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Thread: H2 as dark matter

  1. #1 H2 as dark matter 
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    I am trying to understand the reasons that molecular hydrogen is ruled out as a candidate for dark matter as most stars seem to be made of it. My understanding is that is it pretty much invisible, but can be associated with a CO signature. Is this always the case and if not what are the main reasons it is ruled out as a candidate?
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  2. #2  
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    I can't recall the visibility details, but from cosmology the main reason for denying that DM is H2 is that there are hard limits on how much hydrogen that there can be from primordial nucleosynthesis and from the fluctuation spectrum of the CMB. The amount of baryons in the early universe produce limits on the relative amount of different elements in the universe and on how fluctuations appear at the time when the CMB is released.
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    So if there is no free hydrogen does that mean that no new stars are forming?
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    The hydrogen in most stars is ionized (not molecular).

    H2 interacts with photons, dark matter does not.
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    The reason hydrogen is ionised in stars is because it's hot. Molecular hydrogen is pretty much invisible and does not interact with photons because it has no dipole moment. The 21 cm absorption line of atomic hydrogen does not occur with molecular hydrogen.
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    Quote Originally Posted by Jilan View Post
    So if there is no free hydrogen does that mean that no new stars are forming?
    No, primordial nucleosynthesis only took place in the first 300000 or so years. It ended with the release of the cosmic background radiation.
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    This article suggests that there is a lack of dark matter in the vicinity of the solar system. Would this suggest that dark matter is an inter-galactic phenomenon?
    [1204.3924] Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood
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  8. #8  
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    The reason hydrogen is ionised in stars is because it's hot. Molecular hydrogen is pretty much invisible and does not interact with photons because it has no dipole moment. The 21 cm absorption line of atomic hydrogen does not occur with molecular hydrogen.
    https://www.google.com/search?q=hydr...w=1153&bih=669

    Above shows spectra for atomic and molecular hydrogen.
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    It's all quite a mystery to me. Wiki suggests it is difficult to detect directly so we detect it by the accompanying CO signature instead.
    Molecular cloud - Wikipedia, the free encyclopedia
    However I am confused as to why there should always be a CO signature. Would you not require a star to create CO in the first place?
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  10. #10  
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    It's all quite a mystery to me. Wiki suggests it is difficult to detect directly so we detect it by the accompanying CO signature instead.
    Molecular cloud - Wikipedia, the free encyclopedia
    However I am confused as to why there should always be a CO signature. Would you not require a star to create CO in the first place?
    The article makes a point that it is discussing molecular clouds in our galaxy, so the CO fraction would be fairly constant. In older galaxies it would be less. However, it would be difficult to see molecular clouds that far away.

    You are correct in that C and O were formed previously.
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  11. #11  
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    C and O were originall made in CNO cycle - Wikipedia, the free encyclopedia
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  12. #12  
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    Quote Originally Posted by Jilan View Post
    I am trying to understand the reasons that molecular hydrogen is ruled out as a candidate for dark matter as most stars seem to be made of it. My understanding is that is it pretty much invisible, but can be associated with a CO signature. Is this always the case and if not what are the main reasons it is ruled out as a candidate?
    It's my understanding that the reason dark matter can't be molecular hydrogen or any other gas is the absence of scattering, the phenomenon that causes the sky to be blue.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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    Thanks everyone, that's really useful. I've been digging around and found this article which is interesting.
    Discovery of H2 in Space Explains Dark Matter and Redshift
    It says that unlike other diatomic molecules such as nitrogen and oxygen, hydrogen has no dipole moment so it becomes extremely improbable that photons can be scattered by it. Can this be true?
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  14. #14  
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    Thanks everyone, that's really useful. I've been digging around and found this article which is interesting.
    Discovery of H2 in Space Explains Dark Matter and Redshift
    It says that unlike other diatomic molecules such as nitrogen and oxygen, hydrogen has no dipole moment so it becomes extremely improbable that photons can be scattered by it. Can this be true?
    Comment 1: The article is suspect. If there was any validity, the astrophysics community would have responded.
    Comment 2: Improbable does not mean impossible. H2 does have a spectrum, as I noted above.
    Comment 3: H2 clouds in our galaxy can be measured by CO content.
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  15. #15  
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    Quote Originally Posted by mathman View Post
    The article is suspect.
    Yeah, I thought that when I read:

