Probing the nature of dark matter using gravitational waves

Gravitational wave microlens. Credit: Roshni Samuel / Parameswaran Ajith / ICTS

One of the biggest puzzles in modern cosmology is the existence of dark matter, which makes up most of the matter in the universe. Recent research by an international team of scientists has used gravitational waves to probe the nature of dark matter. This study was recently published in the Astrophysical Journal Letters.

Black matter

Several astronomical observations have established the existence of dark matter, which only interacts with conventional matter through gravity. Dark matter emits no light and therefore escapes direct astronomical observation. Galaxies, including ours[{” attribute=””>Milky Way, are surrounded by a halo of dark matter, whose size extends much further than the visible galaxy.

The Standard Model of Particle Physics describes all the elementary particles that constitute all the normal matter. Particles that are not described by the Standard Model could exist in the universe and could constitute dark matter. Several large experiments have been trying to detect such elusive particles over the past few decades, without success. 

Another possibility is that dark matter comprises a large number of massive and compact objects, such as primordial black holes. Such black holes are different from the black holes that astronomers typically observe, which are produced by the death of massive stars. Primordial black holes are formed in the early universe and could exist in a variety of masses. They could be as light as asteroids or could weigh trillions of solar masses.

However, astronomers havent made a conclusive detection of primordial black holes. Also, various astronomical observations have constrained the abundance of primordial black holes. For example, such black holes could bend light from distant stars; a phenomenon called gravitational microlensing. Till now, scientists have been unsuccessful in observing microlensing produced by such black holes, despite searching extensively. This means that black holes that are much lighter than the Sun, which would have caused microlensing of starlight, are rare. Even if they exist, they would constitute only a very small fraction of the dark matter. Nevertheless, it is quite possible that black holes of some other masses could be contributing to dark matter.  

Microlensing of gravitational waves as a new probe of dark matter 

Recent observations of s theory, gravitational waves are also bent by massive objects in between the source and observer. If a significant fraction of dark matter is in the form of black holes, they should cause microlensing effects in the observed signals. Microlensing will distort the gravitational waves in a manner scientists can calculate exactly. However, the international team could not observe any such distortion in the signals observed by

In the next few years, LIGO and Virgo, along with upcoming detectors such as KAGRA and LIGO-India, will observe thousands of gravitational-wave signals. If scientists do not observe any signatures of microlensing in these gravitational wave signals, they will be able to conclude that only a very small fraction of the dark matter could be in the form of such heavy black holes. On the other hand, if a good fraction of the gravitational-wave signals contains signatures of lensing, this would be a smoking gun evidence of the much sought-after primordial black holes. Either way, microlensing of gravitational waves offers a unique way of probing the nature of dark matter.

Reference: “Constraints on Compact Dark Matter from Gravitational Wave Microlensing” by S. Basak, A. Ganguly, K. Haris, S. Kapadia, A. K. Mehta and P. Ajith, 21 February 2022, Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ac4dfa

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