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Dark Matter, Cluster Dynamics and The Bullet

  1. Apr 4, 2010 #1
    Everyone points to the Bullet Cluster as the best proof of Dark Matter. But I have a few questions about the rationale. There are aspects of the proof I don't understand.

    1) What is the mechanism which makes the interstellar gas visible at such a distance in this particular case? Is all intergalactic gas visible in the same way and at the same brightness levels?

    2) How do we know that all or most of the intergalactic gas collided? How do we know that it wasn't the case that most of the gas stayed with the galaxies and only some collided which was subsequently made much brighter because of the heating due to the collision? So in effect, how do we know that the part that forms the bullet wasn't merely just a visible small fraction of the intergalactic gas that collided while the bulk of the gas passed through the other cluster with the visible galaxies themselves?

    3) What would have caused the gas to slow down so much as it passed through the other gas? At the densities of intergalactic gas, it seems odd that much of it would have actually collided at the atomic level. What am I missing?

    Thanks in advance.
  2. jcsd
  3. Apr 4, 2010 #2


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    I’m going to try to answer your questions, but I suspect the pros to come up with better answers.

    The Bullet Cluster:

    Intergalactic gas is normally invisible to the 'naked eye' (telescopes), except if it’s illuminated by (newborn) stars etc. In this case we are talking about X-rays (from the very hot gas) detected by the satellite Chandra X-ray Observatory. The pink color is applied to show where the gas is located.

    Gravity tends (by time) to concentrate matter more and more. Planet and stars forms out off concentrated gas, to build galaxies. Dark matter makes 23% and ordinary matter (gas/stars) only makes 4%, therefore we should expect the much heavier Dark matter to attract the gas pretty easy.

    Think of normal colliding air in strong wind, or water.

    The Dark Matter Mystery:
    https://www.youtube.com/watch?v=<object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/nJN2X3NrQAE&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/nJN2X3NrQAE&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>
    Last edited by a moderator: May 4, 2017
  4. Apr 4, 2010 #3
    I'm familiar with the fact that the X-rays are the only way we can "see" them. I suppose I should have used a different word than visible as that implies human wavelengths. I really meant detectable via photon emission. Is the intergalactic gas always hot enough to emit enough photons so we can see it with devices like Chandra?

    This is a description of their attraction through gravity, not the likelihood of their collision. The same could be said of the galaxies and stars but they didn't seem to collide. In fact, the highest concentrations of dark matter would have to be in the galaxies themselves, if one is to believe the models, I think.

    With all due respect, I don't see how this answers my question at all.

    Really? We are talking about 10s of orders of magnitude lower density here, are we not?

    The density of intergalactic space is reported to be 10 to 100 hydrogen atoms of hydrogen per cubic meter. In a cubic meter of air you have about 10^25 molecules. The potential for a collision must go up as the square of the number of molecules, so we are talking about hundreds of orders of magnitude less likelihood of a collision for intergalactic molecules when compared with air at STP, unless I'm missing something.
  5. Apr 4, 2010 #4


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    I’m only a layman, but my guess is definitely no. Or maybe better – there is no other (known) intergalactic gas that extremely hot.
    I can’t tell you at what temperature intergalactic gas starts to emit photons (X-rays).

    Let’s start from scratch. Dark Matter is the main gravity attractor for ordinary matter (i.e. intergalactic gas etc). After the Big Bang when the universe expanded there were 'lumps' of Dark Matter that was crucial for the formation of stars and galaxies. The Dark Matter attracted and concentrated the gas, and concentrated gas gave birth to stars etc. This is crucial – this evolution could not take place without Dark Matter – there just isn’t enough ordinary matter in the universe to overcome the expansion of the universe.

    Now we know that the intergalactic gas and the galaxies are 'inside' the Dark Matter, 'caught' by gravity, a long time ago, right? And if two galaxy clusters collide – there’s 100% chance that the gas that actually 'made' these galaxies also collide, right?

    (To be even more specific – the Dark Matter is so dominant, that it actually surrounds the whole galaxy cluster.)

    The reason the galaxies didn’t collide, is the same as why Andromeda and Milky Way don’t collide at present.

    I have no numbers on the gas density. But I do know that the vast majority of the (ordinary) mass is in the intergalactic gas, not the galaxies (as you might have guessed). AND the speed of the collision was of about 6 million miles per hour (2,682,240 m/s ≈ 1% speed of light)!!

    I hope you realize what this means? If not: Imagine driving the hottest open sports car in extremely light rain – at 200 mph... Now, imagine doing the same thing but 30,000 times faster!

    Yes right, big 'problems'...

