter I'm hoping some of you can shed light on.  What makes the scientific community convinced it's matter at all? 

I understand the calculations, the speed of stars around galaxies, I understand why DM needed to be added etc.  And I want it known that none of the questions or theoretical things I'm asking are meant to come off as new theory etc.  I'm just asking the proceeding questions to get a real solid answer rooted in physics and fact, just to help me understand.  I just wanted to give that caveat.

But my question is having that said, understanding the calculations and why the extra gravity is apparent, why must it be a new form of undetectable matter that has no interaction electromagnetically to be the culprit?  Why isn't it called dark gravity instead, merely gravity that we know is present but we don't understand why?  Why must some mystery, as of yet undetected, made of different stuff than we can fathom particle be the solution?  I guess that part just confuses me.  Why the focus on this unimaginable particle instead of a focus on what other things might cause gravity itself to increase without the need for a mass to be present?

For example, we know that spacetime can curve, and that this curvature is directly related to gravity.  Is it not possible that other things we haven't yet thought of can also create this curvature?  I know for example there is strong belief (shown through the microwave background) that dark matter existed first and was the scaffolding that baryonic matter attached itself to in order to form galaxies etc.  We also know the energy that was released in the big bang, and the expansion that took place thereafter.  During this time when physics might not have even existed yet, couldn't that scaffolding have been something like the energy being so hot and dense, so different than anything we can relate to, that as the singularity expanded and spacetime came into being that these were just permanent curves in that spacetime brought on by that energy, almost like flaws, cracks, or another analogy, that kinda were always there and just expanded with spacetime from the start?  And then because of this inherent gravity, this permanent scarring or curvature, real atoms gravitated towards those areas and ultimately formed galaxies etc?

And again, I'm not putting forth a new theory, cause it's totally based on ignorance (my lack of knowing).  It's just I guess one of my mental examples that makes me want to not be as ignorant on the subject, to understand it more.  I guess the main question is why instead of being focused on gravity itself, what else might cause spacetime to curve, that could be attributed to what we see when we measure the speed of galaxies etc and find they don't have enough gravity from ordinary matter to keep them together, do we instead focus on a new form of matter altogether to be responsible?

So summed up, why dark matter instead of merely dark gravity?

And I ask with sincerity.  I'm fascinated with the concept and wanna learn more about it, so that I can think more deeply about it but rooted in as much fact as possible.

Thanks in advance!
« Last Edit: 08/07/2016 18:18:58 by IAMREALITY »


 

Offline dlorde

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Re: Why is it called dark matter instead of dark gravity?
« Reply #1 on: 09/07/2016 13:29:40 »
I think it's just that galaxies behave as if there is more matter in and around them than can be seen. There have been suggestions that it could be that gravity changes at cosmological scales (Modified Newtonian Dynamics or 'MOND'), but that has some theoretical difficulties, serious implications for current theories, and doesn't explain all observed anomalies, so the idea is, that the more obvious solution, i.e. that it is matter that we don't see, should be eliminated before more exotic hypotheses are seriously considered. Nevertheless, there is a good deal of work being done in parallel on other ideas, including MOND, to explain it.
 

Offline IAMREALITY

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Re: Why is it called dark matter instead of dark gravity?
« Reply #2 on: 09/07/2016 15:39:31 »
I think it's just that galaxies behave as if there is more matter in and around them than can be seen. There have been suggestions that it could be that gravity changes at cosmological scales (Modified Newtonian Dynamics or 'MOND'), but that has some theoretical difficulties, serious implications for current theories, and doesn't explain all observed anomalies, so the idea is, that the more obvious solution, i.e. that it is matter that we don't see, should be eliminated before more exotic hypotheses are seriously considered. Nevertheless, there is a good deal of work being done in parallel on other ideas, including MOND, to explain it.
Can you go into a bit more detail for me as to what those theoretical difficulties and serious implications are and also what abnormalities it doesn't explain? That might help me understand more.  I haven't heard those aspects.
 

