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Thread: Dark Matter exists, end of discussion

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    Exclamation Dark Matter exists, end of discussion

    This news article briefly discusses a recent paper that took an exacting look at many spiral and dwarf galaxies, proving that DM was real.

    https://phys.org/news/2019-04-dark-a...lanations.html

    Dark matter exists: Observations disprove alternate explanations
    by International School of Advanced Studies (SISSA)

    As fascinating as it is mysterious, dark matter is one of the greatest enigmas of astrophysics and cosmology. It is thought to account for 90 percent of the matter in the universe, but its existence has been demonstrated only indirectly, and has recently been called into question. New research conducted by SISSA removes the recent doubts on the presence of dark matter within galaxies, disproving the empirical relations in support of alternative theories. The study, published in the Astrophysical Journal, also offers new insights into understanding the nature of dark matter and its relationship with ordinary matter.


    Here is the original paper reference:

    https://arxiv.org/abs/1810.08472

    The Radial Acceleration Relation (RAR): the crucial cases of Dwarf Discs and of Low Surface Brightness galaxies

    Chiara Di Paolo, Paolo Salucci, Jean Philippe Fontaine (Submitted on 19 Oct 2018 (v1), last revised 17 Jan 2019 (this version, v2))

    McGaugh et al. (2016) have found, in a large sample of disc systems, a tight nonlinear relationship between the total radial accelerations g and their components g b arisen from the distribution of the baryonic matter [McGaugh_2016]. Here, we investigate the existence of such relation in Dwarf Disc Spirals and Low Surface Brightness galaxies on the basis of [Karukes_2017] and [DiPaolo_2018]. We have accurate mass profiles for 36 Dwarf Disc Spirals and 72 LSB galaxies. These galaxies have accelerations that cover the McGaugh range but also reach out to one order of magnitude below the smallest accelerations present in McGaugh et al. (2016) and span different Hubble Types. We found, in our samples, that the g vs g b relation has a very different profile and also other intrinsic novel properties, among those, the dependence on a second variable: the galactic radius, normalised to the optical radius R opt , at which the two accelerations are measured. We show that the new far than trivial g vs (g b ,r/R opt) relationship is nothing else than a direct consequence of the complex, but coordinated mass distributions of the baryons and the dark matter (DM) in disc systems. Our analysis shows that the McGaugh et al. (2016) relation is a limiting case of a new universal relation that can be very well framed in the standard "DM halo in the Newtonian Gravity" paradigm.
    Last edited by Roger E. Moore; 2019-Apr-30 at 03:52 PM.
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    Isn’t that subject ATM?

    All I can say, from what I understand, is that DM is still implied and not proven experimentally. “phys.org” has good news in general but sometimes has “tantrum news”.


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    Quote Originally Posted by philippeb8 View Post
    Isn’t that subject ATM?
    Why would something that has been the subjects of mainstream research for nearly 100 years be ATM?

    All I can say, from what I understand, is that DM is still implied and not proven experimentally.
    There are vast amounts of evidence for dark matter. (Nothing is ever really "proven" in science, so I don't know what your expectations are.)

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    Quote Originally Posted by Strange View Post
    Why would something that has been the subjects of mainstream research for nearly 100 years be ATM?



    There are vast amounts of evidence for dark matter. (Nothing is ever really "proven" in science, so I don't know what your expectations are.)
    Because it leads easily to a debate.

    Last week there was another non mutually exclusive research saying exactly the opposite according to observations.


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    Quote Originally Posted by philippeb8 View Post
    Because it leads easily to a debate.

    Last week there was another non mutually exclusive research saying exactly the opposite according to observations.
    Debate and papers with different conclusions is how science works. So those are both good things.

    I would be interested to know what research there was showing that dark matter doesn't exist. Can you provide a link?

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    The wording of the thread title may be a little bit too adamant; certainly the preponderance of evidence supports a DM model and new evidence weakens other models. So...semantics.

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    Quote Originally Posted by Strange View Post
    Debate and papers with different conclusions is how science works. So those are both good things.

    I would be interested to know what research there was showing that dark matter doesn't exist. Can you provide a link?
    I think I was told there needs to be more than 1 paper to change / confirm the mainstream.

