Physicists at Radboud University have outlined a theoretical model suggesting that compact stars and their remnants could, in principle, slowly lose mass through a Hawking-like radiation process even though these objects lack event horizons. The work, by Marcus Wondrak, Walter van Suijlekom, and Heino Falcke, argues that spacetime curvature alone could be enough to generate particle pairs near massive objects, implying an extremely slow energy loss over extraordinarily long timescales. The authors and subsequent coverage frame the idea as a far-future possibility that depends on how the underlying calculations hold up under scrutiny.
Curvature Without a Horizon
Hawking’s original 1974 theory tied radiation to the extreme warping of spacetime at a black hole’s event horizon, the boundary beyond which nothing escapes. The new line of research challenges that link. In a 2023 paper published in Physical Review Letters, Wondrak, van Suijlekom, and Falcke argued that gravitational pair production can occur wherever spacetime is sufficiently curved, not only at horizons. Virtual particle pairs that would normally annihilate each other get pulled apart by tidal forces in the gravitational field, and one particle escapes while the other is absorbed. The net effect is a slow but real energy drain from the massive object.
This mechanism means that neutron stars, white dwarfs, and other dense remnants could radiate particles in a manner analogous to black hole evaporation. As van Suijlekom explained in a Radboud University statement reported by science news coverage, the team argues that Hawking-like radiation is a general phenomenon with broad implications for the universe and its future. The claim reframes a half-century-old idea: rather than being special to black holes, the radiation is a property of gravity itself.
Density Sets the Clock
Building on the 2023 foundation, the team posted a follow-up study on arXiv that applies the framework specifically to compact stellar remnants. In that work, they show that the evaporation timescale for compact objects scales with average density as a power law, with denser objects evaporating faster in relative terms. Mathematically, the characteristic time is proportional to the density raised to the negative three-halves power. In practical terms, a neutron star, which packs roughly the mass of the Sun into a city-sized sphere, would radiate more intensely than a white dwarf of similar mass but larger radius.
The timescales involved are staggering. According to reporting that synthesizes the new estimates, the universe as a whole is expected to fully decay in roughly 10 to the 78th power years, a figure that dwarfs the current age of the cosmos by many orders of magnitude yet remains finite. Earlier ideas about the ultimate fate of matter, such as hypothetical proton decay, often pushed the endpoint even further into the future. The new calculation, if correct, brings that terminal era closer than some previous projections suggested, though it still lies far beyond any conceivable observational test.
For everyday purposes, this changes nothing. Stars like the Sun will exhaust their nuclear fuel on timescales of billions of years, long before curvature-driven radiation could measurably affect them. But for the far future of the cosmos, the distinction matters. Even after stars burn out and their remnants cool, this process would continue, gradually converting mass into escaping particles until nothing solid remains.
Sharp Criticism From Peers
The 2023 Physical Review Letters paper drew swift pushback. A formal comment posted on arXiv by other physicists argued that the consequences of the proposed imaginary part of the effective action are in strong conflict with known results in quantum electrodynamics and cosmology. The critics claimed that when the same mathematical tools are applied consistently to curved spacetime, they do not yield the particle-production rates the Radboud team reported, casting serious doubt on whether Schwarzschild-spacetime pair production actually occurs at the levels described.
This is not a minor quibble. If the effective action does not generate a meaningful imaginary component in the relevant spacetime geometry, the entire evaporation mechanism for non-black-hole objects collapses. The challenge strikes at the core calculation, not at secondary assumptions or approximations. In the critics’ view, what looks like Hawking-like radiation in the Radboud analysis may instead be an artifact of how the formalism is extended beyond its domain of validity.
The Authors Defend Their Framework
Wondrak, van Suijlekom, and Falcke responded through two channels. They published a peer-reviewed reply in the journal’s Comments and Replies section, offering point-by-point rebuttals to the objections. There they maintain that their treatment of the effective action is consistent and that the comment misinterprets key steps in the derivation. The fact that this exchange appeared in the same journal underscores that the editors consider the dispute substantive enough to warrant formal publication.
The team also posted a longer preprint on arXiv that complements the shorter reply. That document lays out additional derivations and context for their use of pair-production analogies in curved spacetime. They argue that the critics applied the formalism under assumptions that do not match the specific gravitational setting of their model, particularly regarding boundary conditions and the global structure of the spacetime. When those aspects are handled differently, they contend, the tension with exact results largely disappears.
Neither side has conceded. The debate remains active and unresolved, which is typical for theoretical physics at this frontier. The radiation these models discuss would be extraordinarily faint, and the authors and coverage emphasize that it would be far beyond practical detectability with present-day observations. For now, the dispute is being fought on blackboards and in preprints, not in laboratories.
What This Means for Cosmic Longevity
Most public discussion of stellar death focuses on relatively near-term events: red giant phases, supernova explosions, and the cooling of white dwarfs. The Radboud proposal, if it survives scrutiny, shifts the conversation to a much longer horizon. In their picture, even the quiet embers of stellar evolution are temporary. Over unimaginable spans of time, neutron stars, white dwarfs, and other compact remnants would slowly shed mass through curvature-induced radiation, eventually dissolving into a bath of low-energy particles.
In this scenario, the familiar “heat death” of the universe takes on a new twist. Classical cosmology already predicts that, given enough time, stars will stop forming and existing stars will fade, leaving a cold, dilute cosmos. The curvature-driven evaporation mechanism adds an additional step: the ultimate erasure of the compact objects left behind. Instead of a universe dotted with inert stellar corpses, the far-future cosmos would be almost entirely featureless, with matter dispersed into extremely diffuse radiation.
Whether that vision is accurate depends on how the current theoretical clash is resolved. If the critics are right that the effective action in the relevant spacetimes lacks the imaginary part needed to sustain particle production, then compact objects may be far more durable than the Radboud group suggests. In that case, black holes would remain the primary sites of Hawking-like evaporation, and the long-term fate of neutron stars and white dwarfs would be governed mainly by other, slower processes such as occasional collisions or accretion events.
If, however, the Radboud calculations withstand ongoing challenges, they would imply a universe in which gravity itself guarantees that nothing gravitationally bound can last forever. Even in the absence of event horizons, spacetime curvature would continually convert mass into radiation, albeit at rates that are effectively zero on human, planetary, and even galactic timescales. The cosmos would still be headed toward a cold, dark end, but it would get there slightly sooner and in a subtly different way than many cosmologists assumed.
For now, the idea that compact stars slowly evaporate like black holes remains a bold extrapolation of quantum field theory in curved spacetime, not an established fact. The ongoing exchange of comments, replies, and extended analyses is part of the process by which such ideas are tested. It may take years of further theoretical work to clarify whether curvature alone can drive the kind of particle production the Radboud team envisions. Until then, the proposal serves as a reminder that even in a seemingly settled field like gravitation, new calculations can still challenge our assumptions about how long the universe, and everything in it, can endure.
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*This article was researched with the help of AI, with human editors creating the final content.
