• This is a political forum that is non-biased/non-partisan and treats every person's position on topics equally. This debate forum is not aligned to any political party. In today's politics, many ideas are split between and even within all the political parties. Often we find ourselves agreeing on one platform but some topics break our mold. We are here to discuss them in a civil political debate. If this is your first visit to our political forums, be sure to check out the RULES. Registering for debate politics is necessary before posting. Register today to participate - it's free!

Rotating Galaxies Could Prove Dark Matter Wrong

Fallenangel

DP Veteran
Joined
Mar 24, 2013
Messages
3,416
Reaction score
1,099
Location
Israel/Russia/UK
Gender
Male
Political Leaning
Undisclosed
M33_rotation_curve_HI.gif




Rotating Galaxies Could Prove Dark Matter Wrong - Forbes.com


"The researchers looked at the observed rotation curves for 153 galaxies, and calculated the radial acceleration at various distances in each galaxy. They then compared these results to the gravitational acceleration as predicted by the distribution of visible matter within a galaxy (technically the distribution of baryonic mass). They found a strong correlation between the two. When the gravitational acceleration was stronger, so was the radial acceleration, and when one was weaker, so was the other. What’s interesting is that this relation holds up in a range of galaxies. It didn’t matter whether most of the visible matter was clustered in the center or not, the relation still held. It’s also a purely empirical correlation, so there is no strong theoretical component to make it work.

So what gives? The researchers propose three possibilities. The first is that the correlation could be due to the dynamics of galaxy formation. It’s not clear how this would occur, but there are aspects of galactic evolution we don’t fully understand. The second is that the distribution of dark matter and baryonic matter within a galaxy are correlated. This would require some new kind of dark matter physics that makes dark matter clump in the same way that regular matter does. The third and perhaps most intriguing idea is that it really is due to some kind of modified dynamics.
"


The actual article on arXiv: https://arxiv.org/abs/1609.05917 (click on the pdf in the link)

Fallen.
 

Tim the plumber

DP Veteran
Joined
Jan 3, 2014
Messages
16,501
Reaction score
3,821
Location
Sheffield
Gender
Male
Political Leaning
Other
I thought we already knew that dark matter was associated with galaxies. That the rotation speed of them and the level of gravitational lensing said that there must be more mass than was accounted for by normal matter and this was why the idea of dark matter is around, the reason we have it.

What's new?
 
Joined
Aug 9, 2016
Messages
162
Reaction score
9
Gender
Undisclosed
Political Leaning
Undisclosed
Wake up people!

Space is all one gigantic hoax created by George Lucas and gene Rodenberry
 

Greenbeard

DP Veteran
Joined
Aug 10, 2013
Messages
16,360
Reaction score
15,190
Location
Cambridge, MA
Gender
Male
Political Leaning
Slightly Liberal
When I was an undergrad I was fascinated by MOND. Not the idea that Newtonian physics worked differently at low accelerations per se, but the idea that inertia itself worked differently at low accelerations. Ties into a lot of bigger cosmological questions about how interconnected distant objects really are. Anyway, interesting finding.
 

Fallenangel

DP Veteran
Joined
Mar 24, 2013
Messages
3,416
Reaction score
1,099
Location
Israel/Russia/UK
Gender
Male
Political Leaning
Undisclosed
I thought we already knew that dark matter was associated with galaxies. That the rotation speed of them and the level of gravitational lensing said that there must be more mass than was accounted for by normal matter and this was why the idea of dark matter is around, the reason we have it.

What's new?


The thing is that despite our best efforts, so far we haven't been able to detect Weakly Interacting Massive Particles (WIMPs), which until recently were considered as the most fitting candidates for the role of Cold Dark Matter (CDM). There were some claims from DAMA collaboration about supposed detection, but as far as I am aware these claims weren't confirmed by other researchers. In fact we are quickly reaching the limit at which we will need to update our theories about WIMP Dark Matter, hence there is currently a lot of buzz about other potential exotic candidates for the role of CDM.
The moment of truth for WIMP Dark Matter (Bertone 2010)

There is a lot of observational, experimental and theoretical research done these days in an attempt to understand why our Dark Matter theories work quite well in theory but so far found little support from astroparticle data. Recently people started playing with other versions of Dark Matter models with a renewed interest (Warm Dark Matter, Fuzzy Dark Matter), as these models might provide an answer for the lack of Dark Matter detection signs. Alternatively, we might need to modify our theories of gravity, of course as long as we can reconcile them with observational data.



