Double or nothing

Gravitational WavesThe Advanced LIGO project was predetermined that if successful then it could only be a merger of two black holes (binary) event.

The finding completed the scientific arc of prediction, discovery and confirmation: first they calculated what they should be able to detect, then decided what the evidence should look like, and then devised the experiment that clinched the matter. Which is why on Thursday scientists around the world were able to hail the announcement as yet another confirmation of their “standard model” of the cosmos, and the beginning of a new era of discovery.

... Thursday’s announcement was the unequivocal first detection ever of gravity waves.
Gravitational waves: breakthrough discovery after a century of expectation | The Guardian

Gravitational Waves

Gravitational waves were predicted even earlier, but detection has been delayed because the quest involves detecting a very elusive effect and again requires large-scale and expensive instruments - a challenge to the many hundreds of engineers and scientists involved in this international collaboration.

Why is the effect so small and elusive? It’s basically because gravity is such a weak force. We only feel its attraction because we are on planet Earth.

The gravitational pull between everyday objects is tiny. If you wave around two dumbbells you will emit gravitational waves - but with quite infinitesimal power. Even planets orbiting stars, or pairs of stars orbiting each other, don’t emit at a detectable level.

Astronomers are agreed that the only sources that LIGO might detect must involve much stronger gravity than in ordinary stars and planets. Objects where gravity is strongest and fastest-changing are the most powerful emitters of gravitational waves. The best bet is that the events involve black holes.

We’ve known for about 40 years that black holes exist: most are the remnants of stars twenty or more times more massive than the sun. These stars burn brightly, and in their explosive death-throes (signalled by a supernova), their inner part collapses to a black hole.
Gravitational waves: Einstein was right | Telegraph

Gravitational Waves

The violent birth of a black hole generates a pulse of gravitational waves. But an even stronger pulse comes if two holes subsequently coalesce. This is a rare event, happening less than once in a million years in our Galaxy. But such an event would give a LIGO signal even if it happened a billion light-years away - and there are millions of galaxies closer than that ... It is this "chirp" - a shaking of space that speeds up and strengthens until the merger, and then dies away - that LIGO can detect. It’s possible to calculate the wave-form of this "chirp": it depends on how heavy the merging holes are, and how they are spinning, and how their orbit is oriented relative to the line of sight. So data of sufficiently high quality can reveal details of what caused the events.
Gravitational waves: Einstein was right | Telegraph

On a graph, the data looks like a symmetrical, wiggly line that gradually increases in height and then suddenly fades away.

"We found a beautiful signature of the merger of two black holes and it agrees exactly - fantastically - with the numerical solutions to Einstein equations... it looked too beautiful to be true," said Prof Danzmann.
Einstein's gravitational waves 'seen' from black holes | BBC

Gravitational waves should penetrate regions of space that electromagnetic waves cannot.
Gravitational Waves | Wikipedia

The particular pattern of brightening observed on September 14 agrees remarkably well with what Einstein's General Theory of Relativity predicts for two massive black holes in the final moments of a death spiral. Fittingly, Einstein's theory of photons has helped to verify Einstein's theory of gravity, a century after its creation.
Gravitational Waves Discovered: Top Scientists Respond | US News

Gravitational Waves

Researchers use descriptions of the expected signal as templates which facilitate the extraction of the signal from LIGO's noisy data. Since no gravitational wave signals had ever been seen before, theorists found themselves unusually relevant to the detection project – only they could provide such data analysis templates.
Gravitational Waves Discovered: Top Scientists Respond | US News

Einstein first thought that objects cannot shed energy in the form of gravitational radiation, then changed his mind. He showed in a seminal 1918 paper which ones could: Dumbbell-like systems that rotate about two axes at once, such as binary stars and supernovas popping like firecrackers, can make waves in space-time.

Still, Einstein and his colleagues continued to waffle. Some physicists argued that even if the waves exist, the world will oscillate with them and they cannot be felt. It wasn’t until 1957 that Richard Feynman put that question to rest, with a thought experiment demonstrating that, if gravitational waves exist, they are theoretically detectable.
Gravitational Waves Discovered at Long Last | Quanta Magazine

gravitational waves

Judging by its shape and size, that first, loudest chirp originated about 1.3 billion light-years away from the location where two black holes, each of roughly 30 solar masses, finally merged after slow-dancing under mutual gravitational attraction for eons. The black holes spiraled toward each other faster and faster as the end drew near, like water in a drain, shedding three suns’ worth of energy to gravitational waves in roughly the blink of an eye. The merger is the most energetic event ever detected.
Gravitational Waves Discovered at Long Last | Quanta Magazine

It took just 20 milliseconds to catch the merger of two black holes, at a distance of 1.3 billion light years, somewhere beyond the Large Magellanic Cloud in the southern hemisphere sky, but it then took months of meticulous checking of the signal against all the complex computer simulations of black hole collision to make sure the evidence matched the theoretical template.
Gravitational waves: breakthrough discovery after a century of expectation | The Guardian