universe is approximately homogeneous and uniformly expanding, the redshift of a galaxy translates into an approximate distance, thus providing the elusive third dimension.

It is not yet known whether the new cosmic structures found in a few selected regions of space are typical. What seems clear is that structures of some kind have been found at the largest possible scale in each survey of galaxies; that is, a survey that looks over a region of 100 million light-years usually finds some chain or wall or absence of galaxies extending roughly 100 million light-years in size; a survey of a 200-million-light-year region finds structures of 200 million light-years, and so on.

Galaxy surveys are now in progress that extend out to a few billion light-years. In addition, surveys with many more galaxies are being planned. The largest redshift surveys to date include only several thousand galaxies and sample only limited directions in the sky. In the coming decade, astronomers hope to initiate surveys of a million galaxies. Such surveys could be accomplished with moderate-sized visible-light and infrared telescopes. If in the future we find filaments and bubbles and voids with sizes of a few billion light-years, several times larger than those now mapped, then there would be a direct contradiction with the uniformity of matter implied by the cosmic background radiation. The Big Bang model might even be called into question.

On the theoretical side, astronomers are attempting to make sense of the observed positions and motions of galaxies by the use of large computer simulations. Such simulations involve 10,000 to several million particles, each representing a portion of a galaxy or a number of galaxies. The particles are placed at initial positions, given an initial outward velocity corresponding to the expansion of the universe, and then allowed to interact via their mutual gravity. The hypothetical galaxies fly around the computer screen, gravitate toward each other, and form clumps and wisps and voids. The largest simulations require the fastest and biggest computers in the world. By comparing computer simulations to the observed large-scale structure of the universe (Plate 2.16) scientists hope to test their assumptions about the initial conditions and forces at work in the cosmos. The current computer simulations, although 10 times larger than those of a decade ago, still do not have enough particles for a decisive comparison between theory and observation. Within the coming decade, larger computers and new methods for using those computers should give more reliable answers. And the new calculations will have more brain as well as brawn. Additional physics will be taught to the computers, and the resulting simulations will be more realistic and believable.

Whatever the outcome of the computer simulations, the ultimate theory of the distribution of matter in the universe must be consistent with all the observations. Astronomers have become increasingly worried about reconciling the smoothness of the cosmic background radiation with the lumpiness of matter



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement