Natural Science Forum / Physics / New Theories / June 2004

Details of the Ekpyrotic Universe theory

Authors: (Justin Khoury, Princeton; Burt Ovrut, UPenn; Paul Steinhardt,
Princeton and Neil Turok, Cambridge):

Our paper proposes a new theory of the very early universe that resolves the
famous puzzles of the hot Big Bang picture -- the horizon, flatness and
monopole problems -- and that generates fluctuations in energy that seed
galaxy formation and produce temperature variations in the cosmic microwave
background. The model is based on the idea that our hot Big Bang universe
was created from the collision of two three-dimensional worlds moving along
a hidden, extra dimension.

The inflationary model of the universe developed in the 1980's by Alan Guth
(MIT), Andre Linde (Stanford), Andreas Albrecht (UC Davis) and Steinhardt
was designed to resolve these very same problems, relying on a period of
exponential hyper-expansion, or inflation.

Conceptually, the Ekpyrotic model is very different. There is no inflation
or rapid change happening at all. The approach to collision takes places
very slowly over an exceedingly long period of time. It is quite fascinating
that rapid change and very slow change can produce nearly the same effects.

The difference results in one distinctive observational prediction, though:
Inflationary cosmology predicts a spectrum of gravitational waves that may
be detectable in the cosmic microwave background. The Ekpyrotic model
predicts no gravitational wave effects should be observable in the cosmic
microwave background.

In the Ekpyrotic model, when the two three-dimensional worlds collide and
"stick," the kinetic energy in the collision is converted to the quarks,
electrons, photons, etc. that are confined to move along three dimensions.
The resulting temperature is finite, so the hot Big Bang phase begins
without a singularity. The universe is homogeneous because the collision and
initiation of the Big Bang phase occurs nearly simultaneously everywhere.

The energetically preferred geometry for the two worlds is flat, so their
collision produces a flat Big Bang universe. According to Einstein's
equations, this means that the total energy density of the universe is equal
to the critical density. Massive magnetic monopoles, which are
over-abundantly produced in the standard Big Bang theory, are not produced
at all in this scenario because the temperature after collision is far too
small to produce any of these massive particles.

Quantum effects cause the incoming three-dimensional world to ripple along
the extra-dimension prior to collision so that the collision occurs in some
places at slightly different times than others. By the time the collision is
complete, the rippling leads to small variations in temperature, which seed
temperature fluctuations in the microwave background and the formation of
galaxies. We have shown that the spectrum of energy density fluctuations is
scale-invariant (the same amplitude on all scales). The production of a
scale-invariant spectrum from hyper-expansion was one of the great triumphs
of inflationary theory, and here we have repeated the feat using completely
different physics.

The building blocks of the Ekpyrotic theory are derived from Superstring
theory. Superstring theory requires extra dimensions for mathematical
consistency. In most formulations, 10 dimensions are required. In the mid
1990s, Petr Horava (Rutgers) and Ed Witten (IAS, Princeton) argued that,
under certain conditions, an additional dimension opens up over a finite
interval. Six dimensions are presumed to be curled up in a microscopic ball,
called a Calabi-Yau manifold.

The ball is too small to be noticed in everyday experience, and so our
universe appears to be a four-dimensional (three space dimensions and one
time dimension) surface embedded in a five-dimensional space-time. This
five-dimensional theory, called heterotic M-theory, was formulated by Andre
Lukas (Sussex). Ovrut and Dan Waldram (Queen Mary and Westfield College,
London). According to Horava-Witten and heterotic M-theory, particles are
constrained to move on one of the three-dimensional boundaries on either
side of the extra dimensional interval.

Our visible universe would be one of these boundaries; the other boundary
and the intervening space would be hidden because particles and light cannot
travel across the intervening space. Only gravity is able to couple matter
on one boundary to the other sides. In addition, there can exist other
three-dimensional hyper-surfaces in the interval, which lie parallel to the
outer boundaries and which can carry energy.

These intervening planes are called branes. The collision that ignites the
hot Big Bang phase of the Ekpyrotic model occurs when a three-dimensional
brane is attracted to and collides into the boundary corresponding to our
visible universe.

The term ekpyrosis means "conflagration" in Greek, and refers to an ancient
Stoic cosmological model. According to the model, the universe is created in
a sudden burst of fire, not unlike the collision between three-dimensional
worlds in our model. The current universe evolves from the initial fire.
However, in the Stoic notion, the process may repeat itself in the future.
This, too, is possible in our scenario in principle if there is more than
one brane and, consequently, more than one collision. We plan to discuss
this possibility in future work, along with further speculations about what
preceded the collision that made our present universe.

As a final remark, we feel that it is important to realize that Inflationary
theory is based on Quantum Field theory, a well-established theoretical
framework, and the model has been carefully studied and vetted for 20 years.
Our proposal is based on unproven ideas in String theory and is brand new.
While we appreciate the enthusiasm and interest with which the paper has
been received, we would suggest some patience before promulgating these
ideas in order to leave time for us to produce some follow-up papers that
introduce additional elements and to allow fellow theorists time for
criticism and sober judgment.

Spud