Exploring the landscape of black hole mimickers

TL;DR

The work investigates a broad landscape of black-hole mimickers by constructing horizonless, geodesically complete spacetimes regular at all curvatures and relaxing $Z_2$ symmetry relative to the Damour–Solodukhin wormhole. It analyzes null geodesics, shadows, and scalar quasinormal modes (QNMs) across four non-$Z_2$-symmetric test metrics using hyperboloidal slicing and a matrix method, revealing a spectrum of observational signatures, including a two-shadow effect and gravitational-wave echoes in several cases. Metric II nearly reproduces Schwarzschild BH ringdown with no echoes, while Metrics I, III, and IV produce echo-rich ringdowns due to inner barriers or walls; the outer BH-like shadow can transition to an inner-shadow-dominated regime depending on throat travel times. The results show that horizonless mimickers can closely mimic BH observables in the exterior while hiding rich interior structure, but current EHT shadow bounds and long-time GW observations provide meaningful constraints and motivate future time-domain tests to distinguish ECOs from true black holes. The framework connects phenomenological mimicker models to observable features such as $R_{ ext{sh}}$, $V_{ ext{eff}}$, and QNM spectra, guiding searches for horizon-scale physics in current and upcoming data.

Abstract

We identify a general class of spacetime metrics that mimic the properties of black holes without possessing a true event horizon. These metrics are constrained by the requirements of being singularity-free and geodesically complete. Specifically, we study metrics that do not possess $Z_2$ symmetry and may deviate slightly or significantly from the symmetric case. Focusing on scalar perturbations propagating on such backgrounds, we analyze the resulting effective radial potentials and their dependence on different corners of the mimicker landscape. We further investigate the corresponding quasinormal modes and explore their characteristic features. Finally, we survey the landscape for potential observational signatures, including shadow properties and the possible presence or absence of echo effects.

Figures (35)

• Figure 1: Metric functions $r(\rho)$ and $g(\rho)$ as functions of the radial coordinate $\rho$.
• Figure 2: The metric function $g_{\text{I}}(\rho)$ for two sets of parameters compared with the function $g_{\text{sch}}(\rho)$\ref{['2']}. The left plot corresponds to the parameters $b = 0.1$ and $a = 0.08$. The right plot corresponds to the parameters $b=0.1$ and $a=-0.08$.
• Figure 3: Left: region in space of parameters $b$ and $\rho_0>0$ for which the metric function $g_{\text{II}}(\rho)$ increases monotonically. Right: the metric function $g_{\text{II}}(\rho)$ for two different values of parameters and the comparisons with the Schwarzschild metric function $g_{\text{sch}}(\rho)$. The top plot corresponds to the monotonic case with parameters $b=0.1$ and $\rho_=0.01$, the bottom plot corresponds to a non-monotonic case with parameters $b=0.1$ and $\rho_0=2$.
• Figure 4: The shape of metric function $g_{\text{III}}(\rho)$ in test metric III for parameters $b=0.1$, $\rho_0=1$ and the comparison with the Schwarzschild metric function $g_{\text{sch}}(\rho)$.
• Figure 5: The effective radial potential for null geodesics for two sets of the parameters: $b = 0.1\, , \ a=0.08$ (left) and $b=0.1\, , \ a=-0.08$ (right).