State of the Cosmos Address Offered
On the occasion of the centennial of Einstein’s theory of relativity, Alan Guth, the father of inflationary cosmology, with colleague David I. Kaiser of MIT, took stock of cosmological theories in the Feb. 11 issue of Science.1 How has inflation fared since its controversial but hopeful proposal in 1981?
“Inflation was invented a quarter of a century ago,” Guth begins (emphasis added in all quotes), “and has become a central ingredient of current cosmological research.” Advances in particle physics have led to a theory, the standard model, that can account for three of the four basic forces – strong and weak nuclear forces and electromagnetism – but not gravity. String theorists, independently, have been working for their own unification of these forces. Guth repeats that amidst all this ferment, “inflation continues to occupy a central place in cosmological research, even as its relation to fundamental particle physics continues to evolve.” From there, he diverges into a primer on inflation. What some had described as a bizarre, untestable, ad hoc invention to get around serious problems in big bang models, he unashamedly portrays as a great success:
Without inflation, this large-scale smoothness appears quite puzzling. According to ordinary (noninflationary) big bang cosmology, these photons should never have had a chance to come to thermal equilibrium: The regions in the sky from which they were released would have been about 100 times farther apart than even light could have traveled between the time of the big bang and the time of the photons’ release. Much like the flatness problem, inflation provides a simple and generic reason for the observed homogeneity of the CMB: Today’s observable universe originated from a much smaller region than that in the noninflationary scenarios. This much-smaller patch could easily have become smooth before inflation began. Inflation would then stretch this small homogeneous region to encompass the entire observable universe.>
Guth points to small-scale perturbations, or ripples, in the cosmic microwave background (CMB) as also supportive of his inflation idea, mainly because other proposals have been ruled out. While “full class of inflationary models can make a variety of predictions,” he says, the simplest model “fits the data beautifully” (see 06/18/2003 and 06/12/2001 entries for contrary views).
With such an admirable track record behind him, Guth turns to how research on inflation has progressed. Some have questioned that, once started, inflation could have ever stopped again: the “eternal inflation” problem. Others wonder how ordinary matter would have arisen when inflation effectively dropped the temperature to zero and diluted the density of ordinary matter to negligible quantities. Particles were created, he explains by oscillations that set up resonances between quantum fields: “Large numbers of particles would be created very quickly within specific energy-bands…. This dramatic burst of particle creation would affect spacetime itself, as it responded to changes in the arrangement of matter and energy.”
Guth also discusses how inflation fits in with brane cosmology (see 04/26/2002 entry) and string theory, insisting it is compatible with either. He seems to like the latter, because it produces a story of two lovers who need each other:
The union of string theory and cosmology is barely past its honeymoon, but so far the marriage appears to be a happy one. Inflation, from its inception, was a phenomenologically very successful idea that has been in need of a fundamental theory to constrain its many variations. String theory, from its inception, has been a very well-constrained mathematical theory in need of a phenomenology to provide contact with observation. The match seems perfect, but time will be needed before we know for sure whether either marriage partner can fulfill the needs of the other. In the meantime, ideas are stirring that have the potential to radically alter our ideas about fundamental laws of physics.
In fact, with brane theory, there seems to be a happy threesome in the offing. The milieu of proposals, each with its suite of variables (some 10500 possible inflating/vacuum states in string theory, for instance) leaves the reader with a sense of an infinite combination of possibilities with little hope for picking the right one to build the universe we know:
Although the rules of string theory are unique, the low-energy laws that describe the physics that we can in practice observe would depend strongly on which vacuum state our universe was built upon. Other vacuum states could give rise to different values of “fundamental” constants, or even to altogether different types of “elementary” particles, and even different numbers of large spatial dimensions! Furthermore, because inflation is generically eternal, one would expect that the resulting eternally inflating spacetime would sample every one of these states, each an infinite number of times. Because all of these states are possible, the important problem is to learn which states are probable. This problem involves comparison of one infinity with another, which is in general not a well-defined question. Proposals have been made and arguments have been given to justify them, but no conclusive solution to this problem has been found.
Guth explains that no one has been able to explain why our universe took the initial state it did: i.e., whether its state was determined or random. Maybe the escape clause is to believe that all possible states exist, and we observe the one that produced observers (the anthropic principle). Guth seems surprisingly warm to this idea that produced a “privileged planet” by chance:
Another possibility, now widely discussed, is that nothing determines the choice of vacuum for our universe; instead, the observable universe is viewed as a tiny speck within a multiverse that contains every possible type of vacuum. If this point of view is right, then a quantity such as the electron-to-proton mass ratio would be on the same footing as the distance between our planet and the sun. Neither is fixed by the fundamental laws, but instead both are determined by historical accidents, restricted only by the fact that if these quantities did not lie within a suitable range, we would not be here to make the observations. This idea—that the laws of physics that we observe are determined not by fundamental principles, but instead by the requirement that intelligent life can exist to observe them—is often called the anthropic principle. Although in some contexts this principle might sound patently religious, the combination of inflationary cosmology and the landscape of string theory gives the anthropic principle a scientifically viable framework.