    Here’s the link:
    https://www.businessinsider.com/astr...-matter-2019-4


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    Quote Originally Posted by philippeb8 View Post
    I think I was told there needs to be more than 1 paper to change / confirm the mainstream.
    It does. It can take years, or even decades.

    Thanks. Most reporting I have seen on this has focussed on the fact that it is good evidence for dark matter as a form of matter. If dark matter were an effect of our current theories of gravity being incorrect then it should affect all galaxies. If there are galaxies that show no signs of dark matter, then it can't just be gravity; it must be that they lack actual dark matter.

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    Quote Originally Posted by Strange View Post
    It does. It can take years, or even decades.



    Thanks. Most reporting I have seen on this has focussed on the fact that it is good evidence for dark matter as a form of matter. If dark matter were an effect of our current theories of gravity being incorrect then it should affect all galaxies. If there are galaxies that show no signs of dark matter, then it can't just be gravity; it must be that they lack actual dark matter.
    Sorry I’ll reread that statement after working hours.


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    Dark Assertion

    Quote Originally Posted by philippeb8 View Post
    Because it leads easily to a debate.

    Last week there was another non mutually exclusive research saying exactly the opposite according to observations.


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    Reference for your claim about last week?

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    Quote Originally Posted by John Mendenhall View Post
    Reference for your claim about last week?
    Same aforementioned link:
    https://www.businessinsider.com/astr...-matter-2019-4


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    Whoops

    Quote Originally Posted by philippeb8 View Post
    Same aforementioned link:
    https://www.businessinsider.com/astr...-matter-2019-4


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    Sorry, others were ahead of me.
    Last edited by John Mendenhall; 2019-Apr-30 at 05:38 PM.

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    Buisness Inside For Comology ??

    Quote Originally Posted by philippeb8 View Post
    Same aforementioned link:
    https://www.businessinsider.com/astr...-matter-2019-4


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    Ah, the link freezes immediately on my computer.

    Maybe Business Inside should work on their software intead of cosmology.

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    this thread title is of course deliberately provocative. I doubt the discussion ends here. Dark matter is a model that fits the observations of distant star motions with the various constants we know about in our solar system. To do better would require another model that fits those observations without calling up dark matter, which cannot yet have the status of matter in the same way as the stuff we can put under a microscope. Maybe the hypothesis has passed enough tests to be a theory but we can be sure more tests will follow in an attempt to falsify. The point is science moves along in observations and interpretations, not by dogma.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by profloater View Post
    this thread title is of course deliberately provocative.
    Yes, true. I was echoing the opinions of the researchers (in this and other papers) and in the opinions of those who studied the research and wrote news pieces about it. It seemed sewn up to them.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Quote Originally Posted by John Mendenhall View Post
    Reference for your claim about last week?
    Not picking at you, just at certain claims being made here by others.

    Look, I am an amateur but I wish to weigh in here, for better or worse. Amateur though I am, one thing I can do is read a science paper. And I found that referenced science paper and even a second paper on another no-DM galaxy, and neither of them refute the concept or existence of Dark Matter. Here they are.

    https://arxiv.org/abs/1803.10237

    A galaxy lacking dark matter

    Pieter van Dokkum, Shany Danieli, Yotam Cohen, Allison Merritt, Aaron J. Romanowsky, Roberto Abraham, Jean Brodie, Charlie Conroy, Deborah Lokhorst, Lamiya Mowla, Ewan O'Sullivan, Jielai Zhang (Submitted on 27 Mar 2018)

    Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio M_{halo}/M_{stars} has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5x10^{10} solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2x10^8 solar masses. We infer that its velocity dispersion is less than 10.5 kilometers per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4x10^8 solar masses. This implies that the ratio M_{halo}/M_{stars} is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.

