The Inflationary Universe: The Quest for a New Theory of Cosmic Origins [Paperback]

During the 1970s Alan Guth helped start a cosmological revolution. Oddly, it has gone largely unnoticed outside professional science, despite the fact that it has scientific, philosophical, and theological implications every bit as mind-boggling as Einstein's revolution of seventy years ago. The idea looks boring, at first. There are certain technical problems with the 'standard' Big Bang theory. So Guth and others develop the idea that the universe underwent 'inflation'-expanding many times faster than the standard theory allows in the first fraction of a fraction of a second of time. This hypothesis, with its fishy, rather ad hoc air, solves the technical problems. But in fact it solves so many problems, and with such spooky neatness, that physicists start to suspect it might be true. As physicists will, they invent an expensive satellite-based experimental test. And the experimental result maps the theory's predictions exactly. By this time, the theory's originators have started to see that this newly confirmed theory has some very, very bizarre consequences. One is that the observable universe (the bubble, radius about 15 billion light years, that we can observe, and which most people confuse with the universe itself) must in fact bear about the same relation to the whole universe as a grain of dust does to the Earth. Another is that this unobserved greater "universe" is, in turn, almost certainly just a "bubble universe" within a creation that is incomprehensibly many orders of magnitude larger and older. Popular science, like liquor, comes in many strengths, and this one is not for the faint of kidney. Although wonderfully lucid (given the subject matter), and charmingly self-deprecating, it tells the recent history of cosmology seriously, carefully, and with a real mission to teach the hard stuff. The result is some long chapters on some very difficult ideas. Magnetic monopoles, false vacuums, and quantum tunnelling in the Higgs field are especially hard on the cranial muscles. But Guth knows what he is doing. One of the best things about the book is the clear explanation of how very speculative theories really are rooted in observation-of how, for example, it's possible to test and judge competing views of what happened 10-to-the-minus-37 seconds after God snapped his fingers. On top of the long, intricate main text, this book has lots of footnotes and four appendices. I guzzled the lot-and I still don't understand energy density in the Higgs field. But if there's a prize for the decade's best science book for the general reader, this should be on the short list.

Alan Guth is one of the outstanding physicists of our times, and it feels great to read this book written about his own discovery. The author reveals one of the deepest secrets about our universe called cosmic inflation. The book documents the drama in his life as a physicist, and his struggle to make a decision about working in the area of monopoles, when he has doubts about his own strength in the field. Being cautious about his calculations and conclusions, feeling low when discouraged, and sometimes ridiculed by his superiors, yet at the same time feeling triumphant when his peers find his work outstanding. The author describes his experience beautifully.

A brief summary of this book is as follows: A natural consequence of the properties of Grand Unified Theories (GUT) is that the universe at a very young age may have gone through a very rapid expansion in a very short time and then returned to a more leisurely rate of growth dictated by the standard big bang theory. Hubble's constant illustrate that the universe is undergoing homogeneous expansion. The first turning event for the author is when physicist Bob Dicke proposed that the value of omega (the ratio of actual mass density to critical mass density) at one second after the big bang was between 0.999999999999999 and 1.000000000000001. If the value was less than this then the universe would have expanded forever and galaxies would never had time to form; on the other hand if the value was slightly more than this, then the universe would have collapsed sooner before it had any chance to grow to this big. This ratio was expected to be very close to one, this is called flatness problem: The big bang theory has no explanation for this ratio. According to general relativity, the mass density of the universe not only slows the cosmic expansion, but it also causes the universe to curve. If the mass density is higher than the critical density; then space curves back on itself forming a spatially closed universe. In such a universe, the sum of the angles a triangle is more than 180°. If the mass density is less than the critical density, then the space is curved in the opposite sense, and the sum of the angles of a triangle would be less than 180°. If the mass density is equal to the critical density, the space is known to be flat, which means that an ordinary Euclidean geometry is valid (the sum of the angles in a triangle is 180°.) The observed universe is remarkably balanced to stay flat!

The second turning point for the author is when physicist Steven Weinberg accounted for the large asymmetry between the baryons (matter) and anti-baryons (antimatter). Since in the early universe protons and neutrons did not exist, but only quarks existed. His calculations showed that for every 300 million quarks there was equal number of anti-quarks less one. At 10exp(-6) seconds after big bang, all matter was annihilated by anti-matter, and the residual matter resulted in the present abundance of matter, but he did not offer an explanation what caused this matter-antimatter inequality. The third problem that needed an explanation is that the big bang theory of Friedman, Lemaitre, Hubble, and others describes the universe as a giant refrigerator cooling and expanding forever, and the remnants of this bang still exists today as an afterglow of 2.7K background microwave radiation. The large scale uniformity of the observed universe is clearly reflected in this, which is known to have been released after 300,000 years of the big bang (before this time the universe was too foggy (dense) for the glow to appear). It has the same temperature in all directions with a remarkable accuracy. Simple calculations show that at this time of the universe, the opposite side of the universe would be separated by a distance 100 horizon distances (90 billion light years), and since light can not travel more than one horizon distance due to specail relativity, it needed an explanation. This is referred to as the horizon problem.

Prior to 10exp(-37) seconds after the big bang, the radius of the universe was only 10exp(-52) meters. At this time GUT predicts that the super-hot matter would have undergone phase transition (sudden change in the behavior of the matter) and the universe went through a tremendous exponential expansion called inflation to spectacular size of 10exp(23) times the size of visible universe. The inflation is driven by the physics of GUT, a patch of false vacuum, volume 10exp(-26) cubic centimeters and a mass of 10exp(-32) solar masses (about 25 grams); this corresponds to a density of (10exp(80) per cubic centimeter; lead to inflation and thus the universe evolves out of nothing at all. The author calls this an ultimate free lunch.

The mechanism of inflation involves false vacuum, which is a peculiar form of matter that existed in the form of fields. This is meta-stable and has negative pressure which creates repulsive gravitational fields that can drive the universe into a period of exponential expansion. After more than 100 doubling time, the inflation stops by forming bubbles. The later theories propose that inflation continues as the fields roll gently towards a minimum energy value, and a single bubble becomes large enough to encompass the entire observed universe. This is called graceful exit problem because end inflation preserves the uniformity. This also solves the horizon problem, the flatness problem and also generates density perturbations that would later become the seeds for galaxy formation. As long as the exponential expansion continues long enough; the value of omega reaches one with great accuracy. As inflation proceeds, the matter that was present at the beginning would be diluted to irrelevance, while space becomes filled with the exquisitely uniform mass density of the false vacuum.

1. The Inflationary Universe: Quest for a New Theory of Cosmic Origins
2. The Infinite Cosmos: Questions from the frontiers of cosmology
3. Particle Physics and Cosmology

This book is an excellent introduction to the theories behind modern cosmology. Although it professes to be accessible to the non-physicist, it must be said that one or two of the topics covered (especially the chapters on field theories) are quite advanced and possibly inaccessible to the lay reader. Indeed the author even advises nonexperienced readers to skip past these chapters. All in all though, the dearth of excellently rendered diagrams and well-thought out narration by the author make this possibly the most enlightening book on cosmic origins since "The First Three Minutes".