Inflationary Perturbation

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!

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56 Responses to “Inflationary Perturbation”

  1. It is not because you believe in Inflation that Inflation is real.

    Faith may be necessary to develop a good theory but it must not be used to promote it!

    Due to the fact that Inflation was based on a number of hypothetical ingredients which remain unfounded after so many years, it becomes more and more a question of faith rather than science as time goes by…

    To elevated a very unlikely theory to a golden standard status is likely to cause a dark age.

    But for me, the main problem with today’s Physics is the faith in a model which is fundamentally probabilistic. This caused a dark age for Physics… All arguments stating that Nature is fundamentally probabilistic are based on circular and short sighted arguments. It is equivalent to believe in magic.

  2. Shantanu Says:

    Peter, Penrose was the first person to criticse inflation back in 1980s long before COBE and I don’t think anyone took Penrose seriously back then. I think one limitation of all inflationary models is that they don’t address the problem of big bang singularity or other problems of standard big bang (dark energy, dark matter, baryogenesis).

    • telescoper Says:

      Well, up to a point. But inflation doesn’t and can’t be expected to account for anything that relates to a very different energy scale, e.g. the structure of the hydrogen atom…

      • Shantanu Says:

        Well, there are models which remove singularities and generate exponential expansion as a consequence. arXiv:1410.3881
        and arXiv:1510.08834

      • telescoper Says:

        You can remove the singularity in GR by simply violating the strong energy condition (which inflation does, actually). But we don’t expect GR to apply at arbitrarily early times anyway, and we don’t know what to replace it with so “We just don’t know” is the only scientifically justifiable position.

    • telescoper Says:

      I don’t think Penrose was the first. In the early 80s there were a great many inflation sceptics.

    • Penrose’s claim is that the cure is worse than the disease. Inflation essentially does away with the need for some types of fine-tuning, but in order to get inflation to work one needs even more fine-tuning.

      Penrose, despite his highly valued contributions to GR, maths, etc, hasn’t done himself a favour by sticking to unorthodox ideas once they have been debunked. This does, but perhaps shouldn’t, colour the perception of his criticisms of inflation.

      Steinhardt has been grinding the same axe for years.

      Interestingly, Andreas Albrecht, one of the early inflationary aficionados, is also rather critical of it these days, promoting his own alternative theory, but is more gentlemanly about it.

      • Anton Garrett Says:

        I always thought that Penrose’s books on consciousness were driven by something like “I’m an expert on quantum gravity, which we don’t understand properly; now, we don’t understand consciousness properly either, so let’s cross our fingers that there’s a link between the two.”

    • Actually, the original motivation for inflation, back in the USSR (cue a song by a popular Beat combo) in the 1970s, was to avoid the singularity.

  3. “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.”

    Indeed. Who questions GR because what it tells us about the inside of black holes is not observable?

  4. “I have to make the confession here that I don’t like the concept of the multiverse”

    The universe does not care what we like.

    What is your objection to it?

    Do you have a better explanation for observed fine-tuning?

    • Anton Garrett Says:

      What’s wrong with saying that you don’t think we have a satisfactory explanation for something at the present time? If we knew everything then there wouldn’t be any research left to do, would there?

      • Nothing wrong with it, but “don’t like” sounds, to me, different than “no satisfactory explanation available”.

      • telescoper Says:

        I don’t see the problem. I like some ideas and don’t like others. I like physics and don’t like inorganic chemistry, for instance. I hope it’s obvious that does not mean that I deny that inorganic chemistry has any validity.

      • Chalk and cheese. Liking physics and not chemistry is a matter of taste. Not liking the multiverse sounds like you don’t take it seriously.

      • Some theories smell good: they seem to get to the heart of the problem and put up a framework which everything fits into, and take the risk to make clear predictions. GR and QED are examples. Or Newtonian gravity in its day. The multiverse doesn’t have that smell: it feels evasive, living in untestable regions of parameter space. Observational disproving it is like nailing down jelly: it evades the nail and just pops up again slightly changed. That says nothing about whether it is correct or not, but it feels an unsatisfactory way to be right.

