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Theory that smashup might end
universe draws support
June 6, 2005
Special to World Science
Cosmologists have come up with some strange end-of-universe scenarios over the past decades, including proposals that
our cosmos might shrink to an minuscule point or burst to bits as every atom explodes at
once.
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A hypothesis that our universe might end in
a cosmic smashup is drawing increasing support among cosmologists. Some of
them hold that the universe might be a cyclic phenomenon, in which
bouncing "branes" -- parallel parts of spacetime -- periodically
collide, destroy everything, recreate themselves and start moving apart
again, as the above schematic diagram shows. (Credit: Lawrence Berkeley
National Laboratory, U.S.A..)
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Now, growing numbers of researchers are leaning toward a new picture: the end could come as the universe we know
slams into
another part of the universe that’s invisible to us.
Three theorists published a calculation this spring showing that this outcome is a possible consequence of string theory—an advanced, speculative model of
nature’s fundamental laws, widely seen as the closest thing science has
to a “theory of everything.”
Some other researchers agree the smashup is possible.
In fact, physicist Paul Steinhardt of Princeton University, N.J., described the new calculation as
potentially “a more rigorous and complete” argument for
a proposal he has also advanced, with Neil Turok of Cambridge University, U.K. Their model, however, also claims the cosmos will be reborn at the instant of its
demise, an issue the newer paper doesn’t address.
In any case, the cataclysm probably won’t happen before another 300 billion years or so, Steinhardt added—20 times the present age of our universe.
The scenario envisions that our cosmos will crash into a vast world of space and time that we can’t see or touch from our own visible universe, but that lies next to it, less than an atom’s width away.
This untouchable zone is sometimes called a parallel or twin universe, although Steinhardt says it’s best considered as part of our own universe, as it’s not quite sealed off from
us. The two worlds interact through gravity, according to the theory. Except for this connection,
though, they are separated by dimensions we can’t enter.
Gary Gibbons of Cambridge University, U.K., and two colleagues described in the April 8 issue of the research journal
Physical Review Letters the way these two regions could end in a “mutual annihilation” as they make contact.
A strange road for strings
The idea that these separate zones exist stems from string theory, the provisional “theory of everything” that originated in 1970.
String theory gained wide currency because it reconciled two pictures of reality that seemed to explain different phenomena with remarkable accuracy, but conflicted with each other. These models, Einstein’s general relativity and the Standard Model of particle physics, describe nature’s fundamental structure over vast cosmic distances and subatomic ones, respectively.
String theory neatly resolved the discrepancies, but created a strange picture
of the universe in the process.
The theory holds that reality ultimately consists of sub-microscopic
strings whose different possible ways of vibrating are the hidden mechanism
behind the many types of particles that make up the universe. But the model works
only if one assumes the strings have several extra dimensions of space in which to
vibrate, beyond the three familiar dimensions. Thus space itself would contain
extra dimensions.
Theorists have traditionally explained the invisibility of the extra dimensions through the rather stupefying idea they are crumpled into
tiny balls at each point in space, big enough to affect the strings but not to
reveal themselves to us.
String theory has drawbacks, the biggest probably being that many physicists
believe it can’t be verified by any test. But its explanatory power has kept it
alive anyway. It has in fact blossomed into a range of different versions.
Complicating the story further, some of these variants have turned out to be
alternative ways of saying the same thing.
Of the different variants of string theory under consideration today, some reduce the need for tiny dimensions by
providing a new explanation of why some dimensions are invisible. In this
proposal, everything we see is trapped in a three-dimensional space, other dimensions
being outside this zone.
Our universe is related to the “extra” dimensions much as a two-dimensional surface, or membrane, would be related to a three-dimensional region within which it existed. For this reason, our visible universe in this scenario is called a “brane” (for membrane).
Branes are in fact the counterparts, in higher dimensions, of the “strings.”
Clash of
the branes
The so-called “brane-world” hypothesis is the starting point for the notion of colliding branes because some theorists believe there may exist one or more branes besides our own. Another of these could exist parallel to ours, and tantalizingly close—a
subatomic distance away, yet untouchable.
Steinhardt and Turok proposed in 2002 that the distance between branes may periodically expand and contract, with branes colliding between each cycle. The Big Bang explosion that gave birth to the universe an estimated 14 billion years ago, some physicists think, may have been a outcome of one such collision.
However, past theoretical studies of brane collisions “have been restricted to approximations,” Gibbons and colleagues wrote in their April paper.
They set out to change this, using a mathematical strategy borrowed from David Kastor and Jennie Traschen of the University of
Massachusetts, who had developed it to study black hole collisions in a 1993 paper. This methodology traced an ancestry back to Einstein’s field equations—formulas that describe how physical objects distort space and time around them, resulting in the effect we call gravity.
Kastor and Traschen’s technique lets the behavior of these colliding objects “be studied exactly,” Gibbons and colleagues wrote, and can also be extended to branes.
Their analysis found that branes will gradually move toward each other. At some point during the history of the universe, a singularity—a point where the laws of physics break down—will appear on one brane.
This singularity will then move toward the neighboring brane, with the result that the branes collide, the physical laws of both crumble, and they disappear. “This is the way the brane world ends,” they wrote, “not with a whimper but a bang.”
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Reference
G.W. Gibbons, H. Lu, C.N. Pope., 2005. Brane worlds in collision. Phys. Rev. Lett. 94,
131602.
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