    (And you must not think of gas particles as 'nice' raindrops – these little 'creatures' moves around, rapidly. And the hotter they get, the more they move = more heat...)

    I can guarantee you that the collision created this enormous heat.
    EDIT: Extremely bad formulation. As a layman I can’t guarantee you anything. Read the "popular press" (=Wikipedia??) and everything else you can find on the subject, and make your own decision.

    Don’t agree? Well, then you better come up with a logical explanation for the gas at 100 million degree Celsius...
    Last edited by a moderator: May 4, 2017
  6. Apr 4, 2010 #5

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    How then do you explain the (many) other x-ray emitting gas regions outside the Bullet Cluster?
  7. Apr 4, 2010 #6


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    Thank you. The simplistic summaries in the popular press, and popularized versions of professional publications are often wildly inaccurate, especially in the the omission of relevant facts. We do not adequately understand the excitation/emissions of some materials, nor do we adequately understand the mechanics behind the formation of jets emerging from very active objects such as quasars and AGNs.

    Once we start integrating our views of our local universe in LOTS of wavelengths, the universe seems a lot more interacting and a lot less discontinuous. Browse through here:

  8. Apr 4, 2010 #7


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    Help! I’m haunted by a PF Mentor!? :smile:

    Seriously, if the universal standard for intergalactic gas is 100 million degree Celsius – I give up.

    If not – you come up with a better 'natural' explanation for this 'behavior' and formation.

    (I asked first, and you are the expert... :wink:)

    (Gas 'dragged' into Black Holes at extreme speed emits x-ray, right?)
    Last edited: Apr 4, 2010
  9. Apr 4, 2010 #8


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    Is this a joke?

    So, when cosmologist Max Tegmark (MIT) says that the Bullet Cluster probably is going to prove DM, or when Sean Carroll (Caltech) writes about the Bullet Cluster as the proof for DM – this "popularized versions of professional publications are often wildly inaccurate" ...??

    And Wikipedia is classified as "popular press" ...!?

    And NASA is wildly inaccurate? Promoting unscientific LIES on their official HubbleSite ...!?!? :surprised

    Good night everybody.

    Last edited by a moderator: May 4, 2017
  10. Apr 4, 2010 #9
    For starters, so we don't get confused with nomenclature here, while the gas within clusters is a subset of the intergalactic medium, it is generally referred to as the intracluster medium (ICM). It is certainly different from the interstellar medium (although it sort of is an interstellar medium).

    The primary emission mechanism for a large cluster is Bremsstrahlung emission -- Bremsstrahlung literally means "braking radiation" and it is so named because it is caused by charged particles which have been accelerated by the electric field of another charged particle. In the case of the ICM, which is so hot that the gas is ionised and emits in X-rays, it is a fast moving electron which is accelerated by a slower moving proton, or positively charged ion. All clusters emit X-rays via Bremsstrahlung, and particularly large clusters have their X-ray emission dominated by Bremsstrahlung. Smaller clusters may have a larger contribution of their X-ray emission from emission due to collisional excitation of the ions.

    Both emission mechanisms are caused by collisions, thus the brightness of the X-ray emission depends on the square of the gas density.

    Well, we know that most of the subcluster gas is with the bright Bullet part because the brightness of the X-ray emission is proportional to the square of the electron density integrated along the line of sight, so it is fairly easy to estimate the gas mass from the ICM, assuming some geometry for the gas. Of the baryonic content of clusters, the ICM clearly dominates making up around 80% of the baryonic mass. Were there a portion of the ICM significant enough to cause the lensing signal coincident with the subcluster galaxies, it would definitely be visible in X-rays. It is not.

    The gas is slowed by ram pressure, which is similar to the pressure felt when you stick your head out of a car window while driving. Your question regarding whether the ICM is collisional is understandable since the ICM is less dense than the best vacuums we can create on Earth (electron densities ~.01-.0001 per cm cubed) so it is hard to see how they will collide. The answer comes by comparing the scale of the system with the mean free path of the particles making up the ICM. When the mean free path is very small in comparison to the scale of the system, the system acts like a collisional fluid. For the ICM, the mean free path is ~1000 parsecs, compared to the size of the system which is of the order of 1 000 000 parsecs. Thus the ICM can be treated as a collisional fluid and we see things like ram pressure stripping of gas from galaxies and subclusters.
  11. Apr 4, 2010 #10


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    Thanks matt.o! Brilliant!!

    You are the first one here who seems to know what he’s talking about (including me :wink:).

