How big is the universe; how far are the most distant starts? The question nags astronomers even though news headlines implied a comfortable certainty when they recently reported that four galaxies had been found to be 10 billion light years away -- the remotest such objects whose distance has yet been measured.
In fact, this latest cosmic ranging exemplifies the ambiguities that beset such questions. For example, the light from these galaxies would have taken 10 billion years to get here, implying that the galaxies themselves are over 10 billion years old. Yet, about a year and half ago, three astronomers -- John Huchra of the Harvard Smithsonian Center for Astrophysics, Marc Aaronson of the University of Arizona, and Jeremy Mould of Kitt Peak National Observatory -- reported data they thought suggested the universe itself is only 9 billion years old, even though most astronomers think the figgure should be more like 15 billion years.
To begin with, Hyron Spinrad and John Stauffer of the University of California and Harvey Butcher of Kitt Peak, who studies the four galaxies, didn't measure distance at all. They measured the so-called "red shift" of spectral lines in the light emitted by the galaxies. That is, they measured the drop in frequency of those lines compared with similar lines emitted by a stationary source in the laboratory.
This frequency drop, or reddening, is usually explained as a velocity effect. The faster a light source moves away from us, the greater the red shift. And for distant cosmic objects, their velocity is believed due mainly to expansion of the universe. This kind of velocity would be proportional to distance. So astronomers translate read shifts, which they can measure, into distances which they have no direct way of estimating.
That's where the 10 billion light years comes from. However, there's no certain relationship between red shifts and distances. The estimates an astronomer makes depends on what is assumed about the nature of the universe -- how fast it expands or how space curves.
The 9-billion-year age came from a different kind of estimate. In this case, astronomers estimated the intrinsic brightness at infrared frequencies of certain distant objects. They then inferred distance by observing how faint these objects appeared. The data implied that red shift interpretations have used models of the universe that overestimate distances and that the universe is smaller and younger than had been thought. This conclusion has not been generally accepted. But if it were true, then the distances of the four galaxies would be recalculated to bring them closer.
The point is that estimating cosmic distances depends on what one assumes the universe to be like. The new galaxy red shift measurements give astronomers more data to test their assumptions. But basic issues of h ow big and how old still elude them.