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CERN | Geneva | Switzerland

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How do we know Dark Matter exists?

First part in a series of four on Dark Matter

Some of you may have heard of dark matter, this mysterious type of matter that no one can see but makes 27% of the content of the Universe while visible matter (you, me, all stars and galaxies) accounts for only 5%. How do we know it really exists? In fact, its existence is confirmed in many different ways.

disk dark matter

Galactic clusters

Fritz Zwicky, a Swiss astronomer, was the first to suspect the existence of dark matter in 1933. He was trying to measure the mass of a galactic cluster (a group of several galaxies) using two different methods. He tried to infer this mass from the speed of the galaxies. Just like kids on a merry-go-round have to hold on to avoid being ejected, galaxies are held together in a spinning galactic cluster by the gravitational force provided by the matter it contains. If there were not enough matter to create this force, the galaxies would simply scatter.

He then compared his result with the mass evaluated from the light the galaxies shed. He realised that there was way more matter in the cluster than what was visible. This matter of an unknown type generated a gravitational field without emitting light. Hence its name, dark matter.

Velocity curves of spinning galaxies

But it was not until the 1970’s that an American astronomer, Vera Rubin, measured the speed of stars in rotating galaxies accurately enough to convince the scientific community. She observed that stars in spinning galaxies were all rotating at roughly the same velocity, no matter their distance to the galactic centre. This is in contradiction with Kepler’s law that describes the rotation of planets around the Sun.

A planet located further from the Sun rotates slower, following the curve labelled A in the graph below. However, Vera Rubin showed instead that stars in a spinning galaxy followed curve B. This was as if the stars were not rotating around the visible centre of the galaxy but around many unknown centres, all providing additional gravitational attraction. This could only happen if huge amounts of invisible matter filled the entire galaxy and beyond.

 velocity-curve

Gravitational lensing

One striking dark matter detection technique is called “gravitational lensing”.  It is based on the way that large concentrations of matter (visible or dark) create gravitational fields strong enough to distort space.

Imagine a stretched bed sheet where we toss a ping-pong ball. The ball will simply roll following the surface of the sheet. But if you drop some heavy object in the middle of the sheet, the ball will still follow the sheet surface but will now move on a curve.

trou-noir

Light behaves the same way in space. An empty space, void of any matter is just like a stretched sheet. There, light moves in a straight line. In the presence of a massive object such as a star or a galaxy, the space is deformed and light follows the curvature of the distorted space.

gravitational-lens

(Adapted from Pat Burchat’s TED talk)

Light coming from a distant galaxy will bend when passing near a massive clump of dark matter as shown above. The galaxy will appear shifted, as if coming from different places (images on top and bottom). In three dimensions, all diverted light will form a ring as seen on the photo below taken by the Hubble telescope. In case the galaxy and the observers are not perfectly aligned, only small arcs form.

 Horseshoe_Einstein_Ring_from_Hubble

(Photo credit NASA)

This technique is now powerful enough to produce maps of the dark matter distribution in the Universe.

Cosmic microwave background

Astrophysicists can even infer how much dark matter exists by studying the cosmic microwave background. This is relic radiation dating back to when the Universe was barely 380,000 years old. This fossil light has been travelling around for more than 13 billion years and now reaches us coming from all and no direction in particular.

The map of the Universe below was drawn using data taken by the Planck satellite. It shows hotter spots corresponding to where first dark matter then visible matter started forming lumps, providing the seeds for galaxies. Nowadays, scientists believe dark matter acted as a catalyst in galaxy formation.

 CMB

(Photo credit Planck experiment)

The microwave background radiation can be decomposed just like sound from a musical instrument can be broken into harmonics. From the features of its “power spectrum”, i.e. the amount of radiation associated to each frequency, astrophysicists can calculate the quantity of dark matter contained in the Universe.

Today, we have numerous and convincing proofs of the presence of dark matter but see it only indirectly through its gravitational effects. How about direct evidence? This will be my next topic. But beware: there’s hot debate in the scientific community on how to interpret the various direct detection results.

Second part in a Dark Matter series:    Getting our hands on dark matter

Third part in a Dark Matter series:       Cosmology and dark matter

Fourth part in a Dark Matter series:    Can the LHC solve the Dark Matter mystery?

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline
 or sign-up on this mailing list to receive and e-mail notification.

