A new theory of the universe suggests that space and time
may not have begun in a big bang, but may have always
existed in an endless cycle of expansion and rebirth.

Princeton physicist Paul Steinhardt and Neil Turok of
Cambridge University described their proposed theory in an
article published April 25 in an online edition of

The theory proposes that, in each cycle, the universe
refills with hot, dense matter and radiation, which begins a
period of expansion and cooling like the one of the standard
big bang picture. After 14 billion years, the expansion of
the universe accelerates, as astronomers have recently
observed. After trillions of years, the matter and radiation
are almost completely dissipated and the expansion stalls.
An energy field that pervades the universe then creates new
matter and radiation, which restarts the cycle.

The new theory provides possible answers to several
longstanding problems with the big bang model, which has
dominated the field of cosmology for decades. It addresses,
for example, the nagging question of what might have
triggered or come “before” the beginning of time.

The idea also reproduces all the successful explanations
provided by standard picture, but there is no direct
evidence to say which is correct, said Steinhardt, a
professor of physics. “I do not eliminate either of them at
this stage,” he said. “To me, what’s interesting is that we
now have a second possibility that is poles apart from the
standard picture in many respects, and we may have the
capability to distinguish them experimentally during the
coming years.”

The big bang model of the universe, originally suggested
over 60 years ago, has been developed to explain a wide
range of observations about the cosmos. A major element of
the current model, added in the 1980s, is the theory of
“inflation,” a period of hyperfast expansion that occurred
within the first second after the big bang. This
inflationary period is critical for explaining the
tremendous “smoothness” and homogeneity of the universe
observed by astronomers, as well as for explaining tiny
ripples in space that led to the formation galaxies.

Scientists also have been forced to augment the standard
theory with a component called “dark energy” to account for
the recent discovery that the expansion of the universe is

The new model replaces inflation and dark energy with a
single energy field that oscillates in such a way as to
sometimes cause expansion and sometimes cause stagnation. At
the same time, it continues to explain all the currently
observed phenomena of the cosmos in the same detail as the
big bang theory.

Because the new theory requires fewer components, and
builds them in from the start, it is more “economical,” said
Steinhardt, who was one of the leaders in establishing the
theory of inflation.

Another advantage of the new theory is that it
automatically includes a prediction of the future course of
the universe, because it goes through definite repeating
cycles lasting perhaps trillions of years each. The big
bang/inflation model has no built-in prediction about the
long-term future; in the same way that inflation and dark
energy arose unpredictably, another effect could emerge that
would alter the current course of expansion.

The cyclic model entails many new concepts that Turok and
Steinhardt developed over the last few years with Justin
Khoury, a graduate student at Princeton, Burt Ovrut of the
University of Pennsylvania and Nathan Seiberg of the
Institute for Advanced Study.

“This work by Paul Steinhardt and Neil Turok is
extraordinarily exciting and represents the first new big
idea in cosmology in over two decades,” said Jeremiah
Ostriker, professor of astrophysics at Princeton and the
Plumian Professor of Astronomy and Experimental Philosophy
at Cambridge.

“They have found a simple explanation for the observed
fact the universe on large scales looks the same to us left
and right, up and down — a seemingly obvious and natural
condition — that in fact has defied explanation for

Sir Martin Rees, Royal Society Research Fellow at
Cambridge, noted that the physics concerning key properties
of the expanding universe remain “conjectural, and still not
rooted in experiment or observation.”

“There have been many ideas over the last 20 years,” said
Rees. “Steinhardt and Turok have injected an imaginative new
speculation. Their work emphasizes the extent to which we
may need to jettison common sense concepts, and transcend
normal ideas of space and time, in order to make real

“This work adds to the growing body of speculative
research which intimates that physical reality could
encompass far more than just the aftermath of ‘our’ big

The cyclic universe theory represents a combination of
standard physical concepts and ideas from the emerging
fields of string theory and M-theory, which are ambitious
efforts to develop a unified theory of all physical forces
and particles. Although these theories are rooted in complex
mathematics, they offer a compelling graphic picture of the
cyclic universe theory.

Under these theories, the universe would exist as two
infinitely large parallel sheets, like two sheets of paper
separated by a microscopic distance. This distance is a
extra, or fifth dimension, that is not apparent us. At our
current phase in the history of the universe, the sheets are
expanding in all directions, gradually spreading out and
dispersing all the matter and energy they contain. After
trillions of years, when they become essentially empty, they
enter a “stagnant” period in which they stop stretching and,
instead, begin to move toward each other as the fifth
dimension undergoes a collapse.

The sheets meet and “bounce” off each other. The impact
causes the sheets to be charged with the extraordinarily hot
and dense matter that is commonly associated with the big
bang. After the sheets move apart, they resume their
expansion, spreading out the matter, which cools and
coalesces into stars and galaxies as in our present

The sheets, or branes, as physicists call them, are not
parallel universes, but rather are facets of the same
universe, with one containing all the ordinary matter we
know and the other containing “we know not what,” said
Steinhardt. It is conceivable, he said, that a material
called dark matter, which is widely believed to make up a
significant part of the universe, resides on this other
brane. The two sheets interact only by gravity, with massive
objects in one sheet exerting a tug on matter in the other,
which is what dark matter does to ordinary matter.

The movements and properties of these sheets all arise
naturally from the underlying mathematics of the model,
noted Steinhardt. That is in contrast to the big bang model,
in which dark energy has been added simply to explain
current observations.

Steinhardt and Turok continue to refine the theory and
are looking for theoretical or experimental ideas that might
favor one idea over the other.

“These paradigms are as far apart as you can imagine in
terms of the nature of time,” said Steinhardt. “On the other
hand, in terms of what they predict about the universe, they
are as close as you can be up to what you can measure so

“Yet, we also know that, with more precise observations
that may be possible in the next decade or so, you can
distinguish them. That is the fascinating situation we find
ourselves in. It’s fun to debate which ones you like better,
but I really think nature will be the final arbiter

For further information and a graphic animation of the
cyclic scenario, see http://feynman.princeton.edu/~steinh/