A long-mooted explanation for the nature of dark matter may have been ruled out by observations made by the ageing Voyager 1 probe — humanity’s most distant spacecraft.
The Voyager 1 probe — launched in 1977 and currently 21.7 billion km from Earth after leaving the solar system six years ago — has assisted theoretical physicists to determine that a long-held theory identifying mini-black holes — left over from the Big Bang — as dark matter may be incorrect.
Alan Cummings, a space scientist at the California Institute of Technology in Pasadena who has worked on Voyager 1 since 1973 and who was not involved in the analysis was surprised at the use of data from the probe in this way: “I never thought we’d be able to contribute in any way to studying dark matter.”
For as long as dark matter has been thought to exist, some astronomers have believed that it may be comprised of black holes. The theory is a slightly unfavourable one as dark matter outweighs regular matter in the Universe by a ratio of 17:3 meaning that there would not have been enough collapsing stars to create the required amount of black holes in the first place.
This means that the connection must be restricted to black holes created in the early Universe by the collapse of dense clusters of particles before the stars even formed. This also means that these black holes must be extremely small and in small clusters or their effects would be apparent from Earth in the form of gravitational lensing (when an area of intense gravity warps a region of space-time to the extent that light is bent as it travels past it).
Bernard Carr, a cosmologist at Queen Mary University of London, who has worked on the idea for 40 years, tells Science that these considerations place restrictions on the possible masses for primordial black holes. The three possibilities that would be allowed are; masses between one and 10 times that of the sun; about one-billionth that of the sun; or below about a quadrillionth that of the sun — 10 billion metric tons. Those smallest black holes would only be as wide as an atomic nucleus.
If these mini black holes are present, they should be radiating so-called ‘Hawking radiation’ at an increased rate than larger black holes. They should also be producing electrons and positrons due to quantum mechanical effects. Whilst this radiation and the charged particles may not be noticeable from inside the solar system due to the Sun’s magnetic field, the heliosphere— it should be observable from outside the solar system.
So from its position outside the heliosphere, Voyager 1 should be in prime position to spot this radiation. That’s the argument of Mathieu Boudaud and Marco Cirelli, theorists at Sorbonne University in Paris, in a paper published in Physical Review Letters.
Since it left the solar system, Voyager 1 has detected small amounts of electron and positrons that could have been created by quantum mechanical effects at the edge of black holes’ event horizons. But even if all of these particles came from mini black holes, it is not enough to justify the amount of mini black holes needed to account for 85% of the matter in the Milky Way. In fact, Boudard and Cirelli calculate it would actually for only about 1% of the Milky Way’s dark matter content.
The lack of detection does not rule out larger primordial black holes as dark matter candidates, as these would not produce nearly as much Hawking radiation or charged particles and are thus, undetectable by Voyager 1.
For one, Carr isn’t too perturbed by the finding: “This low mass window has never been my favourite, it doesn’t personally bother me if the constraints now rule it out.”
Original research: https://arxiv.org/abs/1807.03075