A team of scientists led by the Kavli Institute for the Physics and Mathematics of the Universe (IPMU), Japan has carried out rigorous experiments on Stephen Hawking’s 1974 theory that a large fraction of the dark matter is composed of primordial black holes. The results of the experiments show that the theory proposed by Hawking is not correct and they were published in the Nature Astronomy.
It is very well known to the scientists that 85% of the mass in the universe is made up of dark matter. Because of the presence of the gravitational force of the dark matter, the stars are prevented from flying apart from the galaxies. Phenomenon such as velocity dispersion and gravitational lensing can be explained by taking into account the presence of the dark matter. Dark matter is called so because it does not interact with the electromagnetic radiations such as light making it invisible to the entire range of electromagnetic spectrum and thus it is very difficult to be identified by the normal astronomical equipment which we have today. Past attempts to detect the dark matter through underground, accelerator experiments such as the Large Hadron Collider have failed.
Researchers used the gravitational lensing effect to detect primordial black holes which are present between the Earth and Andromeda galaxy. Gravitational lensing, first suggested by Albert Einstein refers to the bending of the light rays which are coming from a distant object like a star because of the gravitational effect of an intermediate object of a large mass such as a black hole. This bending of the light can make it appear much brighter than it is in many extreme cases.
But gravitational lensing is a rare event as it needs a star in the Andromeda galaxy, a primordial black hole which acts as a gravitational lens and the observer on Earth to be in a line to each other. To increase the possibilities of capturing the event, scientists used Hyper Supreme Cam on Subaru Telescope which could capture the entire image of Andromeda galaxy. As the primordial black holes move very fast in space, scientists captured multiple images so that the flickering of the star can be captured as brightening occurs ranging from few minutes to hours due to gravitational lensing.
Scientists took 190 images over a period of seven hours and they were analysed for gravitational lensing events. If the dark matter had black holes of a certain mass, scientists expected to find almost 1000 events but they managed to find only one event. This shows that the theory is not true. The next aim is to further improve the analysis of the Andromeda galaxy and understand the connected phenomena.