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Baryonic DM?

  1. Jan 28, 2005 #1

    Garth

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    In another thread we discussed the nature and extent of the Giant Galactic Blobs!, and the nature of Dark Matter.

    I advocated a baryonic nature of most or all of the DM following the “Freely Coasting” Cosmology model.

    The question it raises is, "What form does this baryonic matter take and why has it not been observed?" If DM were gas and dust it would absorb light, if bright stars they would be seen, if black holes or dim stars they would be observed through micro-lensing events. Such "MACHO" events have been detected but these observations would only account for about 10% of galactic DM.

    This left smaller red dwarfs, 'Jupiters', smaller planet sized bodies and 'bricks', i.e. planetesimals - ("not-so-MACHOs"!) It would seem impossible to create the smallest objects in a primordial universe and so attention focused on 'Jupiters'.

    However it may be that smaller planet sized bodies are possible after all: see "Earth-mass dark-matter haloes as the first structures in the early Universe".

    Are there a lot of 'earth's' out there? [Note: These would be 'gas-dwarfs' not rocks!]

    Garth
     
    Last edited: Jan 28, 2005
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  3. Jan 28, 2005 #2

    mathman

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    Based on the ratio of deuterium to hydrogen (also considering other species such as He and Li), as well as other tests, cosmologists have estimated that baryonic matter can account for only about 4 % of the universe and non-baryonic matter about 23 % (dark energy being the rest). There is no way to avoid it.
     
  4. Jan 28, 2005 #3

    Garth

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  5. Jan 29, 2005 #4
    Garth, you know that baryonic dark matter doesn't sound right to me, though I try to have an open mind and explore all the possibilities. So in this paper
    http://arxiv.org/abs/astro-ph/0406491
    they cite the possibility that baryonic dark matter can be in the form of "primordial fog particles", that should exist forming clumps, though it seems that this kind of dark matter is explained in a Top-Down scenario, while the vast majority of scientists prefer the Bottom-up scenario of structure formation.
     
  6. Jan 30, 2005 #5

    Garth

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    Thank you for that link meteor I find it very useful.

    Baryonic or exotic non-baryonic DM? Until there is evidence for one or the other, or until a MOND type gravitational theory renders either obsolete, it is all a bit unsatisfactory. Lets keep the options open!

    As far as 'bottom up' or 'top down' structure formation is concerned it is probably a bit of both. Jean's mass gravitational instability is 'top down' and evolving primordial fluctuations are 'bottom up'. I suspect both were going on.

    Garth
     
  7. Jan 30, 2005 #6
    Garth, notice that the article also mentions Cometary knots, like those observed in this image. These Cometary knots are suspected to collapse to form pluto-like bodies. I wonder if these pluto like bodies, in case of forming very far away of the sun and so undetectable, could also account for baryonic dark matter
     
    Last edited: Jan 30, 2005
  8. Jan 30, 2005 #7

    Garth

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    Thank you too for that.

    The problem is, that if the majority of DM (like 20% critical density) is baryonic then only about 10 - 20% of that would be visible matter; 80 - 90% would have to be in such cometary knots, earth-mass objects, 'jupiters', red dwarfs, black holes and IGM gas with 'Freely coasting' BBN metallicity. The latter two have been detected, MACHO micro-lensing and Lyman alpha forest, but how much overall density does it account for?

    One fear of mine is that the theory (standard LCDM model) may be driving the observations, i.e. selectively filtering that which is recognised, and not the other way round. I found this paper a little worrying from this respect, Quasar Lensing Statistics and Omega_Lambda: What Went Wrong?".

    Garth
     
  9. Jan 30, 2005 #8

    mathman

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    Re: Freely Coasting Cosmology

    Looking at the referenced paper, there is one aspect of their analysis which looks peculiar to me. Specifically, their description of nuclearsynthesis. They assert that the process takes place over a long period of time (years), in contrast to a few minutes in standard theory. In order to do this they have protons decaying into neutrons and positrons. They need this to keep a population of neutrons needed for the process. Otherwise no deuterium, helium, or lithium. Furthermore, they have an awkward sounding explanation for the observed ratios to hydrogen for these species.

    Re: baryonic dark matter

    This is a completely different story in the standard model. It is simply a problem of accounting for the fact that only 10% of baryonic matter is visible.
     
