Could the vast universe in which we live actually be the interior of a black hole within a larger universe? The idea, which sounds as if it comes from the realm of science fiction, actually arises from certain paradoxes in modern physics.
Black holes, formed from the explosion of massive star collapses, are areas where the gravitational pull is so strong that no light can escape. The classical view of black holes from general relativity is that of absolute prisons: “For an external observer, when a material body crosses an event horizon all knowledge of its material properties is lost. Only the new values of M, J, and Q remain. As a result, a black hole swallows an enormous amount of information.” as described by French astrophysicist Jean-Pierre Luminet. Black holes are described in greater detail below:
However, the situation becomes more complicated with the advent of thermodynamics principles. Black holes, as a result of their mass, would also require a temperature. Consequently, through the law of thermodynamics, black holes should radiate. It was a major discovery made by physicist Stephen Hawking in the 1970s. Black holes undergo a process where they emit a low-grade heat radiation referred to as Hawking radiation. This process raises a paradox. The evaporation of the black hole is due to radiation. This process raises a paradox because the radiation emitted from the black hole carries no information about what went into it.
But to remove this paradox, the holgraphic principle, coined by Gerard ‘t Hooft and later by Leonard Susskind, came up with the revolutionary idea that the number of degrees of freedom for the whole black hole is proportional to the area, not the volume, of the event horizon. According to the holgraphic principle, the information about the infalling matter, encoded in the two-dimensional surface of the event horizon, is like the information on a hologram. The Bekenstein-Hawking entropy would arise naturally this way, and information would, in theory, be available during the evaporation.
This holographic coding led to speculations of the universe’s own behavior, perhaps mimicking that of a black hole, where all processes can be described in terms of a two-dimensional boundary. Perhaps our own universe might be the interior of a “parent” black hole in a higher-dimensional universe, where the Big Bang might represent the ‘point of collapse’ in the larger universe. Also, the gravity in our universe could somehow ‘emerge’ from the entanglement entropy of the boundary, an idea developed in entropic gravity theories, where the equations of Einstein’s gravity emerge from principles of quantum information.
One interesting numerical coincidence that supports this conjecture is the fact that the Hubble radius for our observable universe is astonishingly close to the Schwarzschild radius for a black hole with the same mass-energy. As Sean Carroll has pointed out, the equality expresses the fact that our universe has a flat geometry, as the Friedmann equation predicts and as is well-known in standard cosmology, but the coincidence supports the black hole interior analogy nonetheless.
Recent breakthroughs in the theory of semiclassical gravity have introduced further complexity. Work based on the entanglement entropy analysis coined from Don Page’s work on the subject commonly referred to as the ‘Page curve’ has shown how information actually does escape black holes without requiring a complete quantum gravity theoretical understanding. This is achieved by the existence of new ‘gravitational configurations’ described by Einstein’s equations that enable “islands” beyond the horizon to communicate with the outside universe. This corresponds well with the wormhole-based escape strategies that indicate the emergent nature of space time itself.
Cosmically, if the spacetime of our universe is emergent, its large-scale structure may be determined by the same informations principles that determine the behavior of black holes. According to the entropic gravity theory proposed by Ginestra Bianconi, the curvature of spacetime is due to the quantum relative entropy between the “geometry” of spacetime and the induced geometry due to matter fields. This theory, besides correctly implementing Einstein’s equations, also predicts a small positive value of the cosmological constant and a G-field, which can mimic the role of dark matter.
The holographic principle, the mechanism of wormhole-mediated escaping of information, and the concept of entropic gravity all suggest one and the same thing: Information is the foundation of the universe, and the geometry of the universe could well emerge as the manifestation of the ordering of the said foundation. If so, the boundary of our universe would hold the details of the universe within, much like the boundary of the black hole called the event horizon.
