Neutrinos near a naked singularity could lose all flavor identity, study suggests

A new theoretical study argues that neutrinos traveling through a self-gravitating gas of ultrarelativistic particles would oscillate between flavors in a way that can be computed entirely from the geometry of spacetime, without the heavy machinery of quantum field theory in a thermal medium. The work, published in The European Physical Journal C by Daniele Gregoris, focuses on a little-known exact solution of Einstein’s equations called the Tolman VI metric, and it arrives at a striking prediction: if neutrinos ever reach the naked curvature singularity at the center of this spacetime, they would be simultaneously in flavor equilibrium, physically overlapped, and quantum-mechanically scrambled, with an equal probability of being detected as any flavor.

The Tolman VI metric describes a static, spherically symmetric spacetime filled with a perfect fluid in gravitational equilibrium, characterized by a single free parameter a that fixes the equation of state of the material. Energy density and pressures fall off as one over radius squared, and the geometry harbors a naked curvature singularity at the center, meaning a point of infinite curvature not hidden behind an event horizon. For the specific choice a = 7/4, the pressure becomes isotropic and the stress-energy tensor tracefree, so the spacetime is supported by a gas of ultrarelativistic particles obeying p = ρ/3, the equation of state of radiation, whether that radiation is a photon gas or an electron-positron plasma with Fermi momentum and temperature far above the electron rest mass. This tuning is what lets one model describe neutrino propagation across very different astrophysical settings, from the early universe to stellar interiors, in a unified way.

The core of the calculation uses the eikonal, or geometric optics, approximation, in which neutrinos, although genuinely wavepackets, are treated heuristically as point particles following geodesic paths. The phase difference between two mass eigenstates is computed as a line integral along the trajectory, and because the spacetime is static and spherically symmetric, the energy and angular momentum of the neutrino are conserved quantities. The result takes a familiar form: the leading-order phase shift equals the mass-squared difference times an effective distance divided by twice the neutrino energy, with the flat-spacetime distance replaced by a curved, gravitationally stretched one. The same phase shift can be re-expressed in terms of the background energy density, the temperature gradient fixed by Tolman’s law, or the Weyl curvature scalar that quantifies the strength of tidal gravity, giving three physically distinct readings of the same effect.

The paper then adds a second layer of refinement borrowed from a recently developed spinoptics formalism, which treats the spin of the neutrino as a vector rather than wrestling with four-by-four spinor matrices. In this picture the eikonal phase tracks the translational motion of the particle while a Berry-like spinoptics phase tracks its rotational or frame motion, arising from the spin connection that couples the neutrino’s internal state to spacetime curvature. A key structural finding is that the spatial-rotation part of this spin contribution vanishes exactly for ultrarelativistic neutrinos on equatorial trajectories whose spins are aligned with their momenta, because the relevant spin expectation differences are proportional to a momentum component that is zero in planar motion. What survives is a boost contribution, and it has a remarkably simple closed form: it scales with the mass difference itself, not the mass-squared difference, and is proportional to the logarithm of the ratio of the starting and ending radii divided by the neutrino energy.

Normally this spinoptics term is subdominant, piling up more slowly than the eikonal phase as the neutrinos travel. The exception is dramatic. The eikonal phase has an integrable singularity at the classical turning point, but the spin-boost phase diverges non-integrably at the central curvature singularity at radius zero. Because the spin contribution scales linearly with the mass difference, it can dominate wherever the absolute mass scale matters or wherever the spin connection is very strong, which in this spacetime means precisely the naked singularity. Numerical comparisons in the paper confirm that away from that central region the eikonal approximation alone tracks the full conversion probability well, with the two curves merging as the crossed distance grows, while near the singularity the spin phase varies so rapidly that only an averaged behavior is physically meaningful.

