All about Cosmic Inflation (Synopsis)

“I don’t think at this point we have any way of knowing where the laws of physics came from. We could hope that when we really understand the laws of physics that they will describe how the Universe came into existence.” -Alan Guth

So, since Monday’s big story — about the BICEP2 collaboration announcing the discovery of the signature of gravitational waves on the cosmic microwave background — I realized that there are simply too many misconceptions and misunderstandings out there about cosmic inflation, the Big Bang, and how the whole story comes together.

Image credit: Bock et al. (2006, astro-ph/0604101); modifications by me.

Image credit: Bock et al. (2006, astro-ph/0604101); modifications by me.

So today, I have a ~4,000 word magnum opus blog post for you, explaining it all.

Here’s everything you’ve ever wanted to know about cosmic inflation, and what the BICEP2 results mean!


  1. #1 rob
    March 19, 2014

    i got 2 Q 1.or that model inflation dont violate thermodynamics law you know dead cold (®for mr.strassler) give extremy hot BB that for me look suspiciously ? we know that we talk about gravitonal wave and not for example sound wave in ultrdensity space can look and work similarly that what is imposible for radiation be easy for sound wave which can free unstopable travel on this conditions ?

  2. #2 Andrew Palfreyman
    San Jose CA
    March 19, 2014

    Gravity wave astronomy looks to be the promising new window for this century, and (apart from Hulse-Taylor) the BICEP2 result its first fruit.

    Actually, it’s been quite the dizzy science decade – oodles of exoplanets, Higgs and now inflation. We live in heady times.

  3. #3 David L
    March 20, 2014

    Ethan, I’m still not clear on the terminology. You wrote “Whenever anyone says that the Big Bang comes before inflation, they are very likely missing this important part of the story!”, which I thought I understood unit I read on to the “History of the Universe” diagram. This puzzled me again until I read your note underneath. (Perhaps you should make that disclaimer large, bold, and red!)

    So on that diagram, “Big Bang” should really be labelled T=0. I know the explosion analogy is lacking, but sticking with it, is the start of the Big Bang (i.e. the instant of “detonation”) the time when the acceleration of the rate of expansion drops to zero? And when did it end, or does it only “end” with the heat death of the Universe?

  4. #4 Sinisa Lazarek
    March 20, 2014

    @ David L

    The start of the BigBang as you see it (the explosion analogy is lacking, but sticking with it) happens once the inflation field starts dropping. It’s not when it reaches zero, since it never reaches zero. That’s also important. It stays at that low state, but it’s not zero. That process is called re-heating of the universe (because the potential energy of inflation field gets transferred to/or creates matter and radiation of our universe. This is the time “matter” gets created , and that’s what most people think about when they think of explosion, but it’s actually not that…

    So in essence, you won’t find a “correct” term for big bang, because the term itself is extremely flawed. Even in remade Cosmos it’s shown as hundred atomic bombs exploding… so very wrong.

    t=0 – creation of our universe… nothing bangs… just empty space filled with huge amount of vacuum energy that expands on itself. No light, no particles, nothing to “show” a bang or anything of sorts.. just emptiness and energy

    t=some time after inflation field starts to drop, in that expanding space, in every point, particles start to pop-out of vacuum fluctuations…. This is where matter as we know it starts taking shape. But this is not starting in one point and that expanding outwards. This is happening in every single point of space…

    so it’s up to you to define BB as whatever time you want to choose. Either t=o or re-heating phase. Just neither of them are big or banging or explosion like

  5. #5 Sinisa Lazarek
    March 20, 2014

    You might not like Susskind, but this is IMO the best 2 hour lecture on what actually is modern BB picture.

    You have to follow because he is dealing with math a bit, but if I as a layman can follow, so can you 🙂

  6. #6 John Duffield
    March 20, 2014

    Ethan: a nice representative and informative article. Well done, and well done for saying mass caused spacetime to curve. But you know, the more I read, the more I think inflation is looking like a solution without a problem. Space isn’t curved now, it wasn’t curved a billion years ago, or a billion years before that. It never ever was. So why do we need inflation to make it indistinguishable from flat today? Magnetic monopoles can’t exist, cosmic strings and domain walls remain speculative, and the leftover glow was uniform, but so what? The evolution of the universe can in some respects be likened to pulling away from a black hole. Spacetime curvature relates to the second derivative of gravitational potential which relates to the coordinate speed of light, which is zero at the event horizon. That’s uniform. Plus there are other possible causes of B-mode polarization such as self-attraction of the CMB photons. Aw, I’m with Peter Coles on all of this. I think he sounds the right note of caution. He says a primordial signal should not vary as a function of frequency, but there was no detection of the B-mode signal at 100GHz on BICEP1. And that the rush to embrace these results as definitive proof of something, is a product of human nature. Sorry.

  7. #7 David L
    March 20, 2014

    @ Sinisa Lazarek

    Thanks, but I don’t think our understanding differs too much. I did say “acceleration of the rate of expansion drops to zero”, not “inflation drops to zero”. My main point was that the purported purpose of Ethan’s post was to clear up the misconceptions circulating following the anouncement, and I’m not sure it does that. Certainly an explanation of things using terms like “after the Big Bang” and “before the Big Bang” seems ambiguous unless the time of the Big Bang is defined.

