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Why must dark matter interact weakly?

  1. Jul 7, 2015 #1
    I've been reading on dark matter and I've never seen a satisfactory explanation as to why dark matter is expected to interact with the weak force. From what I gather, the only reason it's expected to do so is based on assumptions that dark matter had to be created after the electroweak epoch but before the quark epoch because of how it has to decay, is that right?
     
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  3. Jul 7, 2015 #2

    marcus

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    That's an interesting question. I think of the "WIMP" idea as a plausible hypothesis. DM clouds seem to be out there, we can map them by their gravitational lensing effect, they seem to have played a vital role in structure formation. But we do not, I think, KNOW that DM consists of particles that feel the weak force.

    At this point, I think, that is just a (very reasonable) guess. Maybe I'm missing something in which case I hope someone steps in and corrects this---just my impression.

    I like your reasoning--which is a plausibility argument. If an hypothetical massive DM particle did not interact via the weak force then ***how could DM have been formed in the first place?***
     
    Last edited: Jul 7, 2015
  4. Jul 7, 2015 #3
    I think there is also the idea that dark matter is unexpected to exhibit a force that is not yet known. Since it does not react electromagnetically, it is likely to exhibit the weak interaction.
     
  5. Jul 7, 2015 #4
    Why? That's my question, why does it interacting with gravity but not electromagnetism mean it's most likely going to interact with the weak force? Is it simple deduction from the theory of how the fields separated during inflation or is there something more concrete hinting towards it? Furthermore, how? W bosons? Z bosons? Both?
     
  6. Jul 7, 2015 #5
    Like all assertions in science for which there is not definitive experimental support, it is nothing more than an educated guess: a hypothesis. One could argue that it is supported by Occam's Razor.

    There may be additional support in various cosmologies of how the dark matter came to be in the first place, but these lines of reasoning are no stronger than the support for those specific explanations of where dark matter came from.
     
  7. Jul 7, 2015 #6

    Orodruin

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    Nothing guarantees that a WIMP is interacting with the weak interactions, any interaction with cross sections of the order of weak interaction scales would be sufficient for freeze-out production. Of course, other production mechanisms could also be viable.

    When it comes to electromagnetism, we know that DM does not interact via EM interactions (or has a very very small charge). This comes from observations.
     
  8. Jul 7, 2015 #7

    marcus

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    The whole question of how one arrives at hypotheses about DM particle is interesting. I see that axion dark matter is technically different from weakly interacting (WIMP) dark matter.
    ==quote==
    Weakly interacting massive particles
    From Wikipedia, the free encyclopedia
    (Redirected from Weakly Interacting Massive Particle)
    In particle physics and astrophysics, weakly interacting massive particles, or WIMPs, are among the leading hypothetical particle physics candidates for dark matter. The term “WIMP” is given to a dark matter particle that was produced by falling out of thermal equilibrium with the hot dense plasma of the early universe, although it is often used to refer to any dark matter candidate that interacts with standard particles via a force similar in strength to the weak nuclear force. Its name comes from the fact that obtaining the correct abundance of dark matter today via thermal production requires a self-annihilation cross section of [PLAIN]https://upload.wikimedia.org/math/1/2/7/127a07aed49edf56b91a9709ae732625.png, [Broken] which is roughly what is expected for a new particle in the 100 GeV mass range that interacts via the electroweak force. This apparent coincidence is known as the “WIMP miracle”. Because supersymmetric extensions of the standard model of particle physics readily predict a new particle with these properties, a stable supersymmetric partner has long been a prime WIMP candidate.[1] However, recent null results...
    ==endquote==
    It puzzles me that axions are said to interact primarily via GRAVITY AND EM, not via the strong or weak forces! They are even predicted to convert back and forth to PHOTONS in a strong magnetic field, and that has become a way to search for them, using strong magnetic fields!
    So this is very curious--a proposed DM particle that is not a WIMP.
    ==quote==
    Axion
    From Wikipedia, the free encyclopedia

    Interactions Gravity, electromagnetic
    Status Hypothetical
    Symbol A0
    Theorized 1977, Peccei and Quinn
    Mass 10−6 to 1 eV/c2
    Electric charge 0
    Spin 0
    The axion is a hypothetical elementary particle postulated by the Peccei–Quinn theoryin 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interest as a possible component of cold dark matter.
    ...
    ...
    The critical mass is of order 10−11 times the electron mass, where axions may account for the dark matter. The axion is thus a dark matter candidate as well as a solution to the strong CP problem. Furthermore, in 1983, Pierre Sikivie wrote down the modification of Maxwell's equations from a light stable axion [8] and showed axions can be detected on Earth by converting them to photons with a strong magnetic field, the principle of the ADMX. Solar axions may be converted to x-rays, as in CAST. Many experiments are searching laser light for signs of axions.[9]

