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Can dark matter be just a cloud of protons?

  1. Jul 14, 2008 #1
    I have been thinking about this for a while, and I am not sure about how I can visualize this.

    Lets imagine that there can exist clouds of primordial matter, far away in space.

    1. Can it be possible for a cloud to exist and be made out of strictly protons at 0 kelvins?

    I don't suspect they would be very visible, as no electrons would be around to emit EM radiation. I have heard that even protons can emit EM radiation at high velocity, and even in a collision, but at 0K in a cloud with little to know kinetic energy, 2. Would something like this be detectable?
  2. jcsd
  3. Jul 14, 2008 #2
    Protons at 0 kelvin...I don't know how that would work with protons being made of quarks which exhibit some sort of internal jiggling themselves which would produce some kinetic energy above 0k.

    But even so, if they were gargantuan clouds of protons, would they exhibit a slight positive charge? That would be shown false by the Bullet Cluster and DM clouds passing through each other with no real electromagnetic repulsive force acting on them. Granted the distances are far too large, but for sake of argument it doesn't sound very plausible.

    Not to mention the baryonic matter density of the universe itself is constrained by deuterium abundances in the universe. Protons are baryons (eg matter) so all matter, including protons, accounts for only ~2-3% of the energy density of the universe, with DM taking up the other 25% or so (and DE the rest).
  4. Jul 15, 2008 #3
    Here's another answer:

    The universe is suffused with electro-magnetic (EM) waves (light is an EM wave), or oscillating electric and magnetic fields. This means that the EM waves that interact with the cloud would cause the protons to move around since the protons would respond to the oscillating electric field in the EM waves. Therefore, the cloud will affect what we see with our telescopes since telescopes are just EM wave detectors.
  5. Jul 17, 2008 #4
    Sorry, I wasn't being clear. If the protons move around, then they will affect the EM waves by reflecting or scattering or partially absorbing the waves. Then, finally, our EM wave detectors (telescopes) will see the cloud of protons.
  6. Jul 17, 2008 #5


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    What would have happened to all the electrons under this hypothesis?

  7. Jul 17, 2008 #6
    oops, I didn't read your question closely enough.

    You think dark matter could be proton clouds because you say that only electrons (with negative charge) emit EM waves. It turns out that any accelerating charge emits EM waves, including protons (which have a positive charge).

    Don't worry, you weren't totally crazy for thinking this. Indeed, in most situations where both protons and electrons are present, most EM waves come from oscillating electrons. This is because electrons are 2000 times lighter than protons. Think of a bowling ball and a beach ball in the wind, obviously the beach ball will be buffeted (accelerated) more than the bowling ball. Finally, the higher acceleration the charge has, the greater the intensity the emitted EM waves will have.

    Notice I haven't explicitly discussed the case of atoms, electrons bound to protons, but, the same idea is roughly applicable (you really need quantum mechanics to understand exactly what's going on).
  8. Jul 17, 2008 #7
    I can see what you are saying. It is rather frustrating to read in books, primarily classical physics, where the electron is what emits and absorbs photons, because of the electronic cloud.

    I can appreciate the idea that protons can emit photons. But it is still confusing to me to envision a proton at rest emitting anything, close to 0K.

    This may sound stupid, but I guess I will say it anyway. If a kg of protons close to 0K out in space moved 10 meters, how is that any different than those same protons moving right here on earth? How is it different than a kg of water moving 10 meters? Would there be any extra photons discharged because of that, is the question.

    If it takes both electrical and magnetic fields to create and electromagnetic wave, how would it be possible to creating any kind of EM wave with a mass of protons with no negative charge? Is it possible to have an electric field that consists of only protons? The knee-jerk reaction is yes, but to define a positive charge, would mean that it has a potential, energy potential, to store energy, such as a volt. When electrons are not even present, I cant see that there would be a voltage at all.

    You see where I'm going with this? If voltage is not present, then how can you have EM waves even generated? Wouldn't you just have a magnetic field and gravitation field left?
  9. Jul 17, 2008 #8
    Lets say they don't exist near the protons' proximity. I would hate to be around if they ever showed up. :D
  10. Jul 17, 2008 #9
    If you have a cloud of protons, then you will have a net positive charge within the cloud. And you must also keep in mind the natural tendencies of the protons. Like charges repel, so the cloud will expand. This creates accelerating charges, which would then emit photons.

    But you can ignore this and ask if it is at all possible for protons to be the source of dark matter. I think that in order for protons to account for all the extra gravity seen, the universe would have to be practically swimming in a sea of protons. This would have noticeable effects everywhere we look. There would be interactions with the interstellar and intergalactic medium. The protons would interact with any interstellar magnetic field present and emit photons. Then whenever we observe stellar objects, there would be large absorption lines due to the intervening proton clouds. Protons are not neutral, so there is a nonzero cross section for interaction.
  11. Jul 21, 2008 #10
    Your asking good questions protonic_mass.

