## Dark Matter or Modified Gravity ?

What are your personal opinions ??, Do you guys think gravity is just not fully understood that is why we cant explain certain events or DM is the probable answer to these discrepancies ?

I personally dont think we need to create a somewhat imaginary matter (dark matter and dark energy) to explain what happens in some parts of space, i think we just dont really understand gravity enough and the current gravitational laws and theories are incomplete.

What do you guys think ?
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 Recognitions: Science Advisor It's not really a matter of opinion at this point. The mounds of evidence in support of dark matter far outweigh any proposed alternative gravity theory. In short, hypothesizing dark matter allows you to solve a bunch of problems at once, whereas with modified gravity you have to hypothesize additional things as well (including dark matter!).

 Quote by Nabeshin It's not really a matter of opinion at this point. The mounds of evidence in support of dark matter far outweigh any proposed alternative gravity theory. In short, hypothesizing dark matter allows you to solve a bunch of problems at once, whereas with modified gravity you have to hypothesize additional things as well (including dark matter!).
Well, if it's not a matter of opinion, then why are there still people defending modified gravity theories? :)

I think it's very simple: dark matter has to be directly observed before the modified gravity theories can be thrown into the bin. Very often one can be mislead by trying to evaluate probability in the ensemble of all possible physical theories as there is no objective measure there, only personal preferences. I would certainly agree that DM seems by far the more natural explanation, but then again, I don't know everything about nature yet...

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## Dark Matter or Modified Gravity ?

wilha,
Astronomy, Astrophysics, and Cosmology are not sciences built on personal opinions. As you must already know, we rely on the scientific method to arrive at our conclusions of how nature works. We ask a question, (as you have), research what’s already been learned, propose new hypotheses, and then test them. In these areas of science it’s difficult for laypersons to do the actual experiments so we rely on the professionals to devise and perform them. As for analyzing the data and reaching conclusions, most of us trust those same professional experts to perform those functions.

So how does our personal opinion fit in? If someone doesn’t like dark matter or dark energy, or thinks they are not necessary, that is not sufficient reason to discard them. Proposing your alternative hypothesis would seem a logical step to take. Do you have some alternatives to suggest?
Cheers,
Bobbywhy

 Quote by clamtrox Well, if it's not a matter of opinion, then why are there still people defending modified gravity theories? :)
For the same reason there are still people defending creationist theories and flat Earth theories. Some people either just don't get it, or else have another agenda.

 Quote by phyzguy For the same reason there are still people defending creationist theories and flat Earth theories. Some people either just don't get it, or else have another agenda.
Can you point me to a peer-reviewed research article on flat earth? I thought that was figured out a long time ago.

 Quote by Bobbywhy wilha, Astronomy, Astrophysics, and Cosmology are not sciences built on personal opinions. As you must already know, we rely on the scientific method to arrive at our conclusions of how nature works. We ask a question, (as you have), research what’s already been learned, propose new hypotheses, and then test them. In these areas of science it’s difficult for laypersons to do the actual experiments so we rely on the professionals to devise and perform them. As for analyzing the data and reaching conclusions, most of us trust those same professional experts to perform those functions. So how does our personal opinion fit in? If someone doesn’t like dark matter or dark energy, or thinks they are not necessary, that is not sufficient reason to discard them. Proposing your alternative hypothesis would seem a logical step to take. Do you have some alternatives to suggest? Cheers, Bobbywhy
Its not that I dont like DM, i have seen the scientific indirect evidence of it proposed existence. But i also came to the conclusion that our understanding of gravity is not complete, for example, Einstein's theory breaks apart in black holes and at the quantum level (i may be wrong). Im not saying he was wrong but rather his work is incomplete. Remember Newton, his theories worked for what was known at the time and it seemed absolutely correct not including Mercury's orbit until Einstein came about and changed that. At present times we have come to learn about quantum mechanics and Einstein's work may need some revision much how Newton's laws were revisited.

I dont have any specific alternative theory but do you understand where im coming from ?, and i know just because we cant detect DM doesn't mean its not there but also take an objective stance towards the gravity problem.

Like I had said before im not bashing on Einstein, but it seems to me that people are somewhat afraid to call Einstein out and challenge his ideas.

