“
I AM a heretic,” Cristiano Germani announced to an audience of osmologists 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.
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.