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Thread: No Dark Matter in our part of the galaxy?

  1. #31
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    Quote Originally Posted by kzb View Post
    ... It actually says that an initially triaxial DM halo will be made more spherical over time. Also a prolate distribution with its axis perpendicular to the disk and q>2 is not considered.
    If this is true, then I read the paper too quickly... which happens. Thanks for the follow-up.
    Forming opinions as we speak

  2. #32
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    The only direct observations that indicate the existence of DM are based on gravitational effects. Beyond that nothing is known through observation about the nature of this dark matter and so far no direct detection has occurred. The presumed characteristics of WIMP-like DM are contradicted by measured mass distributions especially in DM galaxies (studies of nearby dwarf galaxy ). Theoretical interpretations of WIMP-like DM-driven galaxy formation are in contradiction with observations of the local group (a problem which keeps getting worse).

    The idea that nearby dwarf galaxies which contain lots of mass and few stars are made almost entirely of non-baryonic matter is really hard to swallow. It seems much more likely that they are made of very cold baryonic matter but conditions are not favorable to form a lot of stars. Why would non-baryonic DM have such a radically different distribution from baryonic matter in some local dwarf galaxy when most large galaxies seem to have about the same mix?

    I am not convinced that we are able to detect all baryonic matter in all possible states and densities. It seems that extreme scattering events got very short shrift in the literature and are still in need of much study. I'll bet that we have yet to discover most of the normal matter in our own galaxy. However, if it simply is not there, that does not prove that non-baryonic DM exists. Gravitational theory might be wrong or even just misunderstood.

    The real reason for clinging to non-baryonic DM as an explanation comes from LCMD cosmology, not observation. I'll post another thread on a paper that was in the news today. It concludes (if I interpret it correctly) that our satellite galaxies are the result of a early encounter with another large galaxy making the missing dwarf galaxy problem worse.

  3. #33
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    Quote Originally Posted by TooMany View Post
    The idea that nearby dwarf galaxies which contain lots of
    mass and few stars are made almost entirely of non-baryonic
    matter is really hard to swallow. It seems much more likely
    that they are made of very cold baryonic matter but conditions
    are not favorable to form a lot of stars. Why would non-baryonic
    DM have such a radically different distribution from baryonic
    matter in some local dwarf galaxy when most large galaxies
    seem to have about the same mix?
    Offhand speculation: Dark matter tends to break up into
    clumps which are all about the same size, while ordinary
    matter clumps over a much wider range of sizes. So all
    clumps of ordinary matter tend to associate with large
    clumps of dark matter.

    -- Jeff, in Minnrapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
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  4. #34
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    Too Many wrote:
    I am not convinced that we are able to detect all baryonic matter in all possible states and densities. It seems that extreme scattering events got very short shrift in the literature and are still in need of much study. I'll bet that we have yet to discover most of the normal matter in our own galaxy.

    I absolutely agree with you about the extreme scattering events (calculated to be 600/pc^3 of clouds 0.001 M-solar each) being neglected. But on the other hand, the paper under discussion is saying the mass budget of the galactic disk is more or less complete, at least going by the apparent gravitation in the z-direction (perpendicular to the disk plane). It does not differentiate between non-baryonic matter and baryonic matter, so there's no room for large masses of undiscovered baryonic matter either.

    But then we have this massive discrepancy with the gravitational field in the radial direction in the outer disk. So it's a bit of a headache. I think when you come up against an impasse like that, you have to say, well, this is but one paper, and it's only just come out. Probably some other group will come along and re-interpret their data and come to a different conclusion.

    However, if it simply is not there, that does not prove that non-baryonic DM exists. Gravitational theory might be wrong or even just misunderstood.

    I don't like modified gravity either. I mean, this study might mean the gravitational constant is directional !

  5. #35
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    Quote Originally Posted by Jeff Root View Post
    Offhand speculation: Dark matter tends to break up into
    clumps which are all about the same size, while ordinary
    matter clumps over a much wider range of sizes. So all
    clumps of ordinary matter tend to associate with large
    clumps of dark matter.

    -- Jeff, in Minnrapolis
    But what about the (original) Tulley-Fisher relation?

  6. #36
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    Quote Originally Posted by antoniseb View Post
    If this is true, then I read the paper too quickly... which happens. Thanks for the follow-up.
    Having re-read it, I realise I can't understand it because they don't explain their terms well enough. I think it's fairly clear that an initially triaxal DM halo will be circularised in the disk plane, but their diagrams of what happens in the z-direction seem to contradict each other. One shows the DM distribution profile being spread further, the other one seems to show it being sucked in. ?

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    Quote Originally Posted by TooMany View Post
    The only direct observations that indicate the existence of DM are based on gravitational effects.
    Hence the name "dark."

    Quote Originally Posted by TooMany View Post
    The presumed characteristics of WIMP-like DM are contradicted by measured mass distributions especially in DM galaxies (studies of nearby dwarf galaxy ).
    Citation, please.

    Quote Originally Posted by TooMany View Post
    Theoretical interpretations of WIMP-like DM-driven galaxy formation are in contradiction with observations of the local group (a problem which keeps getting worse).
    Explanation and/or citation, please.

