Friday, February 23, 2007

How we are learning cosmic history and destiny

kw: book reviews, nonfiction, cosmology

When I saw the title, The Origins of the Future: Ten Questions for the Next Ten Years, I expected that John Gribbin, astronomist extraoidinaire, to pose a set of problems in the môde of David Hilbert with his 23 problems of 1900. What I found instead is an outline in ten points, beginning with the philosophy of science ("How do we know the things we think we know?") and continuing through the development of cosmology to its ultimate, prognostic aim: "How will it all end?".

I enjoy reading of big issues by the big thinkers. I have probably read a dozen books that cover the same ground, in the past decade or so, by authors from Christian DeDuve to Stephen Hawking. In such a brilliant assemblage, it takes quite a thinker to propound something new. On a couple of points at least, Dr. Gribbin does just that.

I am particularly enamored of his clear exposition of "theory" in the first chapter. He makes it exquisitely clear that a theory is a model. In particular, a scientific theory must be a mathematical model that calculates phenomena we observe, and to have value it must first make predictions of phenomena we have yet to observe, and particularly draw conclusions that can be tested by observation or experiment. Thus, Newton's theory of gravity is a model of the way objects move under gravitational influence. Though it has been superseded by Einstein's general theory of relativity, Newton's "classical mechanics" model is quite adequate to explain all the phenomena of orbital mechanics within the rather low-speed realm of planetary dynamics and stellar motions in clusters. It is not adequate to calculate with high accuracy the exact orbit of Mercury or the gravitational bending of starlight. In such "relativistic" realms, Einstein's theory is a more complete model.

Gribbin states several times that a scientific model does not explain why some phenomenon occurs, but rather it describes what will occur if the system being modeled were to behave as if the particular why were to be so. Describing the wave/particle duality of light, he writes,

Nobody should ever have said (or thought) that light is a wave, or is a particle. All we can say is that under appropriate circumstances light behaves as if it were a wave or as if it were a particle...

It is hard to keep this in mind, and I know of no writer, Gribbin included, who consistently does so. That's OK.

In the midst of developing the history and theory of cosmology (models of the behavior of the universe), Dr. Gribbin explains, better than any other I've read, why we draw the conclusions we do about the content of the visible universe (and perhaps a much larger amount that is too far to be visible at present). To wit, it has been determined that the universe is very, very "flat" in a mathematical sense, with a very slight bias toward being open. That is, the gravity of all mass and energy in the universe is very nearly sufficient to stop expansion and return everything to a "Big Crunch" to match the Big Bang that apparently started it all.

Therefore, step by step, we find that visible stars and nebulae in galaxies comprise less than one percent of the total gravitational potential; cold, unseen "ordinary matter" brings the total to four percent; "dark matter", whether cold, cool, warm, or hot, totals another 26%; and "dark energy", a kind of anti-gravity, makes up the remaining 70%. The expansion of the universe has proceeded to the point that the dark energy is overcoming gravity, and the stretching of space has begun to accelerate, perhaps in the past one or two billion years. This will result eventually in what I call the "Big Poof" (Gribbin likes the term "Big Rip"), maybe half a trillion years from now, when total density drops rapidly toward zero.

I can in no way condense the reasoning behind these ideas. The book does an admirable job. I must say, I didn't really give much credence to "Big Poof" before, but I do now.

Update Feb 25: I wrote the above without fully understanding the last chapter. Upon re-reading it, I can clarify the author's points, and my own. Assuming "dark energy" is a constant per unit of volume, as modern cosmology does, it is evident that its total influence grows as the universe expands. This will continue to what I call the Big Poof. Dr. Gribbin poses alternative scenarios if dark energy is not constant. Firstly, it may attenuate as space stretches. In such a case, the acceleration astronomers now (claim to) observe will reverse itself, and the universe will collapse into a Big Crunch, just as in a closed-universe scenario. Alternatively, it may increase, until it influences not just the distance between superclusters, but their composition, then galaxies, and so on until it stretches the quarks right out of protons in a grand dissolution called the Big Rip.

This is so purely speculative as to be simple mental gymnastics. I am barely converted to Big Poof, and may apostasize at any moment...not being wholly convinced that we are properly adjusting the brightness of distant supernovae. I think it likely that errors are still larger than the effects being posited, that we are building quite a structure on faint "it must be so" reasoning, not far removed from a "just so" story.

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