    I will try and find time to watch the lecture though.

  8. #8 David L
    March 20, 2014

    @John Duffield

    “…the rush to embrace these results as definitive proof of something, is a product of human nature”

    As if the ability to accept a simplistic solution when one is not fully aware of the problem. But the scientific method triumphs in the end!

  9. #9 John Duffield
    March 20, 2014

    David: IMHO it hasn’t been much triumph recently. As a result there’s a desire to present something as a triumph when maybe it isn’t. See Sean Carroll’s blog where he talks about “bumps on a graph” and the similarity with the Higgs boson. I don’t know if you know, but the Higgs mechanism is said to be responsible for only 1% of the mass of matter. And it contradicts E=mc². Is the mass of a body a measure of its energy-content? Or the measure of its interaction with a field? It can’t be both.

  10. #10 algernon
    March 20, 2014

    Thanks, Ethan. I can now qualitatively understand the situation much better.
    So, when the HBB happened, the universe was already huge, much bigger than our observable slice of it, wasn’t it? Perhaps it’s time to change that popular image of a “hot egg exploding” in laymen’s minds, because that’s about as misleading as you can get 😉

    Also: do we actually have quantitative estimates about the inflation epoch – i.e. size of the universe when it started/ended, time when it started/ended, energy scales involved, etc.? E.g. where does that 10^-32 seconds come from…?
    Because I see a lot of graphs describing “precise” timelines (such as the one you posted, from the NSF no less!), when in fact an average layman as me now gets the idea that before BICEP2, nothing quantitive was really known about inflation, and only now when can start talking about finding a good model of it and getting some real numbers in…

  11. #11 CB
    March 20, 2014

    Thanks a lot Ethan, sincerely for the nice explanation, and sarcastically for reigniting my LISA-cancellation-rage!

    JD: No the Higgs mechanism doesn’t contradict Einstein. A body’s potential wrt a field is a *kind* of energy. But this has been said before, you just don’t want to listen, because you’re happier with thinking that the entire science community is face-palming-ly confused about basic concepts which you understand, rather than vice-versa.

  12. #12 Vicki
    March 20, 2014

    The problem is that “Whenever anyone says that the Big Bang comes before inflation, they are very likely missing this important part of the story!” but people tend to look at and remember pictures. Like the mistaken one here: even those of us who often look at captions are likely to stop at “Image credit” unless we want to reproduce the image, or love it so much that we want to thank the artist; it wasn’t until I saw comment #3, and then went back to look at the article, that I saw the buried correction to the large, colorful illustration.

  13. […] Siegel has a nice pair of summary posts on inflation and the BICEPT2 […]

  14. #14 Figuring it out
    March 22, 2014

    I’m trying to figure this out. If mass increases as it approaches the speed of light and mass and energy are interchangeable. Did a strong fast gravitational wave start the universe?

    Inflation was faster than light ,and gravity pulls light and time.

  15. #16 Sean T
    March 24, 2014

    John Duffield,

    A particle with mass is one that acquires energy via an interaction with the Higgs field. Why is that a problematic statement? An electrically charged particle brought into the local region of another electrically charged particle acquires mass. It also interacts with an electric field. Which is it: does it acquire mass or does it interact with the electric field? According to your post, it can’t be both.

  16. #17 Sean T
    March 24, 2014

    Also John, you are correct, the Higgs mechanism is responsible for 1% of the mass of ordinary matter. This is the intrinsic mass of the elementary particles. The rest of the mass is acquired via the interaction of massive particles (quarks) with the strong field. Why is it, though that the acquisition of mass via interaction with the strong field is less implausible to you than the acquisition of mass by interaction with the Higgs field. Admittedly, before the Higgs particle was found, the Higgs field was much more speculative than the strong field. However, now that a particle with the spin and mass predicted by the existence of this Higgs field has actually been detected, it certainly seems that the Higgs field is on much less speculative ground.

  17. #18 Sinisa Lazarek
    March 24, 2014

    @ John Duffield

    The Higgs field in no way breaks E=mc^2. Stop writing nonsense and take your pseudo science theories somewhere else.

  18. #19 Michael Kelsey
    SLAC National Accelerator Laboratory
    May 25, 2014

    @Ethan: I missed this the first time around (I got the link from your latest weekly comments summary). This is an awesome introduction to inflation for a non-astro like me, with enough detail that I feel confident talking about it. Thank you!

    Given your insistence on correcting the historical misconception about inflation vs. hot BB ordering, I’m a tad disappointed that you’ve perpetuated a different historical misconception. Newtonian gravity does predict gravitational lensing of light, using Newton’s own corpuscular model, but the prediction is exactly half of what you’d compute from GR. Eddington’s confirmation of GR, in that sense, was quantitative, not qualitative.

    I *do* like that you called out the ordering problem in BICEP2’s own PR drawing 🙂

  19. #20 Ethan
    May 26, 2014


    That’s a fair enough criticism, although it depends on how you treat Newton’s gravitational theory. Only if you assign the photon a “mass” according to m=E/c^2 do you get any bending of light in his theory, and that leads to the half-the-GR prediction in the case of a star appearing near the Sun’s limb.

    I’m glad you liked the post! 🙂 I tried to make it as comprehensive and accessible as possible, and I hope it gives you a lot to chew on.

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