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Albert Einstein Professor in Science, Departments of Physics and Astrophysical Sciences, Princeton University; coauthor: Endless Universe
Physicist; Albert Einstein Professor of Science, Princeton University; Coauthor, Endless Universe: A New History of the Cosmos

Bullish on Cosmology

I am optimistic that there will be a historic breakthrough in our understanding of the universe in the next five years that will be remembered as one of the most significant of the millennium. I would also give better-than-even odds that there will be more than one discovery of this magnitude.

My optimism is sparked by a remarkable coincidence: the simultaneous maturing of several unrelated technologies, each of which could open a new window on the cosmos. Historically, every new technology is a harbinger of great discovery. Consider, then, that at least a handful of major advances will occur within just five years:

• Directly detecting of dark matter:

After decades of gradual progress, physicists will finally build the first detectors sensitive enough to detect dark matter particles directly, if they consist of weakly interacting massive particles (WIMPs), as many physicists suspect. 

• Discovering the nature of dark energy:

Although their names sound similar, the only quality dark matter and dark energy have in common is that they are both invisible. Dark matter consists of massive particles that gravitationally attract one another and clump into clouds that seed the formation of galaxies.

Dark energy is gravitationally self-repulsive, so it tends to smooth itself out. When it is the dominant form of energy, as it is today, dark energy causes the expansion of the universe to speed up.The composition of dark energy is one of the great mysteries of science, with profound implications for both fundamental physics and cosmology. 

Over the next five years, arrays of novel wide-field telescopes will be constructed that are programmed to rapidly scan large fractions of the sky to search for astronomical phenomena that vary rapidly with time. The arrays will be used to search for distant supernovae (exploding stars), whose brightness and colors can be used to judge the distance and recessional speed of their host galaxies. From these measurements, astronomers can measure precisely the accelerated expansion of the universe, a primary means of distinguishing different theories of dark energy.

At the same time, in the laboratory, physicists will be trying to detect changes in the gravitational force when masses are placed at close proximity or tiny changes in the strength of the electromagnetic force with time, other effects predicted by some theories of dark energy. These measurements will significantly narrow the candidates for dark energy, perhaps identifying a unique possibility.

• Exploring the big bang and the origin of the large-scale structure of the universe:

The conventional wisdom is that the universe sprang into existence 14 billion years ago in a big bang and that a period of exponentially rapid inflationary expansion accounts for its large-scale structure. However, the last decade has seen the emergence of alternative possibilities, such as the cyclic model of the universe. 

In the cyclic model, the big bang is not the beginning but, rather, an event that has been repeating every trillion years, extending far into the past. Borrowing ideas from string theory, the cyclic model proposes that each bang is a collision between our three-dimensional world and another three-dimensional world along an extra spatial dimension. Each bang creates new hot matter and radiation that begins a new perio of expansion, cooling, galaxy formation and life, but space and time exist before and after the bang.

The large-scale structure of the universe and the pattern of galaxies are set by events that occurred about a cycle ago, before the bang, just as events occurring today are setting the structure for the cycle to come. Although the inflationary and cyclic pictures predict distributions of galaxies, matter and radiation that are indistinguishable, their predictions for the production of gravitational waves in the early universe are exponentially different.

Gravitational waves are ripples in space produced during inflation or near the beginning of a new cycle that propagate through the universe and distort space like undulations traveling through jello. These cosmic gravitational waves are too weak to be detected directly, but experimental cosmologists throughout the world are mounting ground- and balloon-based experiments to search for their imprint on the polarization pattern of cosmic microwave background radiation produced in the first 380,000 years after the bang. 

The results will not only affect our view of our cosmic origin, but our future as well. The conventional big bang inflationary theory predicts our universe is headed towards the cold oblivion of eternal expansion—a whimper—but the cyclic model predicts a new hot big bang.

• Direct detecting gravitational waves:

The first window on the universe using something other than electromagnetic waves could be open within the next five years. After decades of developments, the LIGO (Laser Interferometer Gravitational Wave Observatory), with one detector in Livingston, Louisiana, and one in Hanford, Washington, has a plausible chance of directly detecting gravitational waves, beginning a new era in astronomy. 

The observatory is designed to detect stronger gravitational waves than those produced in the early universe, such as waves generated by the violent collision of neutron stars and black holes in our own galaxy. However, this frontier is so fresh and unexplored that there could well be unanticipated cosmic sources of strong gravitational waves to be discovered that could cause us to reassess our understanding of the universe. 

• Breakthroughs in fundamental physics and direct production of dark matter: 

The Large Hadron Collider at the Center for European Research (CERN) in Geneva, Switzerland, is set to begin operation this year. This facility consists of a powerful particle accelerator that will reproduce collisions of the type that occurred within the first pico-second after the big bang, carrying the investigation of fundamental physics over an important energy threshold where new phenomena are anticipated. For example, physicists hope to discover a spectrum of new "supersymmetric" particles, confirming a key prediction of string theory, and also WIMPs that may comprise the dark matter.  

The impact will  be profound.  As we enter 2007, we understand the composition of less than five percent of the universe; we do not understand how space, time, matter and energy were created; and we cannot predict where the universe is headed. In the next five years, we may witness the historic resolution of one or more of these issues. I have my personal bet on what the individual outcomes will be; but the only prediction I will reveal here is that, with the opening of so many new windows on the cosmos, we are sure to discover something unanticipated and astonishing.