A century of relativity

Cosmologist harks back to the beginning in quest for alternative to ‘big bang’

By Steven Schultz

Princeton NJ -- University physicist Paul Steinhardt has been thinking a lot about beginnings — not only the origin of the universe, which has been a focus of his research for 30 years, but also the chain of discoveries in the early 20th century that led to the birth of his field.

In a year when the international physics community is celebrating the 100th anniversary of the “miracle year” in which Albert Einstein produced three of his greatest papers (see related story on page 6), Steinhardt has been particularly aware of how key questions that vexed Einstein have re-emerged with a new focus.

Steinhardt, Princeton’s Albert Einstein Professor of Science, has played an important role in building the modern picture of the universe and the theory of how it evolved from the first micro-fraction of a second after the big bang to today. This 14-billion-year story has become a triumph of modern science. Recent astronomical observations appear to confirm and reinforce the “big bang” theory with spectacular precision.

In the last few years, however, Steinhardt, along with colleagues and students, has set out to provide an alternative theory that explains all the current observations and goes further, looking beyond that first microsecond to the instant of creation and before. This effort, which has resulted in an idea called the “cyclic universe,” addresses questions about the beginning of time that were debated by Einstein and subsequent scientists, but were essentially set aside for most of the last century.

“What is the big bang? Is it the beginning? Could the universe be cyclic?” asked Steinhardt, who has spent the last few months reading papers from the 1920s and 1930s to understand what scientists of the time were thinking about these questions. “Now that we’ve settled certain issues about the later history of the universe, we’re going to be thinking about these earlier questions, and they are tied very much to issues that Einstein introduced.”

Einstein’s bold guesses

Despite the progress in astronomy through Sir Isaac Newton and into the 19th century, the field of cosmology — the study of the history and structure of the entire universe — did not move from philosophy to science until Einstein. In 1905 and over the next 10 years, Einstein introduced his theories of relativity, which revealed that space and time are not fixed quantities and that matter causes space to curve. In 1917, Einstein wrote a paper that applied the concepts of relativity to cosmology. “Everything we do in cosmology today springs from that paper,” Steinhardt said.

To jumpstart the field, Einstein assumed that matter and energy were distributed evenly across the universe despite evidence that humans lived in a cluster of stars surrounded by vast emptiness. This radical simplification was critical in applying the theory of relativity to the universe, Steinhardt said, because trying to calculate how each piece of matter affected space and time was too complicated. Many years later, with the discovery of a uniform background radiation that suffuses all space, Einstein’s assumption proved to be very accurate.

“Sometimes theorists make bold guesses that fly in the face of current observations,” Steinhardt said. “Usually when you do that you turn out to be wrong, but occasionally you are right, and that turns out to be a famous occasion. Einstein had a pretty good knack at being right.”

Einstein made another assumption in the same paper that turned out not to be correct. In part to avoid the messy situation of having to consider a beginning for all space and time, Einstein supposed that the universe was unchanging, that it looked the same as it always had. In 1929, however, astronomer Edwin Hubble showed that the universe was expanding, confirming the predictions of Alexander Friedmann and George Lemaître and giving rise to the big bang theory.

A mysterious force

The idea that all space and time began at a single moment 14 billion years ago has proven to be remarkably consistent with the wealth of modern observations. In the 1970s, trouble with the theory arose in trying to explain why the universe is so flat and uniform, yet has tiny non-uniformities that led to the clumping of matter into galaxies and clusters.

In the 1980s, Alan Guth, Andre Linde, Andreas Albrecht and Steinhardt solved the problem by proposing an instant of hyperfast expansion, called inflation, in the first second after the big bang. They showed that inflation would automatically make the universe uniform except for tiny variations in energy. The theory’s predictions agree well with recent observations by the Sloan Digital Sky Survey, a project in which Princeton astronomers have a major role, and the Wilkinson Microwave Anisotropy Probe, a satellite mission that also was designed and led in part by Princeton scientists. Inflation theory is now a key part of modern cosmology.

