LIGO just detected an 'unknown or unanticipated' burst of gravitational waves so

iONiUM writes: LIGO has detected gravitational waves deep in space. The source is currently unknown. From the article:

Scientists think they have detected an "unknown or unanticipated" burst of gravitational waves coming from somewhere deep in space. The wobble in spacetime was picked up unexpectedly by the LIGO experiment, which was specifically built to detect gravitational waves. Astronomers have a picture of what part of the sky the burst originated from, and will look to find more information about its source by further studying the area. But for now there is very little indication of what could have caused the blast, which sent ripples through the fabric of the universe that were detected by LIGO in recent hours. Errors of this kind are predicted to happen only once every 25 years, indicating that the burst probably did really come from an astrophysical event.

In case anyone is worried, it's already been confirmed Betelgeuse is still there.

Betelgeuse's Girls

[Layzej]

Aldebaran's great, okay, Algol's pretty neat, Betelgeuse's pretty girls, Will knock you off your feet. They'll do anything you like Real fast and then real slow, But if you have to take me apart to get me there Then I don't want to go.

unanticipated

[Martin S.]

Surely all gravitational waves are unanticipated.

One thing that never gets explained about LIGO is how it accounts for relativity. Gravity waves stretch spacetime, that is both space and time. Any stretching of space will see an identical stretching of time, thereby cancelling out any perturbation of the signal. So why does this still work? What is not being explained? What is missing?