Monday, June 25, 2007

Epicycles or a Cyclic Epic

kw: book reviews, nonfiction, cosmology, inflation, cyclic universe

I've been waiting six years since the "Ekpyrotic Universe" proposal of Paul Steinhardt, Neil Turok and others, for a popular treatment of the theory to be published. I am simply not enough of a mathematician to follow the discussion in the monographs. Steinhardt and Turok's new book Endless Universe: Beyond the Big Bang fulfills my expectations and then some.

I was bothered by "inflation" from the beginning, even more than I've been bothered by the most-invoked explanation of the cosmological red shift. Alan Guth's initial hypothesis seemed to solve a number of problems with the older single-phase Hot Big Bang model. Yet it created others, and continued experimentation since the 1981 proposal have tended to confirm some aspects, yet requiring added complications to it. The current situation is quite messy, and resembles a Ptolemaic Epicycle scheme.

Note that Copernicus's heliocentric model also required epicycles because he continued to "believe" in circular orbits. It took Kepler and Newton to eliminate epicycles. It will take something equally revolutionary, and I suspect, equally unforeseen, to de-complicate cosmology.

The Cyclic Universe that these authors propose may just open the way for a new cosmology. The model is quite simple in concept, if one accepts a single non-testable proposition:
that the universe is contained in one 3-dimensional "brane" (an extension of the meaning of "membrane"), which is coupled to a second brane, the separation between these branes cycling over a very short distance along a dimension we cannot sense or measure (yet?!?)
The cyclic motion, with a period of (very) roughly a trillion years, causes the branes to collide, producing a Bang each time, during which (10-30 seconds or so) a universe of new matter is produced. The rest of the cycle is taken up with forming galaxies and clusters thereof, stars, planets, probably life, while dark energy expands the branes in their 3 dimensions, but doesn't affect the extra dimension along which they cycle.

The "flatness" problem that Inflation was produced to solve is dealt with by the dark energy expansion of the branes, which spreads them out to extreme flatness during each trillion-year cycle. Thus an inflation phase is unneeded, ans the extra dollop of inflation-dark energy that only exists for 10-30 seconds is unneeded.

By the way, the branes are imagined as colliding and bouncing off one another. It seems more logical to me that they pass through instead.

In the past 25 years, increasingly precise experiments, culminating with the WMAP spacecraft, have provided support for five characteristics predicted by both inflation and cyclic models. There is a sixth characteristic that must be tested, which can distinguish between them, related to detecting gravity waves, or at least some of their effects. It'll probably take another twenty years or so.

In the meantime, I hope somebody discovers new information in a few areas of interest to me:
  • Concerning the cosmological red shift, currently interpreted to mean the universe is expanding according to the Hubble "contant": What is the real ratio of red shift due to expansion and red shift due to gravitational potential caused by dark matter and dark energy, over the billions of parsecs light crosses to reach us?
  • Concerning the measurement of accelerated expansion by distant Type Ia supernovae: Is the brightness of a Type Ia supernova really the same across all values of metallicity? In particular, in the very early universe, Z was less than 1/10,000th the current value for Population I stars (it actually began as zero, but the first stars were H-giants that quickly blew up, creating a whiff of the oldest metallic elements). It seems incredible to me that the UV-absorbing metallic content of more recent supernovae could allow the brightness to rise to the same level. I expect a slight difference in maximum brightness and time scale up and down the brightness curve. Acceleration might then disappear.
  • Concerning the "nearly scale-free" spectrum of anisotropy found by WMAP, particularly its "red tilt": At what point does the "slight" decrease of amplitude with decreasing size turn into a pink noise (1/f) spectrum? It has to at some time, or the implied energy is infinite per finite volume. White noise and "pinkish" noise imply infinity, while a pink spectrum, or one between pink (1/f) and brown (1/f2) is needed for a finite energy density.
The engaging tone and humble presentation make the book a refreshing alternative to the take-it-or-leave-it arrogance of many "popular" science books. The authors make it clear that the theory they espouse is by no means proven yet, nor has the competing theory been disproved. The evidence is leaning in their favor, just enough that it seems a viable idea.

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