A LIGO in the Darkness

For the few of you who haven’t yet heard: LIGO has detected gravitational waves from a pair of colliding neutron stars, and that detection has been confirmed by observations of the light from those stars.

gw170817_factsheet

They also provide a handy fact sheet.

This is a big deal! On a basic level, it means that we now have confirmation from other instruments and sources that LIGO is really detecting gravitational waves.

The implications go quite a bit further than that, though. You wouldn’t think that just one observation could tell you very much, but this is an observation of an entirely new type, the first time an event has been seen in both gravitational waves and light.

That, it turns out, means that this one observation clears up a whole pile of mysteries in one blow. It shows that at least some gamma ray bursts are caused by colliding neutron stars, that neutron star collisions can give rise to the high-power “kilonovas” capable of forming heavy elements like gold…well, I’m not going to be able to give justice to the full implications in this post. Matt Strassler has a pair of quite detailed posts on the subject, and Quanta magazine’s article has a really great account of the effort that went into the detection, including coordinating the network of telescopes that made it possible.

I’ll focus here on a few aspects that stood out to me.

One fun part of the story behind this detection was how helpful “failed” observations were. VIRGO (the European gravitational wave experiment) was running alongside LIGO at the time, but VIRGO didn’t see the event (or saw it so faintly it couldn’t be sure it saw it). This was actually useful, because VIRGO has a blind spot, and VIRGO’s non-observation told them the event had to have happened in that blind spot. That narrowed things down considerably, and allowed telescopes to close in on the actual merger. IceCube, the neutrino observatory that is literally a cubic kilometer chunk of Antarctica filled with sensors, also failed to detect the event, and this was also useful: along with evidence from other telescopes, it suggests that the “jet” of particles emitted by the merged neutron stars is tilted away from us.

One thing brought up at LIGO’s announcement was that seeing gravitational waves and electromagnetic light at roughly the same time puts limits on any difference between the speed of light and the speed of gravity. At the time I wondered if this was just a throwaway line, but it turns out a variety of proposed modifications of gravity predict that gravitational waves will travel slower than light. This event rules out many of those models, and tightly constrains others.

The announcement from LIGO was screened at NBI, but they didn’t show the full press release. Instead, they cut to a discussion for local news featuring NBI researchers from the various telescope collaborations that observed the event. Some of this discussion was in Danish, so it was only later that I heard about the possibility of using the simultaneous measurement of gravitational waves and light to measure the expansion of the universe. While this event by itself didn’t result in a very precise measurement, as more collisions are observed the statistics will get better, which will hopefully clear up a discrepancy between two previous measures of the expansion rate.

A few news sources made it sound like observing the light from the kilonova has let scientists see directly which heavy elements were produced by the event. That isn’t quite true, as stressed by some of the folks I talked to at NBI. What is true is that the light was consistent with patterns observed in past kilonovas, which are estimated to be powerful enough to produce these heavy elements. However, actually pointing out the lines corresponding to these elements in the spectrum of the event hasn’t been done yet, though it may be possible with further analysis.

A few posts back, I mentioned a group at NBI who had been critical of LIGO’s data analysis and raised doubts of whether they detected gravitational waves at all. There’s not much I can say about this until they’ve commented publicly, but do keep an eye on the arXiv in the next week or two. Despite the optimistic stance I take in the rest of this post, the impression I get from folks here is that things are far from fully resolved.

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5 thoughts on “A LIGO in the Darkness

  1. ohwilleke

    One more big find was regarding how tightly the contents of a neutron star are bound to each other. If they were less tightly bound, the objects would have broken up as the spiraled into each other muddying the signal. The fact that this didn’t happen puts some pretty strict parameters on models of the internal structure of neutron stars, which is pretty much the only observable location that matter than dense can be observed (since, obviously, you can’t observe matter in black holes). So, your really getting an extreme HEP/nuclear physics data point out of it as well.

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  2. BW

    See tonight’s preprint https://arxiv.org/pdf/1711.00347.pdf by two Perimeter Institute physicists with their own analysis of whether the noise of the black hole gravitational events are correlated between the 2 LIGO detectors. Leaving aside for the moment, the last black hole GW event co-discovered by VIRGO, and the latest neutron star merger GW event, does this Perimeter analysis adequately address the questions raised by the Niels Bohr Institute group on noise correlation?

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    1. 4gravitonsandagradstudent Post author

      It’s encouraging, at least. Skimming the paper, it looks like the weaker version of the NBI group’s claims (that the template-subtraction the LIGO people were doing wasn’t correctly separating the signal from the noise, and that a more template-independent method would work better) seems to hold up (what the PI folks are doing isn’t totally template-independent, but it’s more so), while the darker possibility that there really was nothing there is likely false.

      One thing I should mention is that I don’t think the noise correlation is the only worry of the NBI group at present, based on what I’ve heard from them. In particular, they had some distinct objections to the claimed neutron star merger. So we’ll have to see whether that materializes.

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      1. BW

        Similar to the last black hole GW event where there was an independent detection by VIRGO, it would seem objections for the neutron star merger GW event would be especially difficult since there were independent detections by non LIGO instruments across the electromagnetic spectrum ?

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        1. 4gravitonsandagradstudent Post author

          The short answer is, they think glitches that look like neutron star collisions should be common in LIGO’s data, and they only published that one because the gamma ray telescopes were seeing something. But I don’t really want to say more about this until they’ve published or said something publicly, for one because I don’t know their full argument.

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