In a recent study, scientists from Princeton University conducted the first nonlinear study of black hole mimics mergers. The goal is to understand the nature of the gravitational wave signals these objects emit, which could help pinpoint black holes more precisely. Black hole mimics are hypothetical objects that mimic black holes, especially in their gravitational wave signals and their effects on surrounding objects. But they don’t have an event horizon, i.e. a point of no return. The study was carried out by Nils Simonsen, an associate research scientist at Princeton University, who spoke to Phys.org about his work. “Black hole mimics are objects that are very close to black holes but don’t have an event horizon. “Through observations, we may be able to distinguish black holes from objects that mimic most of the properties of black holes by observing gravitational waves,” he said. The study, published in Physical Review Letters, focuses on a type of black hole mimic called boson stars. According to Dr. Simonsen, gravitational waves emitted when boson stars collide and merge. Binary boson stars and merging Boson stars are one potential candidate for black hole mimics, and as the name suggests, they are made of bosons. Bosons are elementary particles such as photons and the Higgs boson. Boson stars are composed of scalar bosons, such as virtual axions, which are spinless bosons. h.They have no intrinsic angular momentum. The scalar fields of the particles form stable, gravitationally bound configurations without the need for strong interactions. Previous studies have shown that the merger of boson binary systems produces gravitational wave signals, where violent processes cause ripples in space-time. These signals are universally identical to those of black hole collapses (or post-merger stages), regardless of the internal structure of the black hole mimic. The difference in the emitted gravitational wave signal is visible after the transit time of light inside the mimic. h.Depending on the time it takes for light to pass through the diameter of the mimic (in this case the boson star). In the case of black hole mimics, this is characterized by repeated explosive gravitational echoes. To improve on previous work, Dr. Siemonsen addressed issues such as the lack of consideration of nonlinear gravitational effects and the elimination of self-interactions within the matter of the object. Nonlinear and self-consistent treatment of black hole mimics To address the limitations of previous work, Dr. Siemonsen used numerical simulations to solve the full Einstein-Klein-Gordon equations, which describe the evolution of scalar fields such as in boson stars. For nuclear fusion, the work focused on scenarios with large mass ratios: h. the merger of a smaller boson star with a larger, more compact boson star, using the Klein-Gordon equations, which describe the head-on collision of a binary system. The Klein-Gordon equations, in combination with Einstein’s field equations describing the gravitational dynamics, allow for the study of the self-consistent evolution of the system. To solve the equations, Dr. Siemonsen developed a Newton-Raphson relaxation technique using a fifth-order difference method. He explained the challenges of implementing these techniques: “Only under certain conditions, the merger of two boson stars forms a black hole mimic. The region in the solution where this occurs is particularly difficult to simulate due to the large difference in scales.” To overcome this, methods such as adaptive mesh refinement and very high resolution are used. High-frequency bursts The simulations showed that the ringdown gravitational wave signal contains a crack-like component with different properties than previously thought, as well as a long-lived gravitational wave component. “None of these components are present in regular black hole mergers or ringdowns. This could serve as a guide for future gravitational wave searches focused on testing the black hole paradigm,” Dr. Siemonsen explained. However, the first gravitational wave signals of the mimic resemble those of rotating black holes, known as Kerr black holes, which merge as primary (or larger) boson stars become more compact and dense. The study found that the timing of the burst depends on the size of the small boson stars involved in the merger. In addition, they also found a long-lived component with a frequency comparable to that expected from a black hole, probably due to vibrations of the remaining object. “Black holes settle into a quiescent state in a very short period of time. “Black hole mimics, on the other hand, are generally thought to re-radiate part of the energy available during the merger in the form of gravitational waves over a relatively long period of time as they collapse,” explained Dr. Siemonsen. Finally, the study found that the total energy released in gravitational waves is significantly larger than expected from a comparable black hole merger.

More: https://dx.doi.org/10.48550/arxiv.2404.14536