The asymptotic checks are where the heuristic method earns its credibility. At low temperature, the eikonal phase reduces exactly to the flat-spacetime result, meaning neutrinos crossing today’s cold Cosmic Microwave Background oscillate as if they were moving through empty flat space, consistent with earlier finite-temperature field theory analyses showing that low-temperature photon backgrounds generate no flavor-dependent refractive effects. At high temperature, the eikonal phase tends to zero, so the eikonal survival probability tends to one, recovering the thermal field theory picture in which a thermal self-energy suppresses the oscillation frequency and frequent scattering produces quantum damping rather than flavor conversion. But adding the divergent spin-boost phase changes that conclusion: near the singularity the flavor-frozen limit is replaced by a Gribov-Pontecorvo regime of rapid phase averaging, in which the oscillating term averages to one half and all flavor information is lost.

The second half of the paper turns to Mikheyev-Smirnov-Wolfenstein resonances, and here the physical identity of the matter filling the spacetime becomes decisive rather than its geometry. If the p = ρ/3 fluid is interpreted as a photon gas, the matter potential vanishes and no resonance can occur at all, a result that holds independent of the gravitational details. If instead the fluid is a self-gravitating electron-positron plasma, the weak-interaction potential scales as one over radius squared, and a genuine, gauge-invariant resonance radius emerges from the gravitational redshift factors of the metric, with the resonance pushed toward the center by Tolman’s law. The angular momentum of the neutrino matters here too: grazing trajectories linger longer near the resonance region through a lensing effect, enlarging the adiabaticity parameter and making resonant conversion more efficient, with non-adiabatic jump probabilities quantified by the Landau-Zener formula. For a superimposed test plasma, the resonance condition becomes independent of radial position, so neutrinos remain in the same mixing state along their whole path.

The paper also derives a coherence length for the neutrino wavepackets, finding that the metric parameter a sets it: a larger a, corresponding to a steeper decay of the tangential pressure, shortens the coherence length. Notably, the wavepacket velocity difference shrinks as neutrinos fall toward the singularity, so gravitational blueshift acts as a coherency preserver, compressing the wavepackets together even as the spin phase blows up. This is the origin of the paper’s headline prediction: neutrinos that hit the curvature singularity are physically overlapped, because their wavepacket separation goes to zero, yet quantum-mechanically scrambled, because their spin-boost phase diverges and averages any detector reading toward equal flavor probabilities. The author proposes this triple signature, flavor equilibrium plus wavepacket overlap plus rapid phase scrambling, as an observational method for identifying naked singularities, complementing existing proposals based on accretion disk images, perturbation echoes, and radiative signatures.

As an order-of-magnitude illustration, the paper treats a supernova envelope as a scale-invariant radiation bath spanning a region of order a few kilometers at a temperature of order ten kelvin, with neutrino energies of order one mega-electronvolt and mass-squared differences of order ten to the minus four electron-volts squared. For a spherically symmetric collapse with zero impact parameter, the estimated eikonal phase comes out of order twenty-two times the square root of a, large enough that the neutrinos propagate in an averaged oscillation regime where flavor conversion is governed by mixing angles rather than by the distance traveled. The author notes this averaged result is consistent with state-of-the-art simulations of slow neutrino modes, and flags supernova envelopes and the cosmic epoch after positron-electron annihilation as the natural next targets for quantitative predictions. As a purely theoretical work with no new data, the study’s weight rests on its exact analytical closed-form results and their cross-checks against quantum field theory in the known limits.

Subject of Research: A theoretical analysis of neutrino flavor oscillations and resonances inside the Tolman VI spacetime supported by ultrarelativistic matter, including spinoptics effects near its naked singularity.

Article Title: Neutrino oscillations inside ultrarelativistic matter: on the role of the Tolman VI spacetime

Article References: Gregoris, D. (2026). Neutrino oscillations inside ultrarelativistic matter: on the role of the Tolman VI spacetime. The European Physical Journal C, 86(10), Article 1238. https://doi.org/10.1140/epjc/s10052-026-16462-w

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16462-w

Keywords: neutrino oscillations, Tolman VI metric, naked singularity, spinoptics, eikonal approximation, MSW resonance, general relativity, electron-positron plasma, cosmic microwave background, supernova, flavor equilibrium, theoretical physics