Offline PhysBang

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Re: Why is it called dark matter instead of dark gravity?
« Reply #3 on: 09/07/2016 16:39:53 »
I think it's just that galaxies behave as if there is more matter in and around them than can be seen. There have been suggestions that it could be that gravity changes at cosmological scales (Modified Newtonian Dynamics or 'MOND'), but that has some theoretical difficulties, serious implications for current theories, and doesn't explain all observed anomalies, so the idea is, that the more obvious solution, i.e. that it is matter that we don't see, should be eliminated before more exotic hypotheses are seriously considered. Nevertheless, there is a good deal of work being done in parallel on other ideas, including MOND, to explain it.
Can you go into a bit more detail for me as to what those theoretical difficulties and serious implications are and also what abnormalities it doesn't explain? That might help me understand more.  I haven't heard those aspects.
One thing that MOND had a hard time doing was accounting for the proper amount of gravitational lensing that we can observe around galaxies and galaxy clusters. It was also difficult finding a relativistic version of MOND. Something called TeVeS did a decent job of doing both, for the most part.

However, it is not clear that a modified gravity can account for the agreement that different cosmological tests have shown in measuring the amount of mass in matter that is in matter that is not made of nuclear matter, i.e., in dark matter. It is also hard to see how an alternative gravity could account for the observed mass distribution around at least one pair of collided galaxies.
 

Offline IAMREALITY

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Re: Why is it called dark matter instead of dark gravity?
« Reply #4 on: 09/07/2016 18:10:06 »
Thanks.  I guess that makes sense to me.  I guess it just for some reason doesn't sit right with me that there's some mega abundant form of matter that isn't bound by EM at all, can't be seen, detected or found to interact in any way except for gravity. Just feels like we're missing something don't it?  What I love about physics is learning about the magic happening around us all the time, how everything is possible, how weird so much in reality is.  But even with that, something just feels wrong when it comes to dark matter.  Everything we know about particles and the fields that rule them would dictate that we should be able to interact with it somehow! Just blows my mind that with all we're capable of now that it eludes us as it does.  Just fascinates the heck out of me! But then so much about this universe does anyway.  I can't wait to see the news release about some ground breaking discovery about it the day it happens.  Can't wait for the blanks to be filled in a bit more with it.  And anyone here who has any more insight into it please share! I'd love to learn more about it and understand it more, and would also be interested in hearing what some of you might feel it is!
 

Offline PmbPhy

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Re: Why is it called dark matter instead of dark gravity?
« Reply #5 on: 10/07/2016 06:23:49 »
Quote from: IAMREALITY
But my question is having that said, understanding the calculations and why the extra gravity is apparent, why must it be a new form of undetectable matter that has no interaction electromagnetically to be the culprit?
Since the theory of gravity is accurate enough to be able to describe gravitational effects at that scale we know fairly well that it can't be the theory of gravity that is wrong. The galaxy rotation curves behave exactly like there is matter inside the galaxy causing the rotation curves to have the values that they do. Let it be known that cosmologists have ruled out the scenario of the theory of gravity being wrong.

Quote from: IAMREALITY
Why isn't it called dark gravity instead, merely gravity that we know is present but we don't understand why?
Because the term "dark matter" is the name given to the actual matter that is the source of gravity in galaxies. You can't call matter "dark gravity' because that's the name of a force, not the name of matter.

Quote from: IAMREALITY
  Why must some mystery, as of yet undetected, made of different stuff than we can fathom particle be the solution?
Because that's just the way it is. I.e. the galaxies behave as if there is matter present that doesn't reflect or give off light so  that we can't see it.

Quote from: IAMREALITY
  I guess that part just confuses me.  Why the focus on this unimaginable particle instead of a focus on what other things might cause gravity itself to increase without the need for a mass to be present?
Why do you think it should be something else? These cosmologists know what they're doing. They've already discounted every other scenario out because nothing other than matter fits the observations.

Quote from: IAMREALITY
For example, we know that spacetime can curve, and that this curvature is directly related to gravity.  Is it not possible that other things we haven't yet thought of can also create this curvature?
What you don't understand is that the term matter is defined to mean "the source of gravity" and that means all sources of gravity including stress, energy and momentum. By the way; Curved spacetime is nothing more than tidal gradients, i.e. tidal forces.
 