Fallen.
 

Tim the plumber

DP Veteran
Joined
Jan 3, 2014
Messages
16,501
Reaction score
3,821
Location
Sheffield
Gender
Male
Political Leaning
Other
The thing is that despite our best efforts, so far we haven't been able to detect Weakly Interacting Massive Particles (WIMPs), which until recently were considered as the most fitting candidates for the role of Cold Dark Matter (CDM). There were some claims from DAMA collaboration about supposed detection, but as far as I am aware these claims weren't confirmed by other researchers. In fact we are quickly reaching the limit at which we will need to update our theories about WIMP Dark Matter, hence there is currently a lot of buzz about other potential exotic candidates for the role of CDM.
The moment of truth for WIMP Dark Matter (Bertone 2010)

There is a lot of observational, experimental and theoretical research done these days in an attempt to understand why our Dark Matter theories work quite well in theory but so far found little support from astroparticle data. Recently people started playing with other versions of Dark Matter models with a renewed interest (Warm Dark Matter, Fuzzy Dark Matter), as these models might provide an answer for the lack of Dark Matter detection signs. Alternatively, we might need to modify our theories of gravity, of course as long as we can reconcile them with observational data.

Again; dark matter, we know it's out there, what the hell it is is anybodies guess.

What's new?
 

Tim the plumber

DP Veteran
Joined
Jan 3, 2014
Messages
16,501
Reaction score
3,821
Location
Sheffield
Gender
Male
Political Leaning
Other
When I was an undergrad I was fascinated by MOND. Not the idea that Newtonian physics worked differently at low accelerations per se, but the idea that inertia itself worked differently at low accelerations. Ties into a lot of bigger cosmological questions about how interconnected distant objects really are. Anyway, interesting finding.

Eh???

Are you saying that there is some sort of evidence that F=ma does not apply to low accelerations??? News to me!
 

Greenbeard

DP Veteran
Joined
Aug 10, 2013
Messages
16,360
Reaction score
15,190
Location
Cambridge, MA
Gender
Male
Political Leaning
Slightly Liberal
Eh???

Are you saying that there is some sort of evidence that F=ma does not apply to low accelerations??? News to me!

Well, yes. The need for the dark matter hypothesis in the first place could be taken as evidence for that (which is the concept behind MOND, Modified Newtonian Dynamics).

We've known since the '30s that stars orbiting further out from the center of galaxies aren't moving in the way Newtonian physics would predict--they're orbiting too fast. When you get that kind of discrepancy between observation and known physics, you can do one of two things: 1) get creative about how to make your observations work in the context of known physics, or 2) come up with new physics.

This is a problem that came up with observations of planetary orbits in the 19th century. Anomalies in the observed orbit of Uranus led not to abandoning Newtonian physics but instead to using Newtonian physics to predict an unseen planet was responsible; Neptune was discovered not long after where the mathematicians predicted it would be based on known physics. Similarly, anomalies in Mercury's orbit led to the prediction of a planet even closer to the sun than Mercury--the mystery planet was dubbed Vulcan but no one could ever find it. As it turned out, explaining the anomalies in Mercury's orbit required new physics.

If you assume we've got the physics right, then we have to figure out why galaxies act as though there's significantly more mass in them than we see. The answer to the missing mass problem is then that there's some unseen--dark--matter out there that likely doesn't interact electromagnetically at all.

Or you could start by not assuming we've got the physics quite right, and instead work on modifying the equations that describe how masses move. That's what MOND does. But broadly speaking there are two ways to go about doing that: either by thinking of the change as to the laws governing gravity at low accelerations (this is where most of the work has focused) or as a change to inertia itself at low accelerations (this one is less developed). Both are thought-provoking, but the latter gets into some very interesting territory.

Anyway, the article in the OP is taking the new observations as a point in favor of MOND-like thinking and a strike against the dark matter hypothesis.
 

