International Space Agencies – Missions and Discoveries

Can dark matter be composed, even partly, of black holes?


From UPSC perspective, the following things are important :

Prelims level : Dark Matters

Mains level : NA

A recent hypothesis says that dark matter comprises a large number of compact objects such as primordial black holes.

What are Dark Matters ?

  • Astronomical observations suggest that a significant part of the universe is made up of dark matter which interacts with the rest of the universe only through the gravitational pull.
  • Many large lab experiments have tried to detect elementary particles that could be candidates for dark matter.
  • However, such dark matter particles have not been detected until now.
  • Several astronomical observations suggest that all galaxies are embedded in a “halo” of dark matter.
  • The “visible” galaxy is like a disc embedded in a dark matter halo that is much larger in size.

What is the recent proposition?

  • When the universe was very young, hot and dense – soon after the Big Bang, it must have had quantum fluctuations of its density.
  • This, in turn, would have caused some regions to become extremely dense, and therefore, to collapse under their own gravity to form the primordial black holes.
  • While we have no conclusive evidence of spotting these objects, some of the binary black hole mergers detected by the LIGO gravitational wave detectors might be primordial black holes.
  • The question is open there is good reason to believe that primordial black holes did form in the young universe.

Observing dark matter: Gravitational Lensing

  • The paper explores what happens when such objects get in the way of gravitational waves traveling towards the Earth from the distance.
  • It invokes a phenomenon called gravitational lensing that is used regularly in astronomy.
  • When light travels through space and passes near a massive or compact body – a star, a galaxy or a black hole, for example, the intense gravity of that body may attract the light towards it.
  • This causes bending it from its rectilinear (straight line) path.
  • This phenomenon is known as gravitational lensing and was first observed by Arthur Eddington in 1919.

How intense are they?

  • Massive objects like galaxies can bend light significantly, producing multiple images, this is called strong lensing.
  • Lighter objects like stars or black holes bend light less, and this is called micro-lensing.
  • A similar lensing can happen to gravitational waves travelling towards the Earth, and this would leave signatures in the detected gravitational waves.
  • This can be used to detect the presence, or the existence, of primordial black holes.

Assessing dark matter

  • Until now, individual black holes have not marked out these signatures on gravitational waves detected by the LIGO-VIRGO detectors.
  • However, if all of the dark matter is made of primordial black holes, they should have produced detectable signatures on the gravitational wave signals.
  • The researchers use the non-observation of the lensing signatures to assess what fraction of the dark matter could be made of black holes.

Way ahead

  • This provides a new way of constraining the nature of dark matter.
  • The study concludes that black holes in the mass range from a hundred to a million solar masses can contribute only up to 50-80% of the dark matter in the universe.
  • This is an upper limit and the actual fraction can be much smaller.
  • These upper limits will get better and better with more and more observations.


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