    Quote Originally Posted by TooMany View Post
    The idea that nearby dwarf galaxies which contain lots of mass and few stars are made almost entirely of non-baryonic matter is really hard to swallow.
    Argument from incredulity.

    Quote Originally Posted by TooMany View Post
    I am not convinced that we are able to detect all baryonic matter in all possible states and densities.
    Are you aware of all detection methods in use?

    Quote Originally Posted by TooMany View Post
    The real reason for clinging to non-baryonic DM as an explanation comes from LCMD cosmology, not observation.
    I would say that's incorrect. It's more due to discrepancies between the mass of large astronomical objects determined from their gravitational effects, and mass calculated from the "luminous matter" they contain; such as stars, gas and dust. This is a major discrepancy for which a solution is needed. Dark matter appears to be that solution, even moreso since the weak gravitational lensing study on the Bullet Cluster.
    Everyone is entitled to his own opinion, but not his own facts.

  8. #38
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    Quote Originally Posted by Cougar View Post
    I would say that's incorrect. It's more due to discrepancies between the mass of large astronomical objects determined from their gravitational effects, and mass calculated from the "luminous matter" they contain; such as stars, gas and dust. This is a major discrepancy for which a solution is needed. Dark matter appears to be that solution, even moreso since the weak gravitational lensing study on the Bullet Cluster.
    Focusing only on galaxy rotation curves, I don't doubt that the missing matter is dark, I just doubt that it is non-baryonic. How much dark matter is needed seems subject to debate in the literature. If one assumes a spherical distribution, consistent with non-interacting, non-baryoic matter (e.g. WIMPs), then a lot is needed. If one assumes an extended disk of dark baryonic matter, the mass required is smaller. (Admittedly these are just my interpretations of papers that I've read.)

    The popular argument based on the Bullet Cluster has recently crashed into the Train-Wreck Cluster. It's difficult to draw conclusions from cluster interactions when they seem so inconsistent.

    Moreover, the resolution of rotation curves for galaxies does not require resolution of cosmic observations. If there actually is enough normal matter to explain the rotation curves, then other cosmic issues are irrelevant to the rotation issue.

    What is very dark is also very hard to detect. Almost everything we know depends on emitted light (although matter can also be detected by absorption). In the majority of galactic studies, only emitted light is available because there is no measured background that can be used to exploit absorption. Thus a dark baryonic matter component may go undetected.

    I'll try to respond to your other remarks later.

  9. #39
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    When looking at galaxies and trying to work out what is around them we have a very useful backlight - the galaxy itself. You would also expect, for clusters of galaxies, that the hot thin ICM and the X-rays it is throwing around would cause issues for molecular hydrogen clouds. Ionised hydrogen we are very good at spotting. We would also see more signatures of extensive baryonic matter structures when we see galaxies colliding or interacting with each other. Then there are the other species you would expect to find in these clouds, CO and so on. We can spot them pretty easily too. So you end up having to posit some strange properties of this 'normal' matter to explain why it is so good at hiding.

    Astronomers have got pretty inventive in ways to hunt for matter out there. Which is why dark matter has been accepted as the simplest explanation.

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    Quote Originally Posted by Shaula View Post
    When looking at galaxies and trying to work out what is around them we have a very useful backlight - the galaxy itself.
    Spirals have dust lanes. When seen on edge the opacity in the plane of the disk is striking. It appears as if the dust extends beyond the illuminated parts but that could be an illusion.

    Quote Originally Posted by Shaula View Post
    You would also expect, for clusters of galaxies, that the hot thin ICM and the X-rays it is throwing around would cause issues for molecular hydrogen clouds. Ionised hydrogen we are very good at spotting.
    We can't conclude that all molecular clouds are ionized because star formation would stop. Whether we would detect molecular clouds may be quite dependent on their size/density/distribution as well as the actual state of the matter in them.

    Perhaps the hydrogen is frozen. The freezing point is 14.01 K. It's not hard to imagine that temperatures fall well below that in the outskirts of galaxies. In this state H2 is immensely more dense than a molecular cloud and also (I suspect) very difficult to ionize.

    Here's a S&T article that mentions the subject: A Case for Frozen Hydrogen.

    Quote Originally Posted by Shaula View Post
    We would also see more signatures of extensive baryonic matter structures when we see galaxies colliding or interacting with each other.
    Perhaps we do see such signatures: missing matter

    Quote Originally Posted by Shaula View Post
    Then there are the other species you would expect to find in these clouds, CO and so on. We can spot them pretty easily too. So you end up having to posit some strange properties of this 'normal' matter to explain why it is so good at hiding.
    There is some doubt about the use of CO as a tracer in the outer regions because the metallicity is so low.

    Quote Originally Posted by Shaula View Post
    Astronomers have got pretty inventive in ways to hunt for matter out there. Which is why dark matter has been accepted as the simplest explanation.
    But we haven't detected that non-baryonic matter at all yet. If it is there, it apparently does not have the presumed properties (cuspy halo problem, dwarf galaxy problem). Recent studies indicate that those problems are not going to disappear.

