We’re familiar with four dimensions, three spatial and one temporal. But is there a fourth dimension to space? If so, it implies a new way of looking at gravity. So say physicists Lisa Randall (Harvard University) and Raman Sundrum (Johns Hopkins), who have offered a mathematical description of how gravity’s actual effects might differ from those predicted by Einstein’s General Theory of Relativity. That fourth spatial dimension follows from the theory these two have developed called the type II Randall-Sundrum braneworld gravity model. It suggests that the universe is a membrane, or ‘braneworld,’ embedded within a much larger universe.
Centauri Dreams admires robust theorizing but has always hoped to see solid observational clues that would make such hypotheses testable. And it may be that one has now emerged, in the hands of Charles Keeton (Rutgers) and Arlie Petters (Duke University), who used the Randall-Sundrum model to predict certain cosmological effects that could provide answers, effects that may be susceptible to testing via satellites scheduled for launch within the next few years.
For the braneworld model would have notable consequences. The hypothesis predicts that small black holes from the early universe — with a mass similar to that of a small asteroid — would have survived to the present. Such objects would be part of the dark matter that seems to pervade the universe, exerting gravitational force but reflecting or emitting no light. General Relativity says that such primordial black holes would have evaporated away by now, so finding them would make the braneworld hypothesis tenable.
It would also change our view of nearby space. For remarkably, if such black holes do exist, they may be close. Says Keeton: “When we estimated how far braneworld black holes might be from Earth, we were surprised to find that the nearest ones would lie well inside Pluto’s orbit.” And Petters makes an even more mind-boggling statement:
“If braneworld black holes form even 1 percent of the dark matter in our part of the galaxy — a cautious assumption — there should be several thousand braneworld black holes in our solar system.”
So the object is to look for the effects that these braneworld black holes would exert on electromagnetic radiation coming to Earth from other galaxies. Any such radiation would be subject to gravitational lensing if it came near a black hole. One good place to start is with gamma ray bursts, whose path would be impeded by a black hole to produce an interference pattern. The scientists have worked out the bright and dark ‘fringes’ in the interference pattern that would result; these would offer information on the characteristics of the black holes, and by inference just might change our notions of space and time.
As to missions, the Gamma-Ray Large Area Space Telescope (GLAST), scheduled for launch next summer, may be able to measure such interference patterns. So here is a case where an exotic theory may actually be put to an observational test, and much sooner than many of us would have thought possible. The paper, which appeared in the May 24 online edition of Phyical Review D, is Keeton and Petters, “Formalism for testing theories of gravity using lensing by compact objects. III: Braneworld gravity,” available here.