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 Quote by wilha Its not that I dont like DM, i have seen the scientific indirect evidence of it proposed existence. But i also came to the conclusion that our understanding of gravity is not complete, for example, Einstein's theory breaks apart in black holes and at the quantum level (i may be wrong). Im not saying he was wrong but rather his work is incomplete. Remember Newton, his theories worked for what was known at the time and it seemed absolutely correct not including Mercury's orbit until Einstein came about and changed that. At present times we have come to learn about quantum mechanics and Einstein's work may need some revision much how Newton's laws were revisited. I dont have any specific alternative theory but do you understand where im coming from ?, and i know just because we cant detect DM doesn't mean its not there but also take an objective stance towards the gravity problem. Like I had said before im not bashing on Einstein, but it seems to me that people are somewhat afraid to call Einstein out and challenge his ideas.
Surely Einstein's GR is not the full story as far as gravity is concerned; as you note, we have no quantum theory of gravity as of yet. However, the corrections from whatever QG theory you want to consider are almost certainly very small. QG is important roughly at the planck scale, $l_p \sim 10^{-35} {\rm m}$, while DM is a problem observed on roughly galactic length scales $l_g \sim 10^{5} ly \sim 10^{21} {\rm m}$. That's over fifty orders of magnitude of discrepancy, so it's very difficult to believe that quantum corrections are the cause of the DM phenomenon.

Instead, modified theories of gravity change its behavior in the low energy regime, precisely where the DM phenomenon is observed (see, for example, the most naive versions of MOND). These theories are a different game altogether, and as I said, a vanilla DM model seems to fit the data much better than any of these alternatives. That's not to say that they're wrong, or that work on them is completely useless, but simply that at present observations lend much more support to a DM-only model.

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 Quote by wilha Its not that I dont like DM, i have seen the scientific indirect evidence of it proposed existence. But i also came to the conclusion that our understanding of gravity is not complete, for example, Einstein's theory breaks apart in black holes and at the quantum level (i may be wrong). Im not saying he was wrong but rather his work is incomplete. Remember Newton, his theories worked for what was known at the time and it seemed absolutely correct not including Mercury's orbit until Einstein came about and changed that. At present times we have come to learn about quantum mechanics and Einstein's work may need some revision much how Newton's laws were revisited. I dont have any specific alternative theory but do you understand where im coming from ?, and i know just because we cant detect DM doesn't mean its not there but also take an objective stance towards the gravity problem. Like I had said before im not bashing on Einstein, but it seems to me that people are somewhat afraid to call Einstein out and challenge his ideas.
It isn't as people haven't tried to come up with such alternate theories of gravity, its that they can't be made to fit all the observational data.

One observation in particular is the Bullet cluster. It is the collision between two galaxy clusters. During the collision, interactions between "normal" visible matter slows the matter down so that it separates slower than it came together. DM, on the other hand doesn't suffer from these interaction and sails on unaffected. IOW, the DM should separate itself from the visible matter.

Now we cannot see DM directly, but with can see its gravity silhouette, by the gravitational lensing it causes. So what we would expect to see in such a situation is the visible matter and its gravity lensing and then a separate locus of gravitational lensing that has no visible matter associated with it, caused by the DM that has been "knocked loose" from the cluster in the collision.

This is exactly what we see in the Bullet cluster.

This can't be explained by a modified theory of gravity alone unless it incorporates DM.

 Quote by wilha I dont have any specific alternative theory but do you understand where im coming from ?
Yup.

Lot's of people have come up with specific alternative theories. There are two problems, dark matter and dark energy. Dark *matter* is thought not to be the result of alternative gravity, because what you do is to measure the "lumpiness" of the universe, and we've only be able to get the right amount of "lumpiness" with dark matter.

The other thing is that with gravitational lensing, we've been able to "map" dark matter.

Now *dark energy*. The field is wide open since modified gravity is as good as any other of the ideas.

Here is one paper that reviews them.

http://arxiv.org/pdf/0805.1726v4.pdf

Note that it has over 600 citations.

Here is another review paper with 277 citations.....

 i know just because we cant detect DM doesn't mean its not there but also take an objective stance towards the gravity problem.
Yup. One problem with popular science presentations is that they just mention the final result. They don't mention the hundreds of alternatives that were tried but didn't work.

The way that most alternative gravity models work is that the are f(R) models. If gravity worked very different at solar system distances, we'd know about it fast. So you come up with a model that works very much like GR at short distances, but are different at large distances.

 Like I had said before im not bashing on Einstein, but it seems to me that people are somewhat afraid to call Einstein out and challenge his ideas.
That's because popular accounts of science aren't very good at explaining the process of science. It's not that people worship Einstein, but rather that for dark matter we have good reasons for thinking that it's not modified gravity.

