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The Big Bang scenario with inflation is
the prevailing standard model of cosmology, the theory that best accounts for
the origin and evolution of the universe.
It describes the early universe as arising from a singularity that at
first expanded exponentially at super-light speeds, then slowed to the more
moderate expansion that we presently observe.
It has some problems that will be described below, but it is the best
theory we have, and it fits the observations.
However, in the past year a new
alternative to inflationary Big Bang cosmology has been proposed that is
receiving considerable attention. It is
called "ekpyrotic cosmology", and it describes an early universe in
which there was no Big Bang at all, but instead a collision of
higher-dimensional "branes".
This new cosmology uses as its starting
point recent work on millimeter-scale gravity (see my column #98, "Millimeter
Gravity and the Superstring Wall", in Analog,
December-1999) that comes from "M-theory" (what the physics community
used to call "superstring theory").
The millimeter-scale gravity ideas describe each 4-space location in our
universe as confined to a thin extra-dimensional "D-brane wall" on
which the strong, weak, and electromagnetic interactions are confined, while
the gravitational interaction is allowed to expand away from this wall in two
or more extra dimensions. Gravity, in
this picture, is weak because most of its lines of force are dissipated into
these extra dimensions. The other three
forces, because they are confined to the D-brane wall, are not similarly
weakened. Here the word
"brane" is derived from the word “membrane” and means a thin
membrane-like planar structure embedded in a multidimensional space.
This new ekpyrotic cosmology
derives its name from the ancient Greek stoic philosopher’s notion that the
universe was cyclically destroyed and re-created by fire. It suggests that our universe originated
from the fiery collision of two of these “branes”. In other words, our universe, which perhaps had been cold and
featureless for an indefinite time, was struck by another brane, like two hands
coming together to produce a Big Clap.
The effect of this collision of branes was to produce the energy,
matter, and structure that we find in the present universe. In this scenario, the universe did not begin
with an infinitely hot Planck-scale singularity. Instead, it started at a finite size and temperature that was
initially static, but after the collision it expanded.
To understand the implications of these
new ideas, let’s begin with a review of what is presently the “standard model”
of cosmology, the inflationary Big Bang scenario. The simple Big Bang model describes our universe as having
exploded from a “singularity”, a point-like region of space that was
supersaturated with energy, forming an ultra-hot ultra-dense medium in which
the gravity was so strong that it curved space back on itself in a distance of
about 10-34 m. As the
universe became larger it cooled from its ultra-dense ultra-hot origins. The
fundamental interactions sorted themselves out into the strong, weak,
electromagnetic and gravitational forces. The resulting mixed soup of matter, neutrinos, and radiation cooled and
separated, and the components went their separate ways. The radiation component we see today as the
cosmic microwave background. The matter
congealed into dust clumps that became galaxies. Stars formed, exploded in supernova violence, and formed again,
repeatedly recycling matter into heavier elements. Planets formed around some of the stars and became infested with
life. We came along very late in the
game to try to piece together all that had previously happened. The result is what we call the Big Bang.
However,
this simple version of the Big Bang model has a number of problems which are
called the problems of homogeneity, flatness, inhomogeneity,
and monopoles. For example, a
hypothetical observer looking at the sky immediately after the Big Bang would
see the horizon of the visible universe (the distance at which light is Doppler
shifted to zero energy) only about 10-34 m away. Immediately after the Big Bang, each spatial
region of this size became causally disconnected from the many other similar
regions. Today, however, the horizon of
the visible universe combines 1090 of these disconnected regions,
some of which are only now coming into causal contact with the rest of our universe. There is no particular reason why these 1090
regions should resemble one another at all, yet we know from the COBE microwave
background measurements they vary at most by one part in 100,000. The remarkable smoothness of the universe is
a fundamental mystery. Why are the 1090
independent pieces of today's universe so similar? This is the homogeneity problem.
The
flatness problem is raised by the remarkable lack of curvature, either
positive or negative, of the present universe.
There is a nearly precise balance between the expansion energy and the
gravitational pull on the universe.
Gravity and expansion are within about 1% of perfect balance. The inhomogeneity problem concerns
the origins of the structure observed in the cosmic microwave background
radiation and in the large-scale structure of the universe. The monopole problem concerns the
observed absence of magnetic monopoles in our universe, when they should have
been produced in great numbers in the early Big Bang.
The
currently accepted solution for these problems, the "inflation
scenario", assumes that in the very early stages of the Big Bang, for
reasons not well understood, the universe expanded at an exponentially
increasing rate, with the radius growing much faster than the speed of light. The problem with the inflation scenario is
that while it cures the ills of simple Big Bang cosmology, it seems rather
contrived and raises unresolved questions of its own. (See my column #94, "Before
the Big Bang", in Analog, March-1999).
