“…an understanding of the infinite tree of universes seems to be needed in order to make statistical predictions about the properties of our own universe, which is assumed to be a typical “branch” on the tree.” -Alan Guth

The Big Bang is commonly regarded as the start of it all, but that’s only the birth of what we call our observable Universe. There must have been something compelling to set it up, complete with the initial conditions that our Universe began with. An idea called Cosmic Inflation fits the bill perfectly, providing those conditions and making six explicitly new predictions.

The expanding Universe, full of galaxies and complex structure we see today, arose from a smaller, hotter, denser, more uniform state. Image credit: C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47).

The expanding Universe, full of galaxies and complex structure we see today, arose from a smaller, hotter, denser, more uniform state. Image credit: C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47).

Despite the fact that five of them have been observationally verified, there are still detractors of inflation, who go as far as to proclaim that inflation isn’t even a scientific theory. It’s not a crazy claim, but it’s not entirely fair, either.

The fluctuations in the CMB are based on primordial fluctuations produced by inflation. In particular, the 'flat part' on large scales (at left) have no explanation without inflation. Image credit: NASA / WMAP science team.

The fluctuations in the CMB are based on primordial fluctuations produced by inflation. In particular, the ‘flat part’ on large scales (at left) have no explanation without inflation. Image credit: NASA / WMAP science team.

Come find out what the controversy is all about, and learn what it means if cosmic inflation is wrong!


  1. #1 John
    May 11, 2017

    Assuming inflation to be true also suggests to me that it is an impenetrable boundary for the physics describing this universe – GR & QM.

  2. #2 Sinisa Lazarek
    May 11, 2017

    That is sadly very true. Albeit, if inflation is correct, and some day we get some more solid notions of the conditions that ought to have existed in order for inflation to occur (assuming inflation in itself is not “the default” state), then we might be able to shed some more light on what the things that preceeded it might have been.

    But just by reversing the clock so to speak.. we get to inflation and all prior history is erased

  3. #3 Sinisa Lazarek
    May 11, 2017

    p.s. a correction.. not all history is erased.. CMB fluctuations are a proof of that… so in a sense.. what ever was before inflation was still at least partially rooted in the physics that we know, and that’s a powerful tell-tale.

  4. #4 Patrice Ayme
    May 11, 2017

    For decades, cosmologists have screamed from every rooftop that cosmic inflation was right. I was dubious, because they made an hypothesis which was astounding, to explain something rather mundane.

    The short of it is that the universe is huge, at least 45 billion light years across, but looks everywhere the same, as if it originated from just one place (or as if it were immensely old). To explain the discrepancy, some cosmologists, starting in the USSR, assumed the universe expanded at enormously faster than light speed. The speed of light along loops in space is locally limited. The speed of space, though is not limited (a curved manifold of dimension n embeds in one of dimension (2n +1) so that the curvature of the former is a trace of the flat one of the latter).

    This was a huge hypothesis to explain a smaller problem. Basically, it looked as if cosmologists had got the temporal dimension of the universe wrong. Or maybe the Big Bang was wrong. Or maybe both, a bit.
    Instead, cosmologists assumed a completely new force, Cosmic Inflation, and thus a completely new source of energy. They went for a phantasmagoric “explanation”, instead of modestly admitting that they did not really witness the proverbial “First Three Minutes”, from their position, on the right of God.

    If CI existed, why should CI appear just once? Why not here, there, and everywhere, now, yesterday and tomorrow? Could one make universes out of nothing? Yes, yes and yes, screamed hysterical cosmologists from all rooftops.
    A rule in thinking is that when one has a problem with a ready class of explanations one should not explain it with supernatural explanations from the get-go, before the more obvious explanations have been proven wrong.

    Here conventional theories of the Hot Big Bang may not be not quite correct, so hysterical physicists decided that everything-we-know makes no sense, to start with. It turns out that their arguments amounted to hand waving (OK, a gas cools down when expanding; however a quantum fluctuation is not a gas!) Penrose claims that obtaining a flat universe classically without any recourse to inflation out of a quantum fluctuation is 10^100 more likely.

    A casual look shows that conventional Big bang theory makes a lot of assumptions we have no proof of (for example in astrophysics). Absence of logical contradiction is no proof of experimental existence. Especially when, in the end, the theory one gets (the conventional Hot Big Bang) seems incorrect (because nothing can solve the flatness problem, short of immense age!)
    The philosophical problem problem became even more acute when an experimental cosmological inflation was discovered, Dark Energy. The conjunction of CI and DE made the universe expand tremendously, brake down, and then re-accelerate. Weird. Both inflations differ by a factor of 10^27 in their energy density.
    So why not go with Dark Energy alone? Then the universe maybe hundreds of billions of years old.
    Why not? Just to say that can’t possibly be true, because one has seen the universe expands very fast, in a tremendous cosmic inflation, amounts to starting with one’s conclusion.

