Archive for Alan Guth

The Early Stages of Cosmic Inflation

Posted in The Universe and Stuff with tags , , , , on July 13, 2017 by telescoper

When asked to cite the article that first presented the theory of the inflationary Universe most cosmologists would probably offer the famous paper published by Alan Guth in 1981.

However, I recently stumbled across a paper by Demosthenes Kazanas that was published (in the Astrophysical Journal Letters in 1980. I hadn’t seen this paper before a few days ago, and I don’t think it is very well known. Here is part of the front page:

You can get the full paper here.

I know that there were other papers floating around in 1980 that got part of the way to the theory of inflation, but this one seems very close to the theory, e.g. talking about exponential expansion in the context of the cosmological horizon problem.

Interestingly, while Guth (1981) has garnered many thousands of citations, Kazanas (1980) has been cited fewer than 300 times.

Does anyone know the story of this paper, and why it has largely  been overlooked by the exponentially-expanding literature on cosmic inflation? And,while I’m on the topic, can anyone suggest other early contributions to the theory that have been similarly neglected? Please let me know through the comments box below.




Inflationary Perturbation

Posted in The Universe and Stuff with tags , , , , , , on May 11, 2017 by telescoper

I thought I’d just draw the collective attention of my vast readership (Sid and Doris Bonkers) to a bit of a row that has broken out between two groups of cosmologists concerning the theory of cosmic inflation.

This kerfuffle started with an article entitled Pop Goes The Universe in Scientific American by Anna Ijjas, Paul Steinhardt, and Avi Loeb that (among other things) asserts that inflation “cannot be evaluated using the scientific method” and is consequently not a scientific theory. Another group of cosmologists (including Alan Guth, the author of the paper that launched the inflationay universe model) penned a response that was signed by a long list of leading scientists, thirty-three of them to be precise. The original authors then issued a response to the response. Sean Carroll (who was one of those who signed the response the original paper has written a nice blog post summarizing the points of disagreement.

I’m not going to attempt to post a detailed response to every issue raised in this correspondence, but I will make a few points.

First, I think it’s important to realize that there isn’t a single simple definition of `the scientific method’: there are lots of scientific methods, each of which may employed to a greater or lesser degree in different disciplines. Most scientists would probably agree that some notion of `testability’ has to be included if a theory is said to be scientific, but it seems to me that testability is not an absolute, in the sense that not all predictions of a theory need to be observable for the theory as a whole to be testable to a degree. A theory might predict the existence of a phenomenon A that is impossible for all practical purposes to observe, but if that theory also has another necessary consequence B that is observed then the theory does not deserve to be dismissed as unscientific.

One aspect of modern inflationary theory that is singled out for criticism has been the incorporation of the idea of a multiverse. I have to make the confession here that I don’t like the concept of the multiverse, nor do I like the way it has become fashionably mainstream in the field. I’ve never seen it as a necessary (or even useful) addition to inflation theory. However, suppose you have a model of inflation that leads to something like Linde’s version of the multiverse. Causally disconnected domains of this multiverse may indeed not be observable, but if the theory has other necessary implications for things we can observe in our local universe then it is testable to a degree.

My position (such as it is) is that I like the idea of inflation, largely because: (a) it’s very neat; and (b) it provides a simple mechanism for generating fluctuations of the right form to account for so many of the observable properties of our universe, especially the fluctuations we measure in the cosmic microwave background seen by Planck:

These observations don’t prove that inflation is right, nor do they narrow down the field of possible inflationary models very much, but they do seem to be in accord with the predictions of the simplest versions of the theory. Whether that remains true for planned and future observations remains to be seen. Should someone come up with a different theory that matches existing data and can account for something in future data that inflation can’t then I’m sure cosmologists would shift allegiance. The thing is we don’t have such an alternative at the moment. Inflation is the preferred theory, partly for want of compelling alternatives and partly because we need more data to test its predictions.

That said, there are one or two points on which I agree with Ijjas, Steinhardt and Loeb. In particular there has developed what I consider to be a pathological industry dreaming up countless variations of the basic inflation model. There is now a bewildering variety of such models, few of which have any physical motivation whatsoever. I think this is a particularly a grotesque manifestation of the absurd way we measure scientific `success’ in terms of counting publications and how that has driven unhealthy research practice.

No doubt many of you disagree or wish to comment for other reasons either on the original communications or on my comments. Please feel free to offer your thoughts through the box below!

