Space-Time Expands

Posted by jns on June 9, 2009

This is^* author John R. Gribbin (1946– ), a science writer who started life as an astrophysicist. (His website.) I’ve read and mentioned a few of his books here in the last year or so, and I’ve been enjoying them so far.

The one that I most recently read and enjoyed is John Gribbin with Mary Gribbin, Stardust : Supernovae and Life—The Cosmic Connection (New Haven, Yale University Press, 2000. xviii + 238 pages). Here is my book note.

The book is all about stellar nucleosynthesis: how the elements are made in stars and supernovae. As you may have realized, this is a subject I find fascinating, particularly the history of the discovery of nucleosynthesis. I’m especially keen on the late nineteenth controversy about the age of the sun, a controversy starring two big names in science, Darwin and Lord Kelvin, a controversy that couldn’t be settled until the invention of quantum mechanics and the discovery of nuclear fusion.

That problem was finally cracked by Hans Bethe in two papers he published in 1939 in Physical Review (“Energy Production in Stars”, first paper online, and second paper online. A very nice short, nontechnical summary of the importance of these papers (“Landmarks: What Makes the Stars Shine?“) is also available online. I may have to write more about these sometime.

But the excerpt from Stardust that I wanted to share here has to do with a different question, one that came up in a discussion we had elsewhere (“Long Ago & Far Away“) following a question Bill asked about the size of the expanding universe.

This doesn’t address that question directly, but does answer another related question. I was unclear at the time whether celestial red-shift should be interpreted as the result of actual motion of objects in the universe apart from each other, or as the result of the expansion of space-time itself, or some combination.

The answer is unequivocal in this excerpt: red-shifts are due to expanding space-time. That is, the geometry of space-time itself is stretching out and this is what causes the apparent motion of cosmic objects away from us (with some actual relative motion through space-time going on).

Hard though it may be to picture, what the general theory of relativity tells us is that space and time were born, along with matter, in the precursor to the Big Bang, and that this bubble of spacetime full of matter and energy (the same thing—remember E = mc²) has expanded ever since. The galaxies fill the Universe today, and the matter they contain always did fill the Universe, although obviously the pieces of matter were closer together when the Universe was smaller. Since the cosmological redshift is caused not by galaxies moving through space but by space itself expanding in between the galaxies, it is certainly not a Doppler effect, and it isn’t really measuring velocity, but a kind of pseudo-velocity. Partly for historical reasons, partly for convenience, astronomers do, though, continue to refer to the “recession velocities” of distant galaxies, although no competent cosmologist ever describes the cosmological redshift as a Doppler effect. [p. 116]

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^* The source of the photo is an (undated) article from American Scientist, “Scientists’ Nightstand: John Gribbin“, by Greg Ross, which has an interview with Gribbin and gives this thumbnail biography:

John R. Gribbin studied astrophysics at the University of Cambridge before beginning a prolific career in science writing. He is the author of dozens of books, including In Search of Schrödinger’s Cat (Bantam, 1984), Stardust (Yale University Press, 2000), Ice Age (with Mary Gribbin) (Penguin, 2001) and Science: A History (Allen Lane, 2002).