Do gravitational-wave observatories measure distance
October 17, 2017 5:33 AM Subscribe
Do gravitational-wave observatories give us a new way to measure distance?
This article about gravitational astronomy contains the following:
The discovery gave scientists a chance to measure the expansion of the universe, too. Since astronomers knew which galaxy the latest gravitational waves came from, they could calculate the distance between that galaxy and Earth and then plug it into equations for the rate of expansion, known as the Hubble constant. Good news: The answer matched up with previous estimates from other methods.
The article seems to be claiming that gravitational wave observations gave us some new way to infer the distance to the wave source and that this guess matched up with the distance we guessed from the red shift. What is the excerpt above trying to say?
This article about gravitational astronomy contains the following:
The discovery gave scientists a chance to measure the expansion of the universe, too. Since astronomers knew which galaxy the latest gravitational waves came from, they could calculate the distance between that galaxy and Earth and then plug it into equations for the rate of expansion, known as the Hubble constant. Good news: The answer matched up with previous estimates from other methods.
The article seems to be claiming that gravitational wave observations gave us some new way to infer the distance to the wave source and that this guess matched up with the distance we guessed from the red shift. What is the excerpt above trying to say?
Best answer: This may help, as may this depending on how technical you feel. You can indeed use the gravitational wave signal to infer a distance subject to the (startling good so far) assumption Einstein was right. You need to look in detail at the signal from the inspiral phase and there's a fair chunk of maths in there, but by the standards of such things perhaps not too much. And the redshift comes from the optical counterpart.
There's some uncertainty based on the angle from which we view (hear?) the source, but allowing for that the measurements do match up well.
posted by edd at 6:02 AM on October 17, 2017 [1 favorite]
There's some uncertainty based on the angle from which we view (hear?) the source, but allowing for that the measurements do match up well.
posted by edd at 6:02 AM on October 17, 2017 [1 favorite]
Best answer: Yeah, my understanding is that judging just by the strength of the signal and our knowledge of these events we get another luminosity distance independent of EM measurements.
posted by vacapinta at 6:04 AM on October 17, 2017
posted by vacapinta at 6:04 AM on October 17, 2017
Best answer: https://www.preposterousuniverse.com/blog/2017/10/16/standard-sirens/ . A nice article on "standard-sirens".
posted by nickggully at 6:06 AM on October 17, 2017 [1 favorite]
posted by nickggully at 6:06 AM on October 17, 2017 [1 favorite]
Best answer: I mentioned this in the NS-NS GW GRB news thread, and Edd linked to the relevant paper preprints above. (Including Holz & Hughes 2008 - I hadn't seen that before. Nice!)
From the Nature paper abstract:
GW170817 can be used as a standard siren, combining the distance inferred purely from the gravitational-wave signal with the recession velocity arising from the electromagnetic data to determine the Hubble constant. This quantity, representing the local expansion rate of the Universe, sets the overall scale of the Universe and is of fundamental importance to cosmology. Our measurements do not require any form of cosmic ‘distance ladder’; the gravitational-wave analysis directly estimates the luminosity distance out to cosmological scales. Here we report H0 = 70.0+12.0-8.0 kilometres per second per megaparsec, which is consistent with existing measurements, while being completely independent of them.
So yes, a "standard siren" allows a distance to be inferred. I'm sure there will turn out to be various complications in the "standard" when we dig deeper - maybe it'll turn out to depend on the composition and density profile of the neutron stars, not just the mass ratio? I have no idea. But with a few assumptions (and assuming General Relativity is a good-enough description of nature, which is uncontested so far), you can get a decent distance estimate from the amplitude and envelope of the detected gravitational waves.
posted by RedOrGreen at 6:45 AM on October 17, 2017
From the Nature paper abstract:
GW170817 can be used as a standard siren, combining the distance inferred purely from the gravitational-wave signal with the recession velocity arising from the electromagnetic data to determine the Hubble constant. This quantity, representing the local expansion rate of the Universe, sets the overall scale of the Universe and is of fundamental importance to cosmology. Our measurements do not require any form of cosmic ‘distance ladder’; the gravitational-wave analysis directly estimates the luminosity distance out to cosmological scales. Here we report H0 = 70.0+12.0-8.0 kilometres per second per megaparsec, which is consistent with existing measurements, while being completely independent of them.
So yes, a "standard siren" allows a distance to be inferred. I'm sure there will turn out to be various complications in the "standard" when we dig deeper - maybe it'll turn out to depend on the composition and density profile of the neutron stars, not just the mass ratio? I have no idea. But with a few assumptions (and assuming General Relativity is a good-enough description of nature, which is uncontested so far), you can get a decent distance estimate from the amplitude and envelope of the detected gravitational waves.
posted by RedOrGreen at 6:45 AM on October 17, 2017
If you'll permit a (very) lay person's side question: I learned in simpler times that the speed of light cannot be exceeded; if so, why did the the light from this event arrive 2 seconds after the gravitational wave?
posted by baseballpajamas at 9:35 AM on October 17, 2017
posted by baseballpajamas at 9:35 AM on October 17, 2017
it takes approx 2 seconds for the light from the explosion to 'break out' of the explosion, while the gravitational waves are emitted instantaneously. The same thing happens with neutrinos in supernovae...the neutrinos 'break out' first.
posted by sexyrobot at 11:29 AM on October 17, 2017 [2 favorites]
posted by sexyrobot at 11:29 AM on October 17, 2017 [2 favorites]
Sure, the same way a submarine calculates contact range from only a sound bearing and frequency. Now that we have some data on this leg, we just need to steer the Earth onto a contrasting course and speed and analyze what changes.
posted by ctmf at 10:00 PM on October 17, 2017
posted by ctmf at 10:00 PM on October 17, 2017
Best answer: To expand: the frequency and rate of orbital decay gives you an estimate of the size of the neutron stars & once you know their sizes you can calculate the strength (luminosity) of the gravitational waves they emit as they orbit each other. The gravitational wave detectors give you a measure of the amplitude of the waves as they pass through the earth & the difference allows you to calculate the distance between us and the source event.
Make sense?
posted by pharm at 2:22 AM on October 18, 2017 [1 favorite]
Make sense?
posted by pharm at 2:22 AM on October 18, 2017 [1 favorite]
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posted by nickggully at 6:01 AM on October 17, 2017