An update for Section 13.3f (pp. 352-355) in Pasachoff & Filippenko, The Cosmos, 4th ed:
In what many think is the most important step in astronomy since Galileo first turned a telescope on the heavens in 1609, ripples in space-time known as gravitational waves were detected on September 14, 2015, and reported to the world at large on February 11, 2016. Scientists gathered all over in auditoriums at the National Press Club in Washington, at Caltech, at MIT, in Moscow, and in many other places for the epochal event. “Ladies and gentlemen, we have detected gravitational waves,” said the executive director of the LIGO Laboratory. “We did it!”
The observational existence of gravitational radiation had been established by the changing period of the Hulse-Taylor “double pulsar” at a rate that matched the energy loss expected from the emission of gravitational waves (Section 13.3f, pp. 352-354), but the gravitational waves themselves – distortions of space-time – had not been detected directly until the Advanced LIGO (Laser Interferometer Gravitational Wave Detector) picked up a signal in its engineering run on September 14, 2015. The signal, which could be heard as an upwards “chirp” in the audio range, resulted from two black holes 1.3 billion light years away, each containing about 30 solar masses, spiraling rapidly into each other and merging to become a single black hole with almost the same total mass but with 3 solar masses of material converted into energy in the form of gravitational waves.
Advanced LIGO has about 3 times the sensitivity of the earlier LIGO, and this event was within that advance of sensitivity. A further gain of 3x is expected with Advanced LIGO in the future.
A few days later, I [JMP] attended a two-hour evening session at Caltech’s main auditorium to hear the principals of the LIGO project speaking, and I had the feeling that it was just like being at a meeting of the Lincei Academy in Florence in 1610 to hear Galileo speak about his new discoveries with the [optical] telescope.
There are many links to discussions and animations available. Here are some of them:
Popular and academic press:
New York Times coverage (includes movie); also NYT Opinion piece and an NYT Editorial a few days later, justifying scientific inquiry; a congressman’s letter, and sound bites from scientists.
Nature coverage (with explanatory graphics)
Science magazine in depth (requires subscription to access full text)
New Yorker article (with an account of LIGO’s inception and development)
Physics Today comparison of gravitational waves and sound waves
The Wall Street Journal and Michio Kaku’s desciption.
Societies and organizations:
American Physical Society Viewpoint
The Kavli Foundation Scientific spotlight
Caltech (press releases, illustrations, movies and animations)
NSF press release
AAPT resources on gravitational waves
STFC (UK) press release
Cornell Chronicle and media statement
University of Texas Rio Grande Valley press release
CSIRO (Australia) news release