The observable universe
One word is the difference between calculation and infinity.
Last week we started counting stars, ending with the estimate of 1011 in our own Milky Way galaxy. That’s a lot, but not all there are. Other galaxies are out there, some larger than ours. In fact more are visible the deeper we look. So have we simply exchanged an infinite universe of stars for an infinite universe of clumps of stars?
Two interrelated facts stand in the way of that. First, light travels at a finite speed, nearly instantaneous on human scales but terribly slow once we leave the Solar System. Second, the universe has a finite age. Side-stepping questions of the Absolute Beginning, we can say that stars could not have existed before about 13.5 billion years ago. Our observable stellar universe is limited to those places from which light could get to us in that time.
That does not mean that a static sphere of radius 13.5 billion light years is the observable universe. The universe is expanding, a statement that requires a certain amount of explanation to avoid mistakes. For now we’ll just point out that light that has traveled for, say, 10 billion years left that distant object when it was closer to us than 10 billion light-years, and arrives here when it is actually farther away. Also, as we look farther out we look farther back in time, eventually to an epoch when the universe was much different. If we want to estimate stars by counting galaxies, we have to take into account that they were smaller in the distant past, among other things. We cannot see “now.”
But with these and other complications taken into account, it is possible to arrive at a number for the total of stars in the observable universe. Astronomers have done so. It’s a very large number, and has large uncertainties due to some things not being very accurately known, but it falls short of infinity.
It is possible for the number of stars in the whole universe to be finite. One way would be for everything outside a certain radius to be empty, that is, for our illuminated part of creation to be highly unusual. Another would be for the universe itself to be finite in volume though without edges, the three-dimensional analog of the surface of a sphere (which has a finite area without any edges). But to a good approximation and as far as we can see, the three-dimensional space of our universe is flat, so it is indistinguishable from one of infinite volume; and there is no sign of an ending to the kingdom of stars.
Se we cannot say that the number of stars in the universe is finite, much less give a figure for it. But, alas, popular science abhors qualification. When a figure is quoted for the “observable universe” the phrase is routinely shortened to “universe.” We’ve even seen it done in a university astronomy textbook. Led by our astronomer, we will continue to fight this incorrect usage, this conflation of big numbers with infinity.
But we’re afraid we’ll be no more effective than with our contention that “datum” is singlular and “data” plural.
