GW150914: Welcome to the New Era of Gravitational Wave Astronomy

Dave's Take: WOW! What a day yesterday! The long-awaited announcement of LIGO's first detection of gravitational waves was a doozy. LIGO detected a merger in a region of parameter space nobody had expected to dominate the event rates: the merger of two $\sim 30 M_\odot$ black holes about 400 Megaparsecs away. The main paper discovery paper is here, with the astrophysical implications here. Most of the figures below are from the discovery paper.

Cartoon of a merger and estimated gravitational wave strain from numerical relativity. The two ~30 solar mass black holes merged, reaching more than 1/2 the speed of light, and radiating off ~3 solar masses in gravitational waves. 

Parameters of the BH-BH merger GW150914 detected by Advanced LIGO, deduced by comparison to numerical relativity models. Quoted errors are 90% confidence intervals.  

This is the culmination of years of work done the LIGO team and we are incredibly proud to know and work with some of them (including blog contributors Andy and Jocelyn). I got a bit choked up while watching the press conference from UMD, thinking about how long Kip Thorne and Rai Weiss have been going at this. The sheer level of vision that was required to even begin this ludicrous scheme to detect gravitational waves is astonishing.

How Big of A Deal Is This?

Short Answer: This is probably the greatest scientific achievement so far during our lifetimes. Several non-scientist friends have asked me how this compares to the discovery of the Higgs. Personally, I think this easily tops the Higgs. Both have been capstone detections on top of well-confirmed theories. But while the Higgs, in some ways, closed the book on standard-model physics, the first gravitational-wave detection opens an entirely new chapter for astronomy. We can now start to probe physics of the most extreme masses, speeds and densities in the universe. It's truly a new frontier for astrophysics.

A Signal Detectable By Eye

The GW150914 event, measured from the two LIGO detectors. The measured strain shown in the top panel has only had the sharp known line noise removed (whitening) and been filtered by a bandpass between 35-350Hz, and you can pick the signal out by eye from the data. This is ridiculously impressive. 

This is the most gorgeous figure I've ever seen. Almost nobody expected the first signal to be this strong. Everyone who has been working on gravitational-wave astrophysics was expecting for the first signal to be laboriously teased out of the noise by careful matched-filtering algorithms, with integration built up over hundreds of cycles. Because these masses were so large, and it was relatively close, in terms of the LIGO detection horizon, the signal is enormous. In the figure above the top panels only had a  wide bandpass filter applied, along with whitening (this is where you remove strong noise spectral lines, like the 60Hz power-grid line and harmonics). And you can see the signal by eye! This is beyond extraordinary, and beyond the wildest dreams of those of us who have been yearning for a detection.

You might have read in the discovery paper that the signal to noise ratio of the detection was only (!!) ~24 or so. How can this be with such a massive whopper of a signal? This is another consequences of the unexpectedly large masses of the black holes involved. In this merger dominated signal, the peak of the chirp was at ~130 Hz, and the majority of the inspiral was at frequencies lower than the optimum LIGO band in the 10s to few hundred Hz. With only about 10 cycles until the merger, this means there wasn't as much matched-filtering integration time over the inspiral as you'd expect for less massive events like NS-NS mergers. For those of you who are computationally inclined, LIGO has released an open iPython notebook where you can do the data analysis (or at least matching to the numerical relativity signal) yourself! Good times!

This signal was so loud and so unexpected I think everyone assumed it was a blind injection. They had to be reassured by the few involved in blind injections that it wasn't, allowing them to check all the injection channels. On a personal note, after hearing more secondary rumors about the parameter details in October, I had bet a colleague a bottle of wine that it must have been an injection because of the odds of such an event by existing population synthesis was so low! This will be the best bottle of wine I will ever spend.

I'm sure we'll each have much more to say about the science surrounding this detection in the next few days. For instance I'd like to make a post later discussing where such a system could come from, as well as discussing a weakish detection that may have been seen by the Gamma-ray burst monitor on the Fermi spacecraft, which, if associated with the GW150914, would be completely unexpected.

Detection Claims, Rumors and Community Insecurity

The crowd here at UMD gave a heartfelt round of applause to Kip's mention of late UMD professor Joseph Weber. Kip, being Kip, was very gracious to the pioneering work done by Weber and his early (but sadly unsuccessful) attempts at building a gravitational-wave detector using resonant bars. For the modern gravitational-wave community Weber's ultimately discredited claims of detection were a strong source of group self-consciousness and insecurity. Those working on gravitational-wave detection (using interferometric techniques like in LIGO, or more recently begun pulsar timing methods) have been paranoid about making detection claims that they would be forced them to roll back, because of the perceived damage that was done to the reputation of the field in the 70's when Weber's detections were discredited.

This was exacerbated by the debunked BICEP2 claims of primordial gravitational-wave just 2 years previous. The press-release announcement and subsequent drawn-out identification of the dust signal contamination served to once again undermine public trust in gravitational wave claims. The overblown coverage and YouTube stunts only served to make it worse.

The result of this unfortunate history in gravitational wave "detections" is that LIGO was super-paranoid about making any sort of claim before things were checked a thousand times over, and the results were peer-reviewed. We can only imagine how badly Lawrence Krauss's self-aggrandizing rumor mongering was taken. I think Krauss has burned a lot of the capital he had within the scientific community for this self-promoting stunt, though he seems oblivious to it, (RUMOR HAS IT) sending the LIGO collaboration an email "apologizing" but implying they should be thanking him.

Leo's chirping Kip the Thrush for #DrawABirdDay