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LIGO, High Gear Science Run
antoniseb
post May 26 2019, 11:11 PM
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Interesting. Is the GraceDb (https://gracedb.ligo.org/superevents/S190521g/view/) reporting distance in comoving megaparsecs? That seems odd, but would explain why this event was observed at all.
Can you point me to any place where they say or imply that they are reporting in comoving Mpc? Thanks in advance!

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fredk
post May 27 2019, 02:17 AM
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Do you mean comoving vs proper distances? It's standard in cosmology to use comoving distances, since it avoids the problem of separations being smaller in the past. For close distances (redshift z << 1) proper distances are well defined and are very close to comoving ones.
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Lucas
post May 28 2019, 12:24 PM
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It appears that they are using luminosity distances (see http://www.astro.ucla.edu/~wright/cosmo_02.htm#DL for definition) based on https://lscsoft.docs.ligo.org/ligo.skymap/l...p/bayestar.html (look at min_distance parameter) and https://emfollow.docs.ligo.org/userguide/glossary.html (look at “burst range”). In these units the CMB is at ~15000 Gpc.
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fredk
post May 28 2019, 05:44 PM
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Yes, the observed gravitational wave amplitude (and frequency and its rate of change) gives the luminosity distance (once you've got a handle on the polarization), but at this level of accuracy that will agree with any other distance measure. Units, though, are comoving. The CMB is at just under 14 comoving Gpc, not 15 000 - see, eg, sect 3.4 of Planck Inflation. So at very crudely a Gpc, this merger is considerably closer.
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