![]() Then, in 2015, the LIGO-Virgo collaboration detected the first gravitational signal from two merging black holes. Previous searches for black holes via gravitational effects ruled out the existence of large numbers of primordial black holes with masses ranging from 1 0 down to 1 0 − 8 times that of our Sun, restricting possible models that allow black holes to explain dark matter. ![]() To see such a black hole, researchers therefore look for its gravitational effect on light. However, primordial black holes, if they exist, formed before the first atoms, leaving them disc-free and completely dark. But so uncertain is this era of the Universe that our best theories make no solid prediction for the mass of primordial black holes: if they exist, they could be as light as 1 0 − 5 g, roughly the same weight as an eyelash, or as heavy as a billion Suns.īlack holes that originate from a collapsed star usually have a visible “halo” of gas and debris swirling around them. However, current theories suggest that huge numbers of black holes could have formed in the early Universe from the gravitational collapse of dense regions of matter and that the total mass of these so-called primordial black holes could be enough to account for dark matter (see accompanying Feature: Controversy Continues over Black Holes as Dark Matter). Black holes can form when massive stars implode, but there haven’t been enough such stars to account for dark matter. The idea quickly attracted the attention of other physicists, as it didn’t require some, as yet, undiscovered particle. Stephen Hawking first proposed the idea that black holes might contain the Universe’s missing matter in 1974. Their analysis applies to all black holes with masses greater than 0.01 times that of our Sun and indicates that such objects can account for at most 40% the Universe’s dark matter. But when Miguel Zumalacárregui and Uroš Seljak from the University of California, Berkeley, searched for this effect in over a thousand supernovae, they came up empty handed. A black hole of sufficient mass passing in front of a supernova should act as a magnifying lens and make the star appear brighter, allowing the black hole to be spotted. A new analysis of supernovae makes this possibility seem very unlikely. But whether the total mass of black holes is sufficient to account for all the dark matter is unclear. ![]() ×īlack holes are one of the oldest candidates for dark matter-the unidentified “stuff” that underpins galaxies and makes up 85% of the matter in the Universe. The black hole’s gravity distorts the path of light emitted by the supernova, acting as a lens that magnifies the light. APS/ Carin Cain Figure 1: A supernova explosion of a massive star appears brighter to an observer on Earth if a black hole sits between the explosion and the observer. ![]()
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