Of Particular Significance

More on Dark Matter and the Large Hadron Collider

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/15/2015

As promised in my last post, I’ve now written the answer to the second of the three questions I posed about how the Large Hadron Collider [LHC] can search for dark matter.  You can read the answers to the first two questions here. The first question was about how scientists can possibly look for something that passes through a detector without leaving any trace!  The second question is how scientists can tell the difference between ordinary production of neutrinos — which also leave no trace — and production of something else. [The answer to the third question — how one could determine this “something else” really is what makes up dark matter — will be added to the article later this week.]

In the meantime, after Monday’s post, I got a number of interesting questions about dark matter, why most experts are confident it exists, etc.  There are many reasons to be confident; it’s not just one argument, but a set of interlocking arguments.  One of the most powerful comes from simulations of the universe’s history.  These simulations

  • start with what we think we know about the early universe from the cosmic microwave background [CMB], including the amount of ordinary and dark matter inferred from the CMB (assuming Einstein’s gravity theory is right), and also including the degree of non-uniformity of the local temperature and density;
  • and use equations for known physics, including Einstein’s gravity, the behavior of gas and dust when compressed and heated, the effects of various forms of electromagnetic radiation on matter, etc.

The output of the these simulations is a prediction for the universe today — and indeed, it roughly has the properties of the one we inhabit.

Here’s a video from the Illustris collaboration, which has done the most detailed simulation of the universe so far.  Note the age of the universe listed at the bottom as the video proceeds.  On the left side of the video you see dark matter.  It quickly clumps under the force of gravity, forming a wispy, filamentary structure with dense knots, which then becomes rather stable; moderately dense regions are blue, highly dense regions are pink.  On the right side is shown gas.  You see that after the dark matter structure begins to form, that structure attracts gas, also through gravity, which then forms galaxies (blue knots) around the dense knots of dark matter.  The galaxies then form black holes with energetic disks and jets, and stars, many of which explode.   These much more complicated astrophysical effects blow clouds of heated gas (red) into intergalactic space.

Meanwhile, the distribution of galaxies in the real universe, as measured by astronomers, is illustrated in this video from the Sloan Digital Sky Survey.   You can see by eye that the galaxies in our universe show a filamentary structure, with big nearly-empty spaces, and loose strings of galaxies ending in big clusters.  That’s consistent with what is seen in the Illustris simulation.

Now if you’d like to drop the dark matter idea, the question you have to ask is this: could the simulations still give a universe similar to ours if you took dark matter out and instead modified Einstein’s gravity somehow?  [Usually this type of change goes under the name of MOND.]

In the simulation, gravity causes the dark matter, which is “cold” (cosmo-speak for “made from objects traveling much slower than light speed”), to form filamentary structures that then serve as the seeds for gas to clump and form galaxies.  So if you want to take the dark matter out, and instead change gravity to explain other features that are normally explained by dark matter, you have a challenge.   You are in danger of not creating the filamentary structure seen in our universe.  Somehow your change in the equations for gravity has to cause the gas to form galaxies along filaments, and do so in the time allotted.  Otherwise it won’t lead to the type of universe that we actually live in.

Challenging, yes.  Challenging is not the same as impossible. But everyone one should understand that the arguments in favor of dark matter are by no means limited to the questions of how stars move in galaxies and how galaxies move in galaxy clusters.  Any implementation of MOND has to explain a lot of other things that, in most experts’ eyes, are efficiently taken care of by cold dark matter.

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49 Responses

  1. Shalom to Matt Many thanks for yours articles regarding the universe. Some comments are over my head but I appreciate all that you share. Question: The numerous pictures of the stars and galaxies are exciting and depict vivid colors. Are these colors part of the original photographs or are the photos in black and white and then “colorized” or touched-up for publication? I have a feeling that the original photos are b/w and then colorized according to “someone’s” idea of what elements are there. IF true, any idea what color is supposed to depict which element? Best regards, Boyce Fitzpatrick Date: Wed, 15 Apr 2015 12:35:47 +0000 To: fitzpatrick1@msn.com

    1. This is a complex issue for many reasons. In the main all color is false. The best example is images of the sun which, in their ‘native color’ would be blindingly white-yellow. They are often colored orange to fit our ideas of a ‘hot’ sun, and the contrast is enhanced. You will also see many images of the sun in say UV or x-rays which of course cannot have their ‘natural’ color.

