Joseph Silk is Professor of Astronomy at Oxford, and in The Infinite Cosmos , he offers a layman’s summary of what’s happening in cosmology.

One of the central principles of modern cosmology is the “cosmological principle,” the theory that the “universe is approximately the same in all directions. In contrast to pre-scientific mythical cosmologies, this principle is based on solid empirical evidence. Admittedly, it’s “not quite the evidence you can hold in your hand,” but it is “the evidence you can see through a telescope.” In fact, it “has been verified to exquisite precision, in so far as this can be achieved from our vantage point.”

The evidence is points to starts with imagining looking out at the universe through a telescope. The universe looks much the same everywhere we look, provided we factor out the Milky Way galaxy of which we are a part. If one “smooths out the images, by, for example, peering at [galaxies] through an out-of-focus lens: the details are degraded,” then “the general pattern of light, due to galaxies, remains,” and “the universe now seems nearly uniform and the same in all directions.” As my son commented, if we factor out all the variation, the universe looks the same everywhere. Well, it’s not as bad as that: Silk is saying that if we factor out everything else, the distribution of light remains constant in every direction we look. This shows that the universe is uniform in every direction. No matter where you stand - on earth, on a star in the most distant visible galaxy - the universe is going to look the same.

Now, imagining that we’re looking through an out-of-focus telescope hardly counts as verifying the principle with “exquisite precision.” The actual evidence that Silk cites amounts to the “cosmic microwave background” that “has demonstrated homogeneity and isotropy to a level of the order of 1 part in 100,000.” We can infer from the light spectrum of galaxies that they are not at rest, since the light is shifting to redder wavelengths than for nearby galaxies. No matter how deep one penetrates the universe “the density of galaxies is found to be uniform,” and this means we “do not inhabit a universe of vanishingly low density in the mean,” but that we can “set a limit on any large-scale non-uniformities of around 10 per cent.”

Granting that Silk is writing of a layman’s audience, and granted that there’s a lot of things going on here that I don’t grasp, this principle still strikes me as hugely problematic. For starters, Silk begins his book exploring the question of whether the universe is infinite, and he includes the usual astounding statistics about the distances between galaxies. But if the universe is as huge as he says, isn’t it a bit pre-mature to determine that we know it’s uniform through and through?

Plus, Silk himself hedges a bit about the principle. While saying that it has been “verified to exquisite precision” he also admits that “we cannot prove the cosmological principle,” calling it “a matter of philosophy or even faith, albeit faith that is highly motivated.” No problem there: Scientific “proof” is rarely attained. But the admission that the principle is a matter of “faith” makes it appear that the cosmological principle is not so wholly different from earlier cosmologies as Silk first indicated.