The New Synthesis

In recent years, scientists have been working on a synthesis combining inflationary theory, dark matter, the dark energy supposed to exist from the observed accelerated expansion of the universe, and the pattern of anisotro-pies revealed in the early universe by balloon detectors and by WMAP. There is a feeling among some theorists that a coherent picture of the early and evolving universe is finally coming together.

Before 1998 it was customary to distinguish three cosmological models for an expanding universe, all based on the general theory of relativity.

1. A closed universe, in which the average density of matter is large enough so that gravity is eventually able to overcome expansion. The expansion slows down, eventually stops, and at some time, contraction begins. Relativistic theory implies that the geometry of this universe is Riemannian curved space. The universe is finite.

2. An open universe, in which the density of matter is small enough so that expansion continues forever. The geometry of this universe is hyperbolic, analogous to a saddle-shaped surface. The universe is infinite.

3. A flat universe, in which the average density of matter is the critical density. If the density were smaller than this value, the universe would be closed, and if it were greater than this value, the universe would be open. In a flat universe the expansion is steadily slowing down but never stops: it is the slowest rate of expansion with this property. The geometry of a flat universe is Euclidean, like ordinary school or surveyor's geometry. The universe is infinite.

In cosmology the symbol Q (Omega) is used to denote the ratio of the actual density of mass in the universe to the critical density. The three pos sibilities outlined above correspond to the cases Q > 1, Q < 1, and Q = 1. In all three of these models it is assumed that the cosmological constant is zero. The universe was believed to be described by one of these three models, the case Q = 1 being favored by many cosmologists.

With the discovery of acceleration it has been necessary to radically revise the standard scheme. Much interest focuses on models in which the cosmological constant is positive, although other solutions have also been sought. It is now believed that the universe contains ordinary matter, dark matter, and dark energy. The introduction of dark energy implies the existence of a much larger class of cosmological models than the three given by the possibilities Q > 1, Q < 1, and Q = 1. The density Q will be made up of a component Qm due to matter (ordinary and dark) and a component Qv due to dark energy (also known as the vacuum energy): Q = Qm + Q . The nature of the cosmological model will be determined by the value of Q as well as the relative contributions of Q and Q to Q. Current observation indicates that the density Q of the actual v J

universe is the critical density, Q = 1. It is believed that within a small margin of error the contributions to the critical density consist of 5 percent ordinary baryonic matter, 25 percent dark matter, and the rest dark energy. Hence Q = 0.3 and Q = 0.7. In our universe it is not the case that the expansion is mv slowing; to the contrary, it is expanding at an ever-increasing rate.

If the critical density is equal to one, then the geometry of the universe is Euclidean. That this is indeed the case is supported by several pieces of empirical evidence. Calculations of the frequency of deuterium and measures of the masses of galaxies at different times in the history of the universe imply a flat geometry. The map of anisotropies in the cosmic background radiation has revealed that the distance between successive peaks in intensity is one degree, exactly the value theoretical calculations predict if Q is assumed to be one and the universe is flat.

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