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Featured I Merging neutron stars

  1. Oct 25, 2017 #81
    I have been unsuccessfully trying to find a definition of the term "collosional dark matter" or the term "collosionless dark matter". I assume these terms are opposite in meaning so that that finding one definition would therefore be sufficient. I searched the Internet for these terms and found quite a few articles, but in none of the abstracts was a definition given.

    Merriam Webster gives the definition of "collisionless" as
    of, relating to, or being a plasma in which particles interact through charge rather than collision
    https://www.merriam-webster.com/dictionary/collisionless
    The Free Dictionary gives the definition of "collisional" as
    A brief dynamic event consisting of the close approach of two or more particles, such as atoms, resulting in an abrupt change of momentum or exchange of energy.
    https://www.thefreedictionary.com/collisional

    If these are correct definitions in the context of "dark matter", then presumably "collosionless dark matter" means dark matter that interacts via EM, but everything I have read so far about dark matter says this is inconsistent with the lack of any observational evidence for such interaction.

    Here is a quote from the P Salucci and N. Turini paper:
    Moreover, the analysis of the CMB fluctuations spectrum
    and a number of cosmological measurements unavoidably point to a scenario in which a
    Dark Massive Particle is the responsible for the mass discrepancy phenomenon in Galaxies
    and Clusters of Galaxies ( Planck Collaboration (2016)).​
     
    Last edited: Oct 25, 2017
  2. Oct 25, 2017 #82

    SciencewithDrJ

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    What's next in GW research, I wonder.
     
  3. Oct 25, 2017 #83

    phyzguy

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    I think the term "collisionless dark matter" is a shorthand for "dark matter that only interacts with ordinary matter through gravity, and has no other interactions", while collisional dark matter is dark matter that has some other interaction, not necessarily electromagnetic.
     
  4. Oct 25, 2017 #84

    phyzguy

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    My understanding is that LIGO has gone down for about one year to increase the sensitivity. This will allow it to look deeper into space and see events at a higher rate. KAGRA is planned to come on line in Japan next year, and Indigo in India after that. This Wikipedia entry shows some of the plans.
     
  5. Oct 25, 2017 #85

    SciencewithDrJ

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    How do GW detection findings help further refine the calculation of the age of the Universe?
     
  6. Oct 25, 2017 #86

    phyzguy

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    We already know this value to about 1% or better. How accurate do you need it to be? Did you notice a change when Planck refined this value from 13.7 billion years to 13.82 billion years?
     
  7. Oct 25, 2017 #87

    SciencewithDrJ

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    Thanks for the speedy response. What I meant is (not being a physicist) what is it in the findings from GW detection that enables us to get a more accurate calculation. What's the connection?
     
  8. Oct 25, 2017 #88

    Jonathan Scott

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    I'm hoping for mergers with masses heavier than the recent neutron stars to help establish the physics for the threshold for collapse into a black hole. Or, even more exciting, more mergers of objects which ought to be black holes but which are accompanied by electromagnetic radiation (as initially appeared to be the case for the first detection), suggesting the need for new theory!
     
  9. Nov 27, 2017 #89

    PAllen

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    I recently attended a talk by Edo Berger on this, and, in person, he expressed strong confidence that the remnant became a BH very quickly. The data and argument are based on the following paper. However the paper’s conclusion on the final state is stated more weakly than the in person presentation.

    https://arxiv.org/abs/1710.11576

    The discussion of the nature of the remnant begins at the end of p.9.

    A comment in the talk was that the remnant would either be the heaviest known neutron star or the lightest known BH, so a first either way. The suggestion is that a 2.7 solar mass BH is now known.
     
    Last edited: Nov 27, 2017
  10. Nov 28, 2017 #90

    phyzguy

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    From my reading and discussions, it seems that the preferred model is that the SGRB, which occurred approximately 1.7 seconds after the time of merger inferred from the GW signal, is when the BH formed. Was this discussed in the verbal presentation? When you say "very quickly", do you mean on the time scale of seconds?
     
  11. Nov 28, 2017 #91

    PAllen

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    The 1.7 second delay was explained as primarily due to the last stage of inspiral producing GW of too high frequency to be detected. The occurrence of an SGRB per se says nothing about the nature of the remnant. On the other hand, a remnant NS is expected to be accompanied by a very strong neutrino flux (no, I don’t know why this is so, other papers are referred to; neutrino flux in NS formation from collapse is obvious, but why a merger resulting in NS would have one, I do not know). Then, prior work establishes (again, papers given) that a strong neutrino flux would suppress lanthanide production by the r process. The amount and timing of observed lanthanide production suggests that any NS remnant lasted less than 100 milliseconds before collapsing to a BH.
     
  12. Nov 28, 2017 #92

    PAllen

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    Oh, another finding from the talk (and the paper I linked): the neutron star radii were likely 12 km at most. In the talk, this was said to rule out a number of NS equations of state.
     
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