    Just one question:
    Doesn’t the high velocity (~2,600,000 m/s) of the gas make some difference? If I stick my head out of a car window while driving 50 km/h, or 150 km/h, makes a pretty big difference?
  12. Apr 4, 2010 #11
    Yes, it sure does. In fact, the ram pressure [tex]\propto \rho v^2[/tex] where [tex]\rho[/tex] is the density and [tex]v[/tex] is the velocity.
  13. Apr 5, 2010 #12


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    WOW! Haha!! And I used the popularized version of my unprofessional silly little brain to figure that one out completely by myself!! :biggrin: :biggrin: :biggrin:

    Hehe! Victory is sweet! matt.o if you are a girl I want to marry you!! You are my hero!! (noo, just a silly joke, sorry) :rofl:

    Thank you so very very much matt.o!!!

    EDIT: Sorry for the 'madness'... It’s very late, and yes, I was somewhat 'frustrated'... But I’m cool now (and perfectly normal, I promise :smile:). Peace to everyone. Good night.
    Last edited: Apr 5, 2010
  14. Apr 5, 2010 #13


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    Well, I definitely don't think it's the best proof of dark matter. The best proof of dark matter is the confluence of the multiple lines of converging evidence. The best single piece of evidence is debatable. But what the Bullet Cluster most definitely is is the most visceral piece of evidence, the one that most obviously jumps out and screams, "Dark Matter!"

    There are other pieces of evidence that may, on balance, be better evidence (such as the CMB), but tend to be rather complicated and messy to explain. The Bullet Cluster has the virtue of being rather simple and obvious.

    It's really freakin' hot. The heat of the gas is driven by an incredibly simple process: as matter falls into the potential well of a galaxy cluster, it speeds up. The speeds of all these individual bits of matter make for a temperature of many millions of degrees, which makes for emission in the X-ray range. This hot cluster gas is a property of galaxy clusters, makes up most of the mass of galaxy clusters, and can be seen in at least nearly all of them, if not all (I suppose it's possible that collisions have stripped the gases out of some clusters).

    I think matt.o did a good job with the rest of it.
  15. Apr 5, 2010 #14
    matt.o and Chalnoth,

    Thanks for the explanations. That clears some things up.
  16. Apr 6, 2010 #15


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    I agree, and this lively "pink-hot-colliding-speedy-gas-discussion" is rather excessive. We really only need to know 3 basic facts:

    1) Where is the gas located?
    2) Where are the galaxies located?
    3) Does the gas make up for ~80% of the baryonic (ordinary) mass?

    I could be wrong, but to my understanding these questions can be answered with rather high precision by the cosmologist.

    Now, we use http://en.wikipedia.org/wiki/Gravitational_lens" [Broken] to find where the majority of mass is located.


    And it’s not in the gas, where you would expect it to be.

    One could argue that this doesn’t prove anything – this is only one strange phenomenon in the gigantic universe. It could be extremely fat green aliens in tremendously massive spaceships, hiding behind the galaxies, just as the photo is taken...

    We can’t reject the "Massive-Alien-Theory" – but if this is the case, there must be a whole community of "massive aliens" out there, because now there’s another proof of "unknown massive objects" in http://en.wikipedia.org/wiki/MACS_J0025.4-1222" [Broken].

    In the case of MACS J0025.4-1222, even a layman can see where the gravitational lensing is dominant:


    MACS J0025.4-1222 in 60 Seconds Plus
    https://www.youtube.com/watch?v=<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/rn_CBHvq29k&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/rn_CBHvq29k&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>

    If this was the only indication of Dark Matter, the discussion could be extensive whether it’s Aliens, or MOND 2eV hot neutrinos, or Dark Matter, or something else. But it isn’t:
    • http://en.wikipedia.org/wiki/LCDM" [Broken], the standard model of Big Bang cosmology, includes DM.
    • The http://www.sarahbridle.net/lectures/uclgrad07/lss_and_cmb.pdf" [Broken] shows clear indications of DM.
    • In computer simulations of the large-scale structure of the universe DM is needed, to make it work with the current theory of gravitation (to get a picture that fits real observations).
    Large-scale structure of the Universe
    https://www.youtube.com/watch?v=<object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/FFlzyxSQhTc&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/FFlzyxSQhTc&hl=en_US&fs=1&rel=0&color1=0x2b405b&color2=0x6b8ab6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>

    Last edited by a moderator: May 4, 2017
  17. Apr 6, 2010 #16


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    I don't think anybody would ever seriously consider a "massive alien theory".
  18. Apr 6, 2010 #17


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    No, of course you’re right. It’s was more kind of a (obvious) 'joke'... :wink:
  19. Apr 8, 2010 #18
    How about a "fat Americans from the future theory." That I could believe.
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