The
problem with the inflationary scenario is that it is "put in by
hand". In other words, it is
inserted into the theory without explanation or physical mechanism in order to
solve certain problems. The origin of
the enormous force that produced the exponential expansion is not explained,
nor is the requirement that it operated for a time and then stopped. Further, the inflationary Big Bang model
compels us to try to understand the laws of physics at the Planck Scale near
the initial singularity, where energy densities are so enormous that we have no
usable theories and no potential experiments.
The
ekpyrotic scenario provides an interesting alternative to inflation. It describes our universe as a "visible
brane", the D-brane hyper-surface that we live in and experience. It also hypothesizes the existence of a
nearby “hidden brane”, another universe on a D-brane that is parallel to ours but
separated at a constant distance from our universe across two or more of the
extra dimensions. Initially, perhaps
for a very long time, the visible brane remains cold, static, and empty. At some time, however, the hidden brane
sloughs off a lighter “bulk brane” which travels across the extra dimensional
separation and violently collides with our visible brane. There are ripples in the bulk brane, so that
the collision happens at slightly different times in different regions of the
visible brane. There are gravitational
and other forces acting between the branes before and after collision, and
these cause the length scale in the visible brane to contract before the
collision and expand after the collision. This shrinking, collision, and expansion produced the expanding universe
that we presently observe.
The
colliding branes are initially flat (in the sense of space curvature,) and this
flatness is retained by our universe after the collision. The ripples in the bulk brane produced the
balance between homogeneity and large-scale structure that we presently
observe. And the ekpyrotic universe,
even at the instant of the Big Clap, was never hot enough or close enough to a
singularity to produce the flood of monopoles that the simple Big Bang model
predicts. Therefore, the ekpyrotic
scenario deals with all of the problems of the simple Big Bang scenario without
invoking inflation. It has the
advantage that no Planck-scale physics or mysterious forces that switch on and
off are involved. It becomes identical
to the Big Bang model at some extremely high temperature, so that the
subsequent evolution of the universe, in particular the differentiation of the
four forces, the synthesis of light elements, and the production of the
background radiation, is the same in both models.
The
crucial test of any theory is whether it makes predictions that can be tested
experimentally. In the present case,
the question is whether there are tests that can distinguish inflationary
cosmology from ekpyrotic cosmology. The
answer seems to be "perhaps".
The main differences between the models are in their production of
primordial gravity waves. Inflation
tends to produce a "red" gravity-wave spectrum that decreases in
strength with decreasing wavelength, while ekpyrotic cosmology produces a
"blue" gravity-wave spectrum that increases in strength with
decreasing wavelength. Therefore, a
study of the primordial spectrum of gravity waves would be a crucial test of
the models.
Unfortunately,
theses effects are too small to be detected by the NSF's LIGO gravity detector
that is now coming into operation in Louisiana and Washington State, or by the
space-based LISA detector project planned by the European Space Agency. The remaining possibility is the detection
of optical polarization in the cosmic microwave background radiation induced by
long wavelength gravity-wave effects.
If such polarization were detected, it would tend to support
inflationary cosmology and to falsify ekpyrotic cosmology. However, no such polarization has yet been
observed.
Finally,
since this is a science fiction magazine, let's consider the science-fictional
applications and implications of this new cosmology. If our universe was indeed produced by a collision with a
traveling brane in a Big Clap, there's no particular reason why that would
happen only once. That has the makings
of a universe-scale disaster scenario, with unexpected forces first detected
weakly, then building up as the bulk brane approaches our universe through the
extra dimensions. The Clap when the
branes collide would restart the Big Bang and bring an end to all life and
civilizations that did not have an extra-dimensional escape hatch.
Another
scenario might involve travel to another brane, a parallel universe that is
separated from ours only by a small distance in the extra dimensions (as in my
hard SF novel Twistor.)
Are there many of these brane universes? Would each brane-universe resemble ours, or would it be
different? Are its laws of physics the
same? Is it dominated by matter (as is
ours) or by antimatter (a convenient energy source if one could fish some
antimatter across the inter-dimensional gap)? Does time run in the same direction?
Is the speed of light the same, or can one go faster over there and then
return to our universe at a different location? Do the other branes contain stars? Planets? Life? Civilizations?
All
of these questions depend on whether the Big Clap of ekpyrotic cosmology and
M-theory actually describe our universe.
Stay tuned for further developments.
John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.
SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.
Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: https://www.npl.washington.edu/av .
References:
Ekpyrotic Cosmology:
"The Ekpyrotic Universe: Colliding Branes and the Origins of the Hot Big Bang", J. Khoury, B. O. Ovrut, P. J. Steinhardt, and N. Turok, preprint hep-th/0103239 v3
"From Big Crunch to Big Bang", J. Khoury, B. O. Ovrut, N. Seiberg, P. J. Steinhardt, and N. Turok, preprint hep-th/0108187 v3.
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