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doi:10.1038/nindia.2017.113 Published online 28 August 2017

How to hunt down a bare black hole

Biplab Das

What happens when a star, tens of times more massive than the Sun, runs out of fuel? Its gravity increases inexorably, pulling all its matter inwards, and shrinking the star millions of kilometers in diameter to a pinprick, smaller than a dot on an ‘i’.

Such a super dense mass, known as singularity, is covered by a boundary (or event horizon) that traps everything including light, giving birth to a black hole.

The scientists (clockwise from top left): Pankaj Joshi, Sudip Bhattacharyya, Prashant Kocherlakota, Chadrachur Chakraborty & Mandar Patil.

But what if the boundary doesn’t form? Physicists argue that in such case a 'bare black hole' or  'naked singularity' is born. With the help of a mathematical model, physicists led by Pankaj S. Joshi from the Tata Institute of Fundamental Research (TIFR) in Mumbai and Institute of Mathematics of Polish Academy of Sciences, Poland have now shown how to detect such naked singularity1. The Big Bang, a massive explosion through which the universe was born, itself is a naked singularity. Thus understanding the physical phenomena near a naked singularity will throw much light on the physics of the early universe, 

According to Einstein’s theory, any massive rotating object such as a black hole twists the fabric of space-time around it. This makes celestial objects moving around a black hole spin like a gyroscope, changing the orientation of their rotational axis (or precessing). Using the model, the physicists show that it is possible to measure the rate at which a celestial object precesses when placed around a rotating black hole versus around a bare black hole. This would help distinguish a black hole from a bare black hole.

“In reality, one may consider two-object celestial systems of comparable masses such as a black hole-neutron star or a black hole-pulsar. If the pulsar goes around the black hole, the pulsar rotates fast like a gyroscope,” Joshi told Nature India. The pulsar, he said, behaves in one way close to the event horizon of a black hole. The same pulsar’s behaviour differs considerably when there is no event horizon, signalling the presence of a naked singularity.

In some cases, a collapsed star eats material from a companion normal star. This material emits x-rays just before being swallowed, and the nature of these x-rays can provide ways to find out if the collapsed star is a black hole or a naked singularity, said physicist Sudip Bhattacharyya, a  member of the TIFR team.

At low energies general relativity describes the physics well. However, close to the space-time singularity, at high energies and extremely small length scales, quantum effects are highly important. “Naked singularity without an event horizon radiate signals which necessarily contain quantum gravity signatures, offering us clues to constructing a future quantum gravity theory – a unified theory of forces may emerge through observing such ultra-high density regions,” contends Joshi. 

Astrophysicists, who are not involved with this research, are skeptical about this model. I do not think singularity will remain naked, says Sandip Chakrabarti from the S. N. Bose National Centre for Basic Sciences in Kolkata.“If singularity remains naked, the luminosity would be huge, about 16 times higher than what we see and would not go undetected. The only solution is that the singularity is quickly covered by the event horizon keeping the luminosity trapped inside.” 

The model is suitable for theoretical or mathematical studies but not for studying astrophysical systems observed by sophisticated telescopes, says Rajesh Kumble Nayak from the Indian Institute of Science Education and Research in Kolkata. Understanding of naked singularity might only be possible when one considers all physics including the quantum effects taken into account, Nayak adds.

Joshi and his teammates, however, are optimistic about their model. The luminosity of a collapsed object cannot be used to distinguish a black hole from a naked singularity, because the luminosity of most of these objects varies greatly with time, explained the lead author Chandrachur Chakraborty.

Moreover, the consideration of quantum effects is not required to identify the nature of the collapsed object using the model, which uses various properties sufficiently far from the centre where the quantum properties of the central collapsed object does not have any impact, he added.


References

1. Chakraborty, C. et al. Distinguishing Kerr naked singularities and black holes using the spin precession of a test gyro in strong gravitational fields. Phys. Rev. D.95, 084024 (2017)