      For stars and galaxies the issues are the same. Most ‘true color’ images of galaxies would be a rather plain whitish blue or yellow given that they average a large number of stars. Colors and contrasts are often added to show information (Young vs old stars, hot gas vs cold gas, etc.) As such most astronomy photos can be called ‘false color’ but I like to think that they show what is really there and what our own eyes are too poor to see.

  2. It is the distance between the galaxies wich is expanding, not the size of the galaxies themselves (though some may have eaten neighbours).

    1. There would be a problem with that; the stars wouldn’t be orbiting but also moving out. We would see this almost as if the galaxy was exploding. Then would be the question as to what would be causing this expansion. There’s also the problem that wherever we see galaxies the same problem occurs, no matter when in time. Indeed our own galaxy has this anomaly, occurring now.

  3. Looking at the rotation curve of a typical spiral galaxy (ref. Wikipedia, subject Dark Matter) as observed, “dark matter can explain the flat appearance of the velocity curve out to a large radius.

    This I do not understand: to me it seems that there isTa speed increase out to a large radius. Is this abnormal?

    1. It’s simple. The predicted (blue) curve shows the velocity dropping quickly. This you would expect, if you are twice as far from the sun you have to move four times as slow, 3 times out, 9 times as slow. That’s the law of gravity.

      But what we *see* is the velocity is almost the same! It even dips a bit but then *increases*. That doesn’t make sense! It would be as if the planet Mars raced around the sun faster than us. The answer is that there must be some unseen mass spread throughout the galaxy.

      The dip then increase is important too; if you just said ‘Ah, gravity works different’ you would expect to see all decrease, just not as fast. But that increase suggests something more complex is afoot.

  4. Ok, I have to read more carefully the posts on this blog. On April 6, Matt said, in ‘LHC restarts’ :
    “dark matter could easily be of a form that the LHC cannot produce, (for example, axions, or particles that interact only gravitationally, or non-particle-like objects)”

    So, I have my answer, it is : No.

  5. I have a question for Matt : Can axions be produced at LHC ? I am not sure…

    If this is the case, could they be indirectly detected by missing energy, as you explain for ‘dark matter’, but perhaps you suppose it is composed of wimps ?

  6. Matt, let me rephrase my question–what physical substance might dark matter be composed of if not particles? What else is available for making matter?

  7. We can observe the effects of dark matter on the scale of galaxies/galaxy clusters, but has anyone found evidence for/against it on a stellar scale? For instance, is the observed normal matter in a globular cluster sufficient to explain the star’s orbits? Or has anyone found binary stars orbiting a bit faster than they should? It would seem that would be a useful way to discriminate between “plain” dark matter ideas (sterile neutrinos, axions, LSP’s, etc) and theories which would allow richer physics (like mirror matter).

    And as a purely off-topic question, I read an article a few weeks ago about detecting relativistic spacecraft via its interaction with the CMB, and it made me wonder. Is there any unusual physics that would occur when the beam at LHC interacts with the cosmic neutrino background? Or is it moving fast enough to do so?


    1. We have seen dark matter effects on the scale of globular clusters. However when it comes to binary stars we have very strong evidence *against* dark matter. The orbital periods of binary stars, especially ‘extreme’ ones like neutron star pairs can be measured very accurately. (In the case of extreme systems the decay of their orbit due to gravitational waves can be measured.) The individual stars’ masses can be measured via their spectra. So far we have not detected any systems that behave oddly in that they look lighter than they act.

    2. I can answer the off-topic question. The link concerning spaceships is amusing, but it refers most certainly to a well known effect, the GZK cutoff (Greisen – Zatsepin -Kuzmin, the three discoverers of this effect in the sixteen, after the discovery of the CMB). I am surprised that it is not cited in this link.