  5. This argument over inflation is concerning. One of the biggest problems with academia – certainly on my experience – has been twofold: first, the way that some (not all) academics conflate their own personal self-worth with either their status in the field or a particular ideology or theory; and second, there is a collision between our methodologies of analytical philosophy – particularly the mechanisms of analysis and the need to classify everything within specific theoretical frameworks – and what we are observing. Right now cosmology seems to face some pretty substantial issues that question some of the theoretical orthodoxies of the past century. This doesn’t mean Einstein was wrong – I think it is very clear indeed he nailed the determinist universe. However, as he said himself over the quantum-determinist debate, maybe we were missing something. Right now, it seems to me that cosmologists need more empirical data. I’d hope that meanwhile the debate over our current knowledge of it doesn’t get personalised, but it does look very much as if that’s where it’s going when leading figures in the field start making public declarations basically bagging each other’s positions. Ouch. This sort of behaviour is a problem in the humanities (I keep running into it) but aren’t scientists meant to be above that?

  6. […] Update: For a sensible take on this that I think likely reflects well the views of most mainstream cosmologists, see Peter Coles. […]

  7. Kudos on your rational and level headed thoughts, particularly about evolving scientific methods of discovery and progress in science. Everybody has a different idea on what science is and how it works, but most agree when it moves away from experimental empiricism it gets farther afield from science and closer to philosophy. I’m entirely new to this field and my hypotheses concerning these subjects were only recently committed to print, but to my mind the ‘inflationary era clearly preceded the primordial leptogenesis, baryogenesis and axion production eras and involved radically different quantum spacetime expansion properties than we observe now, That’s really all I can say about it because I hesitate to do much modelling or theory until I get a handle on the basics. I have personally decided that ‘gravitational axions’ are the basics here, and until I personally get a handle on that I’ll demure to the experts. Small steps, Ellie. Patience, Grasshopper. Build axion haloscopes and they will come, in a mass distribution somewhere near the cosmic microwave background radiation and He (I/II)

  8. One of the basic problems I see with the Big Bang Theory goes to the original patch; That when they discovered redshift is directly proportional to distance in all directions, making us appear to be at the center of the universe, the premise of spacetime was invoked to argue that space itself must be expanding, making every point appear as the center.
    This would seem to overlook the central premise of Relativity, that the speed of light remains Constant to the distance. If the light is redshifted due to taking longer to cross an expanding space, it is necessarily not Constant to this distance. More lightyears, not expanding lightyears.
    So two metrics of space are being proposed for the same light; One based on its spectrum, that is expanding and one based on its speed, that is the unspoken denominator.
    Now we are at the enter of our point of view, so possibly some future efforts could go to using an optical explanation for redshift and see how many observed effects could be explained by it.
    Such as if it compounds, redshift would start of gradually and eventually go parabolic. Possibly explaining the curve in the rate currently requiring dark energy.
    Or that after all light is shifted off the visible spectrum, only radiation travels over this horizon line, explaining the cmbr and Olber’s paradox.

    • Conventional cosmology explains all your puzzles. I recommend Edward Harrison’s Cosmology: The Science of the Universe.

      • Phillip,

        Given the topic at hand appears to be cracks in conventional cosmology and this is science, not religion, it is at least worth considering alternate explanations. Among other things, there are second generation stars in our own galaxy that are approximately as old as the universe.
        It’s not as though questions haven’t been raised for years;
        http://www.americanscientist.org/issues/pub/2007/9/modern-cosmology-science-or-folktale/99999

      • While there might be some interesting puzzles, the nature of the cosmological redshift is not one of them.

        Michael J. Disney is the author of the linked article; he is notorious for criticizing cosmology in general, usually with arguments like “faint objects by definition convey little explanation”.

