There’s a new popular book about cosmology now out on bookstore shelves, Endless Universe by Paul Steinhardt and Neil Turok. The authors are the inventors of a competing model to inflationary cosmology, variously called “ekpyrotic” or “cyclic” cosmology. They describe coming up with the same idea simultaneously during a lecture at Cambridge on M-theory by Burt Ovrut back in 1999. Ovrut was describing his work on the Horava-Witten scenario, which involves two parallel branes (we live on one of them), and during the lecture both Steinhardt and Turok started wondering about whether one could explain the big bang as a collision of branes. They went up to discuss this with him after the talk, and continued the discussion on a train ride to London that evening to see a performance of the play Copenhagen. This train ride was a central part of a 2002 BBC TV program Parallel Universes and a recent play Strings by Carole Bugge that I saw performed here in New York late last year.
The book is very much an advertisement for cyclic cosmology, and devotes a lot of space to doing something which is rarely done, explaining the problems with inflationary cosmology. Steinhardt has worked extensively on coming up with viable inflationary models, and a large part of the book explains the story of this research. Perhaps the most interesting aspect of the book is the story that Steinhardt and Turok each tell about their careers and how they ended up working together on this alternative to inflation. The problems with inflation are described in the context of promoting their cyclic model of branes colliding, moving apart and then back together in a repeating pattern. They heavily sell the idea that a cyclic model with no beginning of time is conceptually much preferable to a standard inflationary model in which the universe emerges at a given time. Reading this made clear to me why, at the recent String Cosmology meeting here in New York, Turok was so persistently questioning one speaker about whether everything he was doing didn’t just depend on an unmotivated choice of initial conditions.
Steinhardt and Turok do a pretty good job of demolishing the inflationary multiverse and the associated Anthropic Landscape philosophy that has become so popular in recent years. They correctly describe the main problem with the inflationary multiverse as the lack of any way to test the idea, even in principle, making it not really a scientific idea at all. As for the testability of their own theory, they devote an entire chapter to the question of contrasting its predictions for the CMB with those of inflation. They claim that both cyclic and inflationary cosmology make the same predictions for the WMAP results (implicitly criticizing the commonly made argument that WMAP provided strong support for inflation). The one possible test that they point to that could distinguish the inflation and cyclic scenarios is the expected more sensitive measurement in coming years of a possible B-mode polarization signal due to gravity waves in the CMB. They claim that inflation predicts a significant amount of B-mode polarization, whereas the cyclic model doesn’t. Unfortunately, from what I can tell by looking at the recent literature on “string cosmology” (e.g. here), various inflationary scenarios can give a wide range of amounts of such polarization, with stringy models like “brane inflation” and “modular inflation” leading to essentially none, just like in the cyclic case. So, I guess the cyclic model is in principle falsifiable, if next generation CMB experiments turn up measurable B-mode polarization. But if this doesn’t happen, I don’t see how one is ever, even in principle, going to distinguish experimentally between the cyclic and inflationary scenarios, which will make this whole area of research highly problematic.
On the whole the book seems to me to be too much of an advertisement for a very speculative idea, and I don’t think the public needs more of this in this kind of format. I didn’t notice anything in the book about what the case against cyclic cosmology might be, so anyone who wants to find out the other side would have to go on a search of the scientific literature, something most members of the public might not be able to do. Most strikingly, since the cyclic model is based on brane ideas motivated by string theory, the book contains endless hype about string theory, without so much as a word about its problems. One would have to read extremely carefully to realize that there is not a shred of experimental evidence for string theory. As for recent public debates about the problems of string theory, the authors just pretend they don’t exist. They give a long list of recent popular books in this area, such as those of Susskind and Vilenkin promoting the Landscape, but somehow neglect to include the two such books that have taken a critical point of view on string theory.
Associated with publication of the book, it looks like there will be various stories in the media promoting the cyclic vs. inflationary debate, trying to make it into a modern version of the old steady-state vs. Big Bang controversy. On the Edge web-site there’s a recent piece by Turok, which gives a good explanation of his current research and point of view. From NPR, there’s a very recent radio show about the cyclic model, entitled Forget the Big Bang Theory, where “renegade physicist” Turok’s model is described as “fighting words in the halls of science.”
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