arXiv subjects

Camilla Lucamarini

Publications and source records attributed to Camilla Lucamarini.

3 recordsLinked to original sources

arXiv2026

Photon surfaces extensions for dynamical gravitational collapse

The equations for the photon surface in spherical symmetry are worked out, starting from arXiv:gr-qc/0005050, in the most general dynamical setting. We show that the condition for a timelike hypersurface to be a photon surface can be reformulated as a non-autonomous dynamical system, whose analysis reveals that the same condition also holds when the surface is generated by a null radial geodesic. As an application, we consider a well-known model of a spherical dust cloud undergoing gravitational collapse. In the marginally bound case the photon surface condition reduces to an exact planar dynamical system, whose analysis shows that the extension of the exterior photon surface $r=3M$ into the cloud is unique, and null, if and only if the central singularity is covered, an open set of timelike extensions being available otherwise. Comparing our findings with those in arXiv:1910.13758, we then investigate analytically whether the null extension covers the singularity developing in the Lemaitre-Tolman-Bondi (LTB) model.

gr-qc↗

arXiv2026

Photon surfaces extension in general spherical dust collapse

We extend the analysis of photon surfaces in spherical dust collapse to the general, non-marginally bound case, i.e.\ allowing the \textit{energy function} $k(x)$ of the Lema\^ıtre--Tolman--Bondi (LTB) model to be non-zero. Starting from the dynamical-systems formulation developed in our previous work for the marginally bound case (arXiv:2509.01368), we derive the photon surface equations for the LTB metric with $k(x)\neq 0$, and we recast the geometric condition for a timelike hypersurface to be a photon surface as a non-autonomous first-order dynamical system. Even though the LTB evolution equation does not integrate in closed form when $k(x)\neq 0$, the implicit solutions available in the literature, together with comparison theorems for ordinary differential equations, are sufficient to show that the only physically acceptable extension of the exterior photon sphere $r=3M$ into the collapsing dust cloud is a null hypersurface, generated by outgoing radial null geodesics. Combining this fact with the known results on the causal structure of the general LTB model, we then establish that the photon surface reaches the central singularity if and only if the singularity is naked, thereby extending to the general case the picture already known in the marginally bound regime. The structural dichotomy between the naked and covered end states is also discussed in connection with possible observational signatures in the early-time evolution of black-hole shadows.

gr-qc↗

arXiv2026

Emergent order spectrum for transitive homeomorphisms

The Emergent Order Spectrum $Ω(x,y)$ is a topological invariant of dynamical systems providing order-types induced by the limit order of order-compatible nested $\varepsilon_n$-chains (with $\varepsilon_n\to 0$) from $x$ to $y$. In this paper, we investigate how rich these spectra can be under natural dynamical hypotheses. For a transitive homeomorphism $f$ of a compact metric space $X$ without isolated points and of cardinality $\mathfrak{c}$, we show that the global spectrum $Ω_f(X^2)$ is universal at the countable scattered level: every countable scattered order-type together with the order-type of the rationals appears in $Ω_f(X^2)$. More precisely, there exists a comeagre subset $M\subseteq X^2$ such that, for every $(x,y)\in M$, the individual spectrum $Ω_f(x,y)$ already realizes all countably infinite scattered order-types; moreover, the order-type of the rationals belongs to $Ω_f(x,y)$ for every pair $(x,y)\in X^2$.

math.DS↗