 For more information:

Hangout with CERN: The Dark Side of the Universe

TED Ed clip: Dark matter: The matter we can’t see

TED talk by Pat Burchat: Shedding light on dark matter

 

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71 Responses to “How do we know Dark Matter exists?”

  1. veeramohan says:

    What was the “black body radiation” observed by Max Planck ?

    • CERN says:

      Hello,
      this was a completely different matter. It had to do with the radiation emitted by a black object kept at constant temperature. It emits electromagnetic radiation at all frequencies, and the exact spectrum depends on its temperature. the easiest is to look it up on wikipedia. https://en.wikipedia.org/wiki/Black-body_radiation.

      The two are completely unrelated. I hope this helps.

      Cheers, Pauline

    • veeramohan says:

      All matter emits electromagnetic radiation when it has a temperature above absolute zero – but dark matter is not ?

    • CERN says:

      Indeed, stars for example emit light and even us humans, we emit infrared radiation (heat). But dark matter does not.

      Cheers, Pauline

    • veeramohan says:

      There may be a problem in Boltzmann’s statistical interpretation – which explains and predicts how the properties of atoms (such as mass, charge, and structure) determine the visible properties of matter – which was used in the quantization of electromagnetic energy, by Max Planck ?

      physicists like Rayleigh, Jeans, and Lorentz set Planck’s constant to zero in order to align with classical physics, but Planck knew well that this constant had a precise nonzero value. “I am unable to understand Jeans’ stubbornness — he is an example of a theoretician as should never be existing, the same as Hegel was for philosophy. So much the worse for the facts if they don’t fit.”

      I mean, gravity (dark matter) was also some form of radiation, which had broken its symmetry with electroweak force at some stage ? or veiled by 3D spacetime ?

    • veeramohan says:

      .. at the point of electroweak symmetry breaking, by mass (may be by Nambu–Goldstone boson?), like neutrinos, it (dark matter radiation + momentum = gravity) may interacting with weak force ?

  2. Randy Harrison says:

    So Kepler was right but unaware of the added matter affecting the calculation of velocity and distance?

    • CERN says:

      Hello Randy,

      I am not an astrophysicist but my understanding is that the difference stands in the scale. I believe that Kepler’s law is just fine on the small scale of a planetary system (so his calculations there are still valid) but on the large scale of a full galaxy, where there is lots of dark matter at play, it no longer holds as shown on the curve.

      I hope this helps, Pauline

    • Vinay says:

      Hi,

      Would you say that dark matter is an interpretation of the unexpected findings that fit the mathematical pattern that would suggest that our physics or understanding of it would apply to the universe? If so, is it like a suggestion that happens to fit in perfectly (or near perfection) with our understanding of the universe?

    • CERN says:

      Hello,

      your question is almost more on a philosophical level than a scientific one. But, no, I do not think so. I’d say the existence of dark matter is simply a clear indication that there is more to the Universe than what we know. Our understanding of the the Universe is incomplete. I think the Standard Model now explains nearly everything we have observed. Dark matter is one major exception. That indicates that our understanding of particle physics is incomplete. We need now to figure out what else is there. It might be supersymmetry, it could be anything else. Only experimental proofs will tell us what is right.

      I hope this answer your point, at least partially… That’s a big open question. Pauline

  3. Ed Earl says:

    Does gravitational lensing, or any observation, indicate the shape of DM around a galaxy? For example, does DM spiral around a spiral galaxy?

    • CERN says:

      Hello Ed,

      gravitational lensing makes it possible to get a map of dark matter in the Universe. But I do not know the answer to your specific question about the exact distribution of dark matter in a spiral galaxy. That could be another topic for the future.

      Pauline

  4. What’s up with astrophysicists and Comic Sans?!

  5. Martin says:

    One question I have is do black holes count as part of the known mass of galaxies or unknown (i.e. dark matter). Just how much does a black hole weigh? is that calculated by the speed of objects rotating around it or does the mass of the galaxy not change because if the matter is outside or inside of a black hole it’s still part of the known matter of the galaxy?

    • CERN says:

      Black holes are formed by regular matter so they would count into the 5% of regular matter. As for the weight of a black hole, I am not an astrophysicist myself, but know that it weighs about like a massive star. Now don’t ask me how much a massive start weighs… wikipedia would be a better source!

      I hope this helps you some, Pauline

  6. Vidya Shankar Aiyar says:

    So, all known physics only applies to 5% of the universe?