    One reason for favoring the exotic dark matter over normal is the BB conclusions about how much baryonic matter their can be.
    Last edited by TooMany; 2012-Apr-28 at 04:14 PM. Reason: Fix link

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    In this state H2 is immensely more dense than a molecular cloud and also (I suspect) very difficult to ionize.
    It is just as easy to ionise - the only difference would be that if the lumps were large enough then some of the Hydrogen would be protected from ionisation. You also now have the issue that formation models of galaxies are in trouble, that these snowball clouds should behave differently under gravity to the dark matters (which means the model for the evolution of galaxies is even more badly broken than it is now). You also have to fine-tune the size of these lumps to get around the limits imposed on them by microlensing studies, or propose a mechanism to get rid of them from near us because otherwise we'd see evidence for them.

    But we haven't detected that non-baryonic matter at all yet. If it is there, it apparently does not have the presumed properties (cuspy halo problem, dwarf galaxy problem). Recent studies indicate that those problems are not going to disappear.
    And we have not detected the baryonic stuff either - which we should have. Yes there are some issues with the CDM models, hence the use of mixed dark matter models. Yes it is still under development. But so far invisible baryonic matter has proven to have more problems. Which is why it has been discarded at the moment. Who knows, thing may change. But nothing you are bringing up is new. It has bee though of. It has been tried. It did not make the grade. And all of these ideas are still being refined and tested all the time - scientists don't tend to all coalesce around one idea and ruthlessly ignore everything else. It is just that so far the best answer remains a dark matter model.

    There is some doubt about the use of CO as a tracer in the outer regions because the metallicity is so low.
    Which is why I said and so on. Even with ultra-pure hydrogen you should get some tracers. H3+ for example. And given the size of these clouds required even small amounts of tracer should show up - the path length through it is long.

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    Quote Originally Posted by Shaula View Post
    It is just as easy to ionise...
    These guys seem to agree: INTERSTELLAR SOLID HYDROGEN .
    I'd thought vapor pressure would prevent solid hydrogen from existing pretty much anywhere in space. Perhaps not, perhaps there is some out there.
    Last edited by Squink; 2012-Apr-28 at 08:05 PM. Reason: interstellar is spelled with 1 i.

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    Quote Originally Posted by Shaula View Post
    It is just as easy to ionise - the only difference would be that if the lumps were large enough then some of the Hydrogen would be protected from ionisation.
    I don't mean that the ionization potential is different, just that solids are different from isolated gas molecules. Ionization at the surface of a solid would leave it charged potentially making further loss of electrons more difficult. This is just a guess I don't know whether anyone has tested this under suitable conditions (which are very difficult to create).

    Quote Originally Posted by Shaula View Post
    You also now have the issue that formation models of galaxies are in trouble, that these snowball clouds should behave differently under gravity to the dark matters (which means the model for the evolution of galaxies is even more badly broken than it is now). You also have to fine-tune the size of these lumps to get around the limits imposed on them by microlensing studies, or propose a mechanism to get rid of them from near us because otherwise we'd see evidence for them.
    But how confident are we about these formation models anyway? Do they predict compact massive galaxies with very high rates of star formation at z>=2?

    --------------------------

    Consider another way of looking at the likelihood of condensed matter in the galaxy. We believe that our solar system formed in a cloud of dust, molecular hydrogen and other gases. We have detected some quite large solid objects beyond the orbit of Pluto with periods in the thousands of years. We believe that there is a significant amount of condensed matter called the Oort cloud which extends nearly a light-year from the sun and is the presumed source of long period comets. Somehow, all this frozen matter condensed right in the vicinity of a star and has persisted for at least 4.5 Gy!

    "The outer Oort cloud is believed to contain several trillion individual objects larger than approximately 1 km (0.62 mi)."

    We believe the Oort cloud matter exists and yet we are unable to directly detect it, even with microlensing (AFAIK).

    We still don't know all that much about the process in which stars form, but since they do form, we do know that molecular clouds can condense to form solid bodies, even with a star in the middle tending to ionize gas molecules. So what do we know about condensation of matter more generally? We know about star formation because stars are prodigious generators of light so we can see them. Do we really know that the only type of condensation that commonly occurs is star formation? That seems to me unlikely. We may still have a lot to learn about the baryonic matter in galaxies.

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    Quote Originally Posted by Squink View Post
    These guys seem to agree: INTERSTELLAR SOLID HYDROGEN .
    I'd thought vapor pressure would prevent solid hydrogen from existing pretty much anywhere in space. Perhaps not, perhaps there is some out there.
    Interesting. My statement about the freezing point of hydrogen was inaccurate. I neglected pressure.

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    We believe the Oort cloud matter exists and yet we are unable to directly detect it, even with microlensing (AFAIK).
    We have seen a few of them.

    It is not surprising that this matter condensed around a protostar and is stable around a dim star like the Sun.

    OK, I get it - you don't like dark matter. I repeat my previous - the models that best fit what we see use it. Maybe someone will come up with a better model and we can do away with it, maybe not. But people have tries all of the things you are suggesting and they have done a worse job than the dark matter models. Again, you really think that in the entire scientific community no one thought "hey, I will try to explain what I am seeing with comet-like bodies".

