Return of the cosmological constant.

Return of the cosmological constant 

When Albert Einstein wrote down the equations for his theory of general relativity more than 70 years ago, he felt compelled to add a mathematica term representing an unknown, repulsive force to counter the gravitational attraction of mass. The introduction of this "cosmological constant" gave Einstein a way to reconcile his theory with the then-current belief that the universe is neither expanding nor contracting. However, the subsequent discovery that the universe actually is expanding led cosmologists to abandon the cosmological constant. Einstein himself eventually repudiated the notin, describing its introduction as the biggest blunder of his life.

A group of astrophysicists at the University of Oxford in England has now resurrected the idea to solve a different problem. They introduce a cosmological constant to show how matter in an expanding universe dominated by cold dark matter could lead to the formation of great walls, great attractors and other huge aggregations of galaxies.

In its simplest form, the cold-dark-matter theory holds that gravity amplified tiny fluctuations in the distribution of matter in the early universe to produce the collections of galaxies that astronomers observe today. Moreover, more than 99 percent of the mass in the universe is dark, consisting of as-yet-unindentified particles that interact only weakly with ordinary matter. The model also assumes a universe having a "critical" density of matter -- the density separating a universe that expands forever from one that would eventually contract.

Although such a model can account for structures on the scale of individual galaxies and large clusters of galaxies, theorists have considerable difficulty using it to explain the formation of structures on even larger scales. At the same time, measurements by the Cosmic Background Explorer spacecraft indicate that the distribution of matter in the early universe was incredibly smooth.

"The [cold-dark-matter] model is attractive because it links the formation of cosmic structure to plausible physics of the early universe, so it would seem reasonable to retain the basic picture as far as possible," George Efstathiou and his colleagues argue in the Dec. 20, 1990 NATURE. "A positive cosmological constant could solve many of the problems of the standard [cold-dark-matter] model and should be taken seriously."

To save the cold-dark-matter model, Efstathiou and his colleagues introduce such a cosmological constant, which, in effect, endows the vacuum of space itself with a small energy density. Because the cosmological constant affects the overall geometry and expansion of the universe, its introduction allows the researchers to assume a lower value for the density kf the universe -- only 20 percent of that usually assumed.

Calculations based on these new assumptions show that this model can prduce sufficiently large galactic structures and still account for the smoothness of the microwave background. In such a universe, cold dark matter would dominate the expansion for about half of its history. Then the force represented by the cosmological constant would take over.

Efstathiou and his co-workers "present what may be the strongest case to date for a nonzero cosmological constant," Edmund Bertschinger of the Massachusetts Institute of Technology comments in the same issue of NATURE. However, he says, a nonzero cosmological constant would have profound and . . . disturbing implications for fundamental physics."

The new cold-dark-matter model will remain speculative until researchers furnish direct, empirical evidence supporting the notion of a positive cosmological constant, Bertschinger adds. That may require studying the distribution and motion of distant galaxies formed when the universe was young, or determining more precisely the age of the universe and the rate at which it is presently expanding.