  7. elmo says:

    Hello, is dark matter affected by other laws of physics? Is it likely for cern to produce dark matter in the near future? Is there any proposed ideas to detect or create dark matter?

    • CERN says:

      Hello Elmo,

      what we know at this point is that dark matter behaves like regular matter with gravitational interaction. But for the rest, we still have absolutely no clue. This is still a completely open question. We know it does not emit electromagnetic waves but we still does not know how it behaves with respect to the other forces of nature: strong and weak nuclear forces or Higgs field. This is all being investigated.

      Indeed, at CERN, we certainly hope to be able to create dark matter with the LHC. There are lots of efforts in this direction but nothing has been found yet. But keep in mind the LHC will operate for a good 20 years and we will keep increasing the energy and the intensity of the beams, meaning we will have more and more chances to create dark matter particles. I will address that and discuss other experiments where it could be found in a future blog so keep posted.

      Pauline

  8. devansh says:

    Well i want to know why is there grivational force and how is it formed

    • CERN says:

      Ah… this is a question with no answer. It is as if you ask me: why is there a Universe? Or matter? All I know is that it is a property of matter to create a gravitational field. The is the observation we can make as scientists. Asking “why” is hard to answer. Scientists are better at answering “how”. We know that all matter generates a gravitational field. And other matter will react to this field by experiencing a force.

      I hope this helps you some, Pauline

    • Craig Evans says:

      Regarding this excellent question, we know that gravity exists. How do different objects with mass affect each other over long distances? Does this require some medium for the effect to be propagated = Sort of like sound requires air to travel. Could dark energy be the medium by which gravity is transmitted, or is the effect instant, sort of like quantum entanglement for a simplistic example?

    • CERN says:

      Hello Craig,

      not all waves require a medium to propagate. Electromagnetic waves such as light can travel in vacuum. You can test that yourself with a basic physics class demo apparatus: a vacuum pump under a bell jar. As the air is being evacuated, light keeps going through the glass. A more accurate test was the Michelson and Morley experiment over a century ago. They proved there was no ether, a substance thought to be filling the empty space to provide a support for light.

      Dark energy is something completely different. Astrophysicists have shown that the Universe not only is expanding but this expansion is accelerating. Think about it. When you are driving your car (or your bike), if you want to accelerate, you need to provide energy to do so. What is fueling the acceleration of the expansion of the Universe? Nobody has a clue. This mysterious form of energy was dubbed “dark energy”. It has nothing to do with dark matter, except that in both cases, we are completely in the dark.

      Try to imagine a gravitational field as a distortion of the space, like in my example with a stretched sheet. Look at the 3rd image in the blog. An object moving on that curved space has no choice but to follow the curvature of space (like the ping pong ball will follow the surface of the sheet).

      I hope this helps a bit. Pauline

  9. benno ejsing says:

    As Ive allready posted this question to steve goldfarb on hangouts i hope they will be partly or fully xplained ia tjank you benno ejsing

    • CERN says:

      Hello Benno, dark matter will be the topic of the next Hangout with CERN on Thursday June 27. So don’t miss it.

  10. milan_BRKIC says:

    Excelent post. It seems that we are still in phase of answering “how”, but not “why”. What is relation between dark mater and anti mater?

    • CERN says:

      Hello Milan,

      glad to hear you liked the post. Antimatter is part of ordinary or visible matter. Antimatter works exactly like matter and reacts to the same forces. You could say that antimatter is just like a mirror image of matter. Dark matter, on the other hand, seems to have nothing in common with matter. It is something completely different. Hard to describe it since nobody knows what is is yet. It is really the biggest enigma left in particle physics.

      Pauline

  11. Hello,
    It seems that the key is in the way, with a high intention in the least known. And the spiral is the way we see the movement of the galaxies. The form is the identity, and this has to do with the concept of the gravitational field. I know my approach is more philosophical, but my studies start from the individual, we are also mass, occupy a space-time, and we are our own observer of the laws of the matter. I know what is behind the form…and is very spacious, offers answers, for example the issue of gravity.
    The dark matter does not have to be compensating for this event may well be that the statement is based on what already exists by default.
    We open doors, so that the chaos is known. I hope the next post… I also have research that seek to contrast. Thanks for the contribution.

  12. Amir Livne Bar-on says:

    Great introduction.