    Believe whatever you want to - at the moment the evidence favours exotic dark matter. Is that the answer? Well, we don't know. As has rightly been said until we have some of the stuff it is hard to be confident that it is. But as of now, the best models we have been able to develop use it.

  16. #46
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    Quote Originally Posted by TooMany View Post
    [...]

    --------------------------

    Consider another way of looking at the likelihood of condensed matter in the galaxy. We believe that our solar system formed in a cloud of dust, molecular hydrogen and other gases. We have detected some quite large solid objects beyond the orbit of Pluto with periods in the thousands of years. We believe that there is a significant amount of condensed matter called the Oort cloud which extends nearly a light-year from the sun and is the presumed source of long period comets. Somehow, all this frozen matter condensed right in the vicinity of a star and has persisted for at least 4.5 Gy!

    "The outer Oort cloud is believed to contain several trillion individual objects larger than approximately 1 km (0.62 mi)."

    We believe the Oort cloud matter exists and yet we are unable to directly detect it, even with microlensing (AFAIK).

    We still don't know all that much about the process in which stars form, but since they do form, we do know that molecular clouds can condense to form solid bodies, even with a star in the middle tending to ionize gas molecules. So what do we know about condensation of matter more generally? We know about star formation because stars are prodigious generators of light so we can see them. Do we really know that the only type of condensation that commonly occurs is star formation? That seems to me unlikely. We may still have a lot to learn about the baryonic matter in galaxies.
    A suggestion: do some BoT (back of the envelope) calculations.

    For example: what's the estimated total mass of the Oort cloud, expressed in sols*? What's the estimated mass of hydrogen and helium in the Oort cloud**? What, then, is its estimated metallicity? How does that compare with the estimated metallicity of the observed outer disk and halo stars (in our galaxy)? How many "Oort clouds" would you need to have, in our galaxy's halo, for them comprise a significant proportion of our galaxy's estimated DM (for 'significant', pick 30%, say)?

    What you may find is that you have a serious problem with your 'DM is largely "Oort clouds"' hypothesis, if you also wish to maintain consistency with just a small subset of other, directly relevant, observations.

    * 1 sol = the mass of the Sun; a sol can also be the total energy, per unit time, the Sun emits, in the form of electromagnetic radiation (i.e. its luminosity), you can tell the two apart from the context
    ** a reasonable approximation might be to assume that Oort cloud objects are made up of equal amounts of water, methane, and ammonia, with no trapped (or atmospheric) helium

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    Quote Originally Posted by Nereid View Post
    A suggestion: do some BoT (back of the envelope) calculations.

    For example: what's the estimated total mass of the Oort cloud, expressed in sols*? What's the estimated mass of hydrogen and helium in the Oort cloud**? What, then, is its estimated metallicity? How does that compare with the estimated metallicity of the observed outer disk and halo stars (in our galaxy)? How many "Oort clouds" would you need to have, in our galaxy's halo, for them comprise a significant proportion of our galaxy's estimated DM (for 'significant', pick 30%, say)?

    What you may find is that you have a serious problem with your 'DM is largely "Oort clouds"' hypothesis, if you also wish to maintain consistency with just a small subset of other, directly relevant, observations.

    * 1 sol = the mass of the Sun; a sol can also be the total energy, per unit time, the Sun emits, in the form of electromagnetic radiation (i.e. its luminosity), you can tell the two apart from the context
    ** a reasonable approximation might be to assume that Oort cloud objects are made up of equal amounts of water, methane, and ammonia, with no trapped (or atmospheric) helium
    The mass of the Oort cloud is thought to be about 5 earth masses which is a drop in the bucket when you consider the sun itself.

    I brought this up to make two points 1) matter can condense into solids even in the presence of radiation and 2) even though these objects are macroscopic (e.g. 1km) and quite nearby on the galactic scale, we cannot currently detect them. We have only detected the very largest and closest by reflected light from the sun.

    When we survey for dark baryonic matter we generally look for emissions or absorption spectra that is the result of the presence of diffuse gas. We base our estimate of how much matter exists on the intensity of these emissions or depth of absorption. Perhaps we add some factor for dust as well. That method neglects any unseen highly condensed matter. Can it detect relatively small clouds (on the order of AUs) that are quite dense?

    Could such matter exist in the cold outskirts of the galaxy? What will happen to a 1 pc diffuse cloud of hydrogen, helium and a trace of metals at 2.7K in the course of a billion years? As far as I have been able to tell it will collapse and break into clumps. How big will the clumps be? What prevents them from condensing at 2.7K? What is the equilibrium state? I'm trying to find information about this but so far I have found only a few papers. Perhaps I need the right search keys.

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    Quote Originally Posted by Shaula View Post
    Again, you really think that in the entire scientific community no one thought "hey, I will try to explain what I am seeing with comet-like bodies".
    No at all. I found a few papers on the subject but not yet the ones that claim to prove such matter does not or cannot exist. Still looking of course. If you have any suggestions I'd be grateful. Can molecular clouds only collapse into visible stars or not collapse at all?

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    No at all. I found a few papers on the subject but not yet the ones that claim to prove such matter does not or cannot exist.
    Because you cannot prove that. What you can prove is that it doesn't not fit observations made as a primary dark matter component. Which is all that is required.

