Dark matter (DM), an elusive and invisible substance, constitutes ~85% of the Universe's matter content. DM plays a key role in the Universe, including holding galaxies such as our own Milky Way together. Despite its profound significance and decades of efforts to detect its non-gravitational manifestations, the underlying nature of DM remains one of the greatest mysteries in science. Previous DM search methods have typically focused on DM consisting of a specific type of new particle and its interactions, calling for more general and extensive searches especially for scenarios where DM is composed of macroscopic and microscopic objects.

Since the groundbreaking discovery of gravitational waves from a merger of two stellar-mass black holes in 2015 by the international LIGO experiment, leading to the 2017 Nobel Prize, dozens of such events have already been detected thus far. Intriguingly, the gravitational wave observations from merging black holes further indicate that there are many black holes in the Universe and have sparked suggestions on an intimate link to DM if such black holes have non-astrophysical origin. In particular, primordial black holes (PBHs) that have been theorized for decades by cosmologists, including the late Stephen Hawking, to have been formed in large numbers a few seconds after the Big Bang, are intriguing DM candidates and are dramatically distinct from new particles that have often been the focus of traditional DM searches. However, finding a way to detect and identify these PBHs and distinguish them from textbook astrophysical black holes has been a major challenge. On the other hand, it is difficult to explain DM by PBHs with celestial masses alone, and it could be likely that it is composed of multiple components, including new particles. In such a scenario, the PBHs are expected to become “dressed” after their creation, gradually enveloping their surroundings in halos of DM particles. However, there has been no method to test this scenario definitively.

In this new study, an international team of researchers from Japan and Korea have established an innovative approach to search for DM halos around PBHs using observations of unusual gravitational wave signal patterns from diffractive gravitational lensing effects imprinted by DM objects located along the line of sight in response to traversing gravitational waves emitted from a distant merging compact object system, such as two astrophysical stellar-mass black holes (see Figure 1, left panel).

The proposed method exploits unique frequency-dependent amplitude variations of gravitational waves generated during celestial mergers from gravitational lensing of DM objects. The effect is highly sensitive to the presence and distribution of the extended DM halos surrounding the encountered black holes. This study not only provides a way to detect PBHs, but also allows us to clearly distinguish between PBHs with and without surrounding DM halos (referred to as “dressed” and “bare” PBHs, respectively) (see right panel in Figure 1). This distinction is crucial since various detection approaches put forth thus far have failed to identify observables that make a non-ambiguous difference between black holes with and without halos. This allows not only directly probing the theory that PBHs coexist with new particles as DM components, but also can establish definitive support for it.

“Our novel approach paves the way for definitive discoveries of dark matter in upcoming observations should it be composed of macroscopic black holes and microscopic particles, as suggested by various theories. This is quite exciting. However, much remains to be done to comprehensively explore the landscape of dark matter.” says Volodymyr Takhistov, a principal investigator and associate professor at the International Center for Quantum-field Measurement Systems of the Universe and Particles (WPI-QUP) at KEK.

The findings offer a novel perspective on the DM composition and establish new tools for its exploration when it is composed of a mixture of macroscopic and microscopic objects, including particles and ancient black holes. If confirmed experimentally, this could provide new insights about the key components of the cosmos, the early Universe's conditions as well as the nature of gravitational wave events detected by observatories such as LIGO opening new pathways for fundamental discoveries.

This study was published in Physical Review Letters on September 5, 2024.

Figure: [Left] Illustration of the diffractive lensing of chirping gravitational waves originating from binary merger by “dressed” PBHs in particle DM halos at cosmological distances. Diffractive lensing distorts gravitational wave wavefronts along a broad region of space, which is summed up to produce frequency-dependent amplification of the wave in contrast to the discrete images resulting from geometrical optics lensing. [Right] Frequency dependence of the lensing magnification and phase shift for typical assumed parameters. Characteristic amplification by “dressed” PBH (solid) is exhibiting non-trivial frequency dependence at low-frequency diffraction regimes and illustrates discrimination from a bare PBH without halo (dashed) as well as un-lensed observations (straight dot-dashed). Regimes of geometric optics (GO), transition regime beyond geometric optics (bGO) and weak diffraction (WD) are shown. Figures reproduced from publication.

Paper details
Journal: Physical Review Letters, 133, 101002 (2024)
Title: Coexistence Test of Primordial Black Holes and Particle Dark Matter from Diffractive Lensing
Authors: Han Gil Choi (1), Sunghoon Jung (2,3), Philip Lu (2), and Volodymyr Takhistov (4,5,6,7,§)

Author affiliations:
1 Cosmology, Gravity and Astroparticle Physics Group, Center for Theoretical Physics of the Universe, Institute for Basic Science (IBS), Daejeon 34126, Korea
2 Center for Theoretical Physics, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea
3 Astronomy Research Center, Seoul National University, Seoul 08826, Korea
4 International Center for Quantum-field Measurement Systems for Studies of the Universe and Particles (QUP, WPI), High Energy Accelerator Research Organization (KEK), Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan
5 Theory Center, Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan
6 Graduate University for Advanced Studies (SOKENDAI), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
7 Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan

DOI: https://link.aps.org/doi/10.1103/PhysRevLett.133.101002

Related link
Press release from KEK
https://www.kek.jp/en/press/202409201400qup