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Featured I Merging neutron stars

  1. Oct 17, 2017 #61

    mfb

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    The SM, by definition, doesn't include gravity. And just because both photons and hypothetical gravitons are massless doesn't mean the phenomena would be similar in any way. We know they are not, otherwise we would have a consistent theory of quantum gravity by now.

    @ohwilleke: You underestimate the number of events we will get. Let's say conservatively 1 NS event in 1 year of running, ignoring that the sensitivity improved over time. In a year, with twice the sensitivity, we would expect 8 events per year. In ~3 years, with the full sensitivity, we might get something like 20-30 per year. Add KAGRA and the number will get even larger. Add INDIGO and we might get more than 100 events per year. And then we can build the Einstein telescope which will see these events routinely.
    Of course we might have been extremely lucky with this NS event, but that is unlikely. For binary black hole mergers the situation is even better, with 4 events observed already we can be quite sure they are not extremely uncommon.

    That was said by the scientists during the press conference.
     
  2. Oct 17, 2017 #62

    phyzguy

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    So apparently the GW signal was too weak to get any information on the ring-down phase. At least I haven't seen it mentioned anywhere. Did I just miss it? Has anyone seen any information on a ring-down phase?
     
  3. Oct 17, 2017 #63
    Sure. But currently we have a quantum theory of light, but not gravitation. Isn't there anything in these findings of observed gravitational waves that we can use to try to confirm or rule out some of the current proposals for a quantum theory of gravity, whether strings or quantum loop or whatever?
     
  4. Oct 17, 2017 #64

    Vanadium 50

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    No.
     
  5. Oct 17, 2017 #65
    As the inspiral progressed, the frequency of the gravitational waves increases to higher than the frequency range the detectors are sensitive to. I think the merger and ringdown were missed because of this.
     
  6. Oct 17, 2017 #66

    ohwilleke

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    This is very encouraging. I knew there would be some improvement in sensitivity and the number of detectors over time, but I had no idea that it would be so dramatic.
     
    Last edited: Oct 17, 2017
  7. Oct 17, 2017 #67
    How long after merger did visual spectrum start to show lines (whether emission or absorption)?
     
  8. Oct 17, 2017 #68

    OmCheeto

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    I believe physzguy kind of answered that in Borek's "splinter" thread:

    ps. I've been thinking of starting a "splinter" thread myself, after a bunch of people on the internet started whining that; "Those aren't real pictures!" to NASA, et. al.
    My very bad maths told me yesterday that it would take a telescope that is 3 light years in diameter to get a clear visual image of the event.
    I googled feverishly to get confirmation that my maths was wrong, and found a web site* that said an optical telescope of that size would be so massive, it would collapse into a black hole.

    [edit] *found it in my browsing history: http://quarksandcoffee.com/index.php/2015/07/08/aliens-and-dinosaurs/
     
  9. Oct 17, 2017 #69

    phyzguy

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    The optical pictures you see are not resolved images of the merger. They just capture the total light emitted. Optical photos of distant stars are never resolved. The stars are just points of light.
     
  10. Oct 17, 2017 #70

    OmCheeto

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    I know that. But how do you explain to someone who might find out that all of the data comes from a pin point, how the [FAKE!] pictures are created?

    ps. I really wish that Goddard had put in time stamps for their little movie.


    I like watching it at 1/4 speed, at max resolution.
     
  11. Oct 17, 2017 #71

    phyzguy

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    I see your problem. Some of the movies are artists' conceptions and some are computer simulations. It's hard to know which are which without digging into it.
     
  12. Oct 17, 2017 #72

    phyzguy

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    Thanks, that makes sense. That explains why the frequency/time graph disappears off the top. I wonder if the upgrades will help with this.
     
  13. Oct 17, 2017 #73

    stefan r

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    That is only if you wanted to look at a T-rex. A Texas sized pixel would make a very nice picture/video. A 10au diameter solid glass lens could collapse into a black hole. A mirror only needs to be a few atoms thick. There are a lot of photons coming out of 1044J events. An array orbiting the sun could get the resolution.
     
  14. Oct 17, 2017 #74

    Urs Schreiber

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    At least all theories involving violation of Lorentz invariance (e.g. doubly-special-relativity, Horava-gravity, many non-commutative-spacetime models) now face stronger constraints by up to ten orders of magnitude.
     
  15. Oct 17, 2017 #75

    mfb

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    The 11 hours are the time until an optical telescope found the source. That doesn't mean there was no optical light emission before, we just don't have data about this time. Telescopes that can find both dim sources and cover a large area in the sky are rare, and the localization based on the gamma ray burst and the gravitational wave still left a large area to search in the sky (relative to the field of view of telescopes).
    Be careful with overly general statements, they might be wrong. Okay, you could argue Antares is not that distant...