    Waiting eagerly for the next post :)

  13. Martin says:

    If dark matter interacts with a gravitational field, inside our earth planet would be dark matter. So, our planet would have a real mass upper than the theorethical mass. A kind of hight density created by the dark matter, and for example, by the keepler laws, our orbit araund the sun would be more longer than the teorethical orbit considering the mass “withouth” the dark matter. It is posible ?

    • CERN says:

      Hello Martin,
      I think you are mixing things slightly here. It is as if you are saying because the Earth is soaking in a sea of dark matter, it weighs more. That would be equivalent to saying that a porous object floating in a heavy gas weighs more. The two are still two distinct entities: the object and the gas, even though the gas permeates the object. But you are right to say that inside our planet and all stars and galaxies, there is a lot of dark matter.
      Pauline

  14. Chris Cartwright says:

    Is it possible that dark matter, after falling into a black hole, might in some sense be converted into “regular” matter as the black hole evaporates through an eternity of Hawking Radiation? Have scientists ruled out the possibility that dark matter interacts with itself via one or more forces that have no effect on regular matter? Lastly, are scientists mostly sure that dark matter is much more evenly dispersed throughout any particular volume than normal matter (basically, is everyone pretty sure that there are no dark matter stars burning mass through some fusion-like process while they blast dark-photon type particles that we’ll never see)? Sorry for three questions. Thanks.

    • CERN says:

      Hello Chris,

      good questions! You should think about writing science fiction! When matter falls into a black hole, it is simply submitted to greater gravitational fields and accelerated. We do not know if and how dark matter can be converted into regular matter at this point. This is precisely what we are studying right now, trying to figure out how this matter interacts. I will write more about this shortly.

      If dark matter interacts only with itself and not with regular matter, we would not see other effects than gravitational effects as we see now. But nothing has been ruled out yet. I am preparing another blog on this so stay tune! It will be out shortly. Of course, it is possible the special forces act only on dark matter and not regular matter, but that would be hard to detect since by definition, there would be no effect…

      Just as regular matter, dark matter is not evenly spread over the Universe but is rather found in clumps. The map obtained from the cosmic microwave background shows how non uniform it is. It is believed that non uniformities appeared in the dark matter distribution before regular matter after the Big Bang. Dark matter then attracted visible matter to this small clumps, that then grew bigger due to the gravitational force attracting more and more matter, to eventually form stars and galaxies. Gravitational lensing techniques also show that dark matter is accumulated in places (hence the lenses).

      Regarding dark photons, this is indeed one of the many avenues that are explored right now at the LHC at CERN but also other experiments like AMS onboard the International Space Station. We are trying to see if dark photons could decay into a pair of electron and positron. Then we could see dark matter directly. But so far, it has not been observed. It could be that it is extremely rare. If that’s the case, having more data with the LHC restart in 2015 will give us another chance to find it.

      I hope this helps. I will go more on direct detection of dark matters in upcoming blogs.

      Pauline

  15. Laura says:

    What is dark energy and how do you know it exists?

    • CERN says:

      Hello Laura,

      sorry for leaving this point uncovered in the blog. I will come back to it eventually. Please see the answer I just gave to Craig below on this.

      Dark energy is something completely different. Astrophysicists have shown that the Universe not only is expanding but this expansion is accelerating. Think about it. When you are driving your car (or your bike), if you want to accelerate, you need to provide energy to do so. What is fueling the acceleration of the expansion of the Universe? Nobody has a clue. This mysterious form of energy was dubbed “dark energy”. It has nothing to do with dark matter, except that in both cases, we are completely in the dark.

      Cheers, Pauline

  16. devansh says:

    Hello and congrats u r doing a vry good job and i think we both r astrophysics lover well the question is i have read the brief history of time and is said the planets dont move in elliptical orbit but in a move in a mulitdimensional space moving along a straight line can this be true

    • CERN says:

      Sorry, this is beyond my expertise as a particle physicist and cannot comment. Maybe anther reader will know? I will also see if I can find an expert opinion on this.

      Cheers, Pauline

  17. ian says:

    If dark matter exists
    1 does it exist everywhere
    2 if so why don’t particles in the lhc collide
    with it or show deviation

    • CERN says:

      Hello Ian,

      yes, it is just about everywhere. Since it is interacting so little, it acts pretty much like “ghosts”, going through matter without being affected (a bit like the neutrinos). The hope is that is acts also a bit like neutrinos and that once in a while it will interact with matter and we will be able to detect it. I will have a second blog on this very shortly.