      Charged particles (mostly protons), above an energy of 5×10^19 GeV can interact with the photons of the cosmic wave background (at 2.7 K), they have enough energy in the center of mass to produce pions, that decays themselves in two photons. So they loose energy, and after multiple such interaction, their energy drops below 5×10^19 GeV where they can no more interact with the CMB. It happens after around 1 milion year light of travel or so.
      In consequence, we should not observe cosmic rays above 5×10^19 Gev, coming outside the galaxy. This is the GZK cutoff. And we don’t know in the galaxy a mechanism that is capable to produce such energy per proton. See for example :

      So, in your link, they apply this limit to a fictionary spaceship. Why not, but there is something more real and observed, the GZK cutoff for high energy cosmic rays arriving to earth.

      1. But spaceships are cool and particles… aren’t. As it sits the link uses ridiculous ships. Why go at .99c and suffering such things when you don’t get much benefit over .95c which is nearly infinitely safer?

        As it is the GZK limit’s violation is curious, how can this be happening?

  8. Yes BBC report on possible dark matter drag/self-interaction in a group of galaxies quotes one of the authors as non-committal.

    Dr Thomas Kitching, a co-author of the study from University College London, said: “What we measured, with high significance, is an offset between the light emitted from the galaxies in this cluster, and the total mass.
    “It’s too early to say if this is a dark matter effect, or caused by normal astrophysical processes. What we need to do now, is make simulations of these collisions to distinguish these possibilities.”

    1. And you can imagine how non-committal other experts would be if the authors themselves aren’t convinced of the dark-matter interpretation of their results.

      It’s almost certainly an astrophysics effect.

  9. To get galaxies and their filamentary structure, to explain observed gravitational lensing, and to explain the motions of galaxies in galactic clusters, my toy idea has been made. [Link removed by host. This is not an advertising site.]

  10. What accounts for the asymmetry of the filaments? Should it not be more like skewed lattice but still one of consistent and uniform structures. I see a ridge of high intensity of energy (mass) much like the edges of the continental plates floating in the earth’s lava.

    Question I have is, is there an unknown field(s) that are creating these ruptures, which could be spewing “out” matter and it’s this matter that create gravity? Looking at this simulation, seems like gravity is too weak to hold such huge structures in such a huge universe. Is there another field (theory) which we are not aware of yet that could explain the effects of dark matter and gravity together?

    1. If cosmic inflation occurred, then the initial distribution of dark (and ordinary) matter was slightly non-uniform, and the non-uniformities were random — driven by quantum effects that are intrinsically random. (There have been other proposals that the non-uniformities were random due to thermal effects.) It is this randomness that leads to the non-symmetry of the filaments.

      If the non-uniformities had been laid down in a lattice structure, then yes, the filaments would be lattice-like. Said another way, the fact that they aren’t is post facto evidence that the non-uniformities weren’t regular, but instead, random.

      1. Yes, “intrinsically random”, like in the “mexican hat”? It’s just that I have a problem accepting the mexican hat, seems contradictory.

        As for the second question, is it possible that the gravitation field is driven by matter (quantum resonances of a “field”) and likewise, the EMF field is driven by charge (quantum rotations of the same “field”)?

        I know you wrote about even though G and Q have similar equations there is no relationship between them but could they both be driven by the same field at the opposite side of the energy density spectrum?

        1. I’m not sure about the ‘Mexican hat’ but a random, asymmetric distribution is what I would expect. It is what is left over when there’s nothing particularly special organizing things. If DM were in some sort of regular or semi-regular structure we’d need a reason behind that.

      2. I suppose I could have pose the question differently. If there was a single big bang and regardless what the inflation mechanism was (is) one should see a more uniform distribution of the visible matter. The structures we see today would suggest more than one “bangs” (and the after effects of each), which when superimposed over existing matter would create the non-symmetry.

        I see symmetry as the “natural” direction of change (energy) and non-symmetry is an effect caused by a disturbance of a natural phenomenon.

        So, I guess the next question could be, can the simulation results be reached in more than one way?

