Merging neutron stars

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  • #76
ohwilleke
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  • #77
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.
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.
 
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  • #78
ohwilleke
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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.
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:

Motivated by these results, we identify the theories that avoid this constraint and thus can still be used to explain DE (see a summary in Fig. 3). Within Horndeski’s theory, the simplest models such as quintessence/kessence, Kinetic Gravity Braiding or Brans-Dicke/f(R) are the ones surviving. Beyond Horndeski theory, viable gravities can be obtained in two ways. One can apply a derivative-dependent conformal transformation to those Horndeski models with cg = 1, since it does not affect their causal structure. An example of this is the derivative conformal transformation of GR. Alternatively, one can implement a disformal transformation, which does alter the GWs light-cone, designed to precisely compensate the original anomalous speed of the theory. Specific combinations of Horndeski and GLPV Lagrangians are representatives of this class.

In more general grounds, the bounds on cg severely restrict the kinetic term of gravity to be canonical (of the Einstein-Hilbert form), up to field redefinitions that preserve the causal structure. This requirement provides a strong selection criteria for viable modified theories of gravity, applicable also to theories other than scalartensor gravity. Massive gravity [21], bigravity [55] and multi-gravity [56] all fall in the safe category as long as matter couples minimally to one of the metrics. Note that in the cases with more than one dynamical metric(s), tensor perturbations of the auxiliary, uncoupled metrics will in general travel at a different speed. In minimally coupled scenarios this effect is only detectable by graviton oscillations with the physical metric [57].
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).
 
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Sounds to me like we need another "Einstein" to figure out the cosmological situation. That was his biggest "blunder", as he put it.
 
  • #80
Urs Schreiber
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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.
Thanks for the detailed comments. I'll be interested in your take on Boran17.

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).
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.
 
  • #81
Buzz Bloom
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Paolo Salucci and Nicola Turini, "Evidences for Collisional Dark Matter In Galaxies?" (July 4, 2017).
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)).​
 
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  • #82
SciencewithDrJ
What's next in GW research, I wonder.
 
  • #83
phyzguy
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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 Internet searched the Internet for these terms and found quite a few articles, but in none of the abstracts was a definition given.
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.
 
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  • #84
phyzguy
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What's next in GW research, I wonder.
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.
 
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  • #85
SciencewithDrJ
How do GW detection findings help further refine the calculation of the age of the Universe?
 
  • #86
phyzguy
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How do GW detection findings help further refine the calculation of the age of the Universe?
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?
 
  • #87
SciencewithDrJ
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?
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?
 
  • #88
Jonathan Scott
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What's next in GW research, I wonder.
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!
 
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  • #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.
 
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  • #90
phyzguy
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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.
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?
 
  • #91
PAllen
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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?
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.
 
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  • #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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Jonathan Scott
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