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    Quote Originally Posted by Shaula View Post
    What you can prove is that it doesn't [not] fit observations made as a primary dark matter component. Which is all that is required.
    This paper suggests that observations do fit and fit better than CDM: Simulations of galactic disks including an additional dark baryonic component

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    Quote Originally Posted by kzb View Post
    But on the other hand, the paper under discussion is saying the mass budget of the galactic disk is more or less complete, at least going by the apparent gravitation in the z-direction (perpendicular to the disk plane). It does not differentiate between non-baryonic matter and baryonic matter, so there's no room for large masses of undiscovered baryonic matter either.
    More precisely it's saying the mass budget in this part of the disk is complete.

    Quote Originally Posted by kzb View Post
    But then we have this massive discrepancy with the gravitational field in the radial direction in the outer disk. So it's a bit of a headache.
    Maybe not if M/L grows sufficiently.

    Quote Originally Posted by kzb View Post
    I think when you come up against an impasse like that, you have to say, well, this is but one paper, and it's only just come out. Probably some other group will come along and re-interpret their data and come to a different conclusion.
    Right, it certainly needs to be debated.

    Quote Originally Posted by kzb View Post
    I don't like modified gravity either.
    Neither do I, but I'm also not too keen on undetected non-baryonic matter comprising 80% of the universe.

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    Quote Originally Posted by TooMany View Post
    This paper suggests that observations do fit and fit better than CDM: Simulations of galactic disks including an additional dark baryonic component
    No, it says that putting more baryonic matter in the disk helps solve some of the problems with the LCDM formation models. It leaves the halo where it is, as explicitly stated several times in the paper.

    In addition to this extra dark baryonic component, a non-baryonic, spheroidal pressure-supported dark halo containing most of the large-scale dark mass is conserved.

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    Quote Originally Posted by TooMany View Post
    The mass of the Oort cloud is thought to be about 5 earth masses which is a drop in the bucket when you consider the sun itself.

    I brought this up to make two points 1) matter can condense into solids even in the presence of radiation
    If the baryonic matter in the observable universe, averaged over a sufficiently large chunk of space, is totally dominated by H and He, that's irrelevant, isn't it?

    The question, surely, would be whether matter hydrogen and helium can condense into solids even in the presence of radiation. Is there any evidence, of any kind, that they can?

    and 2) even though these objects are macroscopic (e.g. 1km) and quite nearby on the galactic scale, we cannot currently detect them. We have only detected the very largest and closest by reflected light from the sun.
    And, of greater relevance to your idea, is whether any of the solid objects that we have detected are composed of ~90+% H&He (by mass). Are they?

    When we survey for dark baryonic matter we generally look for emissions or absorption spectra that is the result of the presence of diffuse gas.
    And dust, don't forget dust.

    Also, microlensing surveys look for MACHOs (MAssive Compact Halo Objects).

    We base our estimate of how much matter exists on the intensity of these emissions or depth of absorption.
    Um, partly, yes.

    But there's much more to it than that ...

    Perhaps we add some factor for dust as well.
    I think you should heed Celestial Mechanic's advice ("get the to a library!").

    You can search for dust independently.

    That method neglects any unseen highly condensed matter.
    MACHO searches explicitly aim to find exactly this kind of matter (for example).

    Can it detect relatively small clouds (on the order of AUs) that are quite dense?
    Depends on what you mean by "it"!

    There's a thread - started by quotation, IIRC - now in the S&T section which is about the composition of the ISM (among other things) in which there are posts on this; the answer is, yes (with caveats).

    And don't forget that baryonic matter which clumps tends to form clumps with a rather characteristic distribution of clump sizes (broadly, there are more smaller ones than larger ones, and the distribution can be approximated by a function of the form ...). What implications do you think this has?

    Could such matter exist in the cold outskirts of the galaxy? What will happen to a 1 pc diffuse cloud of hydrogen, helium and a trace of metals at 2.7K in the course of a billion years?
    A billion years ago it wasn't at 2.7K (the CMB had a higher temperature then), and won't be even today (cosmic rays, and the radiation field from the home galaxy will raise it by ~1K or so).

    And what is this small cloud surrounded by? Hotter, less dense plasma? Will shock waves from galactic supernovae reach it? How will it behave when there's a major merger? a minor merger?

    Better sharpen your pencil!

    As far as I have been able to tell it will collapse and break into clumps. How big will the clumps be? What prevents them from condensing at 2.7K? What is the equilibrium state? I'm trying to find information about this but so far I have found only a few papers. Perhaps I need the right search keys.
    Perhaps you do.

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    Quote Originally Posted by Shaula View Post
    No, it says that putting more baryonic matter in the disk helps solve some of the problems with the LCDM formation models. It leaves the halo where it is, as explicitly stated several times in the paper.
    That is true. However if you read many of the other papers Pfenniger has published you will see that he has long advocated that a substantial amount of very cold dark baryonic matter exists but is not seen. In this paper he makes very modest claims allowing for additional dark baryonic matter up to nearly 1/3 of the CDM halo.

    He also mentions in passing that moving mass from the halo to the disk does not affect rotation curves. However he avoids even addressing more extreme models in which even less mass is in the the halo and much more in the disk. Hmm... He also stresses trying to keep his claims within feasible limits of LCDM assumptions in his conclusions.