    QUOTE: Regardless of the formation history of NGC1052–DF2, its existence has implications for the dark matter paradigm. Our results demonstrate that dark matter is separable from galaxies, which is (under certain circumstances) expected if it is bound to baryons through nothing but gravity. The “bullet cluster” demonstrates that dark matter does not always trace the bulk of the baryonic mass, which in clusters is in the form of gas. NGC1052–DF2 enables us to make the complementary point that dark matter does not always coincide with galaxies either: it is a distinct “substance” that may or may not be present in a galaxy. Furthermore, and paradoxically, the existence of NGC1052–DF2 may falsify alternatives to dark matter. In theories such as MOND27 and the recently proposed emergent gravity paradigm a “dark matter” signature should always be detected, as it is an unavoidable consequence of the presence of ordinary matter. In fact, it had been argued previously that the apparent absence of galaxies such as NGC1052–DF2 constituted a falsification of the standard cosmological model, and evidence for modified gravity. For a MOND acceleration scale of [lots of math here] the expected velocity dispersion of NGC1052–DF2 is [even more math], a factor of two higher than the 90% upper limit on the observed dispersion.

    [emphasis ALWAYS mine -- REM]

    ===

    https://arxiv.org/abs/1901.05973

    A second galaxy missing dark matter in the NGC1052 group

    Pieter van Dokkum, Shany Danieli, Roberto Abraham, Charlie Conroy, Aaron J. Romanowsky
    (Submitted on 17 Jan 2019 (v1), last revised 15 Mar 2019 (this version, v3))

    The ultra-diffuse galaxy NGC1052-DF2 has a very low velocity dispersion, indicating that it has little or no dark matter. Here we report the discovery of a second galaxy in this class, residing in the same group. NGC1052-DF4 closely resembles NGC1052-DF2 in terms of its size, surface brightness, and morphology; has a similar distance of D=19.9±2.8 Mpc; and also has a population of luminous globular clusters extending out to 7 kpc from the center of the galaxy. Accurate radial velocities of the diffuse galaxy light and seven of the globular clusters were obtained with the Low Resolution Imaging Spectrograph on the Keck I telescope. The velocity of the diffuse light is identical to the median velocity of the clusters, v sys =⟨v gc ⟩=1445 km/s, and close to the central velocity of the NGC1052 group. The rms spread of the observed velocities is very small at σ obs =5.8 km/s. Taking observational uncertainties into account we determine an intrinsic velocity dispersion of σ intr =4.2 +4.4 −2.2 km/s, consistent with the expected value from the stars alone (σ stars ≈7 km/s) and lower than expected from a standard NFW halo (σ halo ∼30 km/s). We conclude that NGC1052-DF2 is not an isolated case but that a class of such objects exists. The origin of these large, faint galaxies with an excess of luminous globular clusters and an apparent lack of dark matter is, at present, not understood.

    ==============

    Those here probably recall the recent and somewhat tedious series of questions I had for moderators and others about whether Dark Matter could literally be dragged out of a galaxy by a larger galaxy nearby; I was given this impression by a paper I was reading.

    https://forum.cosmoquest.org/showthr...-larger-galaxy (link to discussion)

    The answer was yes, DM could be removed from a galaxy by simple gravitation.

    So the fact that a galaxy is found with no Dark Matter in it has absolutely NOTHING to do with whether Dark Matter exists. In fact, according to many researchers, finding a galaxy with no DM in it tends to falsify non-DM theories, according to the above papers.

    Back to everyone else.
    Last edited by Roger E. Moore; 2019-Apr-30 at 06:49 PM. Reason: adds
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Quote Originally Posted by Roger E. Moore View Post
    Not picking at you, just at certain claims being made here by others.

    Look, I am an amateur but I wish to weigh in here, for better or worse. Amateur though I am, one thing I can do is read a science paper. And I found that referenced science paper and even a second paper on another no-DM galaxy, and neither of them refute the concept or existence of Dark Matter. Here they are.

    https://arxiv.org/abs/1803.10237

    A galaxy lacking dark matter

    Pieter van Dokkum, Shany Danieli, Yotam Cohen, Allison Merritt, Aaron J. Romanowsky, Roberto Abraham, Jean Brodie, Charlie Conroy, Deborah Lokhorst, Lamiya Mowla, Ewan O'Sullivan, Jielai Zhang (Submitted on 27 Mar 2018)

    Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio M_{halo}/M_{stars} has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5x10^{10} solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2x10^8 solar masses. We infer that its velocity dispersion is less than 10.5 kilometers per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4x10^8 solar masses. This implies that the ratio M_{halo}/M_{stars} is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.

