Scientists have found a new way to detect a bare or naked singularity – the most extreme object in the universe where the usual laws of physics break down. When the fuel of a very massive star is spent, it collapses due to its own gravitational pull and eventually becomes a very small region of arbitrarily high matter density, that is a ‘singularity’.
If this singularity is hidden within an event horizon, which is an invisible closed surface from which nothing – not even light – can escape, the object is called a black hole.In such a case, we cannot see the singularity and we do not need to bother about its effects, researchers from the Tata Institute of Fundamental Research (TIFR) in Mumbai said.
However, Einstein’s theory of general relativity predicts that the event horizon does not form when massive stars collapse at the end of their life-cycles. In this case, we are left with the tantalizing option of observing a ‘naked singularity’. Researchers, including those from the Institute of Mathematics of Polish Academy of Sciences in Poland, investigated how to observationally distinguish a naked singularity from a black hole.Einstein’s theory predicts an interesting effect – the fabric of spacetime in the vicinity of any rotating object gets ‘twisted’ due to this rotation.
This effect causes a gyroscope spin and makes orbits of particles around these astrophysical objects precess (the axis on which the body rotates changes its orientation).
The team argued that the rate at which a gyroscope precesses (the precession frequency), when placed around a rotating black hole or a naked singularity, could be used to identify this rotating object.
“If an astronaut records a gyroscope’s precession frequency at two fixed points close to the rotating object, then two possibilities can be seen,” researchers said. “The precession frequency of the gyroscope changes by an arbitrarily large amount, that is, there is a wild change in the behaviour of the gyroscope; or the precession frequency changes by a small amount, in a regular well-behaved manner,” they said.
In the first case, the rotating object is a black hole, while the second is a naked singularity.Researchers showed that the precession frequency of a gyroscope orbiting a black hole or a naked singularity is sensitive to the presence of an event horizon.
A gyroscope circling and approaching the event horizon of a black hole from any direction behaves increasingly ‘wildly,’ that is, it precesses increasingly faster, without a bound. However, in the case of a naked singularity, the precession frequency becomes arbitrarily large only in the equatorial plane, but being regular in all other planes.
Researchers have found evidence of a white dwarf star orbiting a likely black hole at a distance of only 961,000 km — just about 2.5 times the distance between the Earth and the Moon.
In a tightest orbital dance ever witnessed for a black hole and a companion star, the star whips around the black hole at an astonishing speed — about two orbits an hour, said the study published in the journal Monthly Notices of the Royal Astronomical Society.
“This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disk of matter around the black hole before falling in,” said study lead author Arash Bahramian, affiliated with the University of Alberta in Canada and Michigan State University in the US.
“Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit,” Bahramian said. Although the white dwarf does not appear to be in danger of falling in or being torn apart by the black hole, its fate is uncertain.
The stellar system, known as X9, is located in the globular cluster 47 Tucanae, a dense cluster of stars in our galaxy about 14,800 light years away from the Earth. “For a long time astronomers thought that black holes were rare or totally absent in globular star clusters,” study co-author Jay Strader from Michigan State University said.
“This discovery is additional evidence that, rather than being one of the worst places to look for black holes, globular clusters might be one of the best,” Strader added. For the study, the researchers used data from the Australia Telescope Compact Array as well as NASA’s Chandra X-ray Observatory and NuSTAR telescope.Link to original