Hunting for extra dimensions in the shadow of M87*

Abstract: The Event Horizon Telescope has recently provided the first image of the dark shadow around the supermassive black hole M87*. The observation of a highly circular shadow provides strong limits on deviations of M87*'s quadrupole moment from the Kerr value. We show that the absence of such a deviation can be used to constrain the physics of extra dimensions of spacetime. Focusing on the Randall-Sundrum AdS$_5$ brane-world scenario, we show that the observation of M87*'s dark shadow sets the limit $\ell \lesssim 170\,{\rm AU}$, where $\ell$ is the AdS$_5$ curvature radius. This limit is among the first quantitative constraints on exotic physics obtained from the extraordinary first ever image of the dark shadow of a black hole.

Constraining Extra Dimensions with M87*'s Shadow

In the paper entitled "Hunting for extra dimensions in the shadow of M87*," the authors investigate the possibility of constraining the physics of extra dimensions, specifically within the context of the Randall-Sundrum (RS) AdS$_5$ brane-world model, using observational data from the Event Horizon Telescope (EHT). The EHT's recent achievement in imaging the shadow of the supermassive black hole M87* provides an unprecedented opportunity to explore fundamental aspects of gravitational theories beyond General Relativity (GR).

Overview of Research Context

The RS model posits the existence of a non-compact fifth dimension within a five-dimensional Anti-de Sitter space (AdS$_5$). This framework suggests that gravitational effects can deviate from the predictions of four-dimensional GR due to the influence of extra dimensions. These deviations could manifest in observable quantities such as the quadrupole moment of black holes (BHs), which, according to the no-hair theorem, should only depend on mass, charge, and angular momentum in traditional GR scenarios.

The observation of M87* by the EHT revealed a dark shadow, characterized by its circular shape, which aligns with the predictions for a Kerr BH. Deviations from circularity suggest modifications in the BH's quadrupole moment potentially indicative of new physics, such as extra dimensions.

Numerical Results and Implications

The paper leverages the high degree of circularity in the observed shadow of M87* to place constraints on the AdS$_5$ curvature radius (denoted $\ell$) in the RS model. The authors derive the modified Newtonian potential in the presence of a fifth dimension and show that the deviation in the quadrupole moment, due to this extra dimensional influence, is $\Delta Q \approx \frac{2M\ell^2}{3}$. Using the EHT's constraint that the relative deviation $\epsilon$ in the quadrupole moment must satisfy $\epsilon \lesssim 4$, they deduce the upper limit $\ell \lesssim 170\,{\rm AU}$.

While this constraint on $\ell$ is significantly weaker than current limits from other experimental approaches, such as precision tests of gravity at millimeter scales, it represents an independent check using astronomical observations. This result underscores the potential of using imaging techniques of black hole shadows to probe deeper into the structure of spacetime and the viability of theories predicting extra dimensions.

Future Directions

This work opens avenues for future research by providing a methodology for using astrophysical observations to test aspects of the RS model and potentially other theories involving extra dimensions. As VLBI technology improves and additional data from sources like Sgr A* become available, more stringent constraints on the properties of extra dimensions can be expected. Continued synergy between observational astrophysics and theoretical physics holds promise for advancing our understanding of gravity's behavior on large scales and exploring the fundamental nature of our universe.

In summary, the analysis of M87*'s shadow has taken a significant step in utilizing novel observational insights to investigate exotic physics proposals, such as the existence of extra dimensions—offering a promising complement to experimental pursuits in particle physics and gravitational studies. Further developments in imaging techniques will refine these constraints and enhance our exploration of potential new physics beyond the standard model of cosmology.