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How Our Universe Could Emerge as a Hologram

The fabric of space and time is widely believed by physicists to be emergent, stitched out of quantum threads according to an unknown pattern. And for 22 years, they've had a toy model of how emergent space-time can work: a theoretical "universe in a bottle," as its discoverer, Juan Maldacena, has described it.

The space-time filling the region inside the bottle—a continuum that bends and undulates, producing the force called gravity—exactly maps to a network of quantum particles living on the bottle's rigid, gravity-free surface. The interior "universe" projects from the lower-dimensional boundary system like a hologram. Maldacena's discovery of this hologram has given physicists a working example of a quantum theory of gravity.

But that doesn't necessarily mean the toy universe shows how space-time and gravity emerge in our universe. The bottle's interior is a dynamic, Escheresque place called anti–de Sitter (AdS) space that is negatively curved like a saddle. Different directions on the saddle curve in opposite ways, with one direction curving up and the other curving down. The curves tend toward vertical as you move away from the center, ultimately giving AdS space its outer boundary—a surface where quantum particles can interact to create the holographic universe inside. However, in reality, we inhabit a positively curved "de Sitter (dS) space," which resembles the surface of a sphere that's expanding without bounds.

Ever since 1997, when Maldacena discovered the AdS/CFT correspondence — a duality between AdS space and a "conformal field theory" describing quantum interactions on that space's boundary—physicists have sought an analogous description of space-time regions like ours that aren't bottled up. The only "boundary" of our universe is the infinite future. But the conceptual difficulty of projecting a hologram from quantum particles living in the infinite future has long stymied efforts to describe real space-time holographically.

[...] Patrick Hayden, a theoretical physicist and computer scientist at Stanford who studies the AdS/CFT correspondence and its relationship to quantum error correction, said he and other experts are mulling over Dong, Silverstein and Torroba's dS/dS model. He said it's too soon to tell whether insights about how space-time is woven and how quantum gravity works in AdS space will carry over to a de Sitter model. "But there's a path—something to be done," Hayden said. "You can formulate concrete mathematical questions. I think a lot is going to happen in the next few years."

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