Steinhardt, however, is not resting. For one thing, observations now show that the universe is not only expanding, but is also accelerating, which requires some unknown, anti-gravity-like force to be driving everything apart. Cosmologists, including Steinhardt, have speculated about the nature of this mysterious new force, but their ideas are more like patches rather than natural outgrowths of the big bang theory, suggesting the need for a new all-encompassing theory, Steinhardt said.

In addition, the big bang theory itself still leaves the thorny question of what happened at the very beginning. Tracing the universe back in time, the equations show that the temperature and density would have been infinitely high about 13.7 billion years ago — “a sign of sickness in the theory,” Steinhardt said. “It’s a mathematical paradox — a moment when we can’t apply our known laws of physics and reach sensible conclusions.”

“You don’t have to look that closely before you discover that the foundations of the standard story are pretty weak because they rest on assumptions about the existence of the big bang and the way the universe emerged from the big bang for which we have no evidence,” Steinhardt said. “In fact, as Einstein understood and warned, you need to replace the current theory with a new picture that is based on an improved theory of gravity and avoids mathematical paradoxes.”

Intertwining threads

That replacement theory is Steinhardt’s goal. With Neil Turok of Cambridge University, Steinhardt proposed in 2002 the cyclic universe idea, in which the universe undergoes periods of expansion and contraction at regularly repeating intervals. Something that looks like the big bang still occurs at the beginning of each cycle. But, unlike the conventional theory, the temperature and density remain measurable. The new theory also builds in the accelerating force that is now being observed, making it a natural part of the cycle.

Steinhardt said he is surprised that the theory did not fall apart as most new theories do when scientists work out the predictions and compare them carefully with the available observational evidence. “Thus far, the idea seems to be getting simpler and better as work goes on,” he said.

If the theory proves to be correct, it may vindicate Einstein’s idea about the universe being uniform over time. Although the universe expands and collapses, the average characteristics are the same from cycle to cycle, Steinhardt said. “It’s interesting that Einstein’s ideas may reappear, rejuvenated by using new ideas in a new context.”

Another appealing aspect of the cyclic theory is that it is testable. The big bang/inflation theory and the cyclic theory make different predictions about how the early universe produced “gravitational waves,” which are subtle distortions of space that propagate through the universe like ripples on a pond. Astronomers are currently attempting to measure gravitational waves, which may offer evidence in favor of one theory or the other.

Remarkably, Steinhardt noted, the existence of gravitational waves also was first proposed by Einstein. “So it connects back,” he said. “These threads connect and intertwine and connect again in interesting ways, and that’s part of the joy of doing physics.”

A sense of joy in physics emerges from Steinhardt’s work as strongly as his scientific results. On his Web site, he makes a special effort to present his findings in language that is accessible to a general audience and includes an animated graphic that illustrates the concepts of the cyclic universe. He has taught the introductory physics course for nonscientists, known among students as “Physics for Poets,” and this year created a new course that teaches principles of physics that will be important for future leaders in business and government (see related story below).

Steinhardt, who received a bachelor’s degree in physics from the California Institute of Technology and a 1978 Ph.D. from Harvard University, was a faculty member at the University of Pennsylvania before coming to Princeton in 1998. He succeeded James Peebles, who retired in 2001, as the Einstein professor. In 2002 he won the Dirac Prize, a major honor in theoretical physics.

In addition to his influence in cosmology, Steinhardt has made important contributions to condensed matter physics, including pioneering work on a new phase of solid matter, called quasicrystals, which have unusual properties that could be of use in superhard industrial coatings and other novel materials.

“I came to physics because I’m very curious about how the world works,” said Steinhardt, noting that he appreciates the unexpected ways in which various fields build on each other. “One of the fun things about cosmology is that it is a place where all these subjects intersect.”