The Scientific American article, Pop goes the universe (LINK), elicited an odd response from our colleagues Alan Guth, Andrei Linde, David Kaiser, and Yasunori Nomura. Rather than following the common practice of writing a Letter to the Editor under their own names, they asked 29 leading scientists to co-sign their criticism of our article.
Because Scientific American could only provide limited space for us to respond (see here), we elaborate in the FAQs section below. We also urge you to read our original Scientific American article, especially the four concluding paragraphs.
Frequently Asked Questions
What is the fuss about? It all concerns the question of whether the inflationary theory makes any predictions or not, and, hence, satisfies the essential criterion of what makes a theory scientific.
How could this be controversial? Because our understanding of the inflationary theory has evolved substantially over the last three decades. What began in the 1980s as a theory that seemed to make definite predictions has become a theory that makes no definite predictions. Nevertheless many scientists judge the theory asssuming the original, now outdated version.
What has changed about the inflationary theory? Originally scientists thought that the outcome of inflation (a smooth, flat universe with a certain spectrum of density fluctuations and gravitational waves) was generic. Now we know it is not.
What do you mean? Inflation has two major problems:
First of all, we have learned that inflation is highly sensitive to initial conditions. This is the opposite of what everyone thought originally.
For example, in the 1990s, by considering different initial conditions and parameters, Linde (and others) championed models of inflation that would lead to an open universe rather than a flat universe, because, at the time, observations seemed to point that way. See, e.g., our Fact Checking section (LINK). We do not hear about these models any more because later measurements showed the universe to be flat.
Second, we have also learned that inflation generically produces a multiverse (“multimess”) of outcomes – literally an infinite number of patches with an infinite diversity of possibilities - and there is currently no criterion to prefer one possibility over another. As Guth has put it, “In an eternally inflating universe, anything that can happen will happen; in fact, it will happen an infinite number of times. Thus, the question of what is possible becomes trivial—anything is possible […] The fraction of universes with any particular property is therefore equal to infinity divided by infinity—a meaningless ratio.” See, highlighted text in the Conclusion section of Guth's paper published in J.Phys. A40, 2007 (LINK). In other words, there is nothing that says that what we observe in our patch is typical or could be predicted a priori on the basis of the theory.
Are the respondents disagreeing that inflation has these two features/problems? No. They admit them. They want us to judge the inflationary theory as originally conceived – before we knew about the two major problems.
Would you agree that inflation inspired lots of important experiments and ideas? Absolutely, including even many of our own ideas. However, just because an idea inspires does not mean that it has predictive power in the scientific sense. This is an old issue, extensively discussed among philosophers of science.
Is that because these are new problems and there has not been time yet to tackle them? No, not at all. These problems have been known for over three decades. They were discovered just a few years after inflation was introduced and, since that time, many theorists (including some of us) have tried all kinds of ideas to avoid or resolve them. But the problems have turned out to be much thornier than expected, and they remain unresolved today.
So, if the respondents agree that there are these problems, as their letter plainly shows, how can there be a conflict with what you wrote? Their objection is essentially to the last section of our paper – which we hope you will reread for yourself – it is four paragraphs and less than one column of text. We write that, unless the two problems cited above can be resolved, inflation makes no predictions and, hence, is immune to empirical tests. Pure and simple.
Their rebuttal letter presents two conflicting sets of statements: (1) inflationary theories make specific predictions; (2) there are the critical problems of initial conditions and the multimess to be resolved.
Who says that, if we ever find a way to resolve the problems, the predictions will turn out the same? No one knows. It is simply an optimistic guess.
Is your article principally about making the case “for a bouncing cosmology, as was first proposed by Steinhardt and others in 2001,” as the respondents claim? No. The paper is almost entirely about the inflationary theory, pointing out that it has two historic problems — initial conditions and the multiverse — that destroy the predictive power of the theory. And furthermore, to show that recent observations by the WMAP and Planck satellites force theorists to consider versions of inflation that make those problems worse. We only briefly mention bounces (the penultimate section) to show that theorists are thinking of ideas that avoid these problems. There are many bouncing cosmologies and we do not mention any particular one.
What about the comparison to the Standard Model of Particle Physics? This comparison is a false equivalency. For the Standard Model, there are definite predictions for any choice of parameters. For Inflation, there is an infinite diversity of outcomes for any choice of parameters (i.e., for any choice of the inflationary energy curve). For example, for any one choice of parameters, an infinite number of patches of space in the multiverse are produced that are not flat, not smooth, and do not have the properties astronomers observe – and there is nothing in the inflationary theory to say that one outcome is more likely than the others. The same does not apply to the Standard Model of Particle Physics.
What about the statement that the “well-defined” models have already made successful predictions? This is not true for the reasons described above – you cannot say what any inflationary model predicts unless you can first resolve the problems of initial conditions and the multimess.
It should also be noted that the so-called well-defined examples are being set by the authors after the fact – that is, after the observations have been made. Was the flurry of inflationary models designed in the early 1990s by Linde and others to give an open universe “well-defined”? Or were those authors sloppy and indefinite? What about the many authors that thought they had inflationary models that predict large non-gaussianity? Large tilt? Or deviations from isotropy? Or bumps and wiggles in the microwave background power spectrum? - All features that the WMAP and Planck Satellite experiments have ruled out.
And how about all the textbook models that were thought to predict B-modes with amplitudes that should have been detected by the WMAP and Planck satellites? The same models celebrated by many of the respondents when they thought that cosmic B-modes had been discovered? Are we supposed to assume that all these other models were poorly-defined and only those that turned out to agree with current observations were well-defined? Or could this possibly be an exercise in 20-20 hindsight?
So, what is your bottom line? As we emphasized in the concluding paragraphs of our Scientific American article, this is not an issue that can be resolved by invoking authority. We urge all our colleagues to focus on solving the outstanding problems of inflation and keeping an open mind about other, yet unknown theories that avoid these problems altogether. The fact that there are these problems should be viewed as exciting: it means there is room for important new discoveries – maybe even a paradigm shift.
Why did you decide to publish this argument in Scientific American? First of all, it is good common practice for the public to be informed about scientific debates. That is one of the roles of popular magazines like Scientific American. This is also not the first time the debate has been aired in public. There are many popular articles about the multiverse, for example, which include a variety of views.
In this case, Scientific American invited us to write the article following the appearance of refereed articles in Physics Letters B. We make it clear at various points in the article that these are our own views and that many cosmologists disagree.