    QUOTE: Regardless of the formation history of NGC1052–DF2, its existence has implications for the dark matter paradigm. Our results demonstrate that dark matter is separable from galaxies, which is (under certain circumstances) expected if it is bound to baryons through nothing but gravity. The “bullet cluster” demonstrates that dark matter does not always trace the bulk of the baryonic mass, which in clusters is in the form of gas. NGC1052–DF2 enables us to make the complementary point that dark matter does not always coincide with galaxies either: it is a distinct “substance” that may or may not be present in a galaxy. Furthermore, and paradoxically, the existence of NGC1052–DF2 may falsify alternatives to dark matter. In theories such as MOND27 and the recently proposed emergent gravity paradigm a “dark matter” signature should always be detected, as it is an unavoidable consequence of the presence of ordinary matter. In fact, it had been argued previously that the apparent absence of galaxies such as NGC1052–DF2 constituted a falsification of the standard cosmological model, and evidence for modified gravity. For a MOND acceleration scale of [lots of math here] the expected velocity dispersion of NGC1052–DF2 is [even more math], a factor of two higher than the 90% upper limit on the observed dispersion.

    [emphasis ALWAYS mine -- REM]

    ===

    https://arxiv.org/abs/1901.05973

    A second galaxy missing dark matter in the NGC1052 group

    Pieter van Dokkum, Shany Danieli, Roberto Abraham, Charlie Conroy, Aaron J. Romanowsky
    (Submitted on 17 Jan 2019 (v1), last revised 15 Mar 2019 (this version, v3))

    The ultra-diffuse galaxy NGC1052-DF2 has a very low velocity dispersion, indicating that it has little or no dark matter. Here we report the discovery of a second galaxy in this class, residing in the same group. NGC1052-DF4 closely resembles NGC1052-DF2 in terms of its size, surface brightness, and morphology; has a similar distance of D=19.9±2.8 Mpc; and also has a population of luminous globular clusters extending out to 7 kpc from the center of the galaxy. Accurate radial velocities of the diffuse galaxy light and seven of the globular clusters were obtained with the Low Resolution Imaging Spectrograph on the Keck I telescope. The velocity of the diffuse light is identical to the median velocity of the clusters, v sys =⟨v gc ⟩=1445 km/s, and close to the central velocity of the NGC1052 group. The rms spread of the observed velocities is very small at σ obs =5.8 km/s. Taking observational uncertainties into account we determine an intrinsic velocity dispersion of σ intr =4.2 +4.4 −2.2 km/s, consistent with the expected value from the stars alone (σ stars ≈7 km/s) and lower than expected from a standard NFW halo (σ halo ∼30 km/s). We conclude that NGC1052-DF2 is not an isolated case but that a class of such objects exists. The origin of these large, faint galaxies with an excess of luminous globular clusters and an apparent lack of dark matter is, at present, not understood.

    ==============

    Those here probably recall the recent and somewhat tedious series of questions I had for moderators and others about whether Dark Matter could literally be dragged out of a galaxy by a larger galaxy nearby; I was given this impression by a paper I was reading.

    https://forum.cosmoquest.org/showthr...-larger-galaxy (link to discussion)

    The answer was yes, DM could be removed from a galaxy by simple gravitation.

    So the fact that a galaxy is found with no Dark Matter in it has absolutely NOTHING to do with whether Dark Matter exists. In fact, according to many researchers, finding a galaxy with no DM in it tends to falsify non-DM theories, according to the above papers.

    Back to everyone else.
    How about lack of consistency in the DM theory?


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  18. #18
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    Quote Originally Posted by philippeb8 View Post
    How about lack of consistency in the DM theory?
    Clarify with references and exact quotes.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Quote Originally Posted by philippeb8 View Post
    How about lack of consistency in the DM theory?
    What lack of consistency?

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    Quote Originally Posted by philippeb8 View Post
    Same aforementioned link:
    https://www.businessinsider.com/astr...-matter-2019-4


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    Maybe a better reference would be the actual paper? I do not know for sure, but it may be this one (link to arXiv abstract).

    philippeb8, earlier in this thread you wrote "Last week there was another non mutually exclusive research saying exactly the opposite according to observations." Can you help me out here please? How does the van Dokkum+ (2019) paper say "exactly the opposite according to observations"?