      • In general, just because some things might be unclear, there is no need to question firmly established facts.

      • I’ve just read Mike Disney’s article. If cosmology were a person, she could sue for libel. If I hadn’t run across similar diatribes from him before, I could almost think that it is supposed to be some sort of satire.

      • Phillip,

        Responding to the issue he raises, the Tolman test, would be a far more convincing rebuttal than hand waving.
        Or for my sake, you could show me exactly what the problem is with the issue I raised, how can the premise of spacetime be invoked, to explain redshift, when the light speed is explicitly not Constant to the distance?
        On a more general level, how can the Big Bang Theory actually be considered science, since there is no way to refute it, given that every time observations don’t match predictions, some enormous new force of nature is proposed and accepted?
        It was one thing when the ancient and medieval astronomers did this with epicycles, because they were not totally ignoring a generally accepted theory of science, that of falsifiability, given it had’t yet been proposed, but today, not only is the BBT unfalsifiable, but the exact same method of adding a new cycle, energy, whatever, is being used again.

        You may as well lump me in with Disney.

        Cheers.

      • “Responding to the issue he raises, the Tolman test, would be a far more convincing rebuttal than hand waving.”

        For this to work, you have to know the intrinsic surface brightness. You don’t. Sandage wrestled with this 50 years ago. Even with SINGLE STARS, namely supernova, it is not trivial to calculate the intrinsic brightness.

        “Or for my sake, you could show me exactly what the problem is with the issue I raised, how can the premise of spacetime be invoked, to explain redshift, when the light speed is explicitly not Constant to the distance?”

        Again, read an introductory cosmology book. I suggested a good one above.

        On these two points, if there really were a problem, don’t you think that someone other than Mike Disney (who is not a cosmologist) would point it out?

        “On a more general level, how can the Big Bang Theory actually be considered science, since there is no way to refute it, given that every time observations don’t match predictions, some enormous new force of nature is proposed and accepted?”

        This is a caricature, not a valid description.

        “It was one thing when the ancient and medieval astronomers did this with epicycles, because they were not totally ignoring a generally accepted theory of science, that of falsifiability, given it had’t yet been proposed, but today, not only is the BBT unfalsifiable, but the exact same method of adding a new cycle, energy, whatever, is being used again.”

        No, it’s not. Read up on the history of cosmology. On the contrary, one might be surprised that all current observations can be fitted with 1920s cosmology.

      • Phillip,

        It has been some years, but the history of cosmology books I did read skipped over that issue quite lightly, because the primary consideration was how to explain redshift and the two alternatives were recession and ‘tired light.’ Tired light being dismissed because there was obviously nothing obscuring or diffusing this light that had traveled extreme distances.
        So since cosmology is not a settled science, or dark matter and energy wouldn’t be referred to as ‘dark,’ nor would there be this current kerfluffle over Inflation, are there alternatives to recession.
        Here is just an idea I’ve come across in my various efforts to study the issue;
        Consider this experiment for the loading theory of light;
        http://fqxi.org/data/essay-contest-files/Reiter_challenge2.pdf
        Then consider that in the light of this experiment, for multi spectrum light ‘packets;’
        http://fqxi.org/data/forum-attachments/2008CChristov_WaveMotion_45_154_EvolutionWavePackets.pdf
        Whether it is photons, or simply the pixels that digital cameras, even the very good ones used for astronomy, detect, light is still detected as packets of energy. Obviously it is emitted as quanta, as well.
        So the question I see as interesting is whether it would travel as quanta, or as waves and quanta are samplings of these wave fronts. Aka loading theory.
        Remember that when we detect light from very distant galaxies, we only are getting a very faint amount, essentially a little more than a few photons worth. So if it was only individual photons traveling the entire distance, they would only convey the specific information of the specific incidence from which they were emitted, but we in fact do manage to extract a lot of information from this very minute amount of light, so it stands to reason, at least to me, we are sampling a multi spectrum wave front and so, according to Christov’s above linked paper, such packets would redshift due to distance alone.
        Given I do have to work fairly long hours and in all the reading I’ve done, have not seen the issue even mentioned, other than by me. So is there any link you can give me, that does address the issue of how light can be both redshifted and constant to the distance, because I’ve been been raising this particular issue for about 15 years and no one has yet pointed out where it was settled.