    Experiments
    The Italian PVLAS experiment searches for polarization changes of light propagating in a magnetic field. The concept was first put forward in 1986 by Luciano Maiani, https://en.wikipedia.org/w/index.php?title=Roberto_Petronzio&action=edit&redlink=1 [Broken] and Emilio Zavattini.[10] A rotation claim[11] in 2006 was excluded by an upgraded setup.[12] An optimized search began in 2014.
    Another technique is so called "light shining through walls",[13] where light passes through an intense magnetic field to convert photons into axions, that pass through metal....
    Several experiments search for astrophysical axions by the Primakoff effect, which converts axions to photons and vice versa in electromagnetic fields. Axions can be produced in the Sun's core when x-rays scatter in strong electric fields. ...
    ...
    Possible detection
    Axions may have been detected through irregularities in X-ray emission due to interaction of the Earth's magnetic field with radiation streaming from the Sun. Studying 15 years of data by the European Space Agency's XMM-Newton observatory, a research group at Leicester University noticed a seasonal variation for which no conventional explanation could be found. One potential explanation for the variation, described as "plausible" by the senior author of the paper, was X-rays produced by axions from the Sun's core.[29]
    ...
    Properties
    Predictions
    One theory of axions relevant to cosmology had predicted that they would have no electric charge, a very small mass in the range from 10−6 to 1 eV/c2, and very low interaction cross-sections for strong and weak forces. Because of their properties, axions would interact only minimally with ordinary matter. Axions would change to and from photons in magnetic fields.

    Cosmological implications
    Theory suggests that axions were created abundantly during the Big Bang.[36] Because of a unique coupling to the instanton field of the primordial universe (the "misalignment mechanism"), an effective dynamical friction is created during the acquisition of mass following cosmic inflation. This robs all such primordial axions of their kinetic energy.

    If axions have low mass, thus preventing other decay modes, theories predict that the universe would be filled with a very cold Bose–Einstein condensate of primordial axions. Hence, axions could plausibly explain the dark matter problem of physical cosmology.[37] Observational studies are underway, but they are not yet sufficiently sensitive to probe the mass regions if they are the solution to the dark matter problem. High mass axions of the kind searched for by Jain and Singh (2007)[38] would not persist in the modern universe. Moreover, if axions exist, scatterings with other particles in the thermal bath of the early universe unavoidably produce a population of hot axions.[39]

    Low mass axions could have additional structure at the galactic scale. As they continuously fell into a galaxy from the intergalactic medium, they would be denser in "caustic" rings, just as the stream of water in a continuously-flowing fountain is thicker at its peak.[40] The gravitational effects of these rings on galactic structure and rotation might then be observable.[41] Other cold dark matter theoretical candidates, such as WIMPs and MACHOs,...

    Axions would also have stopped interaction with normal matter at a different moment than other more massive dark particles. The lingering effects of this difference could perhaps be calculated and observed astronomically. ...
    ===endquote===

    So it seems that hypothetical axions are being considered for DM and that they are not the same as wimps, and they don't even interact significantly via the weak force!!!
     
    Last edited by a moderator: May 7, 2017
  9. Jul 7, 2015 #8

    Orodruin

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  10. Jul 7, 2015 #9


    Oh, horrors, so embarassing...dark matter and dark energy...so recently discovered...so little is known....and according to

    Wikipedia: [Dark matter]
    ".....According to the Planck mission team... dark matter is estimated to constitute 84.5% of the total matter in the universe, while dark energy plus dark matter constitute 95.1% of the total mass–energy content of the universe.....According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of https://www.physicsforums.com/wiki/Subatomic_particle [Broken]. ......."The most widely accepted explanation for these phenomena is that dark matter exists and that it is most probably[9] composed of https://www.physicsforums.com/wiki/Weakly_interacting_massive_particles [Broken] (WIMPs) that interact only through gravity and the https://www.physicsforums.com/wiki/Weak_interaction [Broken]. Alternative explanations have been proposed, and there is not yet sufficient experimental evidence to determine whether any of them are correct. Many experiments to detect proposed dark matter particles through non-gravitational means are under way.[11]...Important as dark matter is thought to be in the cosmos, direct evidence of its existence and a concrete understanding of its nature have remained elusive.

    If anyone has a reference that dark matter does exhibit the weak force, I'd be interested in a link to learn more.

    What I have come to understand, so far, is that it is weakly interacting, meaning it doesn't exhibit any of the familiar three forces but clearly exhibits gravitational effects.

    "Dark matter doesn't give off light...so it likely doesn't interact with the electromagnetic field...." [Lee Smolin, The Trouble with Physics]
    One post from somewhere here on the forums:

    Mfb: "Dark matter has no strong short-range interaction, it cannot "collide", or radiate away energy. There is no way to lose energy to clump together on a scale smaller than galaxies. "

    I don't recall any corrections to that post.

    Under WIMPS, Wiki says:
    "
    The main theoretical characteristics of a WIMP are:
    • Interactions only through the https://www.physicsforums.com/wiki/Weak_nuclear_force [Broken] and https://www.physicsforums.com/wiki/Gravity [Broken], or possibly other interactions with https://www.physicsforums.com/wiki/Cross_section_(physics) [Broken] no higher than the weak scale;[6]........
    That does NOT sound like a firm conclusion.
     
    Last edited by a moderator: May 7, 2017
  11. Jul 7, 2015 #10

    Chalnoth

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