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Dark matter!

  1. May 26, 2005 #1

    Lisa!

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    I want to know more about dark matter.
    Please tell me:
    Why do scientists think that there should be dark matter?
     
  2. jcsd
  3. May 26, 2005 #2

    wolram

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    Try this article Lisa

    http://en.wikipedia.org/wiki/Dark_matter

    Dark matter
    From Wikipedia, the free encyclopedia.

    In cosmology, dark matter consists of elementary particles that cannot be detected by their emitted radiation but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. Estimates of the amount of matter in the universe, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. In addition, the existence of dark matter resolves a number of inconsistencies in the Big Bang theory, and is crucial for structure formation.

    Much of the mass of the universe is believed to exist in the "dark sector". Determining the nature of this missing mass is one of the most important problems in modern cosmology. About 25% of the universe is thought to be composed of dark matter, and 70% is thought to consist of dark energy, an even stranger component distributed diffusely in space that likely cannot be thought of as ordinary particles.
    Science
    Unsolved problems in physics: What is dark matter? How is it generated? Is it related to supersymmetry?

    The question of the existence of dark matter may seem irrelevant to our existence here on Earth. However, whether or not dark matter really exists could determine the ultimate fate of the present universe. We know the universe is now expanding because of the red shift that light from distant heavenly bodies exhibits. The amount of ordinary matter seen in the universe is not enough for gravity to stop this expansion, and so the expansion would continue forever in the absence of dark matter. In principle, enough dark matter in the universe could cause the universe's expansion to stop or even reverse (leading to an eventual Big Crunch).
     
  4. May 26, 2005 #3

    Garth

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    And welcome to these Forums Lisa!!!

    Good question, keep them coming!

    It may be constructive to note that there are other ideas about Dark Matter such as MOND (MOdified Newtonian Dynamics), which may be pertinent because DM has not (yet) been identified.

    The problem with the standard model is that model also predicts that the baryonic density (i.e. ordinary matter, proton electrons, neutrons etc.) should only be about 4% of the critical density. Therefore, if that is correct, most of this DM (23% critical density) must be in some exotic, unknown, form.

    The standard model predicts the overall density is actually the critical density so it also requires that 76% of the mass of the universe is in the form of Dark Energy. (Useful as well to make the universe accelerate in its expansion).

    Thus according to that standard model not only have we not discovered 96% of the mass of the universe but also we have no idea of the form that it may be!

    Some of us have suggested other models that do not require Dark Energy or non-baryonic Dark Matter at all. We shall see...

    Garth
     
  5. May 26, 2005 #4

    Lisa!

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    Thanks both of you.

     
  6. May 26, 2005 #5

    wolram

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    Lisa don't worry, as you read more your understanding will increase, believe
    me i am a numb skull, but please read, read, read, and eventually it will not
    be difficult
     
  7. May 26, 2005 #6
    I know very little about Dark Matter, but my limited reading on it seems to tell me that it (or something else) is needed to explain observed gravitational interactions in and between galaxies, and mostly distant galaxies. I don't recall seeing anything to suggest that there is a similar need to invoke Dark Matter to explain what we observe more locally, that is, to explain gravitational interactions observed in our own galaxy or solar system.

    Can some one tell me about this and set me straight? That is, do we also need a Dark Matter explanation for more local observations? If so, how so (examples)? The Wikipedia site wolram gave said that the Milky Way may contain 10 times more Dark Matter than ordinary matter, but offers no observational basis for this. Do we observe the effect of Dark Matter in our own solar system or on nearby stars? If not, why not -- why would we need such a substantial factor to adjust for / explain distant observations but not need one for more local observations?