    What he claims to demonstrate is that additional dark baryonic mass is not only possible but helps explain galactic structure better than the present LCMD explanation.

    The main result of this work is that, despite having more mass in the disk, these systems are globally stable, the stability
    being ensured by the larger velocity dispersion of the dark gas that dominates the gravity in the outer part of the disk. In addition, the enhanced self-gravity of these disks, due to the presence of the dark clumpy gas, makes them more prone to form spirals extending up to 100 kpc in the dark gas.
    He has proposed in other papers that the HI observed in the outer parts may constitute only an "atmosphere" in which much higher concentrations of matter are embedded.

    I suspect that Pfenniger is trying to get his foot in the door. Papers that outright contradict LCDM about baryonic matter tend to get short shrift. You will notice that the entire last paragraph of his conclusion concerns how ordinary matter may be hiding in the galactic outreaches. He advertises ongoing work soon to be published that will argue for matter in states that are little considered in current theory. He even throws in this thought provoking tidbit:

    It is generally known that H2 condenses in solid form even at interstellar pressures at temperatures close to the 3 K cosmic background.
    He has been working on this stuff for at least a decade, perhaps without getting much attention. My take is that this paper is a clever strategy to gently urge the "mainstream" folks to give this issue the attention it deserves. What happens to cold dark baryonic matter due to self gravitation is extremely difficult to analyze. Simplistic thermodynamic/fluid-dynamic analysis is just not applicable (according to him and others).

  25. #55
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    Quote Originally Posted by Nereid View Post
    The question, surely, would be whether matter hydrogen and helium can condense into solids even in the presence of radiation. Is there any evidence, of any kind, that they can?
    Yes, and some theorist (e.g. Daniel Pfenniger) are trying to figure this out. I'm not forgetting dust. Don't forget that even a very small amounts of dust have a big effect on condensation. Don't snowflakes form that way?

    Quote Originally Posted by Nereid View Post
    And, of greater relevance to your idea, is whether any of the solid objects that we have detected are composed of ~90+% H&He (by mass). Are they?
    I'm not aware of any such detection. However since we cannot detect 1km snowballs in the Oort I think it's probably impossible (just now).

    Quote Originally Posted by Nereid View Post
    Also, microlensing surveys look for MACHOs (MAssive Compact Halo Objects).
    Right. But how big does the MACHO have to be? Also it's only been applied toward the galactic center and at fairly large angles away from the galactic plane (MCs). Study of the outer arms of our own galaxy with this method may be very difficult due to dust and the lack of background stars.

    Quote Originally Posted by Nereid View Post
    And don't forget that baryonic matter which clumps tends to form clumps with a rather characteristic distribution of clump sizes (broadly, there are more smaller ones than larger ones, and the distribution can be approximated by a function of the form ...). What implications do you think this has?
    I think it is generally believed to be fractal but the small-side limits are not known due to limited resolution. My conclusion is that we don't know precisely what can form yet in the sub-stellar range.

    Quote Originally Posted by Nereid View Post
    A billion years ago it wasn't at 2.7K (the CMB had a higher temperature then), and won't be even today (cosmic rays, and the radiation field from the home galaxy will raise it by ~1K or so).
    Nevertheless, condensation occurs or there would not be stars. Moreover we already know that stars formed at z~6 and yet we believe that they can only form at very low temperatures. What was the CMB temperature at z~6?

    Quote Originally Posted by Nereid View Post
    And what is this small cloud surrounded by?... Hotter, less dense plasma? Will shock waves from galactic supernovae reach it? How will it behave when there's a major merger? a minor merger?
    Better sharpen your pencil!
    Survival depends on conditions, rate of formation, density... I'm thinking mostly about the galactic outskirts. SN shocks are rare. Low mass concentrations may well be stable to UV, CMB, CR etc. for long periods during which more concentrations can form.

    Surely you are joking about calculation here. This stuff has barely been addressed in physics. I asked the question about the cloud because I'm not sure that science has a firm answer! Apparently the answer for mainstream astronomers is that nearly all the baryonic matter is in the form of stars, dust and gas. That's an enormous gap in the mass distribution of condensed matter objects (~10^31).

    When you are walking in the dark, it is foolish to declare that what you cannot see does not exist.

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    My take is that this paper is a clever strategy to gently urge the "mainstream" folks to give this issue the attention it deserves.
    So we have gone from "this paper shows proof that baryonic matter can replace non-baryonic matter" to "my reading of this paper implies that the person who wrote it might believe personally that non-baryonic matter could be done away with and is trying to get people used to the idea while he develops a model that will hopefully work". Somewhat less than convincing.

    He also highlights at the start why he keep within the LCDM model bounds - because it does a good job explaining so much else. Including how the galaxies got there in the first place. I know you don't want to discuss all that because it highlights how interconnected things are and weakens your case, but the author acknowledges that right up front.

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    TooMany wrote:

    Here's a S&T article that mentions the subject: A Case for Frozen Hydrogen

    Interestingly, this must be the same Mark Walker who publicised the Extreme Scattering Events. Although the article doesn't connect the topics, I can only think that Walker is quietly positing frozen hydrogen as the source of the scattering clouds.