    Your absolutely right that a proton at rest (and not accelerating) and at 0K (which is sort of saying the same thing) would not emit any EM waves. However, these protons would not remain at 0 Kelvin since EM waves coming from stars etc. would jiggle the protons around with their EM fields. Soon the protons would have a non zero temperature and would be emitting light.

    Now about your question about moving 1 kg of water and protons. First, when you say move, I'm going to assume you mean accelerate, since you must accelerate a charge to get an EM wave. If you accelerate 1 kg of water, it won't emit any EM waves because it has not net charge (it has the same number of electrons and protons), so in the end your not accelerating anything with a net charge. If, however, you accelerate 1 kg of protons, then yes, you will cause the emission of EM waves.

    Now answering the rest of your questions in the last two paragraphs of your last post:

    It does not take both electric fields and magnetic fields to cause an EM wave. An EM wave is caused by an accelerating charge such as a proton or an electron.

    Yes, protons have electric fields that point outward from the protons, and electrons have electric fields that point inward toward the electron. The electric field does not consist of electrons nor protons; it is caused by them.

    Potential/voltage are confusing concepts that aren't needed to understand this situation. Suffice it to say that protons (instead of electrons) can be used to create voltage/potential differences. Again, though, I don't think these concepts help us in this situation.

    I'm puzzled by your last question regarding only having magnetic and gravitational fields left. It looks like you may be thinking that electrons create electric fields and protons create magnetic fields (not a bad guess given the name, "electric" field). As I said above, both protons and electrons create electric fields (and both can create magnetic fields). If the protons or electrons are at rest with respect to you, you will only detect their electric fields. If the protons or electrons are moving at a constant velocity with respect to you, you detect a mixture of their electric and magnetic fields. Finally, if the protons or electrons are accelerating, you will detect their EM waves (you would also detect their mixture of electric and magnetic fields not in the form EM waves).
  12. Jul 21, 2008 #11
    So what you are saying is that if dark matter was a cloud of protons, due to the nature of protons and how the environment around it would affect what we see and how they behave, it is unlikely your original statement is true, for we would have discovered it was protons by now.

    There is a steady steam of theorists who speculate that dark matter could be anti-gravity particles. It would explain the repulsive tendency of dark matter. I find it far fetched. Perhaps one of you could share your opinion.
  13. Jul 22, 2008 #12


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    Who are these theorists? What is an "anti-gravity" particle?
  14. Jul 22, 2008 #13
    Dur! Pardon-moi, i confused my information, as I was getting sources to answer I realized I made a mistake. You were right in being skeptical about my source. I did not mean "anti-gravity" I meant the neutralino.

    Here is a source that talks about the possibility of the neutralino being involved with dark matter. This article also delves into other aspects of finding and defining dark matter which might be helpful.

    http://www.europhysicsnews.com/full/22/article2/article2.html [Broken]
    Last edited by a moderator: May 3, 2017
  15. Jul 31, 2008 #14
    Plus, it's impossible to attain zero K because you would be violating the third law of thermodynamics.
    There's cold dark matter and hot dark matter. I'm wondering if someone could explain to me what the WIMP (Weakly Interacting Massive Particle) and MACHO (Massive Compact Halo Object) are. They are the particles that could describe cold dark matter. I think CDM is more consistent than hot dark matter, but it's just my opinion.
  16. Jul 31, 2008 #15

    T=0 doesn't violate the http://en.wikipedia.org/wiki/Third_law_of_thermodynamics" [Broken].

    A MACHO was (i.e. it's been ruled out) a candidate for Dark Matter in galaxies. A MACHO was simply something large like a stellar mass black hole or neutron star. We know MACHO's aren't dark matter because we would see their signature as they pass in front of stars via gravitational microlensing.

    WIMPs are a possible canidate for Dark Matter. They are simply elementary particles that are massive (probably more massive than protons) but don't interact with normal matter.

    Your opinion on cold dark matter (CDM) is probably correct. The CDM picture accurately predicts (statistically) the evolution of http://en.wikipedia.org/wiki/Large_scale_structure" [Broken].
    Last edited by a moderator: May 3, 2017
  17. Aug 1, 2008 #16


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    This question has already been answered, although not quantitatively.

    protonic_mass, let's assume a cubic metre (or a cubic parsec) of the MW halo contained just 10% of the estimated CDM (density) in the form of protons without electrons. Can you give a back-of-the-envelope estimate of what the Coulomb force a proton at the centre of such a volume would experience? Going a little bit further, and assuming the protons behave classically, what geometric arrangement would they have to adopt, for them to be even close to equilibrium? Finally, is such an arrangement a stable equilibrium?

    Now we know that the MW halo is not empty of 'hot' matter - cosmic rays are zipping through it all the time; we also know that it is transparent to photons of just about every frequency from ~TeV down to the plasma frequency of the solar system (it may well be transparent well below that too, but of course we can't observe in such low frequencies).

    How would all these high speed visitors to a MW halo of crystalline protons affect the crystal structure?

    If you don't know how to work through any of these back-of-the-envelope estimates, the many appropriate sections of PF are at your disposal!

    Finally, if you really want to develop this idea of yours further, why not draft a paper for the IR section?
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