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Thread: dark mass vs visible mass.

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    dark mass vs visible mass.

    I was wondering if there are any theories that predict the amount of dark mass vs visible mass or are the best ideas from the Planck mission which actually measures the amount of dark mass vs visible mass based on theory?
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    Quote Originally Posted by Copernicus View Post
    I was wondering if there are any theories that predict the amount of dark mass vs visible mass or are the best ideas from the Planck mission which actually measures the amount of dark mass vs visible mass based on theory?
    Well, the theory of dark matter "predicts" that there must be 5-7 times (IIRC) more dark matter than visible mass, but this is based on the fact that Newtonian gravitation does not fit the observed motions of the visible mass in most galaxies. The visible mass moves as if there is a lot of unseen mass permeating the galaxy.
    Everyone is entitled to his own opinion, but not his own facts.

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    It is inferred from a range of measurements, not predicted. One of the points in favour of it is that a range of independent phenomena all lead to similar numbers.

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    Quote Originally Posted by Cougar View Post
    Well, the theory of dark matter "predicts" that there must be 5-7 times (IIRC) more dark matter than visible mass, but this is based on the fact that Newtonian gravitation does not fit the observed motions of the visible mass in most galaxies. The visible mass moves as if there is a lot of unseen mass permeating the galaxy.
    Thanks cougar. The Planck Mission seems pretty specific in the amount of dark matter. I'm specifically wondering if a number of 6.4 times the amount of dark matter to visible matter is still possible and a percentage of 4.3 percent visible matter to total mass and energy of the universe is still within accepted theory.
    Thanks for any help.
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    Quote Originally Posted by Shaula View Post
    It is inferred from a range of measurements, not predicted. One of the points in favour of it is that a range of independent phenomena all lead to similar numbers.
    It does seem that cosmologists have this nailed down, although I wonder about the size of the universe, the critical density, and the amount of dark energy are contradictory. I see these theories of the universe being infinite or 90 some billion years in diameter. Are these different theories or all part of the same theory?
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    Quote Originally Posted by Copernicus View Post
    It does seem that cosmologists have this nailed down, although I wonder about the size of the universe, the critical density, and the amount of dark energy are contradictory. I see these theories of the universe being infinite or 90 some billion years in diameter. Are these different theories or all part of the same theory?
    Dark Energy != Dark Matter.

    The theory predicts nothing about the size of the universe, observations can only put a lower limit on it based on the current models used to interpret them. Likewise the models don't predict the curvature of the universe (and hence how close we are to its critical density), that is another observation.

    It's rather like the standard model. That predicts how particles will behave, it doesn't predict all of their fundamental properties. And in fact, like the LCDM model, it would work if those values were different (although there is likely some limit to the range it would work for).

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    I always find the term "dark matter" to be a bit assumptive.

    It is inferred that there is an invisible matter that forms 70-80% of the universe from the motion of the observable galaxies. Since we have no direct observational evidence of what is causing this motion then to call it "dark matter" I feel is a bit presumptuous. I appreciate that its just a label for what ever it is that's causing the un-predicted motion but the label does, in mind, infer that it is "matter" as a fact.

    I had read that the current most acceptable model for this phenomenon is that its likely to be some form of matter that only weakly interacts with "ordinary" matter on a gravitational level but doesn't interact with itself or ordinary matter in any other way. Would this be correct?

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    Yes, that's correct. I think it's safe to say that until some kind of particle is isolated in the laboratory, the term "dark matter" will just be a kind of catch-all phrase for "whatever it is that explains the extra attraction that pervades all scales of galaxy-size and larger", just as "dark energy" will be a catch-all phrase for "whatever it is that explains the extra repulsion that pervades the largest scales we observe in the universe." It seems to work to think of the first as matter and the second as vacuum energy, but until there is some demonstration of either, they are little better than names for a mystery. Still, the lambda-CDM model seems to do fairly well treating dark matter as weakly interacting massive particles, that at least puts some constraints on its behavior that seem to make sense. But it must be found.

    That said, note that the basic principle behind "dark matter" is already well known, because any type of neutral matter already fits the definition of dark matter pretty well. One example is the neutrino, which is about as good of a "dark matter" form as you can get, it just doesn't fit what we need because they don't have enough mass and they move too fast. So we already know there is dark matter, it's just not "the" dark matter that we are searching for.
    Last edited by Ken G; 2019-Apr-09 at 01:59 PM.

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    Quote Originally Posted by Copernicus View Post
    I see these theories of the universe being infinite or 90 some billion years in diameter. Are these different theories or all part of the same theory?
    I think you are mixing up the observable universe (roughly 90 billion light years in diameter) and the "whole" universe which is much large and may be infinite.

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    Quote Originally Posted by cosmocrazy View Post
    It is inferred that there is an invisible matter that forms 70-80% of the universe from the motion of the observable galaxies. Since we have no direct observational evidence of what is causing this motion then to call it "dark matter" I feel is a bit presumptuous. I appreciate that its just a label for what ever it is that's causing the un-predicted motion but the label does, in mind, infer that it is "matter" as a fact.
    It is a little more than that which leads to the label dark matter being widely accepted. Models where dark matter behaves like matter, or at least a tangible something (forming halos, acting like a weakly interacting fluid in structure formation, contributing to acoustic effects etc etc) so far outperform models where dark matter is a field or a modifier to gravity. That's not to say that another, better, model might not come along in which dark matter is not matter - just that in a number of ways dark matter appears to behave like weakly interacting matter.