      • What does “constant to the distance” mean?

      • Phillip,

        The speed of light, as in lightyears, is the conventional astronomical unit.

        Relativity correlates measures of distance to duration, using the speed of light in moving frames to observe that any physical unit of measure would be equally compressed, that relating it to observed light, which travels 186000 miles a second in the vacuum, would still measure the same in the moving frame, because the unit of measure has been compressed, equal to
        the amount the frame

        So my question is; If they are going to say the unit measures of of space are expanding, than wouldn’t the speed of light have to increase, in order to remain Constant?

        Yet this is obviously not so. Consider the alternative explanation for redshift, tired light, which tried to find some medium that would slow the light, so the question is very much about the speed of light relative to this distance between galaxies.

        The point is that if two galaxies are x lightyears apart and the universe expands to where they are 2x lightyears apart, what is the basis of this vacuum through which light travels at C, if it is not space, since space, according to BBT, is expanding, while the speed of light remains Constant. There are more lightyears between galaxies, not expanded lightyears.

        As Einstein said, “Space is what you measure with a ruler.”

        Presumably the cosmological ruler is lightyears, yet this ruler is not expanding, only the distance it is being used to measure.

        More lightyears between galaxies, not expanded lightyears. How does space expand, if the ruler used to measure it doesn’t?

        Is the vacuum through which light travels at C, something other than space?

      • Yes, I tried making the same point several different ways, but it does seem to elusive to other people. So I’ll try to compress it;

        “Space is what you measure with a ruler.”

        The cosmological ruler is lightyears.

        If the light of distant galaxies is redshifted because they are moving away, then there are more of these units of measure between galaxies. That is not expanding space. That is increasing distance, in stable space.

      • To answer the specific question; “What does “constant to the distance” mean?”

        The speed of light in a vacuum is 186000 miles a second. A mile is a unit of distance, as in one dimension of space. If the argument is that space expands and that it is relativistic space, then we still ALWAYS measure the speed of light in a vacuum at 186000 miles a second. C. So if space and thus the mile, is expanding and it is relativistic, then the speed of light would have to increase, with this expanding space, in order to always be 186000 miles a second.
        Yet that is not the argument. Which is that this expansion causes redshift. So that light takes longer to cross this expanding space and if our mile markers are expanding, then the light would be slower than C.

      • telescoper Says:

        I’m sorry but that is simply wrong. The constancy of c is built into general relativity. You need to read some basic physics books.

      • Let me again recommend Harrison’s excellent textbook. All the maths are there, but it is perhaps more accessible than most other books. He also explains common misconceptions well.

      • “The constancy of c is built into general relativity.”

        That is exactly my point!
        If the light is redshifted, because it is taking longer to cross, then the speed of light is NOT Constant to the space between galaxies.
        So using the premise of spacetime to explain why we appear at the center of the universe completely overlooks the central point of GR.

      • telescoper Says:

        Redshift does not arise in the way you appear to think. Perhaps you should learn the basics before commenting further. I endorse Phillip’s suggestion that you read Harrison’s book.

      • So, redshift is not due to the other galaxies moving away and light taking longer to cross. We don’t have to worry about everything eventually moving so far away that nothing beyond our own neighborhood of galaxies is no longer visible?
        I have read a fair amount on the subject, but it is your blog and if that is the limits of your input, I better not push the issue.
        I would comment the current argument over Inflation doesn’t have an easy resolution and issues like dark energy and dark matter are not going to be easily solved, so eventually new generations of cosmologists are going to pursue other ideas.
        As an occasional commenter at phys org, I’d say the percentage of BBT doubters, among interested laypeople, has gone from about 15% to over 50% in recent years.
        There is only so much that chasing multiverses can entertain.

