Is dark matter composed of black holes?

The embarrassing gap in science’s understanding of the universe may soon be filled

The first-ever photograph of  a black hole, taken using a global network of telescopes, conducted by the Event Horizon Telescope  project. Photograph: Reuters

The first-ever photograph of a black hole, taken using a global network of telescopes, conducted by the Event Horizon Telescope project. Photograph: Reuters

 

Ordinary matter (baryonic matter) accounts for only 4.7 per cent of the mass-energy content of the universe while dark matter makes up 24 per cent and dark energy a whopping 71.4 per cent. Amazingly, science understands the nature of only the baryonic matter, composed of protons, neutrons and electrons.

Dark energy, whose nature is entirely mysterious, is accelerating the expansion of the universe. There are various proposals as to the nature of dark matter. A credible hypothesis that dark matter is made of primordial black holes formed at the time of the big bang is described by Juan Garcia-Bellido and Sebastien Clesse in Scientific American (July 2017).

Dark matter is so called because it does not interact with observable electromagnetic radiation, eg visible light, making it very difficult to detect. We must rely on noticing the effects of its gravitational attraction on ordinary matter in order to indirectly detect it.

Dark matter

Astronomer Fritz Zwicky was the first to formally infer the existence of dark matter in 1933. He studied the Coma Galaxy Cluster and he estimated that the cluster had about 400 times more mass than was visually observable. Galaxies are generally observed to rotate too fast to be held together by the gravitational pull of the visible mass in their stars.

Unseen dark matter provides the extra “pull” required to hold galaxies together. This dark matter shapes the largest structures in the universe and determines the origin and growth of the galaxies.

It has long been hypothesised that dark matter is composed of massive particles that interact extremely weakly with electromagnetic radiation, but all searches to detect such particles have proved fruitless. The latest hypothesis proposes that dark matter is composed of primordial black holes.

Most cosmologists believe that our newly-born universe underwent a spectacular inflation in size immediately after the big bang

We know that a black hole can form as a massive star dies, when the centre collapses into a small ball where gravitational forces are so great that nothing, not even light, can escape. Stars with a mass less than 1.45 solar masses cannot form black holes. But primordial black holes formed at the time of the big bang, before any stars existed.

Spectacular inflation

Most cosmologists believe that our newly-born universe underwent a spectacular inflation in size immediately after the big bang, when, in the tiniest fraction of a second, two points sitting closer together than the radius of an atom separated from each other by a distance comparable to the distance from Earth to the closest stars. This inflation greatly magnified tiny quantum fluctuations, seeding the universe with a graininess of matter and energy from which all cosmic structures subsequently emerged.

Physicists Stephen Hawking and Bernard Carr proposed primordial black holes in the 1970s but only considered very small ones of smaller mass than a mountain. Such small holes would have evaporated by now and cannot account for current black matter. However, the present authors have shown it is theoretically feasible that populations of black holes with a large range of masses, ranging eventually from 1/100th to 10,000 times the mass of our sun, emerged less than one second after the big bang.

Such black holes would behave as dark matter and dominate the matter content of the present day universe. These populations each exist in relatively small volumes of space so that the formation of binary black hole pairs and subsequent violent mergers is likely.

Violence of collision

When two black holes merge the violence of the collision shakes the fabric of space-time, sending gravitational waves out through the cosmos at light-speed. Gravitational waves were first detected in September 2015 by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).

Since then LIGO has detected gravitational waves several times from mergers of massive binary black holes. The detection frequency indicates these may well be mergers of primordial black holes and it is hoped future detections by the observatory will settle this matter. A LIGO detection of a merger between two black holes, one of which has a mass less than 1.45 solar masses, would unambiguously signify a black hole of primordial origin.

New observations may soon confirm that dark matter is mostly or entirely composed of primordial black holes, thereby filling in an embarrassing gap in science’s understanding of the universe. As Garcia-Bellido and Clesse put it: “Soon we may no longer be in the dark about dark matter.”

William Reville is an emeritus professor of biochemistry at UCC

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