Imagine walking along the cable of a suspension bridge. You see the cable as a narrow path stretching forward and backward. But an ant crawling along the same cable experiences an extra direction: it can move around the cable’s curved surface, a direction effectively hidden from you. Our universe might work in a similar way. Although we see three dimensions of space, physicists have long considered the possibility of additional dimensions that could exist but are too small for us to notice. Now, researchers suggest that primordial black holes, hypothetical objects formed shortly after the Big Bang, could offer a way to explore this concept.
In a recent study reported in Physical Review D, Luis Anchordoqui of Lehman College, City University of New York, and his collaborators Alek Bedroya and Dieter Lüst, argue that the primordial black holes should be five-dimensional within a theoretical framework known as the “dark dimension” scenario.
Black holes form when massive stars collapse at the end of their lifetime. However, primordial black holes could have formed when an unusually dense region of the early universe collapsed under its own gravity without needing to form stars at first. Although not detected yet, they are one of the possible candidates suggested to explain dark matter.
We usually describe spacetime as having three spatial dimensions – left-right, back-forth and top-bottom – and one dimension of time. The dark dimension scenario, first proposed in a 2023 study, suggests that the universe contains an additional spatial dimension curled up to approximately one micrometre, about one hundredth the width of a human hair. Adding another spatial dimension would make spacetime five-dimensional. The primordial black holes would be small enough to experience this additional 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,” says Anchordoqui. The team’s calculations suggest that the event horizon of primordial black holes, the boundary beyond which even light cannot escape, would be smaller than the extra dimension.
Two potential formation theories
To understand how these primordial black holes may have formed, the researchers considered the temperature of the early universe. In the dark dimension scenario, the radiation-dominated period when radiation, rather than matter, carried most of the universe’s energy began when the universe cooled to around 1 GeV. This threshold is called the “normalcy temperature”. What happened at higher temperatures remains uncertain. One possibility is a period known as “kination”, when 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 explored two different ways in which these black holes could form. The first mechanism considers phase transitions. These transitions could have created unusually dense regions that eventually collapsed into primordial black holes. If these formed above the normalcy temperature, they could initially have been four or five-dimensional. However, any four-dimensional black holes would become unstable and transition into five-dimensional objects by the time the universe cooled below 1 GeV.
The second mechanism considers cosmic strings, hypothetical thin structures that may have formed in the early universe. As first noted by Stephen Hawking, loops of these strings could collapse under their own gravity to form black holes. Unlike those produced by phase transitions, these black holes would be five-dimensional from the beginning, whether they formed above or below the normalcy temperature. Despite these uncertainties, both mechanisms lead to five-dimensional primordial black holes.
Could primordial black holes still exist?
Black holes are predicted to slowly lose energy through Hawking radiation. In the dark dimension scenario, five-dimensional black holes would evaporate more slowly than their four-dimensional counterparts, potentially allowing some of them to survive until the present day.
The researchers considered whether such evaporation might explain 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. However, Anchordoqui emphasizes the uncertainty of this interpretation. “This part I’d say is very speculative, especially because we are in the strong gravity regime,” he clarifies. Instead, he believes electromagnetic signals could provide a more promising way to search for these black holes.
Concerning primordial black holes
In a separate study performed with theoretical physicist Cumrun Vafa, Anchordoqui explores whether these black holes could leave detectable signals in gamma-ray observations from NASA’s Fermi Large Area Telescope. The researchers suggest a possible connection to the so-called “Fermi-halo excess”, an unexplained gamma ray signal, although Anchordoqui cautions that the interpretation remains highly uncertain.
Neither primordial black holes nor the hidden fifth dimension have yet been detected, but future observations could test this idea. Finding evidence of these primordial black holes as predicted could offer a rare glimpse into the early universe and potentially reveal a hidden dimension of space.
