Did the LIGO gravitational waves originate from primordial black holes?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by paddoboy, Oct 27, 2016.

  1. paddoboy Valued Senior Member

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    Shocks in the early universe could be detectable today
    October 27, 2016 by Lisa Zyga feature

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    Simulation showing cosmological initial conditions (left) evolving into shocks (right). Credit: Pen and Turok. ©2016 American Physical Society
    (Phys.org)—Physicists have discovered a surprising consequence of a widely supported model of the early universe: according to the model, tiny cosmological perturbations produced shocks in the radiation fluid just a fraction of a second after the big bang. These shocks would have collided with each other to generate gravitational waves that are large enough to be detected by today's gravitational wave detectors.

    The physicists, Ue-Li Pen at the Canadian Institute for Theoretical Astrophysics in Toronto, and Neil Turok at the Perimeter Institute for Theoretical Physics in Waterloo, have published a paper on the shocks in the early universe and their aftermath in a recent issue of Physical Review Letters.

    As the scientists explain, the most widely supported model of the early universe is one with a radiation-dominated background that is almost perfectly homogeneous, except for some tiny waves, or perturbations, in the radiation.





    Read more at: http://phys.org/news/2016-10-early-universe-today.html#jCp
     
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  3. paddoboy Valued Senior Member

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    https://arxiv.org/pdf/1510.02985v2.pdf

    Shocks in the Early Universe:

    Neil Turok
    Perimeter Institute for Theoretical Physics,
    Waterloo ON N2L 2Y5, Canada
    (Dated: September 8, 2016)


    We point out a surprising consequence of the usually assumed initial conditions for cosmological perturbations. Namely, a spectrum of Gaussian, linear, adiabatic, scalar, growing mode perturbations not only creates acoustic oscillations of the kind observed on very large scales today, it also leads to the production of shocks in the radiation fluid of the very early universe. Shocks cause departures from local thermal equilibrium as well as creating vorticity and gravitational waves. For a scale-invariant spectrum and standard model physics, shocks form for temperatures 1 GeV< T < 107 GeV. For more general power spectra, such as have been invoked to form primordial black holes, shock formation and the consequent gravitational wave emission provides a signal detectable by current and planned gravitational wave experiments, allowing them to strongly constrain conditions present in the primordial universe as early as 10−30 seconds after the big bang.
     
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  5. Q-reeus Banned Valued Senior Member

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    Thread title is completely misleading and the obvious answer to it is no. Also, the news re actual article on primordial shocks is not new - v1 came out Oct 2015 and this is merely a revision v2.
     
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  7. paddoboy Valued Senior Member

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    The article asks the question re whether GW's from the BB era could be detected today.
    from the article:
    "These shocks would have collided with each other to generate gravitational waves that are large enough to be detected by today's gravitational wave detectors".

    "The physicists also showed that, when two or more shocks collide with each other, they generate gravitational waves"

    There are many papers on the subject of GW's from the BB and Inflation era of the early universe, and obviously physicists believe they could be detected.
     
  8. Q-reeus Banned Valued Senior Member

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    The GW emphasis in the cited article is on primordially generated stochastic GW's - overwhelmingly from shock wave collisions, not coalescence of 'primordial BH's' also possibly generated in such collisions, but merging many billions of years later. The thread title encourages conflation of shock generated primordial GW's with 'primordial BH's' merging to form GW's comparatively recently. If you wished to cherry-pick the latter, then the passage dealing with such should have been cited in particular.
     
  9. paddoboy Valued Senior Member

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    27,543
    https://arxiv.org/pdf/1603.08338v2.pdf
    8th August:2016:
    Primordial Black Hole Scenario for the Gravitational-Wave Event GW150914

    Abstract:

    We point out that the gravitational-wave event GW150914 observed by the LIGO detectors can be explained by the coalescence of primordial black holes (PBHs). It is found that the expected PBH merger rate would exceed the rate estimated by the LIGO Scientific Collaboration and the Virgo Collaboration if PBHs were the dominant component of dark matter, while it can be made compatible if PBHs constitute a fraction of dark matter. Intriguingly, the abundance of PBHs required to explain the suggested lower bound on the event rate, > 2 events Gpc−3 yr−1 , roughly coincides with the existing upper limit set by the nondetection of the cosmic microwave background spectral distortion. This implies that the proposed PBH scenario may be tested in the not-too-distant future.
     
