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Thread: for the best Hubble expansion rate: gravitational waves

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    Default for the best Hubble expansion rate: gravitational waves



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    Default Re: for the best Hubble expansion rate: gravitational waves

    In 2014, before LIGO made the first detection of gravitational waves, Vitale and his colleagues observed that a binary system composed of a black hole and a neutron star could give a more accurate distance measurement, compared with neutron star binaries.
    I can see why it would be good to have an independent method for measuring the Hubble constant, but the article doesn't really explain why gravitational waves would give a more reliable or more precise measure than the Planck and GAIA methods.
    Woody

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    Default Re: for the best Hubble expansion rate: gravitational waves

    Quote Originally Posted by stargazer55 View Post
    I can see why it would be good to have an independent method for measuring the Hubble constant, but the article doesn't really explain why gravitational waves would give a more reliable or more precise measure than the Planck and GAIA methods.
    Before answering let me just note that the neutron star merger event measured by LIGO-VIRGO and optical means is halfway between the 67 km/sec/Mpc of Planck and the 73 km/sec/Mpc of GAIA. So it's no help yet in resolving the contradiction. The error associated with this single binary neutron star event is about 14% so.....

    One needs a few events to knock the error bars down. Gravitational waves could be better than the GAIA method since compared with GAIA there is no problem with metallicity dependence or extinction due to dust and gas at the host galaxy, the MW and places in between. the metalliaicy issue goes away because the composition of NSs is fixed, it's fully catalyzed.

    the BH-NS merger is found to have benefits with regard to the binary NS merger scenario in that

    binary NS mergers happen more frequently, more closely and so have uncertainties due to the unknown peculiar velocity of the host galaxy.

    there is a degeneracy in the brightness and orbital inclination for binary NS mergers, but in the BH-NS merger a spinning BH breaks this degeneracy and makes for a more precise distance determination.

    this is the general sense of the published research which is at

    https://arxiv.org/abs/1804.07337

    The detection of GW170817 and the identification of its host galaxy have allowed for the first standard-siren measurement of the Hubble constant, with an uncertainty of ∼14%. As more detections of binary neutron stars with redshift measurement are made, the uncertainty will shrink. The dominating factors will be the number of joint detections and the uncertainty on the luminosity distance of each event. Neutron star black hole mergers are also promising sources for advanced LIGO and Virgo. If the black hole spin induces precession of the orbital plane, the degeneracy between luminosity distance and the orbital inclination is broken, leading to a much better distance measurement. In addition neutron star black hole sources are observable to larger distances, owing to their higher mass. Neutron star black holes could also emit electromagnetic radiation: depending on the black hole spin and on the mass ratio, the neutron star can be tidally disrupted resulting in electromagnetic emission. We quantify the distance uncertainty for a wide range of black hole mass, spin and orientations and find that the 1-σ statistical uncertainty can be up to a factor of ∼10 better than for a non-spinning binary neutron star merger with the same signal-to-noise ratio. The better distance measurement, the larger gravitational-wave detectable volume, and the potentially bright electromagnetic emission, imply that spinning black hole neutron star binaries can be the optimal standard siren sources as long as their astrophysical rate is larger than O(10) Gpc−3yr−1, a value allowed by current astrophysical constraints.
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    Default Re: for the best Hubble expansion rate: gravitational waves

    the degeneracy with binary NS mergers is because the brightness of the source depends on the angle between the total system angular momentum and the line of sight. gravitational radiation is quadrupole radiation so has an angular intensity dependence that is minimized for edge on and face on presentations and greater at other angles.
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    Default Re: for the best Hubble expansion rate: gravitational waves

    Quote Originally Posted by not_Fritz_Argelander View Post
    the degeneracy with binary NS mergers is because the brightness of the source depends on the angle between the total system angular momentum and the line of sight. gravitational radiation is quadrupole radiation so has an angular intensity dependence that is minimized for edge on and face on presentations and greater at other angles.
    You beat me to my question with your answer. Thanks!
    Woody

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    Default Re: for the best Hubble expansion rate: gravitational waves

    Quote Originally Posted by stargazer55 View Post
    You beat me to my question with your answer. Thanks!
    You're welcome. In afterthought I decided some clarification would be good to add.

    Figure 1) of the paper shows the impressive ten fold reduction in distance error when the degeneracy is lifted.
    chas53 and GCoyote like this.
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