      Second point: yes, indeed, we hope to be able to create dark matter with the LHC. More on this in my fourth installment in this series.

      Cheers, Pauline

  18. ian says:

    Sorry, I meant since it’s everywhere its already inside the
    Lhc so why doesn’t it interact with the paticles accelerated into the
    Collision zone

    • CERN says:

      Hello again, Ian,
      the reason is the same for all scenarios: if dark matter interacts with regular matter, it is at best extremely rarely. A dark matter particle is expected to collide with a regular matter nucleus around 0.1 times per year per kg of material. Remember Avogadro’s number? There are 6.023 times 10 exponent 23 atoms for 1 gr of hydrogen or 12 grams of carbon. So compared to a chunk of solid material, there are very few particles in the LHC beam. Hence, it is even more unlikely that a dark matter particle would interact with regular matter there since there are fewer particles (so less chance for a collision). It is simply that the likelihood of a collision with a proton from the LHC beam is even smaller than with a solid detector. And if it happens, we would have to catch it, which is a whole different story. We would need for this to happen often to gather enough evidence that it is there. Given the collision rate in the LHC, there is no chance we could do that.

      I hope this answers your question a bit better. Pauline

  19. ian says:

    Thanks pauline

    • CERN says:

      Thank you for sharing your thoughts on this. It is clear you have your own opinion on this topic but it is widely accepted in the physics community that dark matter is non-baryonic (i.e. not made of protons and neutrons). There is no problem in stating that the cosmic microwave background has been traveling through time despite the presence of dark matter since dark matter does not interact electromagnetically. Hence it is transparent to light.

      Please abide with the general policy of the Quantum Diaries site and stick to short comments.

      Pauline

  20. In this document 3 different cases are discussed as proof that in order to explain certain physical phenomena dark matter is required. The document is correct to claim that there is no direct evidence. However that is not the primary problem. The most important problem is: what is this dark matter? is it baryonic or non-baryonic? I have the impression that the current accepted theory is that it is non-baryonic. For example WIMPS. The problem is that you are only allowed to follow that route if you are sure that the missing mass calculated is not baryonic.
    For example in the paragraph “Velocity curves of spinning galaxies” claims: “This could only happen if huge amounts of invisible matter filled the entire galaxy and beyond.” This is not true. Galaxy Rotation Curve B can be simulated in 3D using small amounts of baryonic matter in the plane of the disc of the Galaxy. With small I mean 20% to 30% of the total visible mass of the galaxy and completely out of scale for the values applicable for the total universe (5 to 27)

    The paragraph “gravitational lensing” also raises certain questions. What you have are two masses. One in the center “close” to the observer and a second one far away. It is this second one which image is gravitational lensed. The problem is the masses of both and the shape of the second are unknown. With masses I mean the total mass i.e. visible plus invisible.
    The problem becomes much easier to understand if you draw inside the top picture also a galaxy.
    A comparable situation are two stars. Stars are much simpler. Because both can be individual studied and the total masses of each are relatif easy to calculate.

    That paragraph also claims: “This technique is now powerful enough to produce maps of the dark matter distribution in the Universe”. What I first would like to see is a map of all the baryonic matter in the Universe.

    The paragraph “Cosmic Microwave Background” also has one serious problem. The text claims: “This fossil light has been travelling around for more than 13 billion years and now reaches us coming from all and no direction in particular.”
    What is also important that this light travels towards us freely in a straight line. As such the CMB gives an imprint of the early universe almost at the moment of the Big Bang. The problem is dark matter (in the hugh amounts proposed) and gravitational lensing (both) make such a free travelling impossible.

    Still one more question: How much dark energy is there in our milky way galaxy.

    See for background information: http://users.pandora.be/nicvroom/

    • CERN says:

      Hello Nicolaas,

      everything I have reported here is widely accepted in the scientific community. You are right to say that the velocity curves could be interpreted differently, by modifying gravitational laws and other such radical approaches. But these do not make a large consensus in the scientific community and I chose not to mention it for this reason.

      Nowhere in the last section on the cosmic microwave background have I suggested that this radiation has been traveling on a straight line. Of course, light any other electromagnetic radiation, it follws the curvature of the Universe.