  10. danshawen Valued Senior Member

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    The more massive the black hole merger, the shorter (and higher pitched) the chirp, At those distances, the inverse square law also comes into play, but our instruments are getting even more sensitive.

    I wonder what it would look like? Great question.
     
  11. Q-reeus Banned Valued Senior Member

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    Wrong. You actually believe GW's from a pair of merging SMBH's would chirp faster than a pair of micro BH's?! It's the other way round.
    Wrong again. Unlike for say regular antenna dishes, aLIGO detector responds directly to signal amplitude, not intensity. Hence, it's the 1/r law that 'comes into play'.
    Can I give you a third wrong in a row? Why not. It wouldn't look like anything, as both the BH's and emitted GW's are invisible. You knew that much Dan, right?
     
  12. danshawen Valued Senior Member

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    Q-reeus, I'm not making this up.

    http://www2.physics.umd.edu/~pshawhan/gw/Shawhan_UMD_Feb2016.pdf

    Read it for yourself.

    Peter Shawhan, a relative of mine, is a principal researcher with LIGO.
     
  13. danshawen Valued Senior Member

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    3,951
    Page 24 is the one you want. Kind of intrigues me that your initial idea was the same as mine, but what Peter's research says actually makes a lot of sense. So tell me, why did you think the chirp should have been lower pitch? Were you thinking it was related to something like a natural resonant frequency?
     
  14. Q-reeus Banned Valued Senior Member

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    I do recall seeing that same claim elsewhere (perhaps it was Shawhan), but it's wrong. Here's the actual physics, covering more than just BH binaries:
    https://www.astro.umd.edu/~miller/teaching/astr498/lecture25.pdf
    Page 1 covers all we need to know. Cole Miller's analysis is correct. The reason I mentioned SMBH binaries is that the size of each one in the pair could be light-days or more in radius. Hence physically impossible for the chirp period to be less than days in duration. A little common sense can go a long way. Have a nice chat with your relative some time, and steer him gently to above.
     
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  15. danshawen Valued Senior Member

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    3,951
    No, Peter is right. The researcher you referenced does not treat the possibility that there is a brief gap when the two EHs coalesce into one, and that event does not take days or even minutes, but typically anywhere from a couple tenths of a second to several seconds, if the first two detections investigated In detail are any indication. After the final stage EH is reached, you will see nothing else forever, other than Hawking radiation.

    I like that we already have some disagreement. This is a sure sign that this field will be receiving the ample attention it deserves.
     
  16. Q-reeus Banned Valued Senior Member

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    4,695
    From your #7:
    Chirp is defined 2nd para here: http://www.sr.bham.ac.uk/gwastro/what-are-we-looking-for
    It coincides with inspiral phase when the two objects are distinct, hence Cole Miller's analysis directly applies.

    You are evidently referring to the ringdown phase which is the settling down of merged entity, after chirp phase, and before a final steady-state object results. Even there, ringdown frequencies should be inversely proportional to combined mass (assume equal mass binaries). The characteristic waveform shown in above article clearly shows ringdown frequencies about equal to that of the maximum chirp phase. For a given binary pair mass ratio, spin (assume negligible in our case), the waveform is a universal one. Meaning the shape stays the same regardless of absolute masses, but the frequencies scale inversely as the masses involved.

    Miller's straightforward analysis makes it clear characteristic frequencies cannot be greater than √(Gρ), which covers both chirp phase and ringdown. To repeat - frequencies at all stages will scale inversely with the system mass. Have you been in contact with your cousin on this?
     
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  17. Q-reeus Banned Valued Senior Member

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    Decided to 'flip the page' of last link I gave, and....Nice specific backing of what should be obvious just from scaling law considerations: http://www.sr.bham.ac.uk/gwastro/what-are-we-looking-for/2
    First sentence, first para. Further down, it does read:
    This cannot be in contradiction to above referenced and what Cole Miller showed, and most likely 'duration' here means bandwidth-limited detector observed cycle count, not absolute time duration.
     
