This page summarizes the main lines of research of our group. If you are interested in our projects and you are considering the possibility of working with us, you may check possible opportunities on the page "Join Us".

Testing Fundamental Physics with Black Holes

Astrophysical black holes are ideal laboratories for testing fundamental physics in strong gravitational fields. Our group mainly works with tests with X-ray and gravitational wave data.

According to Einstein's theory of General Relativity, the spacetime around an astrophysical black hole should be described well by the Kerr solution. However, macroscopic deviations from the Kerr metric are predicted in a number of scenarios beyond classical General Relativity. Our group has developed the reflection model RELXILL_NK [1] and the thermal model NKBB [2] to test the Kerr hypothesis from the analysis of, respectively, the relativistically blurred reflection features and the thermal spectrum of thin accretion disks.  

The plot below summarizes the constraints on the deformation parameter alpha13 of the Johannsen metric from different techniques/observations (for alpha13=0 we recover the Kerr metric predicted by General Relativity while any non-vanishing value of alpha13 would indicate a deviation from the predictions of General Relativity). The labels refer to the names of the sources. In green, there are the three most stringent and robust constraints from stellar-mass black holes obtained with RELXILL_NK. In magenta, there is the constrain from the stellar-mass black hole in LMC X-1 obtained with NKBB (the constraint is weak because of a strong degeneracy between the deformation parameter alpha13 and the black hole spin parameter). In blue, there are the constraints from stellar-mass black holes obtained with RELXILL_NK+NKBB (either because the spectrum presents simultaneously prominent reflection features and a strong thermal component or because we have combined spectra showing prominent reflection features with spectra showing a strong thermal component). In red, there is the most stringent constraint from gravitational wave data. In cyan, there is the most stringent and robust constraint from supermassive black holes obtained with RELXILL_NK. In gray, there are the constraints from the supermassive black holes M87* and SgrA* obtained from the image of their shadows with the Event Horizon Telescope data.  

The model RELXILL_NK and NKBB are public on GitHub here. You can contact us for questions (relxill_nk-at-fudan.edu.cn).

References:

[1] C. Bambi et al., Testing the Kerr black hole hypothesis using X-ray reflection spectroscopy, Astrophys. J. 842, 76 (2017)

[2] Zhou et al., An XSPEC model for testing the Kerr black hole hypothesis using the continuum-fitting method, Phys. Rev. D 99, 104031 (2019)

[3] A. Tripathi et al., Testing General Relativity with NuSTAR data of Galactic Black Holes, Astrophys. J. 913, 79 (2021)

[4] C. Bambi, Testing black hole candidates with electromagnetic radiation, Rev. Mod. Phys. 89, 025001 (2017)

Studying the Astrophysical Processes around Black Holes 

We are interested in the accretion process around black holes, in the properties of the inner part of their accretion disk, and in the geometry and the evolution of their "corona", which is a hotter plasma near the black hole and the inner part of the accretion disk. We run GRMHD simulations like that shown below to predict the properties of the accretion disk and we analyze data from X-ray missions like NuSTAR, Insight-HXMT, XMM-Newton, etc.

References:

[5] C. Bambi et al., Towards precision measurements of accreting black holes using X-ray reflection spectroscopy, Space Sci. Rev. 217, 65 (2021)

Solving the Problem of Spacetime Singularities

According to General Relativity, the complete gravitational collapse of a body (e.g., a heavy star) should produce a black hole with a central spacetime singularity, where the energy density diverges, predictability is lost, and standard physics breaks down. It is widely believed that the formation of a spacetime singularity is a symptom of the break down of the theory and that the singularity should be replaced by something else. Since we do not have yet any theory of quantum gravity, a possible strategy to study the problem of the formation of spacetime singularities and the black hole interior is to investigate phenomenological models exploring modifications to the standard scenario and based on "plausible" assumptions coming from new physics. Within this spirit, we have proposed a few singularity-free gravitational collapse models and singularity-free black holes. 

References:

[6] C. Bambi, D. Malafarina and L. Modesto, Black supernovae and black holes in non-local gravity, JHEP 04 (2016) 147  

[7] C. Bambi, L. Modesto and L. Rachwal, Spacetime completeness of non-singular black holes in conformal gravity, JCAP 05 (2017) 003

[8] C. Bambi, D. Malafarina and L. Modesto, Non-singular quantum-inspired gravitational collapse, Phys. Rev. D 88, 044009 (2013)