Scientist Conducts First Nonlinear Study of Black Hole Mimics

In recent research, a Princeton University scientist has conducted the first nonlinear study of a black hole mimic merger, aiming to understand the nature of the gravitational wave signals emitted by these objects, which could potentially help identify black holes more accurately.

Black hole mimics are hypothetical astronomical objects that mimic black holes, particularly in their gravitational wave signals and their effect on surrounding objects. However, they do not have an event horizon, which is the point of no return.

The research was led by Nils Siemonsen, a research associate at Princeton University, who spoke to Phys.org about his work.

“Black hole mimics are objects that are remarkably close to black holes but lack an event horizon. By observing gravitational waves, we may be able to distinguish black holes from objects that mimic most of their properties,” he said.

The study, published in Physical Exam Lettersfocuses on a type of black hole mimic called boson stars. The key to distinguishing them from black holes, Dr. Siemonsen says, lies in the gravitational waves emitted when boson stars collide and merge.

Binary Boson Stars and Mergers

Boson stars are one potential candidate to mimic black holes, and as their name suggests, they are made of bosons. Bosons are subatomic particles, like photons and the Higgs particle.

Boson stars are made of scalar bosons like the hypothetical axions, which are spinless bosons, i.e. without intrinsic angular momentum. The scalar fields of the particles form a stable configuration that is gravitationally bound without requiring strong interaction.

Previous research has shown that the merger of a boson binary star system leads to gravitational wave signals, which are ripples in spacetime caused by violent processes.

These signals are universally identical to those of a merging (or post-merger) black hole, regardless of the internal structure of the black hole mimic.

The difference in the emitted gravitational wave signals is observed after a light travel time inside the mimetic, which is the time taken for light to travel the diameter of the mimetic, which in this case is the boson star.

In the case of a black hole mimic, this is characterized by repeated “burst” type gravitational echoes.

In an effort to refine previous research, Dr. Siemonsen sought to address issues such as the failure to account for nonlinear gravitational effects and the exclusion of self-interactions between the object’s matter.

Nonlinear and self-consistent treatment of black hole mimics

To address the limitations of previous studies, Dr. Siemonsen used numerical simulations to solve the full Einstein-Klein-Gordon equations, which describe the evolution of scalar fields, such as those in boson stars.

For the merger, the study focused on large mass ratio scenarios, that is, the merger of a smaller boson star with a larger, more compact star, with the Klein-Gordon equations describing the head-on collision of the binary star system.

The Klein-Gordon equation, coupled with Einstein’s field equations, which describe gravitational dynamics, allows the study of the self-consistent evolution of the system.

To solve the set of equations, Dr. Siemonsen used the Newton-Raphson relaxation technique with fifth-order finite difference methods.

He explained the challenges of implementing these techniques: “Only under certain conditions does a black hole mimic form from the merger of two boson stars. The region of the solution where this happens is particularly difficult to simulate because of the large scale separation.”

To overcome these problems, methods such as adaptive mesh refinement and very high resolution have been used.

High frequency bursts

Simulations revealed that the gravitational wave signal from the explosion contains a burst-like component with different properties than previously believed, as well as a long-duration gravitational wave component.

“None of these components are present in a normal binary black hole merger and oscillation. This could guide future gravitational wave searches focused on testing the black hole paradigm,” Dr. Siemonsen explained.

However, the initial gravitational wave signal from a mimic is similar to that of a rotating black hole, known as a Kerr black hole, as the primary (or larger) boson star becomes more compact and dense.

The study found that the timing of the bursts depends on the size of the smallest boson star involved in the merger.

Additionally, they discovered a long-lived component with a frequency comparable to that expected of a black hole, likely due to oscillations of the remnant object.

“Black holes stabilize in their quiescent state on very short time scales. Black hole mimics, on the other hand, are expected to re-emit some of the energy available during the merger as gravitational waves during the merger on relatively long time scales,” explains Dr. Siemonsen.

Finally, the study found that the total energy emitted in gravitational waves is significantly greater than that expected during an equivalent black hole merger event.

Future work

The two components identified in the study could be used as a differentiator between a black hole merger remnant and a black hole mimic.

“However, there are still many unanswered questions about the properties of well-motivated black hole mimics and their merger and resonance dynamics,” added Dr. Siemonsen.

Speaking of future work, he noted: “An interesting future direction is to consider a well-motivated black hole mimic and understand its inspiration, merger, and regression dynamics in the context of a binary.

“Furthermore, analyzing the resonance of these well-motivated mimics using perturbative techniques and connecting these to nonlinear treatments is crucial to guide future tests of the black hole paradigm using gravitational wave observations.”

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