Is cosmic inflation settled science?

Ethan Siegel at Starts With a Bang has a post up arguing that the multiverse must exist.  His reasoning has to do with cosmic inflation.  Inflation is the theory that the universe expanded at an exponential rate in the first billionth of a trillionth of a trillionth of a second of the big bang timeline.  After that, space started expanding at a slower rate along the lines we see today.

Inflation was originally developed as a theory to explain why the universe seemed flat (in terms of spacetime), why temperature fluctuations throughout the early universe were so consistent, and a few other things such as the absence of magnetic monopoles.  These motivations for the theory seem important.  I think they should be remembered when evidence for the theory is being discussed.  In other words, can we cite the motivations for the theory as evidence for it?

Anyway, Siegel notes that, due to quantum uncertainty, the particle physics that led to inflation ending would not have been a strictly deterministic event.  Therefore inflation may not have ended everywhere in space at the same time.  Our universe may be a bubble of non-inflation in a sea of inflating space.   And if there is one bubble, there are likely others, other universes, the multiverse.

But all of this seems to hinge on the idea of cosmic inflation, specifically a variant of it called eternal inflation.  And here’s my issue.  The majority of physicists do seem to accept that the theory of inflation is true, but not all of them.  And the evidence for it seems to be a combination of the original problems the theory was developed to solve and circumstantial evidence.   Some notable physicists, including Paul Steinhardt, one of the theory’s early supporters, seem to think it actually creates more questions than it answers.

So, should we consider inflation settled science?  This seems like an example of an unsettling trend in physics in recent years of accepting theories that can’t be empirically tested.  Given that a huge swath of similar theories in particle physics were reportedly invalidated by the LHC results (or rather, non-results), it seems like a very questionable strategy, an abandoning of a key aspect of scientific investigation that been successful for centuries.

Science has credibility for a reason.  That reason is the value it puts on testing every proposition.  Talking about untestable theories as though they’ve been validated seems to put that credibility in jeopardy.  There’s a danger that the public will see start to see theoretical physics as metaphysical navel gazing.

There is also a danger, identified by Jim Baggott some years ago, that many scientists may simply not look at alternative theories because they think inflation has solved the issue, or that they may eschew some speculative theories just because they’re not compatible with inflation.  But if inflation is still really just a speculative theory, then they’re giving up on one speculative theory because it’s not compatible with another speculative theory, perhaps cutting off a fruitful line of inquiry.

It may turn out that inflation does eventually pass some test we simply haven’t thought of yet.  Or we may eventually figure out a way to test the idea of bubble universes.  But until we do, talking as though these are settled issues makes my skeptic meter jump through the roof.

Physics has a reputation for being a very hard science.  Sometimes I wonder how warranted that reputation really is, at least in the theoretical branches.

Unless of course I’m missing something?

44 thoughts on “Is cosmic inflation settled science?

  1. I characterize science as learning to think the way nature behaves. When I first studied quantum mechanics, I thought it was quite weird, but I admonished myself for allowing the way I was used to thinking to reject the way nature is behaving. Still, we still do not understand quantum mechanics. We can use it spectacularly well but understand it, not so much. I think the same thing is occuring in cosmology. Things have gotten far from the patterns thinking that local, small scale science has taught us in and we haven’t caught up yet. so, much is still up in the air. Living with uncertainty (not just in measurements) is something nature is trying to teach us and we aren’t learning all that well. This, partly, is because you have to be really, really curious to undertake a study of quantum mechanics or cosmology. At this stage of the game I am an “arm charge scientist” who reads about what other people think because I just can’t marshall the effort to keep up.

    Liked by 4 people

    1. I think for all of us have to be armchair versions of the professionals on most subjects. Modern science (or technology, business, or whatever) is simply too complicated for real polymaths anymore. Still anyone who takes the time to educate themselves in a subject knows far more about it than the vast majority of people, even if they aren’t an expert. (It’s always surprising to me how many people have strong opinions on subjects that they obviously haven’t made an attempt to get even the layman version of that education.)

      Liked by 1 person

    2. Hi Steve,

      I take it that you mean “armchair scientist” rather than “arm charge scientist”, unless your arm is charged by some ions or ideas. 😉

      Not only do we have to contend with uncertainty, we also have to face oblivion and obsolescence, as the pace of social change and the increasing human population as well as information overload have caused many things to be cramped out of existence and to recede into the past, into oblivion, into historical junkyards. It would seem that even authors and artists have to build in obsolescence in their stories and characters.

      Even science and technology are not immune, given that portable computer devices are especially plagued with unresolved limitation in functionality, rapid obsolescence and problems of disposal, resulting in millions of these portable devices going to landfills and poisoning the environment every year.

      I have discussed some of these issues in the concluding section of an extensive post called “Conclusion: Change Rules and Moment Matters” at

      Thank you, Mr Mike Smith, for giving us a summary of the state of affairs in matters regarding the expansionary universe.

      Happy April to all of you soon! May your weekend be highly productive and/or enjoyable!