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    Quote Originally Posted by Roger E. Moore View Post
    This news article briefly discusses a recent paper that took an exacting look at many spiral and dwarf galaxies, proving that DM was real.

    https://phys.org/news/2019-04-dark-a...lanations.html

    Dark matter exists: Observations disprove alternate explanations
    by International School of Advanced Studies (SISSA)

    As fascinating as it is mysterious, dark matter is one of the greatest enigmas of astrophysics and cosmology. It is thought to account for 90 percent of the matter in the universe, but its existence has been demonstrated only indirectly, and has recently been called into question. New research conducted by SISSA removes the recent doubts on the presence of dark matter within galaxies, disproving the empirical relations in support of alternative theories. The study, published in the Astrophysical Journal, also offers new insights into understanding the nature of dark matter and its relationship with ordinary matter.


    Here is the original paper reference:

    https://arxiv.org/abs/1810.08472

    The Radial Acceleration Relation (RAR): the crucial cases of Dwarf Discs and of Low Surface Brightness galaxies

    Chiara Di Paolo, Paolo Salucci, Jean Philippe Fontaine (Submitted on 19 Oct 2018 (v1), last revised 17 Jan 2019 (this version, v2))

    McGaugh et al. (2016) have found, in a large sample of disc systems, a tight nonlinear relationship between the total radial accelerations g and their components g b arisen from the distribution of the baryonic matter [McGaugh_2016]. Here, we investigate the existence of such relation in Dwarf Disc Spirals and Low Surface Brightness galaxies on the basis of [Karukes_2017] and [DiPaolo_2018]. We have accurate mass profiles for 36 Dwarf Disc Spirals and 72 LSB galaxies. These galaxies have accelerations that cover the McGaugh range but also reach out to one order of magnitude below the smallest accelerations present in McGaugh et al. (2016) and span different Hubble Types. We found, in our samples, that the g vs g b relation has a very different profile and also other intrinsic novel properties, among those, the dependence on a second variable: the galactic radius, normalised to the optical radius R opt , at which the two accelerations are measured. We show that the new far than trivial g vs (g b ,r/R opt) relationship is nothing else than a direct consequence of the complex, but coordinated mass distributions of the baryons and the dark matter (DM) in disc systems. Our analysis shows that the McGaugh et al. (2016) relation is a limiting case of a new universal relation that can be very well framed in the standard "DM halo in the Newtonian Gravity" paradigm.
    As usual, phys.org is guilty of wild exaggeration.

    The Di Paolo+ 2019 paper (preprint actually) is directly relevant to the McGaugh+ (2016) paper only. Or, more broadly, to an empirical relationship McGaugh+ reported. That 2016 (McGaugh) paper is well worth reading, carefully, because I think it's a good example of how to do astronomy. If you know enough about the subject, it's easy to find "holes" in it; the extent to which those holes weaken the stated empirical relationship is, well, up for discussion. And that's what the Di Paolo+ paper does.

    What gets editors all excited is, of course, how these results can be spun to "prove" or "disprove" the existence of CDM, or support an alternative idea (something like MOND, in McGaugh's case). What so often gets overlooked is the hard fact that galaxies are incredibly complicated systems, and that there are many, many detailed steps between data from the back end of an instrument attached to a telescope and some physics-based explanation.

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    Quote Originally Posted by Jean Tate View Post
    this one (link to arXiv abstract).
    QUOTE (from this paper): "Irrespective of the detailed mass distribution it is clear that Dragonfly 44 has a gravitationally-dominant dark matter halo, similar to many other UDGs (Beasley et al. 2016; Toloba et al. 2018; Martín-Navarro et al. 2019), and in apparent contrast to the UDGs NGC1052-DF2 and NGC1052-DF4 (see, e.g., van Dokkum et al. 2018b, 2019b; Martin et al. 2018; Famaey, McGaugh, & Milgrom 2018; Emsellem et al. 2019; Danieli et al. 2019)."

    If it was this paper, it fits in with the other two by van Dokkum.
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    On the topic of Dark Matter comes a paper on DM-collecting pulsars. It appears that pulsar size depends on how close to the Galactic Center the pulsar is.

    https://arxiv.org/abs/1904.13060

    Can Pulsars in the inner parsecs from Galactic Centre probe the existence of Dark Matter ?

