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Dark Matter: Why doesnt dark matter coalesce 'into' Ordinary Matter - Repost

  1. Oct 4, 2014 #1
    Note: this is a re-post because my initial post had problems, no one could reply to it.

    Ok this is my second in the series about dark matter.

    In a previous thread I asked where is dark matter

    The main response to my question in the other thread was dark matter has no way of losing energy so it doesn't clump basically it just slingshots off other DM and OM (dark matter and ordinary matter) but due to little or no friction can never clump into a large mass like say a dark planet or even a dark star.

    The responses seemed reasonable and made perfect sense.

    But I have had a sleep since then...

    DM interacts with OM due to its mass giving rise to gravity. That is a given. By definition that must mean dark matter which is small and light must be attracted to OM objects. For example planets, stars, galaxies, clusters etc.. As such like neutrinos we must have a constant barrage of DM passing through us all the time.

    Unlike neutrinos DM has considerable mass (relatively) hence when a neutrino passes through the globe or us it virtually passes unhindered. But DM has mass and lots of it! so if a DM particle passes through a massive object like a planet or star or human body it must interact with it. There must be a transfer of energy from the DM particle to the OM object.

    As the DM particle passes through the OM object it must try and drag the OM object along with it thereby losing a large proportion of its momentum (energy) and entering into an orbit around or even within the OM object. The net effect is the OM captures the DM.

    So while I can see individual DM particles dancing around themselves and other OM particles forever in some frenetic dance. Once captured by a more massive object I cannot see it dancing off wildly into the distance.

    As DM outweigh OM significantly these capture events must be very common. Which then implies much of ordinary matter is made up of dark matter.
  2. jcsd
  3. Oct 5, 2014 #2


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    The problem seems to be fixed now.


    Dark matter passes through massive objects in the same way as neutrinos. Those objects look like empty space with a gravitational potential for dark matter. In the same way space probes can fly by planets to change their orbit without colliding with the planet, dark matter particles change their direction and maybe speed a bit, but there is nothing it would collide with (probably with extremely rare exceptions of weak interactions, those would be again similar to neutrinos).
  4. Oct 5, 2014 #3
    Small and light DM is neutrinos, but since our best models of it is CDM, Cold Dark Matter, neutrinos are IIRC only ~ 0.2 % of the total and most DM is relatively large and heavy as particles go. (Unless they are particles of more exotic fields.)

    Gravity is, like fields, conservative. If you want to transfer energy you need tidal effects (akin to near field energy transfer) or gravity waves (akin to far field energy transfer). Just passing another point mass (DM passing a nucleus) won't give any of that, due to the nature of gravity. (E.g. gravity waves need spherical deformations to be generated by a single body, et cetera.)

    If DM is WIMPs, Weakly Interacting Massive Particles, they can hit an atom nucleus now and then. If so it will happen ~ 1 time/year, and leave a heat spike on the order of "forget-about-it". That is why WIMP searches are so hard.

    Most of the DM arriving in the filaments seen in cosmos (which threads galactix clusters) are seeded by energy density fluctuations during inflation, which gets converted to density and velocity fluctuations when the later Hot Big Bang release particles. But as soon as these variations meet (like the light caustics you see at the bottom of a swimming pool), gravity captures them and the results are the filaments seen in large galaxy searches, weak lensing imaging and cosmological simulations.

    I'm not familiar with the details of how such capture works.

    That is not how it works. EM interactions makes ordinary matter way more dense, because it can radiate away energy and bind particles to rigid structures. As a result our solar system has less than an average asteroid worth of DM:

    "So if there’s a sea of dark matter that permeates space where we are — all through the Solar System — the outer planets should see a slightly different (greater) mass than the inner planets. And if there’s enough dark matter, it should be detectable.

    “Let’s calculate it,” the professor said to me, and so we spent the next half-hour doing just that. When we finished, we’d found that about 1013 kg of dark matter ought to be felt by Earth’s orbit, while around 1017 kg would be felt by a planet like Neptune. These values are tiny; the Sun has a mass of 2 ✕ 1030 kg, while values in the 1013 - 1017 kg range are the mass of a single modest asteroid. Someday, we may understand the Solar System well enough that such tiny differences will be detectable, but we’re a good factor of 100,000+ away from that right now."

    [ http://scienceblogs.com/startswitha...matter-affect-the-motion-of-the-solar-system/ ]

    There is a reason why DM went unnoticed for so long, and the is because it is so diffuse. (That it is "dark" doesn't help either, but it doesn't seem to be the main reason. We can see it with weak lensing just fine.)
    Last edited: Oct 5, 2014
  5. Oct 5, 2014 #4
    I was going to put a message in the original thread but it seems to have been removed. Which is OK. Just thought I'd mention it.

    I think you have nailed my problem here. but I am afraid I don't fully understand your response. A mass like a planet or sun is not a point mass. From the outside we can consider it as a point mass but once you are within its confines its no longer a point mass. It is a dense(ish) collection of point masses.

    If we consider the earth a point mass then yes a dark matter particle (relatively light) would swing about it and continue its journey but even then it MUST (conservation of energy) lose some of its energy in deforming the motion of the larger object. if it has just the right trajectory the DM may even increase its own energy but I assume that trajectory has to be just right to achieve that goal.