    "Molecular hydrogen possesses no permanent dipole. Such a perfect coupling is unusual among diatomic molecules. For example, in the cases of nitrogen and oxygen, there are seven and eight electrons per atom, so that when combined, it is not possible to fulfill such a perfect coupling of spins (with zero permanent dipole) for all seven or eight pairs of electrons."

    Other parts read like crackpottery such as:

    "The Doppler interpretation of the redshift is a variation of the Creationist theory, since it claims that the universe was created from nothing, 15 billion years ago, with a sudden Big Bang."
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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  16. #16  
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    Thanks for your comments folks, I did wonder why it was not taken seriously. There are a couple of things that are still bothering me though.
    1) Professor Marmet might have had some non standard views but spectroscopy and scattering appeared to be his speciality
    Paul Marmet - Wikipedia, the free encyclopedia
    2) CO is used to detect H2, but it has to exist already. Would there still be any H2 about that hadn't been processed by a star so didn't have the accompanying CO signature?
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  17. #17  
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    Quote Originally Posted by Jilan View Post
    Thanks for your comments folks, I did wonder why it was not taken seriously. There are a couple of things that are still bothering me though.
    1) Professor Marmet might have had some non standard views but spectroscopy and scattering appeared to be his speciality
    Let's take him at his word and believe him when he writes in that article that, "Consequently, the redshift following the collision of a photon with H2 is indistinguishable from the phenomenon caused by the Doppler effect."

    This being the case, then H2 cannot possibly account for the observations of redshift, because these observations do not match what we would expect from the Doppler effect.

    At greater range, the relationship between redshift and distance changes. For this to be due to H2, there must have been a greater density of H2 in the past and then before that there must have been less density. So there is a strange history to H2 that one must account for in order to allow for H2 to account for redshift.
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  18. #18  
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    Thanks Physbang, I always though that Hubble's law was a straight line relationship, the further away the more redshift. In the H2 proposal I would suppose that the more space the light travels through the more redshift occurs. It would be surprising though if the density of it were the same in each direction, but perhaps it isn't.

    I had been under the impression that all the galaxies has been formed relatively early in the life of the universe, (ours seems to one of the early ones) but it would appear that some relatively new galaxies have been discovered by the GALEX telescope.
    HowStuffWorks "Galaxy Formation"

    I'm not quite sure what to make of this. I can get my head around the existence of clouds of intergalactic hydrogen, but am not sure whether it not we still need dark matter to explain the local effects in our galaxy. Help!
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  19. #19  
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    I think that there is a link to an overview article about the change in redshift over great distances here: Supernova Cosmology Project
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  20. #20  
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    Quote Originally Posted by Jilan View Post
    This article suggests that there is a lack of dark matter in the vicinity of the solar system. Would this suggest that dark matter is an inter-galactic phenomenon?
    [1204.3924] Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood
    FYI
    Jilan

    This paper corrects the analysis of the paper you linked.
    On the local dark matter density
    [1205.4033] On the local dark matter density
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  21. #21  
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    Quote Originally Posted by Jilan View Post
    Thanks everyone, that's really useful. I've been digging around and found this article which is interesting.
    Discovery of H2 in Space Explains Dark Matter and Redshift
    It says that unlike other diatomic molecules such as nitrogen and oxygen, hydrogen has no dipole moment so it becomes extremely improbable that photons can be scattered by it. Can this be true?
    Nothing Paul Marmet claims is scientifically true. Outside his own head.
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  22. #22  
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    Brucep, thanks for your input, but you should have said "claimed" , Paul Marmet died 9 years ago.
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  23. #23  
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    Thats very true. He was well respected for his contribution to experimental physics. Not for his opposition to stuff like relativistic, quantum, and cosmological physics
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