Physicists have proposed that primordial black holes, hypothetical objects thought to have formed shortly after the Big Bang, could reveal whether the universe contains a hidden fifth dimension. The idea, reported in a recent study in Physical Review D, suggests that these tiny objects would be able to sense a spatial direction that remains invisible in everyday life.
The analogy the researchers use is a familiar one. Someone walking along the cable of a suspension bridge sees only a narrow path running forward and backward, while an ant crawling on the same cable can also move around its curved surface. Luis Anchordoqui of Lehman College, City University of New York, and his collaborators Alek Bedroya and Dieter Lüst argue that a similar situation could exist in the cosmos, with an extra dimension too small for humans to notice.
The work is set within the so-called dark dimension scenario, first proposed in a 2023 study. That framework holds that the universe contains an additional spatial dimension curled up to roughly one micrometre, about one hundredth the width of a human hair. Together with the three familiar spatial dimensions and time, it would make spacetime five-dimensional. Primordial black holes would be small enough to experience that extra direction. "These tiny objects see the entire space, the three dimensions we usually see plus the compact one that we do not experience in everyday life," Anchordoqui says. The team's calculations indicate that the event horizon of these objects, the boundary beyond which even light cannot escape, would be smaller than the extra dimension.
Primordial black holes differ from the black holes most astronomers know. Ordinary black holes form when massive stars collapse at the end of their lives. Primordial ones could have formed when an unusually dense region of the early universe collapsed under its own gravity, without first needing to form stars. They have not been detected, but they are among the candidates proposed to explain dark matter.
To understand how such objects might have formed, the researchers considered the temperature of the early universe. In the dark dimension scenario, the radiation-dominated era, when radiation rather than matter carried most of the universe's energy, began once the universe had cooled to around 1 GeV, a threshold the team calls the "normalcy temperature". What happened at higher temperatures remains uncertain. One possibility is a period known as "kination", in which the kinetic energy of a scalar field dominated the expansion of the universe. "The epoch of kination is a possible epoch of the universe for temperatures above the normalcy temperature, but this is just one speculative possibility," Anchordoqui explains.
The researchers examined two mechanisms by which these black holes could have formed. The first involves phase transitions, which could have produced dense regions that later collapsed into primordial black holes. If such objects formed above the normalcy temperature, they could initially have been four- or five-dimensional, but any four-dimensional ones would become unstable and convert into five-dimensional objects as the universe cooled below 1 GeV. The second mechanism involves cosmic strings, hypothetical thin structures that may have formed in the early universe. As Stephen Hawking first noted, loops of such strings could collapse under their own gravity to form black holes. Those would be five-dimensional from the start, whether they formed above or below the normalcy temperature. Despite the remaining uncertainties, both routes lead to five-dimensional primordial black holes.
A further question is whether any of these objects could still exist. Black holes are predicted to lose energy slowly through Hawking radiation. In the dark dimension scenario, five-dimensional black holes would evaporate more slowly than four-dimensional ones, which could allow some to survive to the present day. The researchers considered whether such evaporation might account for an exceptionally energetic neutrino detected by the KM3NeT telescope. The neutrino's energy is close to the maximum particle energy expected near the final stages of evaporation in their model. Anchordoqui stresses the uncertainty of that interpretation. "This part I'd say is very speculative, especially because we are in the strong gravity regime," he says. He considers electromagnetic signals a more promising avenue.
In a separate study with theoretical physicist Cumrun Vafa, Anchordoqui explores whether these black holes could leave detectable traces in gamma-ray observations from NASA's Fermi Large Area Telescope. The researchers suggest a possible link to the so-called "Fermi-halo excess", an unexplained gamma-ray signal, though Anchordoqui cautions that this interpretation is highly uncertain.
Neither primordial black holes nor a hidden fifth dimension has yet been detected. Still, the researchers argue that future observations could test the idea. Evidence of such objects, if found, would offer a rare glimpse into the early universe and could reveal a hidden dimension of space.
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