Future observations of black holes by LISA could reveal the existence of unknown particles much lighter than those detected in accelerators.
The idea is based on a remarkable effect around rotating black holes. Certain hypothetical particles, known as ultralight bosons, could accumulate around them. This concentration would gradually extract some of their rotational energy. It could also produce gravitational waves, meaning tiny distortions of space and time.
Artist’s view of two merging black holes, a source of gravitational waves that LISA will be able to study.
Credit: ESA
These particles have never been detected. Their mass would be extremely small, far below that of ordinary particles. Searching for them directly is therefore difficult. Black holes may offer another method: their mass and rotational speed would retain the signature of the energy lost if these particles exist.
This is where LISA comes in, a future space observatory developed by the European Space Agency with NASA. Three spacecraft separated by millions of kilometers will measure minute variations in distance. The instrument will be particularly sensitive to gravitational waves produced by the merger of black holes far more massive than the Sun.
The researchers studied two complementary methods. The first involves measuring the rotation of black holes before they merge. Certain combinations of mass and rotation would become rare if ultralight bosons efficiently extracted energy from them.
Diagram of a rotating black hole surrounded by a cloud of ultralight bosons.
Credit: Richard Brito, Vitor Cardoso and Paolo Pani / LIGO Scientific Collaboration
The second method takes place after the merger. The newly formed black hole could become surrounded by a cloud of bosons and then emit an almost continuous gravitational signal. LISA could search for this signal after it has already located the merger, reducing the portion of the sky that needs to be examined.
The simulations indicate that rotation measurements could test several ranges of boson masses. For scalar particles, the range studied extends from approximately 5 × 10-18 to 10-14 electronvolts. For vector bosons, it runs from approximately 6 × 10-19 to 2 × 10-14 electronvolts.
These figures remain highly dependent on the actual population of massive black holes. The researchers therefore compared three scenarios describing their formation. Depending on the scenario selected, the probability of detecting a signal after a merger can range from very low to almost certain for certain vector boson masses.
LISA is not yet operational, and the study presents predictions based on simulations. Future data will make it possible to compare these scenarios with actual mergers. Measuring the mass, rotation and persistent signals after the collision simultaneously will then provide several independent tests of the same hypothetical particle.