  10. Jan 30, 2005 #9

    hellfire

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    At high redshifts z > 2 there are lots of baryons in a photoionized medium, which can be observed in the absorption lines in the spectra of distant quasars. Additionally, there must be neutral hydrogen, especially at higher redshifts. But in the local universe at z < 2, there is less amount of baryons than one would expect from the HI absorption at high redshifts. It is postulated that these baryons reside in filaments in a warm phase, which is very difficult to detect. However, this baryons are expected to cover the missing baryonic mass in the standard cosmological model, but not more. To account with [itex]\Omega_b = 0.3[/itex] lots of baryons are needed.

    Garth, you suggest that [itex]\Omega_b = 0.3[/itex] could be possible in a freely coasting universe, but such an universe must be empty. How does such an universe expand linearly regardless of the [itex]\Omega_b = 0.3[/itex]?
     
  11. Jan 30, 2005 #10

    Garth

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    Actually [itex]\Omega_b = 0.2[/itex] or thereabouts. The universe is not empty, that would be required for a GR Friedmann universe, but the freely coasting model does not belong to the GR family of models.

    It is an empirical model based on the realisation that a strictly linearly expanding universe would not require Inflation as there would be no horizon, density or smoothness problems for Inflation to resolve. The freely coasting model is then found to be surprisingly concordant with the other cosmological constraints. In particular it produces the correct amount of helium with 20% baryon density, thereby possibly resolving the DM problem.
    The question it raises is how is this linear expansion achieved?

    My interest in it is Self Creation Cosmology, a modified GR gravitational theory, produces exactly this expansion - in its Einstein conformal frame,[that is the frame in which nucleosynthesis may be calculated] SCC also predicts [itex]\Omega_b = 0.22[/itex] and [itex]\Omega_total = 0.33[/itex]. There is little non-baryonic DM, some neutrino density, and little DE apart from a moderate [itex]\Omega_fv = 0.11[/itex] amount of false vacuum energy determined by the field equations.

    The reason nucleosynthesis is protracted in the freely coasting model is because the universe cools off more slowly,
    T-1 ~ R ~ t rather than
    T-1 ~ R ~ t1/2, not because they need to 'cook the books'! See Nucleosynthesis in a Simmering Universe

    Garth
     
    Last edited: Jan 30, 2005
  12. Jan 30, 2005 #11

    Chronos

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  13. Jan 31, 2005 #12

    Garth

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    Quote from paper:
    Note Cold Dark Matter has little or no pressure and could be baryonic in nature. (Planet sized masses)
    Earth sized masses formed not from a 'top-down' Jeans mass instability process but from a 'bottom-up' evolving primordial fluctuation process?
    Maximum percentage of DM in form of MACHOs 10-20%?
    Note also Constraints on dark energy from Chandra observations of the largest relaxed galaxy clusters
    Quote from that paper:
    (Thank you hellfire for that reference in a PM)

    So as a fraction of the critical density what distribution do we have of:
    1. Luminous observed matter (stars HII regions etc.)?
    2. Baryonic cluster DM (MACHOs, intragalactic X-ray emitting gas)?
    3. Cold intergalactic HI (lyman alpha forest)?
    3. Sub MACHO - earth sized objects?
    4. Unobserved dim dwarf galaxies ?

    Might it be that DM is largely baryonic if these components are summed?

    Garth
     
    Last edited: Jan 31, 2005
  14. Jan 31, 2005 #13
    the x-ray emitting gas in clusters is not dark matter. it is already accounted for, there is still 70-80% of mass lying in DM in clusters.
     
  15. Jan 31, 2005 #14

    Garth

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    I know, but do the sums add up?

    There is so much intra-galactic hot gas, plus the Lyman alpha forest evidence of a substantial amount of cold gas with relatively high metallicity, together with any black holes and planetary sized baryonic DM (see Schild: The detection and nature of the baryonic dark matter) that the visible stars and HII regions can only account for about 1/10 or so of the total baryonic content of the universe. WMAP puts that at 3% critical density, so the normal visible content of the universe, stars etc, account fo only about 0.3% critical density.

    In the 'good old days' before Inflation, DM or DE the density of the visible universe was estimated from star mass distributions to be about 2% critical density - so who is correct, were they mislead then or are we kidding ourselves now?

    Garth
     
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