FieldTheorist

DP Veteran
Joined
Dec 5, 2015
Messages
3,325
Reaction score
2,348
Gender
Male
Political Leaning
Libertarian - Left
M33_rotation_curve_HI.gif




Rotating Galaxies Could Prove Dark Matter Wrong - Forbes.com


"The researchers looked at the observed rotation curves for 153 galaxies, and calculated the radial acceleration at various distances in each galaxy. They then compared these results to the gravitational acceleration as predicted by the distribution of visible matter within a galaxy (technically the distribution of baryonic mass). They found a strong correlation between the two. When the gravitational acceleration was stronger, so was the radial acceleration, and when one was weaker, so was the other. What’s interesting is that this relation holds up in a range of galaxies. It didn’t matter whether most of the visible matter was clustered in the center or not, the relation still held. It’s also a purely empirical correlation, so there is no strong theoretical component to make it work.

So what gives? The researchers propose three possibilities. The first is that the correlation could be due to the dynamics of galaxy formation. It’s not clear how this would occur, but there are aspects of galactic evolution we don’t fully understand. The second is that the distribution of dark matter and baryonic matter within a galaxy are correlated. This would require some new kind of dark matter physics that makes dark matter clump in the same way that regular matter does. The third and perhaps most intriguing idea is that it really is due to some kind of modified dynamics.
"


The actual article on arXiv: https://arxiv.org/abs/1609.05917 (click on the pdf in the link)

Fallen.

Not having read the paper, I would say:

This data doesn't really change or answer the problems associated to other observed phenomena, such as the Bullet Cluster, weak lensing measurements, growth of structure modeling, and especially the acoustic peaks in the CMB.

They discuss MOND as being a possible way as a way to bring all of the data into agreement (that means, "we're just guessing what years of research might bring about," so take that with an immense grain of salt); however, MOND has massive theoretical problems. It's exceedingly difficult to envision placing it into a consistent model in accordance with relativity and quantum mechanics. It's in principle possible, but not at all obvious.
 

Northern Light

The Light of Truth
DP Veteran
Joined
Jan 7, 2013
Messages
7,260
Reaction score
4,461
Gender
Undisclosed
Political Leaning
Undisclosed
Dark matter theory just seems like a way to plug an obvious whole in astrophysics. It'll be nice when they can come up with something better.
 

Tim the plumber

DP Veteran
Joined
Jan 3, 2014
Messages
16,501
Reaction score
3,821
Location
Sheffield
Gender
Male
Political Leaning
Other
Well, yes. The need for the dark matter hypothesis in the first place could be taken as evidence for that (which is the concept behind MOND, Modified Newtonian Dynamics).

We've known since the '30s that stars orbiting further out from the center of galaxies aren't moving in the way Newtonian physics would predict--they're orbiting too fast. When you get that kind of discrepancy between observation and known physics, you can do one of two things: 1) get creative about how to make your observations work in the context of known physics, or 2) come up with new physics.

This is a problem that came up with observations of planetary orbits in the 19th century. Anomalies in the observed orbit of Uranus led not to abandoning Newtonian physics but instead to using Newtonian physics to predict an unseen planet was responsible; Neptune was discovered not long after where the mathematicians predicted it would be based on known physics. Similarly, anomalies in Mercury's orbit led to the prediction of a planet even closer to the sun than Mercury--the mystery planet was dubbed Vulcan but no one could ever find it. As it turned out, explaining the anomalies in Mercury's orbit required new physics.

If you assume we've got the physics right, then we have to figure out why galaxies act as though there's significantly more mass in them than we see. The answer to the missing mass problem is then that there's some unseen--dark--matter out there that likely doesn't interact electromagnetically at all.

Or you could start by not assuming we've got the physics quite right, and instead work on modifying the equations that describe how masses move. That's what MOND does. But broadly speaking there are two ways to go about doing that: either by thinking of the change as to the laws governing gravity at low accelerations (this is where most of the work has focused) or as a change to inertia itself at low accelerations (this one is less developed). Both are thought-provoking, but the latter gets into some very interesting territory.

Anyway, the article in the OP is taking the new observations as a point in favor of MOND-like thinking and a strike against the dark matter hypothesis.

OK. Well written but still at the we don't know stage.

Gven that the gravity density (I saw it on the sky at night TV program...) varies closely but not directly in cinq with the galaxies just making F=ma not work for low accelerations will not do enough.....
 