    Antonino Del Popolo, Maksym Deliyergiyev, Morgan Le Delliou, Laura Tolos, Fiorella Burgio (Submitted on 30 Apr 2019)

    We show that the pulsar mass depends on the environment, and that it decreases going towards the center of the Milky Way due to dark matter capture by those stars. This can be used as a probe for the existence and nature of dark matter. We thus propose that the evolution of the pulsar mass in a dark matter rich environment can be used to put constraints, when combined with future experiments, on the characteristics of our Galaxy halo dark matter profile, on the dark matter particle mass and on the dark matter self-interaction strength.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

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    Quote Originally Posted by philippeb8 View Post
    Isn’t that subject ATM?

    All I can say, from what I understand, is that DM is still implied and not proven experimentally. “phys.org” has good news in general but sometimes has “tantrum news”.


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    Knock off the metadiscussion. NOW! No more about it or there will be infractions. If you think a post belongs in ATM, you Report it. If you want to debate that issue, take it to Feedback.
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    The issue is that we are looking for the most natural explanation of the accelerations we see. That's not the same thing as the "correct" answer, it is just the most natural one, the one science is shooting for (i.e., fewest free parameters and least forcing down the throat of our other theories that would rather be left alone if left to their own druthers). Now, if there is a consistent way to correct the mapping of the acceleration found in galaxies to the acceleration we'd expect from just "regular" baryonic matter (as was reported by McGaugh), that strongly suggests the most natural approach will be to do exactly that-- remap our law of gravity to fit that consistent mapping-- and that's exactly what MOND does. The paper by Di Paolo and others showed that there is not such an automatic remapping that works on galaxies with weaker gravities as well. So this pulls the rug out from the MOND claim that they had a consistent way to do it without any other bells and whistles-- this doesn't make MOND wrong, but remember we aren't looking for what's right (because we never get to know that), we are looking for what works with the most natural assumptions and the least free parameters, thereby allowing us to predict future outcomes without needing additional bells and whistles every time. That's simply the goal of science, we never actually get to know whatever little gremlins might really be in there giving us the impression that we have the "correct" theory. So the Di Paolo paper shows us the MOND folks are going to need more gremlins than they thought, that's exactly what it is is saying.

    Now, again, that means the title of the thread is wrong. What it should say is, MOND needs more gremlins. There's your title. And indeed this has always been the problem with MOND, it has never been "MOND can't do that" because I can come up with a MOND theory right now that does anything you want, it looks like this: "gravity is that force which gives you whatever behavior you see." Done, a successful MOND theory. But it simply isn't what science tries to do, we try to unify and explain quantitative phenomena using theories that have attributes that don't need readjusting for every new context-- theories that can make predictions that generalize. This is what MOND has never succeeded in doing.

    Now, dark matter hasn't either, it should certainly be mentioned. But that's because dark matter is not a finished theory, it is a theory still in the making. All we know now is that the overall concept shows promise for being able to generalize to new situations that we haven't seen yet-- but it's going to require a clearer idea of just what dark matter is. So all we can say at this point is that dark matter seems to need "fewer gremlins" than MOND. That's it, there's your thread headline. Cold dark matter involves WIMPS (weakly interacting massive particles) of unknown attributes and properties but they interact weakly other than gravitationally, and with just a few bells and whistles we seem to be able to understand most of what we see. Do we understand everything? Certainly not, and the dwarf galaxies of this very paper are a perfect example. Di Paolo is flat out saying that we need gremlins in the dark matter theory to understand why the dark matter fraction varies as a function of location in these galaxies, and are different for different types of galaxies. That means we need to understand the full dynamics of the dark matter, in contrast to the baryonic matter, and quite frankly we still don't fully understand the dynamics of the baryons! Maybe the dark matter dynamics are actually easier than baryon dynamics, expressly because they interact so weakly, but we still don't have ab initio calculations of what dark matter will do and how it will interact with baryons, so we don't yet understand the dark matter in these dwarf galaxies. So the idea that these dwarf galaxies prove dark matter exists is downright disingenuous-- the behavior of dark matter in these galaxies is not understood yet, and these galaxies may give us clues that help us understand dark matter better (assuming the currently most natural explanation does turn out to survive into the future of science). But pointing to observations that you cannot yet understand without adding additional gremlins to dark matter doesn't prove dark matter exists, it merely makes the argument that we already know we have to add these gremlins to dark matter-- the MOND camp was claiming they didn't need new gremlins, indeed that is their entire objection to dark matter, that it is a type of unnecessary gremlin. This paper shows we need more gremlins either way, and dark matter seems like a more promising way to get more bang from our gremlin buck.
    Last edited by Ken G; 2019-May-01 at 07:46 PM.