Offline IAMREALITY

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Re: Why is it called dark matter instead of dark gravity?
« Reply #6 on: 10/07/2016 09:06:35 »
Sorry Pete, but I didn't really find that to be helpful much at all, and it actually came off a bit condescending. 

What you seemed to fail to realize is that I wasn't asking because I was trying to imply they don't know what they're talking about, I asked the questions cause I merely didn't understand.  But you replied as if it was wrong to ask the questions, as if everybody is just supposed to 'know' the right answers.  You gave a lot of 'cause that's the way it is" and "don't ask questions cosmologists know what they're doing" fluff, but no actual educational detail, or at least very little. 

But I ask questions cause since I first saw it above the chalkboard in 5th grade, I've lived by the quote "Anybody can know.  The point is to understand".

That is why I ask questions with things that pique my interest.  To understand.  To learn.  I honestly would've rather seen you be a bit more helpful here.  But anyway, I found this blog post to be more in line to the type of reply I had been hoping for.  Was a good read. 
http://askanastronomer.org/bhc/2015/10/21/dark-matter/
 

Offline dlorde

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Re: Why is it called dark matter instead of dark gravity?
« Reply #7 on: 10/07/2016 14:38:00 »
Thanks.  I guess that makes sense to me.  I guess it just for some reason doesn't sit right with me that there's some mega abundant form of matter that isn't bound by EM at all, can't be seen, detected or found to interact in any way except for gravity. Just feels like we're missing something don't it?
Whether something 'sits right' with you isn't necessarily a reliable guide to reality; quantum mechanics is a prime example. But theory allows for a variety of particles that match the required characteristics, e.g. WIMPS (Weakly Interacting Massive Particles).

After all, there are known particles that are 'mega abundant' and hardly interact at all - neutrinos; but they have very low mass.
 

Offline IAMREALITY

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Re: Why is it called dark matter instead of dark gravity?
« Reply #8 on: 10/07/2016 16:03:12 »

Whether something 'sits right' with you isn't necessarily a reliable guide to reality; quantum mechanics is a prime example. But theory allows for a variety of particles that match the required characteristics, e.g. WIMPS (Weakly Interacting Massive Particles).

After all, there are known particles that are 'mega abundant' and hardly interact at all - neutrinos; but they have very low mass.
True, just cause something doesn't sit right doesn't mean it's wrong, especially when dealing with particle physics.  Of course I understand that.  But you'd be surprised how often something not sitting right is actually a realiable guide to reality.  Far more often than not.  And when we talk of neutrinos and the things that make up quantum mechanics, the difference is the direct evidence for their existence, as opposed to merely evidence inferred through something else.

And of course those studying DM know far more than I, and most are convinced it's some sort of new particle, yet most would also admit that when it really comes down to it, they kinda have really no idea what the heck DM is, and are confounded and fascinated by it as well.  That's what I love about it.  And I just can't help but wonder if in the end, when we finally get a firm grasp on it, that it'll end up being more than just a simple particle but instead possibly not only a particle, but some other discovery that redefines our understanding of the universe.  Some other new addition to physics, something about the way spacetime works, or gravity works, or how particles work, or something more than just the particle itself.  But I still look forward to when they nail it down more regardless, even if just a particle, cause the whole thing really does fascinate me.  It just boggles my mind that galaxies have like 5-10 times more DM than actually matter in about roughly the same amount of space.  Yet no DM stars, planets, or otherwise.  All that matter but no trace other than merely it's affect on gravity.  Just mind blowing.  But then that's what makes me fall in love with the universe anyway, and like you said, quantum physics, where pretty much every damn thing about it is equally mind blowing lol.  So yeah, I'm aware how crazy stuff is, and the real magic happening around us all the time, and how downright mindblowing it all is, so I do readily understand that whether mind blowing or not, that DM very well (and admittedly likely) will be exactly what they're thinkning it will be.  But it would also be amazing if it were something else entirely.