      Cheers, Pauline

  21. I suggest that we first differentiate between dark light (dark photons)and light light (light photons).
    second we should differentiate between +mass and -mass.
    the relation between mass and distance for a small universe rotating object in general around another massive one has been governed by a law i have discover it.
    my web site on face book contains many objects in Arabic, skip them.
    thanks
    Anwer Motwadi

    • CERN says:

      Hello,

      if you think you have a good idea, you should share it with the scientific community on the physics arXiv. This is not the place to do it. Also, it is nearly impossible to come up with a new theory unless you are sure it respects every single piece of experimental evidence there is. So working in isolation outside the scientific community is not the way to go in my opinion.

      Cheers, Pauline

  22. Nigel G Tooby says:

    Do you lose sleep at night worrying about MOND?

    • CERN says:

      Fortunately not, I only worry about that during working hours, probably because there are other things to make me loose sleep at night…
      But are you referring to Modified Newtonian Dynamics or Many Other Nerdy Discussions?

      Cheers, Pauline

    • Nigel G Tooby says:

      Modified Newtonian Dynamics. I read about it with interest in Lee Smolin’s recent book…

  23. Nicholas says:

    I can’t help but think there is something completely different than mere matter that only interacts with gravity (or a limited amount of forces). I may be wrong, but the graph showing how galaxies rotate vs solar systems rotation shows a break in how physics works, if it were merely matter that interacted with gravity it would still be concentrated in the galactic center and the graph would still show similar behaviour no? or is it that Dark matter gravity is so powerful and pervasive that the graph is similar, but to show real slowing down of rotation past the galactic center you would have to go past the actual galaxy? It’s almost like we’re saying there is a non-local force pulling all the galaxy around at the same speed and that force interacts with the center of the galaxy equally. Like Black holes/aggregates of dark matter created by the center of the galaxy turn the fabric of the void itself and it’s all at an equal speed. within the galaxy, that in fact this may happen for all galazies for a large amount of space, but it’s not immediately obvious that galaxies are moving this way in relation to each other.

    • CERN says:

      Hello Nicholas,

      we know it is not regular matter because if it was so, we would see it. It would emit light and we would detect it. So it is not that the gravitational field create by dark matter is stronger, it is just that regular matter cannot create it and not be seen.

      Cheers, Pauline

  24. hosam otaibi says:

    i don’t think that the evidences of dark matter are enough
    1- Velocity curves of spinning galaxies : if we suppose that every galaxy have its own cintre dark matter, why it isn’t efect on the other galaxies? and if it is homogeneous distripution,why don’t cancel each other?

    • CERN says:

      Hello,

      up to you to believe the evidence or not. Velocity curves are just one of many proofs. In any case, galaxies are too far from each other for them to feel any substantial attraction at this distance. And for your second point, dark matter is not uniformly distributed. Have a look at the third chapter on cosmological proofs of dark matter and watch the video there.

      Cheers, Pauline

  25. I agree with you and you are right to some limit, but i have to put a little comment. any how just i want to see if you use these scientific terms dark photons and light photons and so on as i used to use to see how and where you are going to see where to put my feet on the steps of your way.
    do you agree with me if we have the right start and academic logic procedure we can see on paper what is happening on nature.

  26. Mike Decker says:

    Thanks for the overview. Had no idea about Fritz Zwicky and the role he played in identifying the undetected gravitational mass that seems to distort the dynamics of ordinary galaxies. Thirty years ago he would have been mainly known for the tired light hypothesis, which was intended to allow Hubble’s Law to be recognized as a form of astigmatism. Kind of ironic that dark matter would be enshrined as an inherently mysterious presence. The earth would almost qualify, if it wasn’t for the heat being absorbed from the sun and radiated out at night, mostly in the invisible spectrum, in additon to a slight amount of internally generated heat.
    The larger universal balance is a bit of a blind spot as well. The sun can’t radiate unless there’s something drawing the energies. Thermodynamic equilibrium requires absorptive radiation sinks, in whatever form that they might take, collapsed stars being the most obvious candidates. Passive heat sinks wouldn’t serve the purpose, because temperatures would increase indefinitely, suggesting a rebuilding process, with the initial glimmerings of the sun, I suppose, as a point of departure.
    No idea about the Higgs field and what distinguishes it from the strong nuclear force, for instance. The nucleus is considered to be incredibly stable. Exhausted stars are known to be capable of collapsing into a form of matter that’s presumed to be more dense than the nucleus, except it requires heavier elements that become unstable eventually, at least under intense pressure.
    The opposite end of the spectrum is interesting as well. The sun, or any other ordinary star, easing into the radiant stage, having been suitably replenished over a long period of time, with solid hydrogen forming, presumably, as the nucleus reestablishes itself. The primordial form of matter that serves as its precursor is something that can only be speculated on, based on current knowledge about elementary particles. It would be kind of a disappointment if dark matter turned out be outside the realm of what must be pretty close to an exhaustive understanding.