Deuce

Outer space potato man
DP Veteran
Joined
Feb 6, 2010
Messages
89,339
Reaction score
42,495
Gender
Male
Political Leaning
Undisclosed
Dark matter theory just seems like a way to plug an obvious whole in astrophysics. It'll be nice when they can come up with something better.

Kind of. It's not so much a plug for the hole as it is a label for the hole.
 

Fallenangel

DP Veteran
Joined
Mar 24, 2013
Messages
3,416
Reaction score
1,099
Location
Israel/Russia/UK
Gender
Male
Political Leaning
Undisclosed
Again; dark matter, we know it's out there, what the hell it is is anybodies guess.

What's new?

Well, first of all we don't know that it's out there. We observe several gravitational effects which can be potentially explained by invoking dark matter, yet, so far we have failed to detect it.

They report on an interesting correlation between the detected radial acceleration and the acceleration predicted by the observed distribution of normal matter (baryons) in those galaxies. As per the paper these new observations can be interpreted in several ways:
1. "It represents the end product of galaxy formation."

2. "It represents new dark sector physics that leads to the observed coupling" - our current understanding of Dark Matter theories and its distribution in galaxies needs to be updated.

3. "It is the result of new dynamical laws rather than dark matter" - this will imply that we need to consider things like MOND rather than invoking dark matter, as long as we will be able to reconcile these new laws with other observational results.


Fallen.
 
Last edited:

Fallenangel

DP Veteran
Joined
Mar 24, 2013
Messages
3,416
Reaction score
1,099
Location
Israel/Russia/UK
Gender
Male
Political Leaning
Undisclosed
Not having read the paper, I would say:

This data doesn't really change or answer the problems associated to other observed phenomena, such as the Bullet Cluster, weak lensing measurements, growth of structure modeling, and especially the acoustic peaks in the CMB.

They discuss MOND as being a possible way as a way to bring all of the data into agreement (that means, "we're just guessing what years of research might bring about," so take that with an immense grain of salt); however, MOND has massive theoretical problems. It's exceedingly difficult to envision placing it into a consistent model in accordance with relativity and quantum mechanics. It's in principle possible, but not at all obvious.

If I knew how to reconcile these data with existing models and observations I probably would've been working on my own paper right now rather than writing stuff on DP. :) However, as I've mentioned earlier we have quite a few issues with existing CDM theories and numerical models, hence I personally always welcome any new data that might help us to explain why detection experiments have so far failed to provide any meaningful results.

The answer might be that the DM halo profiles are much more complex than what we currently assume them to be, especially as only recently we have started to model the effects of baryonic physics on these halos and to consider other DM versions in cosmological models.

Fallen.
 
Last edited:

FieldTheorist

DP Veteran
Joined
Dec 5, 2015
Messages
3,325
Reaction score
2,348
Gender
Male
Political Leaning
Libertarian - Left
If I knew how to reconcile these data with existing models and observations I probably would've been working on my own paper right now rather than writing stuff on DP. :) However, as I've mentioned earlier we have quite a few issues with existing CDM theories and numerical models, hence I personally always welcome any new data that might help us to explain why detection experiments have so far failed to provide any meaningful results.

The answer might be that the DM halo profiles are much more complex than what we currently assume them to be, especially as only recently we have started to model the effects of baryonic physics on these halos and to consider other DM versions in cosmological models.

Fallen.

There's not a shortage of reasons why direct detection experiments have failed to produce results (i.e. basically any model of dark matter that just interacts gravitationally; the WIMP picture is really just a very convenient assumption that one concocts because otherwise every direct detection experiment that doesn't involve experiments the size of the solar system will fail), but there is a shortage of reasons for why MOND's relativistic completions, e.g TeVeS/MOG/etc, have utterly failed to survive basic theoretical tests of consistency, other than the obvious one: MOND and relativity (let alone tossing in QM) are simply not compatible. There's also a shortage of answers for objective, empirical questions regarding weak lensing measurements vs radial velocity, the existence of CMB peaks, MOND's inability to apply to cluster-scale structures, etc. Even if you found billions of galaxies which didn't need dark matter, you can't wash away the other problems raised that aren't answered by MOND or the new problems raised by MOND itself.