    Last edited: Oct 30, 2016
  18. danshawen Valued Senior Member

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    No, I have not discussed his analysis with him yet. That document was some time ago. He may have changed a few things. Interesting.
     
  19. Q-reeus Banned Valued Senior Member

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    Well.... Decided to do a simple check to verify that, apart from that chirp waveform frequencies scale inversely as mass, otherwise it has a self-similar form. From:
    https://en.wikipedia.org/wiki/Gravitational_wave#Power_radiated_by_orbiting_bodies
    P ~ M^5/R^5 - (1) [we are only interested in functional dependencies on R & M, so neglect constants and assume here a given M1:M2 ratio, ellipticity etc.]
    But for BH's (or whatever they are), at region of interest close to merger, we have radii ~ touching, and since BH M ~ R, From (1), P is independent of M.
    Power P represents system mass loss to GW's per unit time. Hence fractional mass loss P/M per orbital cycle will go as
    (P/M)R ~ (1/R)R = constant - (3)
    Which confirms my claim earlier that self-similarity applies so the chirp phase cycle count for a given binary mass ratio, ellipticity etc. is a constant. Thus the absolute time of chirp phase rises in direct proportion to binary mass M.

    [Above is an edit of earlier entry that got the power of M wrong in (1) - and got a bizarre result accordingly!]
     
    Last edited: Oct 30, 2016
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  20. danshawen Valued Senior Member

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    YES! excellent analysis. For those not following, this might help:

    http://nautil.us/issue/34/adaptation/the-gravity-wave-hunter

    and in particular, this:

    "So there’s a difference in what part of the signal that we see in the nanohertz frequency part of the gravitational wave spectrum, and what LIGO sees. So LIGO would see sources that are a few times the mass of the sun, maybe 10 times or more the mass of the sun, and also neutron stars or other compact objects that are inspiraling at the ends of their lives, whereas the nanohertz gravitational wave experiments see much more massive objects, like supermassive black holes, which are a million to a billion times the mass of the sun in their very early inspiral phase; so you don’t hear that final chirp in the nanohertz frequency band, but you do with LIGO."

    Or to make an acoustic analogy of the final phase, if the EH is like a drumhead, more massive objects would correspond to a tighter drumhead, and a correspondingly higher pitch sound when a pebble strikes the drumhead. However the acoustic profile in the case of the first two detailed analysis is just a little more complicated because a drumhead tuned to a slightly higher pitch has been dropped on top of a drumhead with a lower pitch. The jumble of vibrations that are effected in this manner will no doubt be interesting to analyze, but probably anything but musical. The end result is a bigger drum with an even tighter drumhead.

    nano (10^-9) Hertz --> supermassive, before inspiraling phase; very, very low or sub-audible frequency;
    10-300 Hertz --> tens of solar masses, inspiraling phase detected by LIGO; within audible frequency range

    Which bears out Q-reeus's assertion that the situation is a little more complicated or even contrary to what Peter suggested in his colloquia presentation. Too late to change that, unfortunately.

    Looks like GR's principle of equivalence has held up nicely, though.
     
  21. danshawen Valued Senior Member

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    3,951
    So, do heavier objects fall faster than lighter ones, or do they just leave a deeper crater/make a lower pitched noise on impact?

    Would there be any gravity waves generated if they fell directly into each other without spiraling?
     
  22. Q-reeus Banned Valued Senior Member

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    !!?? Evidently I failed utterly to get through to Dan with the actual picture....sigh. The inverse is the case.
    Have tried hard to reconcile what both Peter and the folks at Birmingham Uni claim - shorter detected chirp signal duration (not to be confused with chirp frequency) for higher mass, with my simple to do analysis. Can't. Maybe there is something subtle and counter-intuitive about the detection window of aLIGO and similar, but if so it's not obvious. More likely, your cousin has managed to propagate an error that has caught on elsewhere!
     
  23. Q-reeus Banned Valued Senior Member

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    Err, pass.
    Yes - a burst of axially symmetric GW's would be radiated. Merger and ringdown, dominated by linear quadrupole mode. The fraction of mass lost to GW's is lower than for inspiral case, but I won't try searching for a specific reference to back that up.
     
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