      Liked by 1 person

  2. inflation is an extraordinarily compelling idea, which really does seem to match well with the observed features of our early universe. And it leads to some surprising consequences that Guth himself never foresaw when he first suggested the scenario – including, as we’ll see, a way to make the idea of a “multiverse” become realistic … the question is, why did inflation ever happen? Sean Carroll

    Liked by 3 people

  3. Ethan Siegel is risking a lot here by nailing his reputation to the multiverse mast. Even if it turns out that inflation is right, I think that Ethan is wrong to suggest that it is proven. As you say, when a theory creates more questions than it answers, we have to treat it for what it is – speculation. The example of supersymmetry ought to have been a sobering lesson for theorists. The danger isn’t simply that inflation risks being wrong, but that in accepting it as a theory, it makes it harder to argue with the kind of people who say, “But evolution is just a theory.”

    Liked by 4 people

    1. I agree he’s risking his reputation, although it’s in an internet article rather than a published paper, so he unless he makes it an ongoing theme, he might get away with it. But I’ve noticed in the last year or so that he’s gotten edgier in his writing. Some of it might be pressure from Forbes to keep his traffic numbers up (or increase them) by being more controversial.

      Liked by 1 person

  4. “[C]an we cite the motivations for the theory as evidence for it?”

    Depends on the motivation, doesn’t it? Evidence that supports the theory motivates finding the theory, but “I really think it’s true” is a different sort of motivation!

    “So, should we consider inflation settled science?”

    Absolutely not!

    (I think I’ve mentioned I’m not a fan of Ethan Siegel? It’s partly his writing style and partly how he goes on on topics like this.)

    I am very much not a fan (in fact, I’m a detractor) when it comes to multiverse theories or this ridiculous idea of accepting theories that haven’t been tested. That’s not science; that’s science fiction.

    “Physics has a reputation for being a very hard science. Sometimes I wonder how warranted that reputation really is, at least in the theoretical branches.”

    This is, in many regards, the point of Sabine Hossenfelder’s book, Lost in Math: That theoretical physics has badly lost its way; that theorists have become enamored of “beautiful” mathematics with no physical application; that a kind of groupthink has blinded the field.

    I think she’s right.

    (Speaking of Hossenfelder and Inflation, she recently posted about the current status of Inflation. It’s worth reading. As always, she makes some good points.)

    I worry that our post-factual, post-empirical, post-rational culture is starting to infect science. We were just talking about humanity lasting millions of years… not at this rate!

    Liked by 2 people

    1. Yeah, Siegel’s writing seems to have gotten more aggressively opinionated over time. Or maybe I’m just noticing it more now. I speculated above that he might be under pressure to keep his click volume up. Controversy always does that better than education.

      I agree that Sabine seems right. I might have to read her book at some point, although I really don’t need to be sold on the idea at this point. I actually linked to her inflation post in the post above (as one of the physicists who aren’t necessarily on board), but probably should have labeled it.

      On post-empiricism, I always think about the neo-platonists, the philosophers in the late Roman period, who had largely abandoned the philosophical rigor of the earlier centuries and fallen into self absorbed metaphysical speculation with little grounding in reality. It’s very tempting to wonder how much correlation that might have had with the problems in the late empire years.

      Liked by 2 people

      1. Siegel may have changed over time. I used to read him regularly (long ago), and never had the reaction to him I do know. The last few years, I’ll get a paragraph or so into one of his posts and think, “Oh, no thanks.”

        I did click into his inflation post when I saw it in my newsfeed. Read a few paragraphs and realized there was no point in reading further. 🙂

        Ha! Yeah, I didn’t bother with the disagreeing physicists links because, well, preaching to the choir! I didn’t even hover to see what they were!

        One might also draw some parallels to big empires that become inward focused to the point of distraction and decadence.

        There’s that famous line, “The center cannot hold,” that seems to apply to human pursuits. The entropy of human nature.

        Liked by 2 people

        1. Had to look that term up. Not sure I’d subscribe to never being resolved. In fact, I see conscious as an emergent quality and not really odd at all. In this regard I do like IIT as it shares the same sort of approach. That being said, I do believe we’re missing something rather large.

          Liked by 2 people

  5. Steve, Mike – maybe something less complicated …
    Steve Morris wrote …
    After all, we’re just a bunch of jumped-up apes. Perhaps things we think are false aren’t, and vice versa.
    Just a sideways thought!

    Mike wrote …
    I’ve had similar sideways thoughts. What is “objective” ultimately but subjective perceptions that large numbers of people agree on?

    and I am considering sideways in context: … it’s much harder to measure a star’s motion in the plane of the sky, or side to side.
    This “sideways” motion, called proper motion, …

    Maybe something more about all this …

    Liked by 2 people

    1. I had to do a search to figure out which conversation you were quoting:
      …a discussion under a post about Carroll’s questioning of the value of falsifiability.