    Extreme scattering events and Galactic dark matter

    http://arxiv.org/abs/astro-ph/9802111

  28. #58
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    Quote Originally Posted by TooMany View Post
    Quote Originally Posted by Nereid
    The question, surely, would be whether matter hydrogen and helium can condense into solids even in the presence of radiation. Is there any evidence, of any kind, that they can?
    Yes, and some theorist (e.g. Daniel Pfenniger) are trying to figure this out.
    Can you give some references, please?

    I'm particularly interested in learning how helium can condense into solids.

    I'm not forgetting dust. Don't forget that even a very small amounts of dust have a big effect on condensation. Don't snowflakes form that way?
    As I understand it, certain bacteria can form nucleation centres for snowflakes too.

    However, the mechanisms for the formation of snowflakes in a high-temperature, high density oxygen-nitrogen medium (saturated with water vapour) do not - in and of themselves - tell you much about how hydrogen and helium may form solid bodies in the low-temperature, near vacuum of the ISM, do they?

    And, of greater relevance to your idea, is whether any of the solid objects that we have detected are composed of ~90+% H&He (by mass). Are they?
    I'm not aware of any such detection. However since we cannot detect 1km snowballs in the Oort I think it's probably impossible (just now).
    This is getting rather silly, isn't it?

    We can't detect 1km gold nuggets in the Oort, nor 1km balls of frozen xenon, nor 1km balls of stromatolite DNA, nor ... Perhaps the missing baryonic mass in spiral galaxies (to explain their rotation curves) is in the form of a mixture of these three things?

    Also, microlensing surveys look for MACHOs (MAssive Compact Halo Objects).
    Right. But how big does the MACHO have to be?
    IIRC, down to about the mass of Neptune, to date.

    Also it's only been applied toward the galactic center and at fairly large angles away from the galactic plane (MCs). Study of the outer arms of our own galaxy with this method may be very difficult due to dust and the lack of background stars.
    Are you sure of that?

    Doesn't our galaxy - like a great many other spirals - have a flared, or warped, outer disk?

    And don't forget that baryonic matter which clumps tends to form clumps with a rather characteristic distribution of clump sizes (broadly, there are more smaller ones than larger ones, and the distribution can be approximated by a function of the form ...). What implications do you think this has?
    I think it is generally believed to be fractal but the small-side limits are not known due to limited resolution. My conclusion is that we don't know precisely what can form yet in the sub-stellar range.
    For a BoT, you don't need "precise"; you can reverse-engineer it: what would the size (or mass) distribution function have to look like to satisfy, simultaneously, the 'missing mass' (and its distribution) per rotation curves AND have escaped all attempts at detection, to date?

    If you do the calculation, I think you may find the required distribution function is rather, um, odd.

    A billion years ago it wasn't at 2.7K (the CMB had a higher temperature then), and won't be even today (cosmic rays, and the radiation field from the home galaxy will raise it by ~1K or so).
    Nevertheless, condensation occurs or there would not be stars
    Aren't you forgetting something?

    A mass of hydrogen and helium, with a teensy amount of metals, can contract under its own gravity, and efficiently get rid of the 'contraction energy' through electromagnetic radiation (that's what the metals do). 'Dust' which clumps to form planetismals can become massive enough to hold hydrogen and helium against escape. In either case, the minimum mass of a hydrogen/helium dominated object is ~that of Neptune, isn't it? Otherwise the Earth (and Venus and Mars and ...) would be mini-gas giants (a scaled-down version of Jupiter), wouldn't it?

    . Moreover we already know that stars formed at z~6 and yet we believe that they can only form at very low temperatures.
    I didn't know that.

    Do you have some references to papers on that, which I could read?

    What was the CMB temperature at z~6?
    Good question; do you know how to work out an estimate, yourself?

    What inputs do you need?

    And what is this small cloud surrounded by?... Hotter, less dense plasma? Will shock waves from galactic supernovae reach it? How will it behave when there's a major merger? a minor merger?
    Better sharpen your pencil!
    Survival depends on conditions, rate of formation, density... I'm thinking mostly about the galactic outskirts. SN shocks are rare.
    Are they? How rare?

    Low mass concentrations may well be stable to UV, CMB, CR etc. for long periods during which more concentrations can form.
    Indeed, they may.

    Or they may not.

    How would you go about finding out?

    Surely you are joking about calculation here. This stuff has barely been addressed in physics. I asked the question about the cloud because I'm not sure that science has a firm answer!
    BoTs can be very powerful.

    And they are relatively quick and easy to do.

    Apparently the answer for mainstream astronomers is that nearly all the baryonic matter is in the form of stars, dust and gas. That's an enormous gap in the mass distribution of condensed matter objects (~10^31).
    You've lost me, I'm afraid; what you are you referring to?

    When you are walking in the dark, it is foolish to declare that what you cannot see does not exist.
    Good thing no one is doing that then, isn't it?