  26. #26
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    Quote Originally Posted by Ken G View Post
    But pointing to observations that you cannot yet understand without adding additional gremlins to dark matter doesn't prove dark matter exists, it merely makes the argument that we already know we have to add these gremlins to dark matter-- the MOND camp was claiming they didn't need new gremlins, indeed that is their entire objection to dark matter, that it is a type of unnecessary gremlin. This paper shows we need more gremlins either way, and dark matter seems like a more promising way to get more bang from our gremlin buck.
    IIRC, and it's been a while, one of the early key objections to MOND was that it didn't eliminate DM all together; some DM was needed in conjunction with MOND to explain observed velocities. Has this been changed to a MOMOND (Modified MOND ), or something?
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    IIRC, and it's been a while, one of the early key objections to MOND was that it didn't eliminate DM all together; some DM was needed in conjunction with MOND to explain observed velocities. Has this been changed to a MOMOND (Modified MOND ), or something?
    It’s rather more complicated than that.

    First, we should note that MOND was dead the very day it was written; it is inconsistent with relatively. So “MOND” should be read as something like “some extension which reduces to MOND in the appropriate limit”.

    Next, MOND, in any form, is inconsistent with even early observations of the CMB. As far as I know, there is no published version of MOND which addresses this and is also consistent with “galaxy rotation curves” (GRCs). And much, much more besides.

    The McGaugh paper shows that one set of GRCs is consistent with MOND, and not so easy to demonstrate consistency with CDM models. The paper’s scope does not extend beyond GRCs, and the GRCs of a small number of galaxies only. The Di Paolo paper reports on observations of a small number of other galaxies.

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    It has always annoyed me that there's a ratio of total universal matter to ordinary matter that has been converging on (+/- 1.1 % now using and both the WMAP and PLANCK data) for over 10 years now and nobody mentions 'maybe we made a mistake'.

    The following paper is a rebuttal to the 2012 paper by Moni Bidin et al. where Bidin did not find any local dark matter. It seems that the difference between finding dark matter locally and not finding dark matter locally is due to using the galactic center as the radius of rotation as opposed to using the axis of rotation of the galactic center as the radius of rotation, for non central disk stars.

    https://arxiv.org/abs/1205.4033

    Obviously if you know you're right and you don't have to prove why you are right in any consistent manner you must be right.

  29. #29
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    Quote Originally Posted by LaurieAG View Post
    It has always annoyed me that there's a ratio of total universal matter to ordinary matter that has been converging on (+/- 1.1 % now using and both the WMAP and PLANCK data) for over 10 years now and nobody mentions 'maybe we made a mistake'.

    The following paper is a rebuttal to the 2012 paper by Moni Bidin et al. where Bidin did not find any local dark matter. It seems that the difference between finding dark matter locally and not finding dark matter locally is due to using the galactic center as the radius of rotation as opposed to using the axis of rotation of the galactic center as the radius of rotation, for non central disk stars.

    https://arxiv.org/abs/1205.4033

    Obviously if you know you're right and you don't have to prove why you are right in any consistent manner you must be right.
    According to ADS, that 2012 Jovi&Tremaine paper has 208 (!) citations. Some report estimates of the local CDM density, using a variety of techniques and observations, some are overviews, and some are not directly relevant (this is not at all unusual). It’s a hot astronomy/astrophysics topic. And this is just MW-locally, with all the challenges of trying to get a big picture view from deep inside, of just one galaxy ...

  30. #30
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    Quote Originally Posted by John Mendenhall View Post
    Ah, the link freezes immediately on my computer.

    Maybe Business Inside should work on their software intead of cosmology.
    It works fine on my iPhone. Just FYI, Business Insider is a very respectable general news site. They are not specialists in cosmology, but cover it like other subjects.


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