    Thanks for any insights.

    f3
     
  8. May 26, 2005 #7

    Chronos

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    Hi and also welcome Lisa! Wolram and Garth did a splendid job. Permit me to add another indicator. There is a very important feature of the the universe as a whole called curvature. This curvature feature depends entirely upon the total energy content of the universe, where matter is one component of this total energy - a very dense one as expressed by e=mc^2. This total energy density tells us whether the universe will expand forever, perhaps even tearing itself apart [the big rip] or eventually stop expanding and reverse course collapsing back upon itself [the big crunch]. The term used to describe this energy density is called omega [scientists are fond of using exotic names for things to impress non-scientists]. The magic value for this omega thing, called the critical density, is 1.0000000000... If omega is less than 1.000, the universe is 'open', which means there is not enough gravity to stop the universe from expanding forever. If omega is greater than 1.000, the universe is 'closed', which means there is more than enough gravity to someday stop expansion and pull everything back together again, presumably leading to a repeat performance of the big bang. Now you probably are asking yourself, how in the world do you go about measuring this omega thing? Actually there a number of ways. Astronomers have been busily counting all of the stuff they can see [matter] in the universe and figuring how far apart they are from each other. They have also measure how much non-matter energy [radiation] is floating around in the universe. Add all of this up and you get the average measured energy density [omega] of the universe - which turns out to way less than the critical density of 1.000. Ok, then the universe is 'open' and will expand forever, right? Well, maybe not. Being diabolically clever and resourceful, scientists have also figured out how to physically measure this curvature thing. You will have to trust me on this one. It's complicated, so I will spare you the brutal details, but is quite doable as it turns out. One of the most accurate measurement made of this curvature was done with this experiment called the Wilkinson Microwave Anisotropic Probe [WMAP]. WMAP is [it's still going on] a study of the cosmic microwave background radiation [CMBR]. After analyzing the huge amount of data gathered by WMAP, scientists deduced the universe is as flat as a possum who guessed poorly at when to sprint across the interstate - omega = 1.02 ± 0.02. You can find all kinds of articles about this on the internet. Just do a search for 'WMAP omega' and you should get around 10,000 hits.
     
  9. May 26, 2005 #8

    Chronos

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    Hi Frank3 and also welcome to PF. Truth is, there can be no more than a very small quantity of dark matter in our solar system - less than earth mass. If there was much more than that it would, as you may suspect, mess up the planetary orbits. This would have really messed with Kepler's head when he was deriving his laws of orbital motion, as you might well imagine. Why so little of the stuff in our neighborhood yet supposedly so much in the galaxy as a whole? There is no especially good answer to that question. This is part of the reason Milgrom came up with MOND. Unfortunately, MOND doesn't answer all the hard questions either. So, as usual in the wild world of astronomy and cosmology, we have more questions than answers. The good new is, our revered elder astrophysicists have left lots of cool stuff for aspiring youngsters to discover. I have some related reading material you may find interesting:
    http://www.astro.livjm.ac.uk/~ejk/dark.html
    http://zebu.uoregon.edu/2004/a321/lec18.html
    http://curious.astro.cornell.edu/question.php?number=571
     
  10. May 26, 2005 #9

    Nereid

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    OK, in addition to adding my welcome to PF! to Chronos' and others', may I add my favourite spiel for DM?

    The role of DM in cosmology is real ... and also hotly debated.

    The role of DM in galaxies (well, actually only spirals; but that's another story) is confusing/troubling/clear/whatever ... this is the playground of MOND (also the Pioneer anomaly, and maybe more).

    HOWEVER, there's another regime - not (necessarily) cosmological, and certainly not galactic - where NO ONE has come up with an even vaguely plausible alternative to DM ... rich clusters.

    I've written on this numerous times - both here in PF and elsewhere - and I really don't want to repeat myself ... BUT:
    a) if there is DM, then there's far, far more in the (local) universe in rich galaxy clusters than anywhere else (including the halos of spirals)
    b) there are multiple, independent lines of investigation - yielding approx equal results - that the DM in rich clusters is very real, and very non-baryonic (note in passing, mostly to Garth - this is one set of good observational data which the SCC fans have yet to address, unless I have missed a key paper or three)
    c) MOND explicitly cannot address this rich cluster DM (as Milgrom was among the first to fess up to)
    d) the physics behind the independent 'telescopes' showing this rich cluster DM covers most of modern physics; if there's no DM, then just about ALL of modern physics is (likely) in serious trouble.

    OK, enough for today.
     
  11. May 26, 2005 #10

    Garth

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    Neried, I have followed your posts on the subject and I have been grateful for your considered criticism of baryonic DM. However, exactly why can this rich cluster DM not be baryonic?
    O.K. so it is not observed, but has this ruled out all baryonic possibilities?