  29. #59
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    Quote Originally Posted by Shaula View Post
    He also highlights at the start why he keep within the LCDM model bounds - because it does a good job explaining so much else. Including how the galaxies got there in the first place. I know you don't want to discuss all that because it highlights how interconnected things are and weakens your case, but the author acknowledges that right up front.
    Hmm... Non-baryonic matter is needed to explain galaxy formation because we believe not enough baryonic matter exists to explain it. That's a circular argument.

    You have to assume the LCDM model or nobody will read your paper. Here's a quote from a much earlier Pfenniger paper from 1993:

    In a companion paper (Paper I) we have proposed a new candidate to account for the dark matter around spiral galaxies: cold H2 gas in a fractal structure, supported by rotation, and concomitant with the HI disc. We have shown that this hypothesis is compatible with dynamical and observational constraints about disc galaxies, and explains several conspiracies [e.g. disk/halo] and paradoxes, since the dark matter is then in a form of fresh gas able to produce stars. In this paper we attempt to describe the physical conditions leading to a fractal state of cold gas in outer galaxy discs.
    He has written many papers since then, but as far as I can tell they have not attracted much interest. It's been almost 20 years since this paper was written.

    Mainstream theorists have reached the conclusion that such matter does not exist. They are not highly motivated to find such matter because it would conflict with LCDM which they consider a very satisfactory explaination. Therefore there is a tendency to explain away the possibility that baryonic matter could be there. Here's an early (1986) paper claiming to dismiss the possibility: A case against baryons in galactic halos

    It uses simplistic calculations to show that solid hydrogen could not exist. E.g. it says this in reference to the possible existence of hydrogen snowballs:

    It is clear that a single collision between them [snow balls] would be destructive. We must then require that the time between collisions be longer than the present age of the universe.
    Wow, how did they arrive at that? It seems they are making the assumption that snow balls cannot form to prove they cannot persist! If you continue to read this paper you will see that unjustified assumptions arise again and again allowing the authors to use simple mathematics to prove their point. If this is the type of evidence you refer to as why mainstream astronomers have excluded the possibility of substantial additional baryonic matter, it is disappointing.

    Note that I'm not claiming that there is solid hydrogen in particular. The whole question is what sorts of condensations can occur? We can directly detect stars, gas and dust, but not much else. Can we conclude only stars can form and everything else is diffuse gas? No doubt we would have concluded that nothing exists well beyond Pluto, if the long period comets did not show up in the sky.

    The LCDM argument is top down. It says we know how the universe began and how much baryonic matter was created and therefore galaxies don't have additional baryonic matter beyond that which we have detected. But what about bottom up? We live in a galaxy, what is it actually made of, irrespective of cosmic theory? How can we observe what could be there? Can we form theories about possible states of matter other than those which we detect? This approach seems somewhat neglected/dismissed.

  30. #60
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    Quote Originally Posted by TooMany View Post
    Hmm... Non-baryonic matter is needed to explain galaxy formation because we believe not enough baryonic matter exists to explain it. That's a circular argument.
    No.

    What you assume is gravity; observations then give you mass, and different observations give you baryonic mass.

    If there's a gap, either there's more (undetected) mass, or your assumptions about gravity are wrong (or both). This is a gross simplification of course.

    Where's the circularity?

    Mainstream theorists have reached the conclusion that such matter does not exist.
    What matter? Cold H2 gas?

    TooMany, I often find what you write frustratingly vague and imprecise. Could I ask you, please, to take greater care? Please try to express your ideas as unambigously as you can.

    They are not highly motivated to find such matter because it would conflict with LCDM which they consider a very satisfactory explaination.
    You really don't know many astronomers, do you? In fact, do you know any?

    I mean, how can you possibly know what motivates any astronomer?

    Therefore there is a tendency to explain away the possibility that baryonic matter could be there.
    If I were to start making wild assumptions about your motivations, then drawing conclusions from those assumptions, you'd scream, loud and clear, wouldn't you*?

    Yet you show no qualms about making ridiculous claims about hundreds, if not thousands, of people you have never even met!

    Why, oh why, do you do this (if I may ask)?

    [...]

    Note that I'm not claiming that there is solid hydrogen in particular. The whole question is what sorts of condensations can occur? We can directly detect stars, gas and dust, but not much else. Can we conclude only stars can form and everything else is diffuse gas? No doubt we would have concluded that nothing exists well beyond Pluto, if the long period comets did not show up in the sky.
    I took you to task, mildly, earlier, for over-simplifying things (and for not spending more time in the library).

    If you don't understand what I write, when I directly address a point in a post by you, please, ask me for clarification!

    Repeating arguments we've already been over, and then you ignoring what you yourself have accepted, is not conducive to having a meaningful discussion.

    The LCDM argument is top down. It says we know how the universe began and how much baryonic matter was created and therefore galaxies don't have additional baryonic matter beyond that which we have detected. But what about bottom up? We live in a galaxy, what is it actually made of, irrespective of cosmic theory? How can we observe what could be there? Can we form theories about possible states of matter other than those which we detect? This approach seems somewhat neglected/dismissed.
    Here's another example: we've been over this, and you already at least acknowledged the errors and weaknesses in the presentation of your case. Yet, here we are, mere days later, and you're repeating the same things!

    * unless, by some miracle, the assumptions I made were either correct, or wrong in a way you approved of

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