Emanuele Berti bio photo

Emanuele Berti

Professor, Johns Hopkins University

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Schematic illustration of the cloud formed around a spinning black hole. The black hole loses energy and angular momentum through the growth of the cloud and the emission of gravitational waves. Accretion of gas from the disk transports energy and angular momentum. The balance between these phenomena depends on the mass of the particles forming the cloud, and it determines whether the cloud can grow. [Image Credits: Ana Sousa.]

If dark matter consists of ultralight particles, it could form giant clouds around stellar-mass and supermassive black holes. We just published two papers in Physical Review Letters and Physical Review D shows that these systems would produce gravitational-wave signals detectable by current and future instruments. This work was covered by various news outlets, including UM News, phys.org and egno (in Greek!)

Dark matter is five times as abundant as ordinary matter in the Universe, but after more than 50 years from the first evidence that it exists, its nature remains unknown. The most natural candidate for dark matter are heavy, weakly interacting particles that are not part of the Standard Model of particle physics. However, so far no hint for such heavy particles has been found at the Large Hadron Collider at CERN. Another possibility is that dark matter consists of ultralight scalar fields (similar to the Higgs boson, but much lighter than neutrinos). This type of dark matter is hard to study in particle accelerators, and this is why researchers have started to investigate the possibility of using precise astrophysical observations to probe the microscopic world.

If ultralight particles exist in nature, black holes would behave very differently from what is predicted in the standard theory. Indeed, fast-spinning black holes would be unstable, triggering the growth of a scalar condensate at the expense of their rotational energy. The outcome of this instability is a scalar “cloud” that rotates around the (more slowly spinning) black hole. The “black hole+cloud” system emits gravitational waves, pretty much like a giant lighthouse in the sky.

We studied in detail the gravitational-wave emission from these sources. We found that, depending on the mass of the hypothetical particles, the gravitational-wave signal is strong enough to be detectable by current interferometers (such as LIGO and Virgo) and by the future space mission LISA. Furthermore, the population of those sources that are too weak to be individually detectable would pile up to produce a strong stochastic background of gravitational waves. We suggest that the absence of such background in current LIGO data may already be used to rule out the existence of ultralight scalar fields (for example, axions) with masses roughly a trillion time smaller than the estimated mass of the neutrino.

Even more stringent constraints will be placed in the near future, as gravitational-wave detectors improve in sensitivity. Therefore our paper shows that ultralight dark matter may be ruled out (or maybe detected!) by gravitational-wave interferometers. This is a new, exciting and unexpected frontier in astroparticle physics, that could shed light on our understanding of the microscopic Universe.