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Scientists at AAAS Lecture Expand on
Theories of Creation of the Universe
In an ongoing challenge to proponents of the Big Bang theory of the origins of the universe, Paul Steinhardt of Princeton University recently spoke at AAAS, describing the theory that the universe is in an endless cycle of expansion and rebirth (see Science 2001 April 13).
"This idea of a cyclic universe is not new," said Steinhardt. "What distinguishes this new model is its reliance on string theory which suggests a multi-dimensional structure of space, time and matter."
In Steinhardt's theory, the visible universe exists within a flat, four-dimensional membrane, or "brane," that can be stretched like a rubber sheet. Another, parallel four-dimensional universe, separated from ours by a microscopic distance, contains a "dark matter" universe. It sheds a membrane that slowly makes its way toward our universe. When the two membranes collide, the resulting matter and energy make up our cosmos.
During each cycle, the universe fills with hot matter and radiation, and then expands, accelerates and cools down. The cycle repeats after trillions of years.
"For a half-century," Steinhardt said, "the most widely accepted notion of the universe began in a 'Big Bang,' a phenomenon where all the matter and energy in the cosmos expanded outward from a tiny speck of matter within a fraction of a second."
Today most cosmologists believe in a "consensus theory," which combines the Big Bang with the notion of "Inflation," which suggests that the universe has been expanding since it began with the Big Bang, 14 billion years ago. Inflationary theory explains many of the observed properties of the cosmos, such as its spatial flatness and the way galaxies are distributed. But a few years ago, astronomers were surprised to discover that the universe is expanding at an accelerated rate, rather than slowing as had been predicted.
"To explain this, scientists concluded that the universe contains not only ordinary matter and dark matter, but an ingredient called dark energy that drives the expansion," Steinhardt said.
The scientists theorized that billions of years after the Big Bang, following the formation of galaxies, the Universe was overtaken by this dark energy, causing the expansion rate to accelerate. The Big Bang could be seen then as a bridge to a pre-existing contracting era. "The one shortcoming of this model is that it can't explain the Big Bang itself, or the conditions that created it." Steinhardt noted.
Theories of "bouncing," or cyclic universes, do not predict a beginning or an end of time. Early advocates of a cyclic model thought that the universe must shrink into a singularitya point of infinite density and temperaturebefore exploding in a new big bang. But this idea proved too difficult to explain, and most theorists rejected the concept of a cyclic universe.
"The key difference between the cyclic model and the consensus picture comes down to the nature of time," said Steinhardt. "The Big Bang theory assumes that time has a beginning. According to this model, the universe sprang from nothingness into something, full of matter and energy, and has been expanding and cooling for the past 15 billion years."
In the cyclic model the universe is endless. Time is endless and goes on forever in the past and forever in the future. The four spatial dimensions remain infinite throughout the evolution of the universe.
"The three dimensions we recognize are a surface embedded in a space of an extra dimension," Steinhardt said. "The universe's galaxies and light are confined to this surface. And another parallel surface exists, though we cannot see or touch it, but we know it exists because it exerts a gravitational effect."
Like proponents of the inflationary model, Steinhardt contends that dark energy is speeding the universe's expansion. He predicts that this expansion will continue (like the stretching of a rubber sheet) for trillions of years, until everything is so far apart that space becomes like a vacuum. Then the balance of forces in the membrane will change, expansion will stop and the two membranes will collide.
One possible way to test the alternatives would be to look for gravitational waves from the earlier universe. Inflationary theory predicts that such waves exist; the membrane theory predicts that they don't. Only with a next-generation gravity wave detector might scientists come closer to knowing which theory is more valid.
22 January 2003
For more information, see:
- Space Science Topics to be covered at the AAAS Annual Meeting
- History of the Cyclic Universe, quoted from lecture
- Information on Paul J. Steinhardt and Neil Turok
- An Introduction to String Theory
- Einstein's Theory of Relativity
- A Brief Introduction to the Cyclic Model (for the non-scientist)
- AAAS Dialogue on Science, Ethics, and Religion