(See also 02/05/2002 entry on multiple universes.) One particularly shocking example of anthropic parameters is the energy density of the vacuum (see 09/30/2004 entry) which, according to naive estimates, could be up to 10120 times as high as that which is observed, even with dark energy (see 02/28/2004 entry). Puzzles like the anthropic principle reinforce the necessity of asking cosmological questions:
There are both positive and negative contributions, but physicists have been trying for decades to find some reason why the positive and negative contributions should cancel, so far to no avail. It seems even more hopeless to find a reason why the net energy density should be nonzero, but 120 orders of magnitude smaller than its expected value. However, if one adopts the anthropic point of view, it was argued as early as 1987 by Weinberg that an explanation is at hand: If the multiverse contained regions with all conceivable values of the cosmological constant, galaxies and hence life could appear only in those very rare regions where the value is small, because otherwise the huge gravitational repulsion would blow matter apart without allowing it to collect into galaxies.
The landscape of string theory and the evolution of the universe through the landscape are of course still not well understood, and some have argued that the landscape might not even exist. It seems too early to draw any firm conclusions, but clearly the question of whether the laws of physics are uniquely determined, or whether they are environmental accidents, is anissue too fundamental to ignore.
Guth repeats the usual “precision cosmology” rhetoric that our instruments are nailing down the values of fundamental cosmic properties (see 09/20/2004 entry). But inflation is not such a precise quantity; in his conclusion, he admits that much work needs to be done (see 12/21/2000 and 05/30/2001 entries):
Even with the evidence in favor of inflation now stronger than ever, much work remains. Inflationary cosmology has always been a framework for studying the interconnections between particle physics and gravitation—a collection of models rather than a unique theory. The next generation of astronomical detectors should be able to distinguish between competing inflationary models, whittling down the large number of options to a preferred few.
Hopefully, those detectors will also solve some remaining “major puzzles” such as the nature of dark matter and dark energy, which combined are said to make up 96% of the universe, leaving a mere 4% that we observe (see 06/20/2003 and 12/17/2003 entries). “Whatever its origin, dark energy, much like dark matter, presents a fascinating puzzle that will keep cosmologists busy for years to come.” (See also06/04/2002 entry, and 11/02/2002 entry and commentary.)
1Alan H. Guth and David I. Kaiser, “Inflationary Cosmology: Exploring the Universe from the Smallest to the Largest Scales,” Science, Vol 307, Issue 5711, 884-890, 11 February 2005, [DOI: 10.1126/science.1107483].
We had to show you in their own words what these MIT eggheads are saying. Guth, whose name stands for Grand Unified Theory Huckster, has been propounding his “framework” for 25 years now, and has become famous for it. But what is inflation, other than an untestable, ad hoc proposal invented to get around insurmountable obstacles in the Big Bang cosmology of the 70s? Astronomers were well aware of the flatness problem and the horizon problem; with a sweep of the hand and some abstruse math, con artist Guth in his magic show wagon said “no problem,” we’ll just stretch the universe and the problems will no longer be visible. A viewer objects that he has just diluted the particles to negligible density. “No problem” again; we’ll pick the right vacuum state to make quantum fields resonate, such that their energy produces new particles out of nothing. Another viewer objects that one cannot determine the conditions by chance to rig the outcome. ”Well, then,” the huckster chimes, “if it were not so, we would not be here arguing about it now, would we? Hmmmmm?”
Don’t be fooled by Guth’s shameless claims that observations are confirming his little something-from-nothing trick (see 06/23/2001 entry). When he invented inflation to get around known problems, he cannot turn around and say that his trick predicted that it would solve them. The fact that he turns to the “patently religious” anthropic principle is a clear sign of desperation. His model does not account for the finely-tuned parameters of the universe that permit galaxies, stars and life, and invoking an infinity of universes to keep chance in the running is patently unscientific. Don’t be fooled by the math, either; it just means he got good grades in calculus and knows how to move Greek symbols around according to some rules. No amount of mathematical manipulation can save a proposition from bad assumptions. When your math is off by 120 orders of magnitude and forces you to compare infinities, you have lost all contact with reality; you’re just playing games.
Guth and Kaiser need to take up truck driving. That would get them out of their ivory towers at MIT and into the real world, where they would be forced to look at trees, mountains, weather, ecology and all the other observable things on our privileged planet that are inexplicable by chance: realities that proclaim design, purpose, intention. While driving down the road, Guth should pop in a cassette of Bob Berman’s hellfire sermon (see 10/06/2004 entry).