Radio maps of a band of galaxies 700 million light years long - the largest continuous structure yet charted on the sky - reinforce a growing conviction among astronomers that the universe may be something like a Swiss cheese.
Its material may be clumped in ''super-clusters'' of galaxies, linked together perhaps by narrow bands of galaxies, with vast empty regions in between.
This contradicts the classical assumption of cosmologists that, on the average and on the large scale, the universe is a homogenous blend of matter and energy.
A growing mass of evidence accumulated over the past few years is forcing astronomers to reconsider that assumption. Last June, for example, R.Brent Tully of the University of Hawaii pointed out that evidence shows ''clumps and holes can exist on scales as large as 300 million light years.'' Now the new radio evidence suggests that the Swiss cheese structure exists on a far larger scale.
These radio maps were constructed by Riccardo Giovanelli of the National Astronomy and Ionosphere Center near Arecibo, Puerto Rico, and Martha P. Haynes of the National Radio Astronomy Observatory at Green Bank, W.Va. They show a continuous concentration of galaxies as a band connecting two previously known superclusters - one in the constellations Perseus and Pisces and the other in Ursa Major and Lynx. Dust had hidden the connecting band in surveys with optical telescopes. But it shows up clearly in the radio maps.
Such mapping is often scaled in terms of the light year - the distance light travels in a year in vacuum, or about 6 million-million miles. The universe, as a whole and considered as a sphere, is believed to be 10 billion to 20 billion light years in radius.
Thus a continuous structure of galaxies 700 million light years in extent is on the order of 5 percent of the scale of the entire universe, perhaps larger. Although he does not yet have evidence for it, Dr. Giovanelli says the band may be part of a continuous structure ''that you can trace all the way around the sky.''
Even at 700 million light years, the structure is an order of magnitude larger than the previously estimated size of superclusters. But this was not unexpected. For example, the band as now mapped extends some 200 degrees across the sky. Stephen A. Gregory of the State University of New York at Oswego and Laird A. Thompson of the University of Hawaii had already traced the Perseus-Pisces supercluster as a filament 40 degrees long. In a review of superclusters and voids in Scientific American last March, they noted that, judging from the work of some other astronomers, the Perseus structure ''may occupy an area of the sky 10 times larger than the area we have cautiously proposed.'' This now has been shown to be the case.
Astronomers are cautious about accepting a Swiss cheese universe. They have been particularly skeptical of the holes. But the evidence is forcing the concept upon them. ''The tendency of galaxies to clump is seen to be pervasive, '' say Drs. Gregory and Thompson. They add, ''The existence of voids, which we were initially hesitant to credit, can no longer be doubted.''
There is some mathematical support for the this as well. F. Shandaring of the Institute of Applied Mathematics in Moscow has used a technique called ''percolation theory,'' which analyzes the degree of connectedness, not just the clumpiness, of structures. This analysis of astronomical data suggests a basic structure of strings of galaxy clusters linked in a network with voids between strings.
The new findings interest cosmologists, who are trying to decide between two competing theories of how the present structure of the universe formed. One theory holds that, in the early universe, gravitational forces caused galaxies to form first and then to cluster together. The competing ''pancake'' theory was proposed in 1972 by Soviet astronomers Yakov B. Zel'dovich and Rashid Sunyaev. It maintains that matter first condensed into thin sheets or pancakes that then broke up into galaxies and galaxy clusters.
The emerging picture of galaxies grouped into a network of fairly flat superclusters connected by narrow bands favors the pancake theory. But astronomers are not jumping to that conclusion. J. Brent Tully warns in a review in Sky and Telescope: ''It is tempting to say that the present evidence favors the pancake models. . . . However, the gravitational clustering models cannot be readily dismissed.''