  5. #5 Sinisa Lazarek
    May 12, 2017

    @ Patrice

    You say that the horizon/thermal equilibrium of the Universe is a mundane problem. I am wondering why you say that?

    If we assume lambda-CDM model to be correct…and thus our estimates of the age of the universe to be more or less correct (lets say to couple billion years +/-)… how did universe get to be in such a uniform state.. even hypothetically?

    Sure, it’s easy to wave a hand and say, the universe might be several hundred billions years old… but not according to data.

  6. #6 Patrice Ayme
    May 12, 2017

    Sinisa: Thanks for your comment, much appreciated. You put your finger on the problem.

    Assuming that the Lambda-CDM model is correct, is assuming what some of us want to see proven beyond any suspicion. I do not just observe data in that regard, but also a lot of suppositions about what the data is supposed to mean.

    Let’s concentrate just on CDM. Although I do believe in Cold Dark Matter, I have seen theories of it which are in complete contradiction with my own CDM suggestion. Actually it seems that most CDM theories contradict mine (especially regarding galactic halos). Not to say I am a potential genius, but there are more possibilities out there than generally considered. As far as I can see. And no doubt we will see more very soon. Galactic halos are more readily observable than what supposedly happened 14 billion years ago.

  7. #7 Cleon Teunissen
    May 13, 2017

    I’ve been wondering whether in the history of physics there has been a case with some similarities, enough to make an informative comparison.

    My suggestion is Alfred Wegener theory of continental drift. When it was proposed Wegener’s theory of continental drift did not find support in the scientific community. Now, the evidence in favor of continental drift was remarkable: the shapes of the continents and the similarities/parallels in fossil record between areas that according to the theory were at one time continuous landmass. What counted against the theory of continental drift was that there seemed to be no conceivable physical mechanism for the continent’s motion. (As we know, this counter-argument gave way as plate tectonics came to light.)

    This story illustrates general physics judgement in the following way: even when there is remarkable evidence in favor of a theory, as long as there is still an insurmountable obstacle the theory must be considered a hypothesis. More strongly: as long as there is still an insurmountale obstacle the theory should definitely not be presented as proven beyond reasonable doubt.

    Of course it is possible that at some future point in time the insurmountable obstacle goes away, as has happened in the case of continental drift. The thing is: if the present available knowledge tells you that the obstacle is insurmountable, then that is what you have to go by.

    Ijjas, Steinhardt and Loeb have pointed out that as the depth of understanding of inflationary models has increased, more and more problems have arisen. The way I understand it: IS&L have for example pointed out that it is now understood that once inflation starts the overwhelmingly probable course of events is that the inflation continous forever. As I understand it: while it is _possible_ for inflation to end, that is an exceedingly improblable outcome.

    As I understand it: IS&L state that the current deeper understanding of inflationary models shows that in order to arrive at the universe as we observe it an inflationary scenerio must along many different points proceed according to an exceedingly improbable course of events.

    As we know, cosmic inflation was introduced to address a problem of improbablity. The observed uniformness of the Universe, and other properties: very improbable.

    IS&L argue that as understanding of inflationary models has deepened it has become clear that cosmic inflation scenarios are actually even more improblable than the problem they were supposed to solve; you lose more than you gain.

  8. #8 John Hasler
    Elmwood USA
    May 13, 2017

    > In general, the simplest inflationary models are
    > based on a potential: you draw a line with a
    > trough or well at the bottom, the inflationary field
    > starts off at some point away from that bottom,
    > and it slowly rolls down towards the bottom,
    > resulting in inflation until it settles at its
    > minimum.

    So the inflationary field is not conservative?

  9. #9 Patrice Ayme
    May 13, 2017

    Along the lines of Cleon above:

    In the history of science, and even mathematics, crazy theories which prove correct, and theories long considered correct, which prove crazily idiotic, are legions. To wit:

    (I) Anaximander and other Greeks, more than 25 centuries ago, viewed evolution, and selection, natural or artificial a given. Later Christian Jihadists burned all books and intellectuals to erase the theory of evolution from the collective psyche.

    (II) Theorems of Non-Euclidean geometry both hyperbolic and elliptic were known before Aristotle. However, a century later, Euclid erased it, in the name of simplemindedness.

    (III) Aristotle rolled out a completely idiotic theory of force and motion. Neglecting friction, he considered that, to maintain a motion, one needed a constantly applied force. As Aristotle was pro-monarchy, and monarchs and other plutocrats were the force behind Christian theocracy, Aristotle was saved, reproduced and celebrated. Thus this erroneous physics held sway until Buridan (circa 1350 CE).

