Exploring the fourth dimension

A team of American scientists think that our universe has more than four dimensions. Their new theory of gravity is challenging Einstein's general relativity. They think that the discovery of hidden "black holes" in the "dark matter" of our universe can be done by satellites launched in a near future and well inside Pluto's orbit.

Does our universe have more than four dimensions -- four spatial dimensions plus time? According to some American scientists, the answer is yes. Their new theory of gravity is challenging Einstein's general relativity. They think that the discovery of hidden "black holes" in the "dark matter" of our universe can be done by satellites launched in a near future and well inside Pluto's orbit. NASA's Gamma-ray Large Area Space Telescope, scheduled to be launched in August 2007, might prove -- or not -- their theory. But read more...

As this work is based on a theory named the braneworld gravity model, let's start with some definitions with the help of Wikipedia. First, what is a brane?

In theoretical physics, branes or p-branes are spatially extended objects that appear in string theory and its relatives (M-theory and brane cosmology). The variable p refers to the dimension of the brane. That is, a 0-brane is a zero-dimensional particle, a 1-brane is a string, a 2-brane is a "membrane", etc.

The braneworld theory states that extra dimensions exist all around us. But brane cosmology is not yet considered as a science today, only a protoscience.

The central idea is that our visible, four-dimensional universe is entirely restricted to a brane inside a higher-dimensional space, called the bulk. The additional dimensions may be taken to be compact, in which case the observed universe contains the extra dimensions, and then no reference to the bulk is appropriate in this context.

If you're still following, let's jump to the Duke University news release.

Charles R. Keeton of Rutgers and Arlie O. Petters of Duke base their work on a recent theory called the type II Randall-Sundrum braneworld gravity model. The theory holds that the visible universe is a membrane (hence "braneworld") embedded within a larger universe, much like a strand of filmy seaweed floating in the ocean. The "braneworld universe" has five dimensions -- four spatial dimensions plus time -- compared with the four dimensions -- three spatial, plus time -- laid out in the General Theory of Relativity.

Below is a photo of Arlie Petters posing with a model of the solar system (Credit: Duke University).

Arlie Petters with a model of the solar system

And here is one of the major differences between the braneworld theory and Einstein's theory of relativity.

The braneworld theory predicts that relatively small "black holes" created in the early universe have survived to the present. The black holes, with mass similar to a tiny asteroid, would be part of the "dark matter" in the universe. As the name suggests, dark matter does not emit or reflect light, but does exert a gravitational force.
The General Theory of Relativity, on the other hand, predicts that such primordial black holes no longer exist, as they would have evaporated by now. "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," Keeton said.

If this is true, this would probably surprise lots of physicists. But how can these scientists find these hidden black holes?

Petters and Keeton said it should be possible to measure the predicted gamma-ray fringe patterns using the Gamma-ray Large Area Space Telescope [GLAST,] which is scheduled to be launched on a spacecraft in August 2007. The telescope is a joint effort between NASA, the U.S. Department of Energy, and institutions in France, Germany, Japan, Italy and Sweden.

In the mean time, you can read the articles published by the researchers in Physical Review D under the name "Formalism for testing theories of gravity using lensing by compact objects" in three parts. Here are the links to the abstracts of these papers, Part I: Static, spherically symmetric case (Vol. 72, Issue 10, Id. 104006, November 4, 2005), Part II: Probing post-post-Newtonian metrics (Vol. 73, Issue 4, Id. 044024, February 21, 2006) and Part III: Braneworld gravity (Vol. 73, Issue 10, Id. 104032, May 24, 2006).

Finally, if you have a solid background in mathematics and in physics, here is a link to the full text of the third article (PDF format, 13 pages, 303 KB).

Sources: Duke University news release, via EurekAlert!, May 25, 2006; and various web sites

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