![]() Measuring the distance is the tricky part, and this is where gravitational waves offer a new technique. That’s why the exploding neutron stars were so crucial in this event, GW170817, we can for the first time determine the precise redshift of a distant gravitational-wave source. But with gravitational waves alone, you can’t do it - there isn’t enough structure in the spectrum to measure a redshift. ![]() Usually velocity is the easy part: you measure the redshift of light, which is easy to do when you have an electromagnetic spectrum of an object. You’re trying to compare two things: the distance to an object, and the apparent velocity with which it is moving away from us. The simultaneous observation of gravitational and electromagnetic waves is crucial to this idea. This new kind of gravitational-wave observation is something else entirely, allowing us to completely jump over the distance ladder and obtain an independent measurement of the distance to cosmological objects. Works well, but of course is subject to accumulated errors along the way. You build up distance measures step by step, determining the distance to nearby stars, then to more distant clusters, and so forth. We’re already pretty good at measuring the expansion of the universe, using something called the cosmic distance ladder. But it’s my blog, so let me highlight the aspect of personal relevance to me: using “standard sirens” to measure the expansion of the universe. Apparently some folks are very excited by the fact that the event produced an amount of gold equal to several times the mass of the Earth. Look at all those different observatories, and all those wavelengths of electromagnetic radiation! Radio, infrared, optical, ultraviolet, X-ray, and gamma-ray - soup to nuts, astronomically speaking.Ī lot of cutting-edge science will come out of this, see e.g. ![]() The event was the merger of two neutron stars, rather than black holes, and all that matter coming together in a giant conflagration lit up the sky in a large number of wavelengths simultaneously. This has changed now, as we’ve launched the era of “multi-messenger astronomy,” detecting both gravitational and electromagnetic radiation from a single source. Since our current gravitational-wave observatories aren’t very good at pinpointing source locations on the sky, we’ve been completely unable to say which galaxy, for example, the events originated in. Which is super-awesome, but also a bit lonely - black holes are black, so we detect the gravitational waves and little else. The LIGO observatory, recently joined by its European partner VIRGO, had previously seen gravitational waves from coalescing black holes. Everyone is rightly excited about the latest gravitational-wave discovery.
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