But my original ignorance in the OP was wondering why it couldn't be a clumping of spacetime, this weird created from the big bang type gravity, but I've since had that answered via the blog I found, and understand now why they know it wasn't due to anything like that.  And that was my biggest question.  I hadn't realized they had already ruled that out.  I thought the only evidence they had was merely the fact that galaxies had more gravity than they should, that all the evidence was merely gravity driven, and that's why I wondered why it couldn't be some weird twisting or whatever of spacetime that replicated gravity, that could be the culprit.  But now I understand why not.

One question you might know the answer to though, is how is it that there could be that much DM in the same general space as regular matter in galaxies, without there being actual objects formed from it?  Is it because regular objects like stars and planets in galaxies have so much space between them, but DM would be more equally spread out, thereby being able to 'weigh' that much more yet not have any need to be clumped into objects like stars and planets?  Is it roughly equally distributed throughout all that space?
 

Offline PmbPhy

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Re: Why is it called dark matter instead of dark gravity?
« Reply #9 on: 10/07/2016 16:13:54 »
Quote from: IAMREALITY
Sorry Pete, but I didn't really find that to be helpful much at all, and it actually came off a bit condescending. 
Actually the only thing that's condescending in this thread is this accusation that you made here.

The problem is that you misunderstood what I wrote. Please do me a favor. The next time you read something that I post and think that it's condescending please ask me what I meant by what I said because I can promise you that I never intentionally write something that's condescending. If it ever came across that way it'd be because of a miscommunication.  I'm here for the sole purpose to help people understand physics. People ask questions because they don't know something. Intentionally answering a question in a condescending fashion is the wrong way to post and sends a terrible message to the person you're trying to help. If you thought that I was being condescending then you read something into it that was never meant to be there. So please don't assume things like that, okay my friend?

Take this as an example; you said that nothing that I wrote was helpful. Recall when I wrote Let it be known that cosmologists have ruled out the scenario of the theory of gravity being wrong. By that I was referring to MOND. But I take it that this was not what you thought that I meant by my response, correct?

Quote from: IAMREALITY
What you seemed to fail to realize is that I wasn't asking because I was trying to imply they don't know what they're talking about, I asked the questions cause I merely didn't understand.
Yes. I knew that.

Quote from: IAMREALITY
  But you replied as if it was wrong to ask the questions, as if everybody is just supposed to 'know' the right answers.
That's not true at all. I never believe that someone shouldn't ask any question that they want to. In fact I encourage it. Please don't put words into my mouth. That'd be like me saying that if you don't like what I write then I won't answer your questions. While it fits my belief I'd (1) be wrong in assuming that you don't really want that and (2) it'd be both the wrong action to take even if I were correct in my assumption.

Quote from: IAMREALITY
  You gave a lot of 'cause that's the way it is" and "don't ask questions cosmologists know what they're doing" fluff, but no actual educational detail, or at least very little. 
For the most part I did no such thing. I never gave a "cause that's the way it is" answer. The only thing that even came close to it was my response to Why must some mystery, as of yet undetected, made of different stuff than we can fathom particle be the solution? and the appropriate answer to that actually is "cause that's the way it is" but in this case it's true.
« Last Edit: 10/07/2016 16:36:10 by PmbPhy »
 

Offline hamdani yusuf

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Re: Why is it called dark matter instead of dark gravity?
« Reply #10 on: 11/07/2016 02:21:37 »
I find this article is useful and simple enough to understand for most people.

https://www.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect24/lecture24.html
 

Offline hamdani yusuf

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Re: Why is it called dark matter instead of dark gravity?
« Reply #11 on: 13/07/2016 07:16:53 »
Due to rotational speed of stars in galaxies, dark matter explanation suggests that galaxies contain much more mass than currently visible, which is called dark matter and distributed in dark halo.
How does dark matter affect the movement of planets in solar systems?
 

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Re: Why is it called dark matter instead of dark gravity?
« Reply #11 on: 13/07/2016 07:16:53 »

 

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