        1. Looks are very deceiving my friend. Especially here. The problem that inflation solves is matter is TOO evenly distributed. If there was no inflation we should be nothing but black holes and empty space, very uneven indeed.

          If there were multiple bangs and inflations we have a problem; inflation is BIG. There’s no way our universe would be able to have two ‘inflations’ cross it, not unless it was too slow to even out everything to what we see now. In fact the results we see are very consistent with one bang.

          Symmetry is a high energy phenomena, unstable. Think of a thousand pencils all balanced on their points together. Tip just one and it will knock others until the whole lot fall down. So it is with gravitationally attracting matter; once one small unevenness appears it magnifies. You can in fact see this with water. Splash some on a window. One splash, but you will see as it drains and forms droplets it forms a weblike structure of tiny rivulets; the surface tension of the water magnifies the initial unevenness. Slow motion videos of these things are informative.

  11. After all if the LHC is a potential gravity engine = fusion reaction ( potential gravity engine )

    Our problems are over !

    And if there was a lot of mass in a black hole why is it not a v big sun ? ?

    Because there is non !

    1. I personally don’t think MOND is right, but I respect people who are making a serious, mathematically and physically rigorous effort to formulate a theory of MOND that would work. [My first calculation in my long career was a MOND calculation; but later a graduate student I knew showed the idea it was based on was inconsistent with the behavior of galaxy clusters.] It’s always important to have alternatives because that’s how you check your reasoning and avoid missing conceptual possibilities.

      What I don’t respect is people who insist that dark matter is a dumb idea and who don’t know the data. It’s not a dumb idea and it fits a lot of different types of data. If it turns out to be wrong, that will not be a proof that it was a dumb idea — just that some smarter idea was needed.

  12. Hi matt – i think i have asked this question before but cannot remember if you answered it?

    With all your proton masses travelling at near C with angular momentum and acceleration – One would expect some form of

    Local lateral gravity to be produced? Is this the case? Anything detected? Have you even searched for it ?

    1. Each bunch of protons contains 10^11= 100,000,000,000 protons. Their energy is less than 10^4=10,000 times their mass times c^2, so their gravitational effect is less than the gravity of 10^16 protons. That’s less than one gram. Even the gravity of an entire beam, with over 1000 bunches, is much less than the gravity of a coin. So no, the gravitational effects cannot be observed. I’m going to delete the pure-speculative and long post that you followed this with.

  13. Matt: For the purpose of simulation, do they assume the same law of gravitation (or space-time curvature) for interaction between dark matter and dark matter and between dark matter and ordinary matter? What about G?

    1. Yes, they assume that the laws of gravitation are those of Einstein’s theory of gravity.

      In Einstein’s theory, G has to be the same — that’s the equivalence principle — for all types of matter, and gravity itself.

      Of course the fact that the amount of dark matter they need to get galaxies and their filamentary structure is similar to the amount of dark matter needed to explain observed gravitational lensing, and is similar to the amount needed to explain the motions of galaxies in galactic clusters, is a cross-check that the gravity exerted by dark matter on itself, on ordinary matter, and on light are all consistent with Einstein’s theory.

  14. I was not aware Einstein had an equation for Gravity only its affect ? Describing an understanding when in the presence of it? With so many celestial bodies makes a very complicated 3D timespace

    Map -! Really think Nature is so complicated? I prefer to think everything has a value ( quasi binary ) and each object assigned its value via the Emergent Space phenomena. It too is Dark Energy.

    As it is non participant and quite black in the real world. I don’t know how you are going to detect the undetectable? Other than realize that a photon assumes more mass if you try to make it travel

    Faster than the rate at which new space ( dark energy ) is being produced around it.

    1. http://www.math.fsu.edu/~dli/einsteinfieldeq.jpg

      If you tried to make math for your notion, you’d (a) discover the math is very complicated [what is emergent space in mathematics?] and (b) you’d have a lot of trouble reproducing what we observe. You would not simplify things that way.

      Also, I would advise you to remember that nature — the universe — has no interest in what you “prefer to think”, or in what any other human prefers to think. I don’t prefer anything except equations whose predictions agree with data.