      Stan, why don’t you tell us what you think? Does science require falsifiability, that is, should every scientific proposition be testable, at least in principle, to be scientific?

      Liked by 2 people

      1. because in this case it is only a problem in expression (sideways), in the context of (it’s much harder to measure a star’s motion in the plane of the sky, or side to side. This “sideways” motion,) but if is a problem, I can disappear …

        Liked by 2 people

        1. Ah, ok. The term “sideways” as used in Steve and I’s comments were metaphorical, not literal. In that case, we were just referring to unconventional or random thinking.

          I can’t say I know too much about measuring a star’s proper motion. I imagine the radial velocity can be measured by doppler shifts in the star’s spectral lines, but transverse velocity can only be measured by observing its position relative to background stars across large tracts of time. Of course, any observed motion is going to be relative to our solar system’s motion, although I suppose someone could try to work it out in relation to the galactic center.

          Liked by 2 people

          1. Mike. Thanks. Your patience is really admirable and my stupidity is simply unimaginable. Probably the entire context of this expression (sideways) was missing.
            (While it’s easy for astronomers to measure whether a star is moving toward or away from us, it’s much harder to measure a star’s motion in the plane of the sky, or side to side).
            My fault, so i’m disappearing …

            Liked by 1 person

  6. You don’t judge a hypothesis by how many predictions it makes that you can’t verify. You judge it by how many predictions it makes that you have verified/falsified, or failing that, that you can.

    Liked by 2 people

      1. This quickly leads to complicated territory but, I don’t think you can generally exclude the data that motivated a theory’s development from the list of supporting data. Yes, it’s always easier to retrodict than predict, but that doesn’t make retrodiction worthless.

        Liked by 1 person

        1. I wouldn’t say you should exclude it, but characterizing the motivations as validated predictions, as Siegel is doing, is misleading. It implies the theory has made novel and unexpected predictions, akin to General Relativity’s prediction of curved light, that have proven to be accurate. As far as I can tell, inflation hasn’t done anything like that.

          Liked by 1 person

  7. Imagine a gas is pumped into a room that was previously a vacuum.

    At first, the gas will be concentrated at the point of entry but gradually it will tend to spread out and occupy the entire room. The most probable state to find the gas in the room would be spread out more or less equally through the room.

    However, given enough time, random movements of the molecules of the gas could cause it to cluster and concentrate in one part of the room. From that point it would most likely spread out equally again. If we lived on one of the molecules of the gas after one of these periods of concentrations, it would appear to us that we were living in an inflating universe.

    Liked by 2 people

    1. That’s a good description of the Boltzmann entropy dynamics. And it might be relevant to how the universe got to the pre-Big Bang state, although inflation may actually complicate that initial state (or not depending on which physicist you talk to).

      But just to be clear, “inflation” doesn’t refer to the normal expansion of space, which we can observe and confirm, but to the massive exponential expansion in the early universe, by a factor of at least 10^26 in the first 10^-33 seconds of the Big Bang timeline.
      This would have been prior to the generation of the CMB, so we won’t be able to observe it with electromagnetic radiation, although it might eventually be possible to detect gravitational waves from it.

      Liked by 1 person

  8. Great analysis of such a serious issue.
    Although It seems to me that there is something more intriguing in this text.
    (we do have a very strong suite of evidence that supports a period in the early Universe where it occurred. It set up and gave rise to the Big Bang. eth)
    More serious analysis? … maybe another time ???

    Liked by 1 person

    1. Stan,
      I’m not sure what you’re asking me about here. The verbiage you’re quoting(?) doesn’t appear to be in the post or thread. If it’s something I said somewhere else, could you link to it for context?

      On inflation overall, was there something specific you were asking about?

      Liked by 2 people

  9. There is no physical universe, only a formless flux of snailpaced sensations. From ‘zero’ to ‘infinite’ – all energy values are quantum-mechanically(HUP) superposed to represent a multiverse.
    Our particular universe might not be a contingent one. According to HUP, there is always some non-zero probability for our universe to come into existence.
    One can wonder if the abstract logic of HUP can activate potential field values(‘zero’ by default) with probability distribution initiating entangled information processing. Thus ‘physical’ might be a derivative of ‘logical’, if not a mere linguistic translation of the latter. ‘The asymptotic spectrum of temporal physicality’ might be a quantum symmetrical duality (whether holographic or not) of ‘energy uncertainty’. Even abstract amplituhedron or einselected quantum Darwinism might play some role in creating our so called ‘concrete’ reality.
    Let us leave such speculations aside! My only dream is to turn school level(up to 10th grade) Science and Math textbooks into a window to the cutting edge scientific ansatzs. What if I link academic exam-notes to my relevant (at least as funny mnemonics / imagination-booster/ museum-style exhibition of worldviews) blog posts sharing a lot of facts(actually stranger than fiction) and speculations that otherwise might never surface in kids’ classroom? I invite critical comments and positive suggestions, if any, about my passion.

    I’ll be honored to have you as a visitor of my blog:-

    Liked by 1 person

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