      • telescoper Says:

        I repeat. The cosmological redshift is perfectly compatible with constancy of the speed of light, indeed it is built into the Robertson-Walker metric that underpins the theory.

        I’d like to see a reference to the evidence on which you claim about those who doubt the Big Bang Theory.

        Of course all scientists doubt their theories. That’s why they spend so much time trying to test them using evidence.

        That’s not quite the same thing as dismissing things out of hand without bothering even to attempt to understand them properly.

      • telescoper Says:

        To clarify, your statement “redshift is not due to the other galaxies moving away and light taking longer to cross” is correct, as you would know if you read a basic textbook.

      • Peter,

        Thank you for the advice. Unfortunately I don’t have the time to digest that level of math and so it doesn’t give me a clear explanation for why what was originally assumed to be basic doppler shift, isn’t. Even though all the popular descriptions use terms like expansion, recession, big bang, etc.
        So I’m left with; “Trust us. We are experts.”

        As for those doubting the current model, I have, in my internet wanderings, run across a few, though that comment was inspired by a recent foray into the phys org comments sections, after at least a year away, where there are usually a strong contingent of BBT supporters, to balance the electric universe , Halton Arp fans, etc, but there was only one strong advocate and the ones in the middle seemed to be more willing to question the paradigm. String theory wars and now Inflation issues are starting to take their toll.

        I don’t necessarily follow the subject that closely, because I really got into studying science as a way to make sense of society, rather than technical or mathematical interests. You might say that childhood interest in war machines evolved into a teenage interest in the mechanics of war and thus politics.
        It seemed obvious to me, from an early age, that there are various unspoken physical processes, largely thermodynamic, governing social dynamics and assumed studying the sciences would give me a better handle on it. Unfortunately mostly what I found was excruciating attention to details and not much concern for those larger patterns. Given we evolved in a thermodynamic environment, you would think there would be some study about how these processes, waves. circulation, feedback, etc, govern us, but its mostly background to names and the preferred ideas.

        What I do see as the most basic mistake(pardon the presumption) being made, is that we model time backward.
        We experience reality as flashes of cognition and so think of time as this point of the present, moving along a narrative line, from past to future. Newton’s flow. While spacetime seemingly escapes this, it doesn’t because it simply distills and codifies time as measures of duration. Which really are measures of the timeline.
        What causes time is change, turning future to past. Tomorrow becomes yesterday, because the earth turns. This makes time an effect of action, similar to temperature, color, pressure, etc.
        We could use ideal gas laws to correlate temperature to volume, similar to how GR correlates measures of distance and duration, but since temperature isn’t foundational to our thought process, only our bodily functions, we can be more objective about it.
        I realize you are going to throw a lot of math at me for saying this, but the block time concept does seem muddleheaded. It can’t even explain why time is asymmetric, until entropy is apparent. Yet if we understand time as a measure of action, then inertia explains asymmetry. The earth turns one direction, not the other.
        Different clocks can run at different rates because they are separate actions. A faster clock necessarily uses energy quicker. Like an animal with faster metabolism ages quicker. Witness the twin in the faster frame ages quicker, she doesn’t travel into the future faster.
        The simultaneity of the present was dismissed on the premise different observers will see events in different order. Yet it is the energy which is conserved, not the information it transmits. This is no more remarkable than seeing the moon as it was a moment ago, simultaneous with seeing stars as they were years ago. Energy, not information, defines the present. The flow of information coalescing out of probabilities and receding into residual structures is the flow of time.
        Alan Watts used the analogy of a boat and its wake to describe this view of time. In that the wake, as the past, doesn’t steer the boat, rather the boat creates the wake. Events are first in the present, then in the past. Occurrence yields determination, not the other way around.
        In the east and in native American philosophy, the past is seen as in front and the future behind the observer. While the western view is that the future is in front and the past behind. That is because the eastern view is contextual, while the western view is more object oriented. So while we see ourselves as objects, moving forward, though our context, hence the association with direction and distance, the eastern, contextual view is that events occur, before they are observed, so are in the past.
        I realize you are thoroughly aggravated with my ignorance, by this point, so I won’t press my luck any further, but you did ask about those doubting the current model and so I thought some insight into the thought processes of one particular crank would have to do.
        Cheers.