    Garth
     
  12. May 26, 2005 #11

    Nereid

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    Garth,

    Consider:
    1) SZE: it essentially 'counts free electrons', so its estimate of mass includes all the (baryonic) plasma in the cluster, by default (unless you can construct a model which reproduces the observed effect, AND gives the same estimates of total cluster mass as hydrodynamic IGM equilibrium, Virial Theorem, and GR lensing)
    2) 'distant object/quasar absorption': the lines in the spectra of background objects rule out significant quantities of neutral gas (at least, enough to account for the mass 'missing' in the 3, independent 'total mass' methods
    3) colours of distant objects seen through the cluster: essentially zero 'IGM reddening', so no significant amount of 'missing baryons' in the form of dust
    4) systematic errors (in observations): OK, this may have legs, but to show that the many sets of independent observations are consistent - within rigourously analysed datasets - would be quite a challenge (and AFAIK, no one has even hinted this is where the resolution may lie)
    5) what does that leave us with? red or white or brown dwarfs loose in the cluster potential? pebbles and rocks? rogue Jupiters and comets?? Just to think of how there could be enough of any of these to make up the gap must give one a headache! how did they get there? where did they form? why aren't the transition objects visible (e.g. SNR, PNe)? why are the cluster galaxy stars still primarily H/He (if the missing mass is in the IGM and not primarily H/He)?
    6) ultra-massive (DM?) halos for the cluster galaxies: inconsistent with lensing and X-ray observations (not to mention galaxy velocity dispersions)
    7) some combination of all of the above? This would actually be rather nice! :smile: a giant, cosmic joke ... just the right balance to throw all independent sets of observations off, by just the right amounts :surprised

    Note that, for sure, there is 'missing mass' in the form of plasma clumps, rocks, rogue Jupiters, white/red/brown dwarfs, neutral gas, dust, ... but no way (consistent with the observational results) that all of this added up could amount to more than a small fraction of what we estimate to 'missing' ... from three independent types of observations (just in case I didn't already say that).

    Have I missed anything significant?
     
  13. May 26, 2005 #12
    or in simple terms
    the math doesnot work , something is missing
    so we call it DARK MATTER and DARK ENERGY
    so we need DARK MATTER and DARK energy to balance the books
    on the univerce we see and how we think gravity "works"
    just exactly what it is we have only very a few vauge clues
    and a better idea on what it is not then what it is
     
  14. May 27, 2005 #13

    Lisa!

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    Thanks all of you.

    Oh thank you but you know it was my browser problem.I couldn't go through the link and read it at all.?(I'll give it a try again)
     
  15. May 27, 2005 #14

    Garth

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    Thank you Nereid for that considered reply.

    The hypothesis I wish to test is:
    The DM (23% closure density – Omega = 0.23) is primordially baryonic.

    Here there is no question as to existence of DM, especially rich cluster DM, but, ‘Have you missed anything significant?’ The first response from those holding to the standard model to my hypothesis would be, “It cannot be baryonic, because BB Nucleosynthesis only allows a baryonic 4% closure density; and that is pushing it.” (Before WMAP this value was generally thought to be limited to 2% – 3%)

    However the Concordant “Freely Coasting” Cosmology model produces a baryonic closure density of 23.9% and so this cosmological objection may be overcome, the question is where is all this baryonic matter now?

    You were right to question a “cosmic joke”, that is a contrived mix of just the right ingredients to mislead the set of observations that have placed a low maximum limit on observed baryons.

    But first let us remember how ‘contrived’ the standard model is. Dark energy, or cosmological acceleration, has to be massively ‘switched on’ to provide inflation to overcome the horizon, flatness and smoothness problems of the Friedmann models. Next, it is ‘switched off’ during the nucleosynthesis period to produce the right ‘cosmic mix’. Then it is ‘switched back on’ to account for the distant SN Ia observations, and finally it is ‘switched back off again’ for the recent past period. The result is a model that fits the observations; but forgive me if I am a little sceptical of this scenario and see it as ‘contrived’!