        1. So? There is a much larger infinity of equations whose predictions disagree with data. And while there are an infinity that agree, try to find even one … it’s not easy.

          1. Well said Matt 🙂 …. Science will never know the ultimate truth, the mystery. But True Enough is better then not true at all.

      1. What a beautiful equation! Dr E must have been a fan of Gustav Mahler, checkout a fascinating documentary on Mahler musical interpretation (3rd Symphony) of “What the universe tells me” .

        You will find it on youtube.

  15. Dark matter is a hypothesized matter.
    It is really going to be interesting how your credibility will recover from your statements about dark matter.

    1. Thanks! And yes, the map is quite beautiful. We also have an extremely controversial claim of a sign of dark matter interactions… reported by the BBC as though the matter is settled, but the paper itself makes me very pessimistic that this is a real effect.

    2. Yes, beautiful. But it is not a dark matter map. It is a structure of stronger gravity. Not curved filaments , but stright lines. Just the opposit from random gravity found here in the solar system. Very thin stright lines and always right to the center of a galaxy. Normally about ten times more gravitons on average then the random case. Which explains why the force between two galaxies is ten times stronger and why we detect it as lensing.

      1. Please provide a reference (and not one merely written by yourself alone) that shows this mathematically. The dark matter map has an entire field of mathematical study behind it, going back at least 15 years; see the references in the paper in which the dark matter map is produced.

        1. I am sure we are all trying to learn more. I can not provide a second reference. I do not know if any one else thought of it . But to be fair, take the other side of references, there are 100 different theories on gravity since General Relativity. 99 must be wrong. Therefore most referiences must be wrong. But to help, Space is a particle, just like any other elementry particle. All with the same common properties. Space particles remember what just happen to them, if a graviton passes thru them they aline themselves in timing for a second exact particle. What takes 50,000 years for a graviton to get here from the center of our galaxy, take 15 nanoseconds to get back and another 15 back here. If you average all gravitons over time one comes up with a factor of 2. Just what is needed. Another example, the two electron experiment, the electron does not run into itselve, it runs into a space path from a previous electron from the other slit. There are another 20 more different examples, but you must be willing to look at them. Like why 2 quarks stay together, again a stright line space path. And 3 quarks is even more interesting. And then 6. There is not a single reference on any one of those ideas. !! I suggest you not use GR as a reference. There is a big chance it might be one of the wrong 99.

          1. See, if you disagree with me , you can now reference CrankDetector. Or I can give you 100 more that say the same thing. But back to space, you list its properties. Here they are for a space particle. A space particle is built from three preons, as all particles are. A preon is the next biggest thing to a point. Points don’t exist in this universe, preons do. There are 3 frequences for preons (LHE) Space is (LLL). fermions (LLH) and bosons (LHH) and unit mass is (HHH). (EEE) is super mass and (HHE) contained the first particle at the BB. There are 4 fermions, and every particle in all tables has 3 mass states because there are 3 preons.. There are 4 space particles. 11 have mass and act just like an electron, but with 1/3 and 2/3 charge also, and around 10**15 .more mass. Therefore only found at the center of a galaxy. And that is why that body is stable. And you will always fine a FERMI Bubble with all that charge to offset gravity. I am sure you can find all kinds of other references for other theories as to why there is these bubbles. , .

            1. Ok, I’m sorry, but I have no choice to ban people like you, who have no sense of what makes an appropriate comment. This site is dedicated to mainstream physics, not an advertising site for wild ideas by either professionals or amateurs. It would be best if you could put things like this on your own site.

  16. At some time you said that the hot big bang started when the inflation field energy is transformed into mass/energy of ordinary matter , then , what is the mechanism by which DM was generated ?

    1. The inflaton may have decayed into dark matter as well as ordinary matter. Or it decayed to dark matter and its friends, and its friends may have decayed to ordinary matter. Or it may be that at high energies and temperatures dark matter and ordinary matter interact rather strongly, so high temperatures would have led to both being produced. There are many options. We don’t have a way to say which is right.

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