Inflationary Opinion Poll

Posted in The Universe and Stuff with tags , , , , , on February 28, 2014 by telescoper

Compare and contrast this abstract of a paper on the arXiv from Guth et al. from last year:

Models of cosmic inflation posit an early phase of accelerated expansion of the universe, driven by the dynamics of one or more scalar fields in curved spacetime. Though detailed assumptions about fields and couplings vary across models, inflation makes specific, quantitative predictions for several observable quantities, such as the flatness parameter (Ωk=1−Ω) and the spectral tilt of primordial curvature perturbations (ns−1=dlnPR/dlnk), among others—predictions that match the latest observations from the Planck satellite to very good precision. In the light of data from Planck  as well as recent theoretical developments in the study of eternal inflation and the multiverse, we address recent criticisms of inflation by Ijjas, Steinhardt, and Loeb. We argue that their conclusions rest on several problematic assumptions, and we conclude that cosmic inflation is on a stronger footing than ever before.

and this one, just out,  by Ijjas et al.:

Classic inflation, the theory described in textbooks, is based on the idea that, beginning from typical initial conditions and assuming a simple inflaton potential with a minimum of fine-tuning, inflation can create exponentially large volumes of space that are generically homogeneous, isotropic and flat, with nearly scale-invariant spectra of density and gravitational wave fluctuations that are adiabatic, Gaussian and have generic predictable properties. In a recent paper, we showed that, in addition to having certain conceptual problems known for decades, classic inflation is for the first time also disfavored by data, specifically the most recent data from WMAP, ACT and Planck2013. Guth, Kaiser and Nomura and Linde have each recently published critiques of our paper, but, as made clear here, we all agree about one thing: the problematic state of classic inflation. Instead, they describe an alternative inflationary paradigm that revises the assumptions and goals of inflation, and perhaps of science generally.

I’m not sure how much of a “schism” (to use Ijjas et al.’s word) there actually is, but it seems like an appropriate subject for a totally unscientific Friday lunchtime opinion poll:

Inflation and the Multiverse

Posted in Astrohype, The Universe and Stuff with tags , , , , , , on January 6, 2014 by telescoper

I was quite excited when I discovered, via Twitter, a paper on the arXiv with the title Quantum Fluctuations in Cosmology and How They Lead to a Multiverse, which was written by one of the architects of the inflationary universe scenario, Alan Guth. Despite numerous attempts to understand the argument how inflation leads to a Multiverse I’ve never really succeeded. To me it always seemed like  a version of the Mind Projection Fallacy inspired by a frequentist interpretation of probability: the construction of notional ensembles for the purposes of calculation in quantum mechanics does not imply that such ensembles are realized in nature. In fact I’ve never found much more substance in articles about this issue than the assertion that Quantum Physics = Woo! = Multiverse.

Anyway, since the paper I found is a review article I hoped it would help teach me the error of my ways. Here is the abstract

This article discusses density perturbations in inflationary models, offering a pedagogical description of how these perturbations are generated by quantum fluctuations in the early universe. A key feature of inflation is that that rapid expansion can stretch microscopic fluctuations to cosmological proportions. I discuss also another important conseqence of quantum fluctuations: the fact that almost all inflationary models become eternal, so that once inflation starts, it never stops.

My eye was drawn to the phrase “almost all inflationary models”.  I had hoped to see “almost all” used in its strict mathematical sense, ie “apart from a set of measure zero” with the measure being fully specified. Disappointingly, it isn’t.   Guth discusses the consequences of the tail  the inflationary potential V (for large values of the inflaton field ϕ) on the long-term evolution of inflationary dynamics and then states

Since V3/2/|V ′| grows without bound as ϕ → ∞ for most potentials under consideration, almost all models allow for eternal inflation.

This means, to me, most models people have constructed but doesn’t mean all possible models. I don’t doubt that some inflationary models  become eternal, but would have preferred a more rigorous statement.  This is particularly strange because Guth spends the last section of his paper discussing the “measure problem”:

While the multiverse picture looks very plausible in the context of inflationary cosmology — at least to me — it raises a thorny and unsolved problem, known as the “measure problem.” Specifically, we do not know how to define probabilities in the multiverse.

The measure problem to my mind also extends to the space of all possible inflationary theories.

And then there’s the title, which, I remind you, is Quantum Fluctuations in Cosmology and How They Lead to a Multiverse. Guth’s argument consists of going through the (standard) calculation of the spectrum of cosmological density fluctuations (which does fit a host of observational data). He then states:

Since the density perturbation calculations have been incredibly successful, it seems to make sense to take seriously the assumptions behind these calculations, and follow them where they lead. I have to admit that there is no clear consensus among cosmologists, but to many of us the assumptions seem to be pointing to eternal inflation, and the multiverse.

I have to admit that I get a bit annoyed when I read a paper in which the actual conclusions are much weaker than implied by the title, but that seems to be par for the course in this field.

For the record, I’ll state that I am an agnostic about the multiverse. It may be a correct idea, it may not. I will say, however, that I still haven’t found any article that puts it on a firm scientific footing. That may well, of course, just be a measure of my ignorance. If you know of one, please let me know through the comments box.