    (IV) The Atomic Theory and its constantly moving atoms was viewed as proven 2,000 years ago. Because what is now called “Brownian Motion” was claimed to have been observed repeatedly in peculiar lighting conditions.

    (V) Archimedes accomplished at least one infinitesimal calculus computation, using correct modern method. That was lost, until resurrected by Fermat, 19 centuries later.

    (VI) Aristarchus rolled out the heliocentric theory. The ensuing debate is now lost (see Christian fanatics above).

    (VII) Ptolemy and Al. came out with a crazy theory of the Solar System, and discreetly cheated to impose it. (Count Tycho discovered the cheating 15 centuries later.) The natural theory was the heliocentric theory, be it only because the Sun was known to be pretty far, and it made no sense it would go that fast around, being obviously much bigger.

    (VIII) Buridan introduced “impetus”, inertia, and the notion that, without force applied, an object would go either straight or keep on going around (planet, satellites). Buridan, head of the university, adviser to four French kings, could afford to contradict Aristotle. Buridan discovered much physics later attributed to Newton, born three centuries after. Not only did Buridan anticipate General Relativity, but his treatment of the Cretan Paradox was new, and modern.

    (IX) Laplace suggested a theory of the Solar System and galaxy formation by the flattening, under conservation of angular momentum, of vast dust clouds. This theory was considered completely false later. Instead astronomers prefer to think a passing star had torn material away from the Sun, and the debris created the Solar System.

    (IX) The Cretan paradox was refurbished and digitalized by a number of mathematicians, in the 20 C, including Kurt Goedel, demonstrating incompleteness of standard arithmetic containing logic.

    (X) After 1950, Robinson and Al., using Model Theory, built non-standard arithmetic, by uncovering an axiom implicit in Archimedes (see above). This enabled to make Leibniz’s long standard analysis, rigorous. 250 years after Bishop Berkeley had made fun of it.

    (XI) In the same 1950s, new evidence surfaced that, after all, Laplace was right, and galaxies and solar systems formed from collapse of gas clouds.

    (XII) Huyghens, paid by France’s Louis XIV, suggested the wave theory of light. He was disproven by Newton’s particle theory of light, before being proven right by Young, an MD, around 1800. A century later, Einstein scrambled the whole thing with poorly considered statements, in the photoelectric paper which earned him the Nobel Prize. Those indigestible ideas led straight to the Multiverse madness, in my opinion.

    (XIII) Last, not least: biology research professors Lamarck and Cuvier suggested antinomic theories of evolution, circa 1800. They were made fun of later, and rolled out as bad scientists doing bad science. However, not only they have now been demonstrated to be much more important biologists than Darwin, but, surprisingly enough, their antinomic theories of evolution are correct!

    The evolution of science is the evolution of thinking. Nothing straightforward about it!

  10. #10 Sinisa Lazarek
    May 14, 2017

    @ Patrice re: #6

    I’m not sure DM is that relevant concerning horizon problem, what I initially commented on.

    Because even our crudest estimates/measurements on age of universe (from beginning to middle 20th century) gave the time that’s just to short to reach this state of uniformity. And that’s without any DM. Later things, like lambda and dark matter and dark energy etc.. that just tweaked it down more and more. But 13.7 or 20 or 25 billion.. it’s all in the same ballpark.

    Going back to inflation. I have nothing against something “less” strange.. but what?

  11. #11 Narad
    May 14, 2017

    I’m not sure DM is that relevant concerning horizon problem, what I initially commented on.

    You really have to check out Patrice’s blog to get an idea of the flavor of, ah, things.

  12. #12 Patrice Ayme
    May 17, 2017

    For Sinisa, Narad (;-)!):
    The sort of cosmic inflation which is observed, Dark Energy, is compatible with a universe which would be hundreds of billions of years old. OK, that does not “PROVE” it. But something similar happened with the age of the Earth. Fossils were well-known by Aristotle’s time, and a mystery. Aristotle sent his students to record live species. Also there was this huge meteor which landed in northern Greece and was visited for centuries. Indices, indices…

    The Big Boom theory rests on lots of hypotheses within hypotheses, for example about nucleosynthesis and astrophysics… Not to say that shows it’s wrong. Simply it’s much less strong that people usually assume.

  13. #13 Sinisa Lazarek
    May 18, 2017

    @ Patrice

    I think we’ll stop here, since you don’t seem to want to back your claims with any evidence or further explanation.

    First I asked you about your comment on horizon problem, instead of explaining your position you introduced dark matter. When I pointed the irrelevance of that, and asked why you feel it has a place there, you now again deflect and now talk something about dark energy which again is non-sense…

    If you can’t offer at least simple arguments to issues you yourself raised, then we can’t really have a scientific discussion.