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    Quote Originally Posted by Copernicus View Post
    It does seem that cosmologists have this nailed down, although I wonder about the size of the universe, the critical density, and the amount of dark energy are contradictory. I see these theories of the universe being infinite or 90 some billion years in diameter. Are these different theories or all part of the same theory?
    All part of the same theory or to be more exact the Lambda-CDM model. The model describes an infinite or finite universe. The model uses densities of baryonic and non-baryonic (dark) matter and dark energy rather than trying to calculate amounts in a universe of unknown size.
    We have estimates of the lower limits of the size of the universe. The critical density is a theoretical result. The density of dark energy is a measurement. There are no contradictions.

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    Quote Originally Posted by cosmocrazy View Post
    I always find the term "dark matter" to be a bit assumptive.

    It is inferred that there is an invisible matter that forms 70-80% of the universe from the motion of the observable galaxies. Since we have no direct observational evidence of what is causing this motion then to call it "dark matter" I feel is a bit presumptuous. I appreciate that its just a label for what ever it is that's causing the un-predicted motion but the label does, in mind, infer that it is "matter" as a fact.
    That is not the complete evidence for dark matter, cosmocrazy. Dark matter lists 11 lines of observational evidence. This includes direct observational evidence from colliding galaxy clusters starting with Clowe, Douglas; et al. (2006). "A Direct Empirical Proof of the Existence of Dark Matter". The Astrophysical Journal Letters. 648 (2): L109–L113. arXiv:astro-ph/0608407. Bibcode:2006ApJ...648L.109C. doi:10.1086/508162. The Bullet Cluster is the direct astronomical observation that dark matter separates from normal matter in colliding galaxy clusters.

    The full body of evidence tells us that either dark matter is non-baryonic matter similar to neutrinos or that physics needs modifying . The latter alternative has been explored for almost 40 years by theorists without any credible results. Modifying Newtonian mechanics or GR explains some evidence but breaks elsewhere.

    ETA: What we do not have is clear local observational evidence such as a lab here on Earth detecting dark matter particles. The DAMA collaborations attribute an annual modulation in their results to dark matter but there are negative results from other experiments.
    Last edited by Reality Check; 2019-Apr-10 at 01:42 AM.

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    Thanks folks for the replies and the links

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    Does the recent discovery of diffuse galaxies with no dark matter, whose rotation curves match those predicted using the visible matter alone, show that our theory of gravity is correct, and thus falsify alternative gravity solutions whose purpose was to explain dark matter?

    https://arxiv.org/abs/1901.05973

    https://iopscience.iop.org/article/1...13/ab0d92/meta

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    Quote Originally Posted by speedfreek View Post
    Does the recent discovery of diffuse galaxies with no dark matter, whose rotation curves match those predicted using the visible matter alone, show that our theory of gravity is correct, and thus falsify alternative gravity solutions whose purpose was to explain dark matter?
    Too soon to say, I would think. More research needed.
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    Quote Originally Posted by speedfreek View Post
    Does the recent discovery of diffuse galaxies with no dark matter, whose rotation curves match those predicted using the visible matter alone, show that our theory of gravity is correct, and thus falsify alternative gravity solutions whose purpose was to explain dark matter?
    I think it goes a long way towards it. Another brick in the wall. (Or nail in the coffin.)

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    The problem with MOND theories (that modify gravity instead of adding matter) is that there are so many such theories that you can find several that work in any situation, including those diffuse galaxies, but there isn't one that works everywhere. So those galaxies don't defeat MOND, indeed some versions of MOND have the same issue with those galaxies that dark matter does, but there still isn't just one MOND.

    The reason some MOND versions have no worse of a time with those galaxies than dark matter does is that some versions of MOND have no correction to the Newtonian gravity if there is an "external field effect," since those diffuse galaxies exist in the external field of larger nearby galaxies. So those types of MOND predict a Newtonian outcome, which is what is seen, if we assume the canonical baryonic M/L ratio that is being sold as evidence for absence of dark matter.

    The reason even dark matter theories struggle with those galaxies is, why should a weird galaxy like that have the same baryonic M/L as the Milky Way? That's the question they're not telling you about when they say "look, we get a normal M/L if we simply say there isn't dark matter in those galaxies-- never mind how they got rid of their dark matter, and never mind that whatever did that could easily have also changed the expected baryonic M/L." So the evidence is quite weak that these galaxies are evidence of dark matter elsewhere, instead we should just say these galaxies are difficult to understand in any theory and we have more to learn there.

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    Quote Originally Posted by Ken G View Post
    That said, note that the basic principle behind "dark matter" is already well known, because any type of neutral matter already fits the definition of dark matter pretty well. One example is the neutrino, which is about as good of a "dark matter" form as you can get, it just doesn't fit what we need because they don't have enough mass and they move too fast. So we already know there is dark matter, it's just not "the" dark matter that we are searching for.
    On a related topic, neutrinos might lead us to Dark Matter.

    https://arxiv.org/abs/1904.04355

    Extragalactic neutrinos as tracers of Dark Matter?

    Ana V. Penacchioni, Osvaldo Civitarese (Submitted on 8 Apr 2019)

    Neutrinos produced in extragalactic sources may experience flavor-oscillations and decoherence on their way to Earth due to their interaction with dark matter (DM). As a result, they may be detected in pointer-states other than the flavor states at the source. The oscillation pattern and the structure of the pointer-states can give us information on the characteristics of the DM and the kind of interaction that has taken place. From this perspective, neutrinos can be viewed as DM-tracers.
    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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