    In addition, the event produced jets much larger than stellar objects, it might be possible to see them in the future. ELT with its 5 mas resolution could see structures as small as 3 light years across at this distance. I don't know if the jets are bright enough for that, however.
     
  16. Oct 19, 2017 #76

    ohwilleke

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  17. Oct 23, 2017 #77

    Urs Schreiber

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    Similarly, MOND-like modifications of the laws of gravity are further constrained by the new data, see here:
    • Jose María Ezquiaga, Miguel Zumalacárregui, "Dark Energy after GW170817" (arXiv:1710.05901)
    • Jeremy Sakstein, Bhuvnesh Jain, "Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories" (arXiv:1710.05893)
    • Sibel Boran, Shantanu Desai, Emre Kahya, Richard Woodard, "GW170817 Falsifies Dark Matter Emulators" (arXiv:1710.06168)
    Of course MOND faces bigger problems already,
    • Scott Dodelson, "The Real Problem with MOND", Int. J. Mod. Phys. D, 20, 2749 (2011). (arXiv:1112.1320)
    but still.
     
  18. Oct 23, 2017 #78

    ohwilleke

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    Dodelson (2011) is really just attacking a straw man. Everyone has known since the beginning that MOND-proper is a toy model and needs to be generalized to be relativistic and doesn't capture cluster phenomena. And, it was generalized with TeVeS and a similar approach was made with Moffat's MOG theory (that work for clusters and cosmology). Apparently Boran (2017) is a blow to those relativistic approaches that are more general and not pure toy models.

    Ezquiaga (2017) and Sakstein (2017) are not primarily going after MOND-like modifications. They are instead addressing a different group of gravity modifications usually pushed by GR theorists (e.g. some f(R) theories of gravity) designed only to deal with dark energy and not with dark matter - almost the opposite of what MOND-like gravity modification theories do, MOND-like gravity modification theories often don't address dark energy phenomena at all. Ezquiaga argues that gravity doesn't propagate at the speed of light in TeVeS, but I'm skeptical of that claim (he relies on another paper for this throw away statement in his conclusion) and it is certainly a theory specific argument and not a generalized modified gravity argument. Ezquiaga (2017) also makes clear that some modified gravity theories do make the cut:

    Boran (2017) does place significant limits on the parameter space of MOND-like theories that use gravity modification to explain phenomena attributed to dark matter. But I'll defer further commenting on that paper as I haven't had a chance to really dig into it yet. FWIW, at first glance it looks to me like the case of Boran (2017) is probably overstated, but I'm willing to keep an open mind for now.

    Certainly, nothing in Boran (2017) in any way impairs the approach taken in the following series of papers that involve a massless boson as a force carrier:

    * A. Deur, "A possible explanation for dark matter and dark energy consistent with the Standard Model of particle physics and General Relativity" (2017).
    * A. Deur, "Self-interacting scalar fields in their strong regime" (November 17, 2016).
    * Alexandre Deur, "A correlation between the amount of dark matter in elliptical galaxies and their shape" (July 28, 2014).

    Incidentally, I don't agree that Deur's approach is actually consistent with classical GR as currently formulated, although the tweak that he makes in coming up with his own regime that handles rotation curves, cluster data, elliptical galaxies and cosmology tests, at least at a back of napkin level of precision, are very subtle and very principled. In both results and theoretical motivation it is probably the best of the current gravitational explanations of dark matter phenomena, although it has been ill developed as the author has had to devote most of his work to his day job in QCD and doesn't have the funding, support or following necessary to really kick the tires of this approach.

    The other point to recognize is that dark matter particle theories are in very deep trouble in ways which this data point doesn't address. Truly collisionless dark matter is all but ruled out, and the parameter space of self-interacting dark matter theories is also highly constrained. See, e.g., Lin Wang, Da-Ming Chen, Ran Li "The total density profile of DM halos fitted from strong lensing" (July 31, 2017); Paolo Salucci and Nicola Turini, "Evidences for Collisional Dark Matter In Galaxies?" (July 4, 2017).
     
    Last edited: Oct 23, 2017
  19. Oct 23, 2017 #79
    Sounds to me like we need another "Einstein" to figure out the cosmological situation. That was his biggest "blunder", as he put it.
     
  20. Oct 24, 2017 #80

    Urs Schreiber

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    Thanks for the detailed comments. I'll be interested in your take on Boran17.

    Sure. It seems you are now passing from the question "How does the coincident GW+EM radiation from GW170817 constrain modifications of basic physical laws?" to a general discussion of the problem of DM+DE. How about ultralight axion models? They seem to be in decent shape.
     
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