There's no outstanding issues, to my understanding, with numerical DM models, except that they aren't very predictive. They assume some distribution around galaxies, which is totally misunderstood from first principles so instead they match with data, and then do numerical simulations. A similar issue exists for CDM theories, namely that there's an abundance of theories and an absence of way to rule them out. That's very different, however, than being inconsistent with data or having inconsistencies in the theoretical models.

And despite the very optimistic claims of astronomers and cosmologists, I don't see these issues getting solved anytime soon, unless someone comes up with a modified theory of GR that isn't totally insane nonsense. In other words, right now, I don't see the science case for how experiments are going to meaningful rule on these questions, unless we get absurdly lucky (and for no apparent reason) and LZ/XENON 1T discovers a WIMP. And scientists should never bank on that happening, because it's irrational and scientists are supposed to be rational. (Which is distinct from not trying; you always try.)
 
Last edited:

Masterhawk

DP Veteran
Joined
May 6, 2016
Messages
1,854
Reaction score
431
Location
Colorado
Gender
Male
Political Leaning
Independent
Dark matter is just a theory. The only facts to back it up is that gravity is too weak to hold galaxies and galaxy clusters together.
 

William Rea

DP Veteran
Joined
Aug 23, 2010
Messages
8,949
Reaction score
2,231
Location
UK
Gender
Male
Political Leaning
Socialist
Dark matter is just a theory. The only facts to back it up is that gravity is too weak to hold galaxies and galaxy clusters together.
It's not 'just a theory'. It is either a scientific theory that is consistent with a body of knowledge and facts or it is not. I believe that you meant that it is 'just a hypothesis'.
 

Fallenangel

DP Veteran
Joined
Mar 24, 2013
Messages
3,416
Reaction score
1,099
Location
Israel/Russia/UK
Gender
Male
Political Leaning
Undisclosed
There's not a shortage of reasons why direct detection experiments have failed to produce results (i.e. basically any model of dark matter that just interacts gravitationally; the WIMP picture is really just a very convenient assumption that one concocts because otherwise every direct detection experiment that doesn't involve experiments the size of the solar system will fail), but there is a shortage of reasons for why MOND's relativistic completions, e.g TeVeS/MOG/etc, have utterly failed to survive basic theoretical tests of consistency, other than the obvious one: MOND and relativity (let alone tossing in QM) are simply not compatible. There's also a shortage of answers for objective, empirical questions regarding weak lensing measurements vs radial velocity, the existence of CMB peaks, MOND's inability to apply to cluster-scale structures, etc. Even if you found billions of galaxies which didn't need dark matter, you can't wash away the other problems raised that aren't answered by MOND or the new problems raised by MOND itself.

I don't advocate for MOND and its variations, I am saying that our current research into WIMP DM have failed so far and that the researchers I'm familiar with are starting to get a little pessimistic regarding the prospects of detecting WIMPs. I personally favor the idea that we are simply lacking some key part in our understanding of DM properties and the properties of halos that it supposed to form, hence we don't have any meaningful direct and/or indirect detection results.

There's no outstanding issues, to my understanding, with numerical DM models, except that they aren't very predictive. They assume some distribution around galaxies, which is totally misunderstood from first principles so instead they match with data, and then do numerical simulations. A similar issue exists for CDM theories, namely that there's an abundance of theories and an absence of way to rule them out. That's very different, however, than being inconsistent with data or having inconsistencies in the theoretical models.

I don't really know which models you are talking about here, but as far as I am aware CDM cosmological models such as the Millennium simulation and its later modifications track the formation of structures that evolve in a cube (of set size) containing N number of gravitationally interacting particles. The obtained hierarchical merger trees are then used to fit the structures that have formed in the simulation with semi-analytic models (SAM), which essentially mimic the galaxy and cluster distribution. Despite their high resolution so far these simulations and SAM failed to replicate the observed variety of smaller scale galaxies, only with the recent introduction of the effects of baryonic physics we got closer to mimicking their distribution and properties. Additionally, the interpretation of structures and halos in such models and their predictive power for observational studies (as well as future LSST and DES) sometimes lacks the needed accuracy, such as the need for detailed analysis of the effects of DM halo morphology on cluster mass estimation.

