Researchers have used algorithms to show that primordial black holes are expected to reveal two distinct loop quantum gravity signatures, while larger black holes are expected to reveal one distinct signature. One of the biggest challenges will be simply detecting evaporating black holes. The researchers are also working on specific footprints in the cosmic microwave background might be detected in the future.
This letter aims at showing that the observation of evaporating black holes should allow the usual Hawking behavior to be distinguished from Loop Quantum Gravity (LQG) expectations. We present a full Monte Carlo simulation of the evaporation in LQG and statistical tests that discriminate between competing models. We conclude that contrarily to what was commonly thought, the discreteness of the area in LQG leads to characteristic features that qualify evaporating black holes as objects that could reveal quantum gravity footprints.
Loop Quantum Gravity (LQG) is a promising framework to nonperturbatively quantize General Relativity (GR) in a background invariant way. Interestingly, it has now been demonstrated that different approaches, based either on quantizations (covariant or canonical) of GR, or on a formal quantization of geometry lead to the very same LQG theory. As for any tentative theory of quantum gravity, experimental tests are however still missing. Trying to find possible observational signatures is obviously a key challenge. In this article we address the following question : could there be objects in the contemporary universe whose observation would lead to a clear signature of LQG ? Fortunately, the answer turns out to be positive. Although small black holes have not yet been directly observed, they could have been formed by different mechanisms in the early universe or even by particle collisions. We don’t review here the well-known possible production mechanisms, but instead we focus on how to use the evaporation of microscopic black holes to investigate the discriminating power of the emitted spectrum. Three different possible signatures will be suggested. Although one should be careful when pushing the limits of the LQG approach to black holes to the microscopic limit, our results rely on features of the area spectrum and are rather insensitive to small modifications in the theoretical framework.
Spectrum of emitted particles in LQG, in the pure Hawking case, and in the Mukhanov-Bekenstein approach, from top to bottom.
Number of evaporating black holes that have to be observed as a function of the relative error on the energy reconstruction of the emitted leptons for different confidence levels (the gray scale corresponds to the number of standard deviations). The first row corresponds to the discrimination between LQG and the Hawking hypothesis and the second row between LQG and the Mukhanov-Bekenstein hypothesis.