    Note the freely coasting model does not require inflation or non-baryonic DM, it does not require DE to explain the distant SN Ia observations but it does require a mechanism to deliver that linear expansion. SCC provides that mechanism in the form of a non-minimally connected ‘Machian’ scalar field, and furthermore it does not require significant DE to make up the closure density. The SCC model is conformally flat, yet closed, so that it not only fits the WMAP spectrum peaks but also the WMAP lack of low mode anisotropies.

    The problem with the SCC model is locating the baryonic DM. This could be an irrelevant question if the theory is rubbish, however, should GPB come up trumps for SCC it may then become the most relevant astrophysical/cosmological question of all!

    My scenario:
    1. Out of the freely coasting BB emerges a dense plasma of H He and high metallicity.
    2. A series of over-dense inhomogeneities condense into a series of objects with a ~linear log-log mass function.
    3. A few super-massive BHs (106 – 109)MSolar form.
    4. Massive PopIII stars quickly consume their fuel and go SN. They re-ionise the IGM and leave many intermediate mass BH’s (10 – 105)MSolar.
    5. Finally a dense high metal IGM permeates space.
    6. The super-massive and massive BHs form the nuclei for galaxy formation, which leads to PopII star formation and further metallicity.
    7. Finally PopI stars with planetary systems form. And here we are.


    This ‘hand waving’ scenario would lead us to expect, together with the normal galactic and stellar systems, a high metallicity IGM (observed in the Lyman alpha forest), a number of super massive BHs (observed in galactic centres/quasars) and a huge number of intermediate mass BHs. (I am warming to Smolin’s CNS hypothesis that our universe maximises BH number!)
    Could these intermediate BHs be the unobserved DM?

    All I have to do is put some numbers to this! Is anyone willing to help? Does anyone think it worth discussing this further? Shall we start another thread?

    Garth
     
    Last edited: May 27, 2005
  16. May 27, 2005 #15
    Hi Garth, one thing you need to remember is the hierarchical fromation methods involving Lambda CDM models mimick what we observe very closely. Since baryons collapse in a very different manner to CDM, it would be difficult to derive models that describe what we observe in large scale structure.

    you also need to ask yourself; what would we observe if there were ~10^9 intermediate mass black holes residing in a cluster (since dark matter makes up around 10^14-10^15Msolar in a rich cluster, you would need around 10^9 to account for the unobserved mass)?
     
  17. May 27, 2005 #16

    turbo

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    Can you supply a time-line (approximate epochs) for items 3-7? The reason I ask is that as far back as we have been able to see (z~6.5) quasars and their host galaxies exhibit metallicities similar to or higher than that of our own galaxy. There has been no observed evolution in metallicity with redshift. This is a happy circumstance for an infinite steady-state universe, but puts some constraints on the Big Bang model (galaxies of super-solar metallicity form in less than a billion years). I know that your freely coasting model gets some extra breathing room in this regard (>13.7Gy), but at what point would you regard this metallicity as a constraint on your model? If the Webb or the LBT can push these observations back to z~7, 7.5, 8, etc, with no evolution of metallicity, where do we start questioning the validity of the BB model in SCC?
     
  18. May 27, 2005 #17

    Garth

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    Thank you matt.o Hi! Indeed what would we observe?
    The SCC freely coasting model (SCC's Einstein conformal frame) has the following parameters:
    R(t) = t,
    Omegabaryon = 0.22 (precisely 2/9),
    Omegafalse vacuum = 0.11 (precisely 1/9),
    Omegatotal = 0.333 (precisely 1/3)
    Has anyone run a hierarchial model simulation of these parameters? At the moment I don't feel able to re-run the programme of cosmological research for the past twenty years on my own, but I'm willing to try with help!
    It would be interesting to see what it would come up with and so compare with the LambdaCDM model.

    As far as observing the intermediate BH's is concerned we would need 1012 of 109 MSolar in a rich cluster. So lensing events are to be expected. Are these observed? In our galactic halo microlensing surveys, ( Preliminary results on galactic dark matter from the complete EROS2 microlensing survey ), have shown that MACHOs (< 102 MSolar) (based on 4 microlensing events) can account for only 3% of the halo mass. However these speculative objects could be small BHs, in which case there may exist a population of rarer, more massive, objects that would account for the greater percentage of the halo mass. Should these have been observed? Has anyone looked?