Now for dark energy.......
 Recognitions: Gold Member Science Advisor If you need some amount of non-baryonic dark matter to make MOND work, the fundamental motivation for MOND is fatally wounded.

 Quote by Janus It isn't as people haven't tried to come up with such alternate theories of gravity, its that they can't be made to fit all the observational data. One observation in particular is the Bullet cluster. It is the collision between two galaxy clusters. During the collision, interactions between "normal" visible matter slows the matter down so that it separates slower than it came together. DM, on the other hand doesn't suffer from these interaction and sails on unaffected. IOW, the DM should separate itself from the visible matter.
Why? Why should DM behave any differently under gravitation than normal matter? This is a huge weakness for DM: it's a "just so" type of theory that behaves just the way you want it to behave so that everything works out right. If the galaxy is spinning too fast, no problem: there's more DM (that we can't see). If the galaxy's spinning too slow, no problem: there's less DM (that we can't see). The whole thing verges far too close to untestability.

 Quote by Janus Now we cannot see DM directly, but with can see its gravity silhouette, by the gravitational lensing it causes. So what we would expect to see in such a situation is the visible matter and its gravity lensing and then a separate locus of gravitational lensing that has no visible matter associated with it, caused by the DM that has been "knocked loose" from the cluster in the collision. This is exactly what we see in the Bullet cluster. This can't be explained by a modified theory of gravity alone unless it incorporates DM.
Not true. Bullet Cluster type effects can be explained by several modified gravity theories. See Brownstein & Moffat 2006, and Angus et. al. 2006.

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 Quote by hipparchos Why? Why should DM behave any differently under gravitation than normal matter? This is a huge weakness for DM: it's a "just so" type of theory that behaves just the way you want it to behave so that everything works out right. If the galaxy is spinning too fast, no problem: there's more DM (that we can't see). If the galaxy's spinning too slow, no problem: there's less DM (that we can't see). The whole thing verges far too close to untestability.
DM does not interact electromagnetically. It does not "collide" with itself or normal matter, as such collisions are electromagnetic interactions.
The collisions that occur with normal matter also results in loss of kinetic energy which is radiated away as EM radiation. DM does not interact with or emit EM radiation, so this method of shedding energy is not available.

Thus normal matter is subject to both gravitational interaction and electromagnetic interaction, while DM is only subject to gravitational interaction. Ergo, DM would be expected to lose less KE in the collision of cluster.
 Back to your opening question. I for my part think a modified understanding of gravitation or the dynamics of the universe is needed. In my view neither modified gravity=MOND or DM seems really up to it. MOND is modifying the math to match observations (not adding insight to why) and LCDM adds mass to fit the math. Really not the same level of deep understanding on fundamental interactions like in GR. The quantum approach like loop gravity is not that ecxiting either and are as far as I know trying to reach the same level of explanation as GR which means we are still left with unknown/nondetected particles or unexplained mathematics. My hunch is that we need a theory with a more global approach incorporating what we now know on the overall structure of the universe. But thats just speculation and guessing of course;) Recently there has been an interesting thread in PF about missing DM in our own galaxy. which among others mentions Kroupa et al who challenges the LCDM-model from a MOND-perspective and claims that LCDM is ruled out by recent findings. I dont have enough postings so I cant link to the thread in PF or to their (Kroupas) blog (They discuss amongst other things the bullet cluster). The name of the thread is: Serious Blow to Dark Matter Theories? And the blog is called the-dark-matter-crisis on SciLogs.eu.
 Recognitions: Gold Member Science Advisor The MOND explanation for the bullet cluster is ... somewhat lacking. See "Modifying Gravity: You Can't Always Get What You Want" by Starkman http://arxiv.org/abs/1201.1697.
 Recognitions: Gold Member Science Advisor Pesi, I assume you are referring to the recent paper by Bidin, et al "No evidence of dark matter in the solar neighborhood" http://arxiv.org/abs/1204.3919. This finding was quickly contested by Bovy and Tremaine in "On the local dark matter density" http://arxiv.org/abs/1205.4033

 Quote by Pesj I dont have enough postings so I cant link to the thread in PF or to their (Kroupas) blog (They discuss amongst other things the bullet cluster).
I'll post it, because it's dynamite. Here's the blog.

And here's the discussed paper, which rules out a spherical DM halo in the Milky Way at the 4$\sigma$ confidence level, using dynamical observations of bright stars above and below our position on the galactic plane.

So I guess we're looking for DM that only attracts in one direction?

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