New Wave Modes from Black Holes Discovered

Faculty Asserts Frequencies Can Be Tested Experimentally to Advance Unifying Physics Theory

New types of wave oscillations in black holes have been discovered that can be probed experimentally by gravitational-wave detectors, which in turn could advance scientific understanding of the key elements of a grand unifying theory for physics.

A black hole is formed through the collapse of a star, which causes a massive gravitational force to pull in all objects around it, including light, dust, and gas, thus causing the black hole to grow. These massive and incredibly dense objects have in general three ‘layers’– the singularity at the center, then the inner event horizon, and finally the outer event horizon, where phenomenon take place that challenge the laws of General Relativity. Our galaxy – the Milky Way – is estimated to have several black holes. Moreover, recent research in astrophysics indicates that a supermassive black hole should sit at the center of every galaxy. The mass of such astrophysical objects should be typically of the order of several million solar masses.

Black holes pull objects towards them and they can also attract each other. Like two whirlpools in the ocean, the black holes orbit around each other, radiating gravitational waves as they draw nearer. Eventually they lose energy in the gravitational radiation as their revolutions speed up and get closer, allowing their event horizons to merge. The last phase, before they merge, is called the ‘ringdown’, where the unified black hole system is still ringing and radiating, but progressively less so.

This ringdown phenomenon was first detected in 2016, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) operated by Caltech and the Massachusetts Institute of Technology detected gravitational wave signals from a pair of inspiralled black holes as they merged and underwent the ringdown – discoveries that led to the Nobel Prize in 2017.

“In the ringdown phase, the black hole starts vibrating after interacting with matter.  These vibrations get translated into gravitational waves, in the same way a guitar string translates being plucked into sound waves. It also happens that independently on how you ‘pluck’ the black hole, for example if it is fed by a scalar particle, a photon, or an electron, the resulting gravitational wave will have the same frequency, much like the string,” explained KU Assistant Professor in the Department of Applied Mathematics and Statistics Dr. Davide Batic.

The waves are sent out during the ringdown phase and are composed by many frequencies, called quasinormal modes. Their oscillations become smaller and smaller as time goes by.

“Despite all the knowledge we have on the quasinormal spectrum of black holes, there has been no actual explicit formula to compute them. All computations have been done using numerical methods,” Dr.  Batic added.

Dr. Batic has co-published a paper on the new black hole oscillations he believes he has discovered. The paper titled ‘Some exact quasinormal frequencies of a massless scalar field in Schwarzschild spacetime’, was published in the journal Physical Review D with co-authors Dr. Marek Nowakowski from the Universidad de los Andes, Columbia, and the master student Karlus Redway, from the University of the West Indies.

The team’s research results may also advance the development of a grand unified physical theory, which has a been an ongoing challenge in physics for decades. Such a grand unified theory should merge two of the main pillars of modern physics – General Relativity and Quantum Mechanics. Furthermore, when General Relativity is pushed to the limits, like inside the event horizon of a black hole, it makes an ‘unphysical prediction’ that the core of a black hole would have infinite curvature.  

In Einstein’s General Theory of Relativity, gravity is caused by the curvature of space-time. However, the theory cannot account for ‘unphysical predictions’ — calculations not in accordance with the laws or principles of physics — when applied to what happens inside the event horizon of a black hole.

“Apart from trying to describe how quantum fields interact with black holes – this is what we call quantum field theory in curved space-times – results in this area are of paramount importance in the development of a unified physical theory such as Quantum Gravity because every candidate theory of topics such as String Theory and Loop Quantum Gravity will need to pass a fundamental test, namely it must be able to reproduce on a certain scale all predictions arising from quantum field theory in curved space-times,” Dr. Batic explained.

He is now working to derive a formula to compute the numerical values of the quasinormal wave modes from black holes. This, combined with the experimental data collected by LIGO and the European Virgo interferometer experiment, may be able to show the existence or absence of black holes inspired by noncommutative geometry, thus helping us to better understand the key ingredients of Quantum Gravity.

“We already know that General Relativity is not able to reliably explain what happens inside the event horizon of a black hole. This suggests that we need a better theory unifying General Relativity with Quantum Mechanics, and at the same time black holes may contain the deepest secrets of the universe and its beginnings. Many things can be benefited by further study into black holes, as they provide a unique opportunity to test all of the physical extremes – very large distances, very small distances, very high energies, etc.,” Dr. Batic explained.

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