Gravitational wave detector might detect the merging of authentic black holes with the mass of a planet tens of millions of sunshine years away

Gravitational wave detectors have been part of astronomy for a number of years and have given us a wealth of information about black holes and what happens when they merge. Gravitational wave astronomy is still in its infancy, and we are still very limited in the types of gravitational waves that we can observe. But that could change soon.

Current gravitational wave observatories are sensitive to the merging of black holes with stellar mass. We have seen some mergers with neutron stars, but most of them took place between black holes on the order of ten solar masses. We cannot yet observe the gravitational waves of supermassive black holes in other galaxies, nor can we observe those of planet-sized worlds. Proposed detectors like eLISA will allow us to observe the former, but it takes a novel new idea to detect the latter.

The sensitivity of different gravitational wave detectors. Photo credit: Christopher Moore, Robert Cole, and Christopher Berry

The problem with observing gravitational waves from planetary mass bodies is that they are both very weak and very high frequency. Our current designs using laser interferometry make these waves difficult to observe. The gravitational waves that we can observe are already so weak that they are barely above the background noise. But recently a team proposed a gravitational wave detector that uses resonance instead of lasers.

The idea of ​​using resonance to detect gravitational waves is not new. As early as the 1960s, Joseph Weber tried to discover it with a large aluminum cylinder. When gravitational waves passed through the cylinder, squeezing and pulling them would cause the cylinder to ring at a certain frequency. Weber hoped the ringing caused by gravitational waves would be stronger than that of background noise and heat. But Weber’s experiment failed, which led astronomers to pursue other methods such as the laser interferometry method that we are now using.

Joseph Weber and one of his gravitational wave detectors. Credit: Special Collections and University Archives, University of Maryland Libraries

This new design takes a similar approach to Weber, but uses modern technology. One of the limitations of Weber’s design was that he had to use piezoelectric sensors to measure the vibration of the cylinder, which limited the sensitivity of his experiment. Instead, the team suggests using a hollow cylinder in a strong magnetic field. When gravitational waves pass through the cylinder, they should induce electromagnetic waves in the cylinder that we were able to detect. Because of their design, the team believes they should be able to detect very weak gravitational waves.

Perhaps the most interesting aspect of the idea is that the detector would be sensitive to high frequency gravitational waves such as those created by the merging of primeval black holes. Primitive black holes are hypothetical objects the size of a tennis ball that would have been formed in the earliest moments of the universe. If they exist, they could explain things like dark matter. And this new detector would be perfect for finding them.

Overall, this new design is a bit speculative and is still in the drafting phase. The team has to build one to see if it works, and it remains to be seen if they can distinguish between signal and noise. But if they are successful, it could tell us about black holes, dark matter, and more.

Reference: Herman, Nicolas, et al. “Detection of primordial black holes with planetary mass with resonant electromagnetic gravitational wave detectors.” ArXiv-Preprint arXiv: 2012.12189 (2020).

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