An 11-year hunt for ripples in space-time produced by the merging of two black holes has apparently turned up empty for now.
This means Albert Einstein's theory of general relativity still has at least one remaining asterisk next to it, since the mystery of gravitational waves remains its last unconfirmed prediction.
A new study in the journal Science has found that gravitational waves are more mysterious than perhaps initially thought. Gravitational waves, or literal ripples in space and time, can theoretically be created when two black holes spiral around one another as galaxies crash into each other over the course of billions of years.
Scientists expected to make a direct detection of gravitational waves by now, but this study might cause a re-think for researchers hunting for these signs of the biggest crashes in the cosmos.
More broadly, this could mean scientists need to rethink the mechanics of black hole collisions.
"The main reason we were surprised by our result is that we didn't see the gravitational wave signals predicted by many teams of theorists," Vikram Ravi, a co-author of the study, told Mashable via email.
"That is, we've achieved our design sensitivity that should have yielded a detection of gravitational waves, but didn't. Our result means that theorists need to come up with new models for gravitational waves from binary supermassive black holes."
Gravitational waves are sent out from merging black holes because they actually perturb the fabric of space-time around them.
Think about space-time as a sheet on a bed, and the binary black holes (the two black holes orbiting one another as galaxies merge) as a pair of bowling balls spinning around one another on that sheet.
The sheet would ripple and move, affecting other parts of the bed as well.
Ravi and the research team were looking for those ripples in space-time by probing the universe with a powerful radio telescope.
How to see an invisible wave
Scientists were using the Parkes telescope in Australia to find gravitational waves via a method called "pulsar timing" from Earth. The research team kept a close eye on a set of small stars called pulsars, which regularly send out radio pulses into space.
Scientists were able to clock the timing of when the pulses arrived at Earth to "an accuracy of ten billionths of a second," the Commonwealth Scientific and Industrial Research Organization (CSIRO) wrote in a press release.
If a gravitational wave were to ripple in the space between a pulsar and Earth, it would have changed the distance between the two bodies by approximately 10 meters, CSIRO said.
While that's a tiny distance on a comic time scale, it would have been enough to change when a signal from the pulsar would arrive on Earth, meaning that scientists would know that something extreme like a gravitational wave changed the regular timing of the pulsar.
"The gravitational waves don't affect the pulsars at the level we thought they would," Ravi said. "You can think of pulsars like awesome clocks in space, which emit regular ticks of radio waves each time they rotate. We've measured the arrival times of the pulses from the pulsars over approximately 11 years."
It's not that simple
But the new study isn't a simple open-and-shut case.
While the ripples in space-time are expected to occur whenever massive black holes merge, they still haven't been observed. The mystery of gravitational waves remains the last unconfirmed prediction of Albert Einstein's theory of general relativity, CSIRO said.
The prevailing theory is that pretty much all large galaxies should have a supermassive black hole at their centers, so when those galaxies combine, the black holes should produce a binary, and in turn, send out gravitational waves that scientists can (in theory) detect once they parse out the signal from the noise.
"Most mergers should indeed produce black hole binaries, and it is indeed a big -- one could fairly say: shocking -- mystery why we don't see clearcut evidence for more of them," John Kormendy, a black hole scientist unaffiliated with the study, told Mashable via email.
Why haven't we found them?
The gravitational waves hunted for in this study may not have been found because perhaps black holes merge more quickly than initially expected.
“There could be gas surrounding the black holes that creates friction and carries away their energy, letting them come to the clinch quite quickly,” Paul Lasky, co-author of the study, said in a statement.
Scientists will also need to continue hunting for gravitational waves through pulsar timing for many more years before giving up on the hunt.
"What our result means is that once we've accounted for all that physics (which includes known sources of measurement error), there's no sign of gravitational waves at the level we expected," Ravi said. "They may be weaker and hidden in the data, but that is inconsistent with current models."
It's also possible that other factors confounded the detection of these particular gravitational waves, which might actually be better found at other wavelengths.
"Environmental effects such as coupling with the stellar background -- possibly resulting in large eccentricities -- can very well explain the non detection, even keeping a fairly high merger rate," astrophysicist Alberto Sesana, another researcher unaffiliated with the study, told Mashable via email.
Also, the result isn't wildly outside the range of possibilities predicted by models, meaning that it is possible to reconcile the non-detection with current ideas.
"In short, there is still a lot of room to 'play with the models' in a physically sound way and still be consistent with this result, without invoking a radical change in our understanding of massive black hole evolution," Sesana said.