  27. Eric says:

    Good post, but i always wondered, how do we know matter only takes up 5% of the universe, have we observed the entire universe, I mean all the stars and planets. As far as I know we have seen only a fraction of the universe.

    • CERN says:

      Good point, Eric.

      as you may suspect, this is really complex. I did not know exactly myself and asked a cosmologist, Alex Arbey. Here is a not-so-clear explanation based on what he told me.

      It all comes from measuring the quantity of light atoms in the Universe. This is done using spectral lines of hydrogen, lithium, helium etc. From that, astronomers use a very precise model called the Big Bang Nucleosynthesis or BBN. This model explains how the lighter elements appear and how heavier atoms formed. It has strict constraints so once astronomers get the ratio of the amount of matter and light from the Cosmological Microwave Background (measured by the Planck satellite), then they fit that in and infer the share of matter and dark matter. They also use constraints on the formation of large structures. You need a certain amount of dark matter and matter for it to work according to the models.

      I hope this helps you some. Cheers, Pauline

  28. It’s rather a nice and very helpful piece of facts. I will be content which you provided this handy information here. Make sure you keep us educated like this. Thank you sharing.

    • CERN says:

      Thanks, I am happy you found it useful. All the historical facts were provided by Peter Higgs himself last summer in his talk. This man is the most honest person I ever met!

      Cheers, Pauline

  29. H. Burgmann says:

    I have a question with regards to the velocity curve evidence. Probably way too long after the orignal post, but I found this a great summary so I thought I’d ask here. It says that the problem is a deviation from the predicted Kepler-like rotational speed. But systems like solar systems that Kepler looked at have a very large central mass, and a largely negligible mass of the planets that orbit around them. While a galaxy would have a much broader distribution of mass, so wouldn’t you expect something like “many unknown centres, all providing additional gravitational attraction” anyway? Is there a better explanation how the expected rotational speeds are calculated based on the visible mass distributions somewhere on the net?

    • CERN says:

      Hello Helmut,

      it is never too late to ask a good question. I am not an astronomer and do not know a good site for explanations on rotational curves. But as a physicist, here is a simple explanation: all mass always acts as if it was concentrated in one point, the center of gravity. If you were to look from far away, a galaxy would appear as a point-like object with all its mass concentrated on the center of gravity.

      The fact that the mass is all spread over a large distance for a galaxy is irrelevant. Just like with this nice trick with a spoon and fork. You can balance them on a glass just by supporting their center of gravity. If you don’t know this trick, have a look here: http://www.wikihow.com/Make-a-Fork-and-Spoon-Appear-to-Defy-Gravity

      I hope this helps. Cheers, Pauline

    • H. Burgmann says:

      Wow, thanks for the rapid reply! I guess based on your answer I then don’t understand how adding dark matter would help explain the different rotation curves if it’s not about the distribution of mass in the first place… I guess I need to learn more about the basic physics. I’ll see if can find some more info on astronomy sites. Thanks again for this great post on dark matter. And thanks for the great link, I know what I’ll show the kids tonight…

  30. CERN says:

    I think the main point here is that the dark matter and regular matter do not have the same distribution. Then the stars within a rotating galaxies do not rotate around a unique center, like in the solar system case. Note that our solar system is far away from the galactic center and there is very little dark matter around us. Hence, Kepler’s laws of motion for the planets around the sun are not affected.

  31. PhysicsSuck says:

    Helpful. Next step: find out how to create dark matter and see what happens. Is there any way to do that?

    • CERN says:

      Hello,

      that’s exactly what we are trying to figure out at this point. But nobody knosw yet how and if dark matter interacts with regular matter. So until this remains a mystery, there is no way I can answer your question!

      Cheers, Pauline

  32. CERN says:

    Well… that’s the billion dollar question. Nobody knows!

  33. Superb says:

    Superb article

  34. Ebe says:

    Is it a possibility that gravity does not exist?
    Rather it is a force exerted by dark matter on matter.

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