Cosmological Constraints on 4D Einstein-Gauss-Bonnet Gravity and Kaniadakis Holographic Dark Energy: Implications for Black Hole Shadows

TL;DR

This study explores how 4D Einstein-Gauss-Bonnet gravity coupled with Kaniadakis holographic dark energy affects black hole shadows and cosmological evolution, using observational data to constrain model parameters and assess observable signatures.

Contribution

It provides the first detailed analysis of black hole shadow dynamics within this modified gravity and dark energy framework, constrained by current cosmological observations.

Findings

Best-fit parameters favor a phantom-like dark energy equation of state.

The EGB coupling is consistent with zero, aligning with General Relativity.

Black hole shadow size shows potential deviations up to 6% at redshift 2 compared to ΛCDM.

Abstract

The direct imaging of black holes by the Event Horizon Telescope (EHT) enables strong-field tests of gravity. We study the cosmological evolution and the black-hole shadow radius in 4D Einstein-Gauss-Bonnet (EGB) gravity coupled to Kaniadakis holographic dark energy (KHDE), adopting the future event horizon as the infrared cutoff. Using Cosmic Chronometers, Pantheon+ Type Ia supernovae, and DESI BAO data, we constrain the model with a Markov Chain Monte Carlo analysis. The best-fit values favor a phantom-like equation of state driven by Kaniadakis entropy ($c\simeq 1.18$, $\beta\simeq 2.26$), but $\beta$ remains weakly constrained ($\beta=2.26^{+0.11}_{-2.20}$), consistent with the standard holographic limit $\beta\to0$ at $1\sigma$. The EGB coupling is constrained to $\alpha\simeq -0.004$, also consistent with General Relativity ($\alpha=0$) at $1\sigma$. Guided by the posterior, we define five representative scenarios to probe the dynamical phase space. We find that the accretion history is highly sensitive to the thermodynamic sector: standard holographic cases yield monotonic evolution, whereas phantom-divide crossing leads to non-monotonic behavior in both the black hole mass and the vacuum shadow radius. Including a dispersive plasma medium, refraction dominates over intrinsic mass growth and induces an overall shrinkage of the observable shadow at high redshift; nevertheless, a residual intrinsic deviation of $\sim6\%$ (for our conservative accretion setup) persists at $z\simeq2$ relative to the $\Lambda$CDM prediction. These results indicate that, despite environmental dominance, precision population analyses of black hole shadows may help disentangle subtle dynamical dark-energy imprints from the standard cosmological paradigm.