      • telescoper Says:

        I’m not aggravated by your ignorance, but your refusal to make any effort to learn the basics before commenting at great length on how I’m to blame for not educating you.

      • Duration is the state of the present, as events coalesce and dissolve.

      • I’m certainly not blaming you and thank you for taking the time to read and respond. I am naturally defensive, from past experience.
        It is impossible to be expert in more than a very few fields. As I see it, I’m a generalist and the patterns I see incline me to think what cosmology is actually observing are cosmic convection cycles, of expanding radiation and coalescing mass, which are then mathematically distilled to collapsing and expanding space, over the course of time.
        Space appears overall flat for the simple reason that it is. What expands between galaxies, is balanced by what collapses into them. So what Hubble discovered was actually Einstein’s original cosmological constant, proposed to balance gravity from collapsing everything into a gravitational point. I see black holes as the eye of the storm, not portals into other universes, infinite density, etc.
        I also see math as a mapping device, not the territory, nor the platonic foundation of the territory. As in three dimensions of space are really just the xyz coordinate system and no more foundational to space, than longitude, latitude and altitude are foundational to the surface of this planet.
        Being an expert might make one extremely knowledgable about a particular field of study, but it also naturally leads to biases. Which is why the people running armies are called generals and specialist is an enlisted rank.

        Here is an essay I wrote some years ago, trying to tie a number of subjects together, in ways which could be useful. If you have about ten minutes, it sort of explains where I come from ,emotionally and intellectually;
        http://fqxi.org/community/forum/topic/1981

        Cheers.

  9. telescoper Says:

    I remind potential commenters that I do not accept comments posted without a valid email address.

  10. I guess we are no more used to real models – as opposed to theories – these days. Clearly inflation hit a mark (as you very nicely summarized), but equally clearly the inflaton field is totally ad-hoc. In its current state, i tend to think about inflation like a Bohr-Sommerfeld model. It has its merits and explains observations, but very clearly it needs to be further fleshed out.

    it seems that the timescale for this is longer than a scientific career, so people get nervous. all very understandable, but really more about sociology than cosmology.

    • The Bohr model is a good example. It is wrong, but that doesn’t mean that every aspect of it is wrong. It was a step in the right direction. Also, the fact that it is wrong doesn’t mean that all other models are equally valid.

  11. Tom Shanks Says:

    “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.”

    But I thought the problem was that the multiverse is now realised to be generic to the vast majority of inflation models. So if you appeal to inflation to create one Universe then you get zillions, whether you like it or not. It was when Alan Guth accepted this in 2001 – see https://arxiv.org/abs/astro-ph/0101507 – that it dawned that the multiverse had to be taken seriously.

  12. I find the aspect of an anthropic-multiverse explanation very interesting because of its possible consequence that the multiverse killed the dinosaurs.

  13. My friendly advice to all multiverse sceptics is to read “The World-Thinker” by Jack Vance: I think Lamarck would know how to deal with Multiverse-Thinkers and their perturbations :-).

  14. […] los 33 firmantes y las de Peter Coles, “Inflationary Perturbation,” ItD, 11 May 2017, y Ethan Siegel, “What if cosmic inflation is wrong?” SwB, 18 May 2017. Hay que separar […]

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