    Garth
     
    Last edited: May 27, 2005
  19. May 27, 2005 #18

    Garth

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    Hi turbo-1 I won't engage in any more 'hand waving' in terms of putting ages to these epochs without doing the calculations. However the linearly expanding model gives considerably more breathing space in the earliest PopIII star and galaxy forming epochs.

    The major point though is the “Freely Coasting” model produces high primordial metallicity:
    The problem it has is deuterium, which has to be produced by spallation in this model. Spallation occurs anyway and contaminates the results of the standard model - but that is not often acknowledged!
    So there does not seem to be a problem with high z metallicity, on the contrary such observations support this model.

    Garth
     
  20. May 27, 2005 #19
    i think your numbers are slightly out, a rich cluster has a mass of around 10^14-10^15 Msolar, so 10^12 of 10^9 Msolar BH's doesn't match. But anyway, I dont think this is plausible in a cluster, since the dark matter distributions seem to be smooth in a relaxed cluster (they would not be if the DM was concentrated into BH's). I am guessing we would also observe the effects of these intra cluster BH's, due to their interaction with the other baryons (ie accretion leading to outbursts etc). I don't think observations of the x-ray emitting ICM would be so featureless in nearby relaxed clusters either.

    We would also need to observe quite a number of these intermediate mass black holes at the centers of clusters. I also believe that the to form this many BH's, we would need a very biased initial distribution of density perturbations.

    Please note, I have not laid pen to paper to work out most of the above, and I don't intend to (not enough hours in the day!).
     
  21. May 27, 2005 #20

    Nereid

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    First, a general comment on how well SCC may apply to 'rich cluster DM' - it surely would be good fun to crunch the cosmological (SCC) models! However, it's at the 0-th order that I will concentrate my fire (a.k.a. sharp questions; well, I hope they'll be sharp). So, to that end, let's let the herrings (red) swim away: the ratio of baryonic to non-baryonic DM for rich clusters comes from observations, not a theory in cosmology (no matter how powerful the cosmological models are) ... you take the data from observations (of many different kinds), you plug it into your favourite Virial Theorem/hydrodynamic/GR lensing/SZE/... calculators, and out pops the answer - no cosmological parameters in these calculators! (except H0, and the 0-th order properties of the CMBR).

    Some big nuts for you to crack Garth (matt.o has already mentioned several; apologies for the repetition):

    - the 'missing mass' is not in the galaxies: unless gravity is way off (and in SCC it's essentially the same as GR, to the level we're discussing, right?), we can work out the total mass within spirals from the rotation curves - SMBHs, IMBHs, and all (I'm not so sure how well constrained the total mass in ellipticals is) - and we can count the galaxies ('faint dwarf galaxies' can be estimated, and the estimates constrained, by various techniques) ... so it doesn't matter whether all the stars in all the galaxies are made of Fe, whether there are billions of rogue Jupiters, whatever

    - IF the 'missing cluster mass' is in the IGM, THEN it can't be (mostly) baryonic UNLESS it's not ionized (from SZE observations) and not gas (from absorption lines) and not dust (from absorption spectra and a lack of reddening). Note that analyses of the Lyman forest - so far - are lacking in good 'smoking guns' (AFAIK, there are no Lyman system quasars 'behind' the well observed SZE/lensed/X-ray/virial-theorem-observed clusters), no doubt these will turn up in the next decade or so. HOWEVER, if you'd like to make an estimate of what the IGM plasma/gas/dust composition must be to account for (say) 80% of the 'missing cluster mass', we could then test whether that is consistent with observational data

    - the IMBHs: you may be onto something here. Do you have an OOM mass distribution (if not, you could always just assume a 'reasonable' one)? If there were sufficient numbers to account for the 'missing cluster mass', and spread evenly (relatively speaking) throughout the cluster, what could we expect to see? While there's certainly some work you could do on things like their relaxation time (would they be approx virialised by now?), maybe the first cab off the rank would be things like how many should be passing through a galaxy 'now', how would they interact with the IGM, how often would they collide/merge/form binaries, how would they 'show up' in lensed clusters (and how), ...
     
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