And despite the very optimistic claims of astronomers and cosmologists, I don't see these issues getting solved anytime soon, unless someone comes up with a modified theory of GR that isn't totally insane nonsense. In other words, right now, I don't see the science case for how experiments are going to meaningful rule on these questions, unless we get absurdly lucky (and for no apparent reason) and LZ/XENON 1T discovers a WIMP. And scientists should never bank on that happening, because it's irrational and scientists are supposed to be rational. (Which is distinct from not trying; you always try.)

The thing is that even the discovery of some particle X for the role of WIMP in one of the latest or upcoming direct detection experiments (which gets more and more unlikely, as we get closer and closer to the neutrino range) would probably not mean the discovery of DM. We will need to verify their role with the use of indirect detection experiments and LHC data, which might not support the direct detection results.


Fallen.
 
Last edited:

FieldTheorist

DP Veteran
Joined
Dec 5, 2015
Messages
3,325
Reaction score
2,348
Gender
Male
Political Leaning
Libertarian - Left
I don't advocate for MOND and its variations, I am saying that our current research into WIMP DM have failed so far and that the researchers I'm familiar with are starting to get a little pessimistic regarding the prospects of detecting WIMPs. I personally favor the idea that we are simply lacking some key part in our understanding of DM properties and the properties of halos that it supposed to form, hence we don't have any meaningful direct and/or indirect detection results.

[...]

The thing is that even the discovery of some particle X for the role of WIMP in one of the latest or upcoming direct detection experiments (which gets more and more unlikely, as we get closer and closer to the neutrino range) would probably not mean the discovery of DM. We will need to verify their role with the use of indirect detection experiments and LHC data, which might not support the direct detection results.

This is definitely true, I'm certainly pessimistic, at any rate. It's not that WIMPs are definitely false, of course, it's that they continue to get torn down further every major experiment (much like the TEV-scale supersymmetric Standard Model, which is one of the big sources of inspiration for hoping that DM should be WIMPs).

I don't really know which models you are talking about here, but as far as I am aware CDM cosmological models such as the Millennium simulation and its later modifications track the formation of structures that evolve in a cube (of set size) containing N number of gravitationally interacting particles. The obtained hierarchical merger trees are then used to fit the structures that have formed in the simulation with semi-analytic models (SAM), which essentially mimic the galaxy and cluster distribution. Despite their high resolution so far these simulations and SAM failed to replicate the observed variety of smaller scale galaxies, only with the recent introduction of the effects of baryonic physics we got closer to mimicking their distribution and properties. Additionally, the interpretation of structures and halos in such models and their predictive power for observational studies (as well as future LSST and DES) sometimes lacks the needed accuracy, such as the need for detailed analysis of the effects of DM halo morphology on cluster mass estimation.

More or less these, yes, but also I know some people have worked on modeling DM via various distributions, and the very large question about how one should expect DM is distributed around galaxies. That creates problems when comparing theory to data, because the modeling of DM is a pretty wildly open question. As far as I understand it, that's the primary issue, but I'm not an expert in the astrophysics of DM.
 

Fallenangel

DP Veteran
Joined
Mar 24, 2013
Messages
3,416
Reaction score
1,099
Location
Israel/Russia/UK
Gender
Male
Political Leaning
Undisclosed
This is definitely true, I'm certainly pessimistic, at any rate. It's not that WIMPs are definitely false, of course, it's that they continue to get torn down further every major experiment (much like the TEV-scale supersymmetric Standard Model, which is one of the big sources of inspiration for hoping that DM should be WIMPs).


More or less these, yes, but also I know some people have worked on modeling DM via various distributions, and the very large question about how one should expect DM is distributed around galaxies. That creates problems when comparing theory to data, because the modeling of DM is a pretty wildly open question. As far as I understand it, that's the primary issue, but I'm not an expert in the astrophysics of DM.


Yeah, pretty much this. That is why I've mentioned that the DM density profiles of galaxy scale halos might be quite different from what we expect/approximate them to be, for instance it is quite obvious these days that the simplistic Navarro-Frenk-White profile is a gross oversimplification. Only recently (with the appearance of multiple high-resolution simulations) researchers started studying halo morphology/density profiles in more detail and their implications for observational studies.

Fallen.
 
Top Bottom