Primordial black holes have never been found. A new calculation says that if they exist, they may be the only objects in the universe we can use to see a fifth dimension — and that they may be five-dimensional themselves.
The work, by Luis Anchordoqui of Lehman College at the City University of New York with Alek Bedroya and Dieter Lüst, appears in Physical Review D. It sits inside the "dark dimension" scenario, a 2023 proposal that the universe carries one additional spatial dimension curled up at roughly a micrometre — about one hundredth the width of a human hair. Too small to notice, large enough to matter for very small things.
The analogy the researchers reach for is a suspension bridge cable. You see a line stretching forward. An ant on the cable sees the line and also the surface it can walk around — a direction that exists for the ant and is hidden from you. In the dark dimension picture, the universe is the cable and we are the walker. Primordial black holes would be the ant.
The mechanism is scale. Primordial black holes are hypothetical objects formed when unusually dense regions of the early universe collapsed under their own gravity, without needing a star to die first. They are expected to be small — and the team's calculations put their event horizons below the size of the extra dimension. An object smaller than the compact dimension experiences it as a full dimension. "These tiny objects see the entire space," Anchordoqui told Physics World, "the three dimensions we usually see plus the compact one that we do not experience in everyday life."
The second question is how they formed, and here the paper walks two routes that converge on the same answer. The first is phase transitions: in the hot early universe, a transition could have created overdense regions that collapsed into black holes. Above a threshold the scenario calls the "normalcy temperature" — around 1 GeV, where radiation rather than matter began to carry most of the universe's energy — the physics is uncertain, and the authors consider a "kination" epoch, when the kinetic energy of a scalar field drove expansion, as one speculative possibility. Black holes formed in that regime could have started out four- or five-dimensional; the four-dimensional ones would be unstable and would transition into five-dimensional objects by the time the universe cooled below the threshold. The second route is cosmic strings: loops of these hypothetical filaments can collapse into black holes, as Stephen Hawking first noted, and those would be five-dimensional from birth regardless of when they formed.
Both paths lead to five-dimensional primordial black holes. Which makes them an observational target rather than a curiosity.
The reason they might still be around is evaporation. Black holes lose mass through Hawking radiation, and in the dark dimension scenario a five-dimensional black hole evaporates more slowly than its four-dimensional counterpart — slowly enough that some could survive from the early universe to the present day. That opens the door to looking for them.
The team tested one possible signal and were notably cautious about it: an exceptionally energetic neutrino detected by KM3NeT falls close to the maximum particle energy their model predicts near the final stages of evaporation. Anchordoqui describes that reading as "very speculative," largely because it requires trusting the calculation deep in the strong-gravity regime. He puts more weight on electromagnetic searches. In a separate study with Cumrun Vafa, he examined whether these black holes could leave a signature in gamma-ray data from NASA's Fermi Large Area Telescope, including a possible connection to the unexplained "Fermi-halo excess" — again flagged as highly uncertain.
Nothing here is confirmed. Neither primordial black holes nor the extra dimension has been observed, and every observational hook in the paper is a candidate signal with caveats attached, not a detection. What the work does is convert a piece of theoretical furniture into something with a testable shape: if primordial black holes exist and the dark dimension is real, they should be five-dimensional, they should evaporate on a different schedule than standard theory predicts, and that difference should show up somewhere in the neutrino or gamma-ray sky.
That is a modest claim, and it is the right size for the evidence. It is also the kind of claim that makes an exotic idea useful: not proof, but a place to point a telescope.
