In 2019, LIGO and Virgo recorded something truly bizarre – a gravitational wave event less than a tenth of a second in duration.
Compared to the drawn-out chirps of black hole binaries on decaying orbital spirals, it was a sharp crack. The best explanation of the event, named GW190521, was a chance encounter of two black holes snaring each other in passing.
Now, a new paper has presented an alternative, far more exotic option: the echo of a black hole collision in another universe, reverberating through a collapsing wormhole that formed as a result of that merger.
Related: Two Black Holes Met by Chance, And It Created Something Never Seen Before
To be clear, the black hole collision right here in our own Universe is still the preferred interpretation of the strange signal… but that preference is not strong enough to rule out the wormhole model entirely, writes a team led by physicist Qi Lai of the University of Chinese Academy of Sciences in a preprint uploaded to arXiv.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>If it were indicative of a wormhole, GW190521 and gravitational waves like it wouldn’t just confirm the existence of these wild, hypothetical structures: they would also give us a new tool for probing the physical properties thereof. That’s a very large if.
The gravitational waves we can currently detect are ripples in the fabric of spacetime generated by collisions between massive, dense objects, such as black holes and neutron stars.
But it’s not just the collision itself that creates these ripples. As a black hole pair in a binary orbit spirals in towards each other, their interacting gravitational fields also generate ripples, starting weaker and getting stronger as the objects grow closer together.
The resulting “chirp” therefore has a rising waveform, as seen in the video below.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>GW190521 lacked the inspiral part of the signal, and at the derived mass of the merger – 142 times the mass of the Sun – it should have been detectable. This led to the conclusion that the black holes were not locked in a binary orbit, but were simply passing each other by when they were slurped into a mutual gravity well and merged.
Lai and colleagues thought that there might be another explanation: what if the object that formed in a binary black hole merger was a wormhole that then collapsed to form the final, merged black hole? If this were the case, we might receive just one short burst of gravitational wave data as the wormhole collapsed.
The researchers developed a waveform model of what this gravitational wave signal would look like and compared it to the data from LIGO and Virgo. They also constructed a waveform for a standard binary black hole merger to compare against the observation data.
The binary black hole merger waveform was a slightly better fit for the observed signal than the wormhole waveform – but only slightly. There’s enough room for doubt, the researchers say, that the wormhole scenario is still possible. Which means that GW190521 could be the first time that humanity has glimpsed another universe beyond the borders of our own.
That doesn’t mean that we should all run about declaring wormholes exist, especially because that explanation requires exotic physics. What it does mean is that further investigation down the wormhole avenue could be worth pursuing.
The most massive black hole merger to date, GW231123, produced an object 225 times the mass of the Sun. It was also relatively brief, similar to GW190521. Comparisons between these events and others yet to be detected, the researchers say, present a way to test their findings and probe which scenario is more likely.
Their research is available on preprint server arXiv.
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