At the Heart of the Galactic Centre: Black Hole or Naked Singularity?

A deep mystery is unfolding on the true nature of the Ultra dense object Sgr A* at the galactic centre. Pankaj Joshi, Distinguished Professor of Physics and Director of the International Centre for Space and Cosmology, explains that new studies he led at the Centre provide greater evidence supporting the Naked Singularity theory.

On May 12, 2022, the Event Horizon Telescope collaboration released its data and the photograph and images of Sgr A* in a widely publicised press conference in Washington DC. It showed a wonderful image and a shadow-like structure for this ultra-compact object at the Galactic centre. That very day, the Event Horizon Telescope collaboration also released a series of seven detailed papers outlining the entire procedure and the methodology they used to obtain this image from the given data with the help of an array of radio telescopes across the globe. They also tried to model, explain, and understand the data using the possibility of a black hole sitting at the centre of our galaxy. 

The Black Hole Mimicker
Interestingly the black hole model did not work as well as it should have. Various problems came up in explaining the intensity distribution of light across the image that was obtained by the Event Horizon Telescope. Additionally, the angle or the tilt of the image was quite different as compared to what would be predicted by a black hole model. This situation prompted the scientists to look for scenarios other than the black hole model. In particular, they examined the Naked Singularity model given earlier in 2011 by Joshi, Malafarina, and Narayan, called the JMN model.

Both Black Holes and Naked Singularities model Ultra compact objects where an enormous amount of matter is compacted in a very small region of space. However, the difference between the two is that the Black Hole has a one-way membrane called an Event Horizon which does not allow any information to escape away from its vicinity. It is kind of a Black Box sitting out there in space. On the other hand, the Naked Singularity has no Event Horizon and, therefore, we can get, in principle, the signals and information of the physical phenomena happening in the Ultra compact regions of the universe.

It was seen that the observed data fit well with this model, which one of the EHT papers described as the “best possible black hole mimicker”. This was a significant recognition of the JMN model by modern observational data. In light of these recent developments, our recent paper published in the journal Physical Review D, ‘Comparing thin accretion disks around black holes and naked singularities’, by Tahelayani et al, reports several interesting further results. These results would allow us to decide whether the object sitting at the centre of our galaxy is a Black Hole or a Naked Singularity.

Understanding Accretion Disks and Their Significance
In the new idea we have proposed and supported, we have examined Accretion Disks around Black Holes and Naked Singularities. An Accretion Disk is swirling matter that moves swiftly around a massive body in space. These are captured with the help of several radio telescopes around the world. Our idea proposes that if a suitable Naked Singularity model is used, then that produces emissions of light frequencies in very high frequency and radio bands. In the case of Black Holes, ultra high frequency radiation is ruled out theoretically.

We examined the properties of the circular orbits in these spacetimes. The emitted flux, radiation spectra, disk efficiency, and temperature distribution on the disk surface are then investigated. The efficiency of converting the accreting matter into radiation is found to be substantially higher for Naked Singularities than for Black Holes. We also verified that the flux radiated from the disk surface is greater for classes of Naked Singularities than Black Holes. Hence, the Accretion Disks around Naked Singularities are much more luminous than Black Holes of the same mass and accretion rate. In the luminosity spectra of Naked Singularities, we find that a significant contribution of low-frequency is emitted from the nearby regions of the singularity. Furthermore, the spectral luminosity distribution for the ‘nonzero torque’ at the inner boundary is also analysed by the inclusion of the nonzero torque value at the inner edge of the disk. The slopes of the luminosity distribution with respect to frequency for Naked Singularity spacetimes differ significantly from those of Black Holes.

Light Trajectory and Shadow Shape
In another related research, ‘Light trajectory and shadow shape in the rotating naked singularity’ published in the European Physical Journal C by Vishwa et al, we investigated a case of Naked Singularity with rotation included. The gravitational bending of light was analysed, and its results compared with the Black Hole theory. The Naked Singularity projects an arc-shaped shadow that differs from the contour-shaped shadow cast by a rotating Kerr Black Hole.

These unique features of the Naked Singularities serve as an effective tool to distinguish them from the equally massive Black Holes.