    My point is that DM must interact (due to its mass hence gravity) with OM and while its within an ordinary matter object it must lose energy. hence even without friction it can lose enough energy to slow down to orbit within the much larger. Hence coalesces 'into' the larger object. Even though it never actually clumps (electrical attraction).

    I have pondered that when a particle accelerates towards a larger massive object it gains enough energy to eject itself out the other side but I also consider that will only happen when the BM has a trajectory that passes directly through the centre of gravity of the larger object. Any other trajectory will result in the DM giving the OM object torque. Thus losing energy. Thus entering into an orbit around the centre of gravity thus continuing to slow down (due to the dense field of point masses around it) by trying to apply torque to the massive object until it finally comes to rest within the larger capturing ordinary matter.

    If what you are saying is true then it also means dark matter can pass through black holes unhindered and even ordinary matter could pass through a black hole unhindered provided it doesn't encounter any frictional events within the black hole.

    When it comes to simulations that show the universe evolving into its current state only if you include just the right amount of DM. Could it be that these models erroneously consider all mass as point masses and do not take into account what I have discussed above?
  6. Oct 5, 2014 #5


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    You use qualifiers like "must" all the time, when there is in fact no process that would dictate such a necessity, or even a possibility.
    There is nothing in gravitational interaction that would make two objects exchange or radiate energy.

    What Torbjorn said about it being conservative, means that a particle will gain exactly the same amount of kinetic energy as it falls down towards the other body, as it will lose on the way up and away. Conservation of energy requires that to be true.

    There is nothing to cause DM to slow down while it's inside a large lump of ordinary mass. For a dark matter particle, an ordinary mass is as good as invisible. The only thing that it "is" from the DM perspective is a shape of a gravity well, a curvature of space-time. The spatial extent and composition will only change the shape of the well, nothing more.

    How about next time you're primed to say "must lose energy", you try and formulate how exactly that happens, because so far you seem to be just using your hunch.
  7. Oct 6, 2014 #6
    I use the phrase "must" because that is as I understand it. If DM can somehow not be influenced by the gravity of OM then please explain by what mechanism this would operate.

    For example can a particle of DM pass at close proximity to a particle of OM say Hydrogen without influencing the hydrogen? If that is the case how does it not influence it?
  8. Oct 6, 2014 #7


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    Recent papers on DM are leaning towards the lighter range [<10 Kev]. Heavy DM particle models create tension with observation - like overdensities in galactic cluster and severe core-cusp gradients.
  9. Oct 6, 2014 #8


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    Saying "as I understand it" would be so much better. Saying "must" can imply connotations you are asserting your statements as fact (has to be true).
    Bandersnatched answered that for you:
    Think about this. It isn't like DM has some type of external leverage to pull with, or rocket engines to kick in and add extra energy. If I follow what Bandersnatch indicates, it's an incoming-outgoing type of a dance. Your earlier post mentions "coalescing" into the larger (presumably OM) object. The OM is physically structured ("clumped" if you prefer) so particles aren't just floating aimlessly on their own. When DM passes through, any influence due to gravity and OM will mostly be felt by the DM "on the way in", and given up "on the way out".

    Is that right @Bandersnatch?
  10. Oct 6, 2014 #9
    Are you thinking of gravity assist? "A physics student is entitled to be puzzled ... To accelerate, the spacecraft flies with the movement of the planet (taking a small amount of the planet's orbital energy); to decelerate, the spacecraft flies against the movement of the planet. The sum of the kinetic energies of both bodies remains constant (see elastic collision)." [ http://en.wikipedia.org/wiki/Gravity_assist ]

    I forgot that, because it is a curious 3-body problem, depending on the presence of a dominant star that sets a reference motion for the planet. "A physics student is entitled to be puzzled ... Thus even a tiny spacecraft can extract a finite amount of energy from an overwhelmingly massive planet—provided the planet is moving initially. Here ‘‘finite’’ is contrasted with ‘‘infinitesimal.’’ [ http://community.dur.ac.uk/bob.johnson/SL/AJP00448.pdf ]

    [I can certainly state that I was one of those "puzzled students" that paper tries to teach. :H]

    In my facile classification, gravity assist would be closest to "a tidal force". Anyway, it is negligible in most cases. Compare with neutrinos.

    I don't get how that would be implied. But: no. The trajectories of all particles, including photons, will end up behind the event horizon as soon as they are behind the event horizon. That is what defines a _black_ hole.

    Here it is easier to see why: gravity curves spacetime. It is spacetime itself that curves "too much" eventually. No need for energy losses. (Though there will be such, due to the strong tidal forces and what not.)
  11. Oct 6, 2014 #10


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    Gravity assist needs careful planning to give the desired velocity change. If you just make it randomly (like dark matter), the net effect is nearly zero.

    @curiouschris: If the particles on earth would all be free to move everywhere, then a dark matter particle would attract nearby particles in its path, creating a tiny overdensity behind it, which indeed would act as some sort of friction. However, most atoms on earth are in liquids or solids and cannot move like that, and even if they could the effect would be tens of orders of magnitude (!) too weak to be relevant.
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