A group of physicists from Lehman College (USA) has presented a theoretical model according to which primordial black holes — hypothetical objects that formed in the first fractions of a second after the Big Bang — could have survived the 13.8 billion years of the Universe's existence thanks to interaction with an additional, fifth spatial dimension. The study, being prepared for publication in the journal Physical Review D, offers a new explanation for why some microscopic black holes did not evaporate over cosmological timescales, as predicted by the standard four-dimensional model.
The Brane Universe and a Hidden Dimension
Modern theoretical physics treats the observable Universe as a four-dimensional membrane (brane) embedded in a higher-dimensional space. In this picture, elementary particles and electromagnetic radiation are tightly bound to the brane and cannot leave the confines of four-dimensional space. However, gravity, according to the hypothesis, is not subject to such a constraint: it is able to "leak" into the extra dimensions. The fifth dimension, according to the model, exists in a folded (compactified) state and has a characteristic size on the order of one micron. The key threshold for a black hole is determined by the size of its event horizon: if it is smaller than one micron, the object's gravitational field spreads out across five dimensions, turning it into a five-dimensional object with fundamentally different properties.
The Birth of Primordial Black Holes: Two Scenarios
Unlike stellar-mass black holes, which form as a result of the collapse of massive stars, primordial black holes formed in the first moments after the Big Bang from overdense regions of matter. The model considers two main mechanisms of their formation. The first is linked to rapid phase transitions during the swift cooling of the early Universe: local regions with anomalously high density collapsed into microscopic black holes. The second mechanism involves the collapse of cosmic strings — hypothetical topological defects of spacetime that formed loops, contracting under their own tension. Within the framework of the "dark dimension" model, primordial black holes arising from such strings are born five-dimensional from the outset, since their event horizon is below the micron threshold from the very beginning.
Gregory–Laflamme Instability and the Transition to the Fifth Dimension
Calculations performed by the authors of the work showed that even four-dimensional primordial black holes, formed as a result of phase transitions, would have been unstable. Due to the so-called Gregory–Laflamme instability — a relativistic effect in which cylindrical or spherical objects with a small transverse size lose stability and "unfold" into an extra dimension — such objects would have rapidly transformed into five-dimensional ones. Thus, regardless of the specific formation mechanism, a significant fraction of primordial black holes would sooner or later have ended up in a five-dimensional state.
Slowed Evaporation and Survival Over 13.8 Billion Years
The transition to the fifth dimension radically changes the physical evolution of a black hole. By the Hawking radiation mechanism, black holes gradually lose mass and ultimately evaporate. However, five-dimensional objects evaporate significantly more slowly than their four-dimensional counterparts, because the gravitational field is distributed over a larger volume of space, reducing the intensity of radiation per unit area of the horizon. This slowdown, according to the authors' calculations, is sufficient for some primordial black holes to have survived to the present moment — 13.8 billion years after the Big Bang. This is precisely what makes them potential candidates for the role of "eternal" black holes, which have been discussed in the popular science press in recent months.
Connection to Anomalous Astrophysical Signals
The authors of the model also offer an explanation for a number of anomalous observations. In particular, the final evaporation flash of a five-dimensional black hole could have led to the detection of a high-energy neutrino by the KM3NeT observatory without accompanying photon radiation — an event that is difficult to explain within the framework of standard astrophysical models. If even one primordial black hole is directly detected, physicists will gain a direct tool for studying physical processes and scales that are not accessible to direct observation, including testing hypotheses about extra dimensions and the properties of gravity on sub-micron scales.
Contradictory Data
Several aspects of the presented work require cautious interpretation. First, at the time of publication of the article, the study is in the preparation stage for release in Physical Review D, meaning it has not yet undergone the full peer review cycle, which means that the conclusions may be revised during expert evaluation. Second, there is no single mechanism for the formation of primordial black holes in the scientific community: alongside the phase transitions and cosmic string collapse discussed in this work, other studies (including those published in 2026) point to the possible role of the first stars in the Universe in forming primordial objects. These mechanisms do not exclude one another, but they yield different estimates of the mass and number of primordial black holes. Third, the connection to the detection of a neutrino by the KM3NeT observatory is hypothetical: the authors propose it as a possible explanation but do not claim that a specific event unambiguously confirms the five-dimensional nature of the source. Finally, the very existence of a fifth dimension on the micron scale remains a theoretical hypothesis, not confirmed by direct experimental data.
Significance for Fundamental Physics
The work by Lehman College fits into the broader context of the search for signs of extra spatial dimensions, which is being pursued at the intersection of quantum gravity, cosmology, and high-energy astrophysics. If five-dimensional primordial black holes truly exist and their evaporation can be registered, this will become the first direct observation of gravity spreading beyond the four-dimensional brane. This will open a fundamentally new channel for testing brane cosmology models and may perhaps provide an answer to the question of the nature of dark matter, part of which in some scenarios could be "hiding" in an extra dimension.
