Wednesday, 5 September 2007

'Heretical' cosmologist does away with big bang theory

reposted from NS

'Heretical' cosmologist does away with big bang theory

  • 05 September 2007
  • news service
  • Zeeya Merali

"I AM a heretic," Cristiano Germani announced to an audience of cosmologists last month. Few would disagree, as he is proposing

a radical alternative to standard cosmology: a universe with no big bang creation moment, and no rapid inflation. Rather than a big bang, he suggests a slingshot.

In the early 1980s, Alan Guth at the Massachusetts Institute of Technology proposed that our universe underwent inflation - a period of rapid expansion in the first 10-34 seconds after the big bang. Germani, a cosmologist at the International School of Advanced Studies in Trieste, Italy, says that inflation is beautiful and successful, yet he insists that we need to replace it.

"We don't have any fundamental physical explanation for how or why it occurred,"
he says.
"Yet cosmologists today accept it as though it is a religion."

Germani's alternative, unveiled at a cosmology conference at the University of Sussex, UK, last month, is based on a string-theory model in which the three visible dimensions of space are confined to the surface of a membrane, or brane, floating in a 10-dimensional space. The extra dimensions are wrapped up into a complex shape known as a Calabi-Yau space (see Illustration). The forces and particles in our 3D world are shadows of the motion of branes and strings in the Calabi-Yau space.

The problem with the simplest versions of this model is that the Calabi-Yau space is unstable, constantly vibrating and changing size. Each wobble of the surface creates unwanted particles and extra forces in the universe - none of which have ever been observed. Attempts by string theorists to stabilise the space always warp it, forcing strange spikes and throats to pop out, Germani says. This warping, he believes, is the key to explaining the evolution of our universe.

Germani and his colleagues examined what would happen if a brane containing our universe fell down one of these throats. At first things looked bleak: the universe dropped like a stone, getting squeezed until it was crushed at the tip of the throat, corresponding to a big crunch in which the universe collapses in on itself.

But then Germani considered a spinning universe. "In fact, it is much more realistic that the universe will be rotating as it drops," he says. Something more interesting happens to a rotating universe as it hurtles down the throat. Because it is spinning, it avoids falling into the tip of the throat and whirls round it instead.

Like a boomerang or a stone from a slingshot, it then flies back up again. Germani realised that the second leg of this journey could correspond to the expanding universe we observe today.

Other cosmologists have suggested that our universe went through a superficially similar cycle of big bangs and big crunches. Germani's slingshot mechanism is different from these because it never sends the universe through a big bang singularity. As a result,

the model can solve the so-called "horizon problem" without resorting to inflation.

The horizon problem runs like this. No matter where you look in the universe, the background temperature is about the same, but not enough time has elapsed since the big bang for radiation to travel across the universe and back, exchanging temperature information. Inflation solves this problem because regions of space which sit on opposite sides of the visible universe today could once have been close together, and been blown far apart during inflation.

With the slingshot picture, there is no big bang and so no horizon problem. "We have no beginning of time, so the universe is easily old enough for regions on both sides of the sky to have been in contact in the past," Germani says.

"In the slingshot scenario we could have an ever-existing universe." His team's calculations also show that the apparently finely tuned density of today's universe arises naturally using the slingshot, though inflation is also able to account for this.

We have no beginning of time. In the slingshot scenario we could have an ever-existing universe

Last year,

support for inflation was bolstered by measurements of the pattern of cold and hot spots in the cosmic microwave background (CMB) made by the Wilkinson Microwave Anisotropy Probe, which seem to fit perfectly with the predictions of inflation.
When Germani calculated how temperature imprints would develop in his slingshot universe, he found that they also matched the data. Germani and his colleagues are now working out
what signatures in the CMB could distinguish it from inflation
, in the hope that they might turn up when the European Space Agency's Planck satellite begins more detailed measurements in 2008.

Cosmologist Paul Frampton at the University of North Carolina, Chapel Hill, likes the idea. "They have solved the key problems that inflation solves and have good agreement with the latest observations," he says.

String theorist Damien Easson at the University of Durham, UK, agrees that inflation needs an explanation based on fundamental physics. However, he does not see the cosmological slingshot model as the answer. "It's extremely controversial to claim to have found an alternative to one of the most respected theories in cosmology," he says.

Easson points out that string-theory models usually represent the universe as a stack of branes in the non-warped region of the Calabi-Yau space, and have successfully used this to explain why we see the forces and particles that we do. "It's difficult to see how this can be achieved if our universe is flying down the throat," he says.

Germani accepts that his model still needs work, but he believes that he will eventually meet this challenge. "Remember, inflation theory has been around for more than 20 years, while my theory is still young," he says.

From issue 2620 of New Scientist magazine, 05 September 2007, page 12-13

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