Using a gravitational wave detector to locate primordial black holes

Researchers at Université libre de Bruxelle have developed a novel gravitational wave detector to find primordial black holes.

This innovative technology will be able to locate black holes, as small as tennis balls, that were created in the immediate aftermath of the Big Bang.

“Detecting primordial black holes opens up new perspectives to understand the origin of the Universe, because these still hypothetical black holes are supposed to have formed just a few tiny fractions of a second after the Big Bang. Their study is of great interest for research in theoretical physics and cosmology because they could notably explain the origin of dark matter in the Universe,” commented members of the research group led by Professor André Fűzfa, an astrophysicist at UNamur, when talking about the perspectives of their research.

A collaborative research approach

This study is the result of extraordinary cooperation between the UNamur and Université libre de Bruxelle (ULB), to which the ENS added thanks to the participation of trainee student Léonard Lehoucq.

The concept was to merge the UNamur expertise in the field of gravitational wave antennas, a concept patented by Professor Fűzfa in 2018 and studied by Nicolas Herman as part of his doctorate, with that of ULB in the booming field of primordial black holes, in which Professor Clesse is one of the key players.

The researchers have established an application of this type of detector in order to study ‘small’ primordial black holes. Their findings are published in the journal Physical Review D.

“To this day, these primordial black holes are still hypothetical, because it is difficult to make the difference between a black hole resulting from the implosion of a star core and a primordial black hole. Being able to observe smaller black holes, the mass of a planet but a few centimetres in size, would make the difference,” the group explained. “We are offering experimenters a device that could detect them, by capturing the gravitational waves they emit when merging and which are of much higher frequencies than those currently available”.

What is this novel technique?

This method is composed of a gravitational wave ‘antenna’, comprised of a specific metal cavity, and appropriately immersed in a strong external magnetic field. When the gravitational wave goes through the magnetic field, it produces electromagnetic waves in the cavity. In a way, the gravitational wave makes the cavity ‘hiss’, not with sound but with microwaves.

The device, merely a few meters in size, would be sufficient in detecting fusions of small primordial black holes millions of light years away from Earth. It is much more compact than the frequently used detectors (LIGO, Virgo and KAGRA interferometers) which are several kilometres long. The detection technique makes it susceptible to very high frequency gravitational waves (in the order of 100 MHz, compared to 10-1000 Hz for LIGO / Virgo / Kagra), which are not created by ordinary astrophysical sources such as fusions, neutron stars, or stellar black holes. On the other hand, it is ideal for the detection of small black holes, the mass of a planet and with sizes ranging from a small ball to a tennis ball.

“Our detector proposal combines well mastered and everyday life technologies such as magnetrons in microwave ovens, MRI magnets and radio antennas. But don’t take your household appliances apart right away to start the adventure: read our article first, then order your equipment, understand the device and the signal that awaits you at the output,” the researchers added.

Currently, this patented method is at the stage of advanced theoretical modelling but has the required components to enter a more concrete phase, with the construction of a prototype. The researchers are hopeful that it will facilitate the essential research into the beginnings of our Universe. Additionally, to primordial black holes, this detector could also directly examine the gravitational waves produced at the time of the Big Bang, and therefore probe physics at much higher energies than the ones attained in particle accelerators.

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