Inflation

© Matt Strassler [March 15, 2014]

The inflationary epoch was a possibly very brief but certainly spectacular period when the space inside a region of the universe which includes our own observable patch (i.e., the part of the universe that we can observe today) expanded with blistering speed — completely unbelievable speed. The expansion rate was so big that it sounds totally insane. And the only thing that keeps it from being insane is that the theory of inflation makes predictions which, so far, agree with our measurements of the cosmos. (Including those by BICEP2.) That doesn’t mean it’s right, but it does mean that

  • there are good reasons to think it might be right, and
  • no one can currently show that it’s wrong.

Let me say again: the space expanded. Things didn’t rush into the space: the space simply became much larger. It’s not one bit like an explosion.  Click here to read more about the difference between an explosion of something into space and an expansion of space itself.

How insane is this rate of expansion? A patch of the universe no larger than your computer screen expanded to the size of the observable patch of the universe, or larger, in less than the time it takes for a quark to cross from one side of proton to the other. I won’t even bother to tell you the numbers, partly because we don’t actually know how long inflation lasted, but also the numbers are too big in size and too small in time for humans to think about them. Basically, a giant chunk of universe was created from a tiny one almost instantaneously.

What was the universe like during the period of this expansion? Empty. Extremely empty. Much, much, much emptier than space is now. Extremely cold. Extremely dark. Anything which might have been there before inflation started would have been pulled apart and dragged to great distances in an instant.

What happened before inflation, and how inflation got started, we don’t know. There are a number of reasonable scientifically-grounded theoretical ideas, but they’re all speculation until someone thinks of a way to test them by making measurements. There may not even have been a “before inflation”, either because inflation is always going on somewhere in the universe, or because time doesn’t really make any sense if you go back too far, or for some other reason. But in many contexts it almost doesn’t matter, as I’ll now explain through a set of figures, answering some frequently asked questions along the way.

  • What caused this insane rate of expansion?

The cause was a large amount of what is often called

  • “dark energy” (but it’s not energy, it’s energy and negative pressure in the right combination) or
  • the “cosmological constant” (Einstein’s [non]-blunder: but fortunately it wasn’t constant, or the universe would have inflated forever) or
  • “dark smooth tension” (which is correct but it’s kind of clunky-sounding and not any clearer.)

Anyway, the universe has some of this stuff now, which is why the universe’s expansion rate has started to increase in the past few billion years. But (we suspect!) at some point, for some reason, it had a lot, lot more. And this caused the region containing our part of the universe to expand with a rate that accelerated enormously… i.e., caused it to “inflate”.  See Figures 1,2,3, which contain a wild and surely wrong guess as to how inflation started, but by Figure 4, the details of the guess have become completely irrelevant.

Fig. 1: A completely wild and unjustified guess about what one region of the universe might have looked like before inflation began.  In the grey region, for some unknown reason, there is a substantial amount of dark energy.   I've also drawn a few objects inside the grey region as green and red dots.  I have no idea what's outside the grey region, but as you'll see, it doesn't matter very much in the end.

Fig. 1: A completely wild and unjustified guess about what one region of the universe might have looked like before inflation began. In the grey region, for some unknown reason, there is a very substantial amount of dark energy. I’ve also drawn a few objects inside the grey region as green and red dots. I have no idea what’s outside the grey region, but as you’ll see, it doesn’t matter very much in the end.

Where did this huge amount of “dark energy” come from?

We don’t know. There are various suggestions … some of which have been ruled out by recent data. We hope to learn more about this question in the coming decade.

Fig. 2: Dark energy causes the grey region to begin expanding.  The objects in the grey region (green and red dots) are carried apart as the space with the dark energy expands,  becoming more spacious without actually moving into the exterior region outside the grey region.

Fig. 2: Dark energy causes the grey region to begin expanding. The objects in the grey region (green and red dots) are carried apart as the space with the dark energy expands, becoming more spacious without actually moving into the exterior region outside the grey region.

Why doesn’t the rate of expansion slow down as the dark energy becomes diluted by the expansion?

Curiously and surprisingly, as the universe inflates and its volume grows, the amount of dark energy per unit volume stays the same. That means it will inflate and inflate and inflate, without slowing down, until something makes the dark energy go away.

Fig. 3: Since dark energy, unlike ordinary materials, does not become diluted as space expands, but remains constantly dense, the grey region continues to  expand.  By now all but one of the green and red dots has receded from view.

Fig. 3: Since dark energy, unlike ordinary materials, does not become diluted as space expands, but remains constantly dense, the grey region continues to expand. By now all but one of the green and red dots has receded from view.  Whatever the temperature of the expanding region was to start with, it is becoming extremely cold.

Note how the red and green dots are receding from each other with enormous speed.

Doesn’t that incredible expansion mean that things moved apart faster than the speed of light … the universal speed limit?

Yes it does.

And doesn’t that violate Einstein’s theory of relativity?

No it doesn’t. Einstein’s theory says that if two objects pass each other at the same point, an observer moving with one of them will measure the other to be traveling below or at the universal speed limit, and never faster. But two objects at two different points can move apart faster than the speed of light if space itself expands… which is what happens in the expanding universe.  Read more here about the expansion of space, and how and why it is completely different from an explosion.

Fig. 4: The heart of the inflationary epoch.  By now, inflation has moved all the objects that were in the original grey region of Figure 1 (green and red dots) to extremely great distances from one another.  The grey region has grown to a incomprehensibly vast size, profoundly empty and cold.  And the expansion may go on and on for many stages.

Fig. 4: The heart of the inflationary epoch. By now, inflation has moved all the objects that were in the original grey region of Figure 1 (green and red dots) to extremely great distances from one another. The grey region has grown to a incomprehensibly vast size, profoundly empty and cold. And the expansion may go on and on for many stages. The original guess illustrated in Figures 1 and 2 is now completely irrelevant to the properties of this region of the universe; if we had started with a quite different guess in Figures 1 and 2, we would still have arrived at this same Figure 4.

I thought the Big Bang was all about the universe being really hot… but now you’re telling me it was really cold???

That’s right. The universe became hot after inflation; see below for more on this. Whether it was also hot at some period before inflation is completely speculative; there’s no evidence one way or another.  But during inflation, the temperature dropped to a tiny fraction of a degree above absolute zero.

Fig. 5: The expansion of the inflating region is slowing down.  And what will become the observable patch of our universe has now become big enough to draw.

Fig. 5: The expansion of the inflating region is slowing down. And what will, over time, become the observable patch of our universe has now become big enough to draw, outlined in red dashes.

Why did inflation stop?

We don’t know. But again, there are a number of scientifically grounded suggestions, ones with equations and predictions and ways to test them, at least in part. We may learn more soon from ongoing studies of the cosmos.

What happened when inflation stopped?

The best guess as to what happened (and our equations show this is possible, but don’t tell us the details) is that all that dark energy got turned into particles — including particles we’re made from, and lots of other types of particles we know about, and perhaps lots of particles we don’t know about. And when this happened, the universe became very hot, and very dense — and it continued expanding, though much more slowly.

Inflate6

Fig. 6: As inflation ends, the dark energy that fills the formerly inflating region is turned into the motion-energy and mass-energy of particles, which appear in enormous abundance, making the universe extremely hot. The larger the amount of dark energy per unit volume was during inflation, the hotter the universe can become after it heats up. A large region, extending far beyond what is shown, and including what will become our observable patch, is filled with a nearly uniform hot dense soup of particles. From here, the universe will continue expanding, but much more slowly than during inflation, and it will slowly cool. The Hot Big Bang is described in a separate article.

This was what is the origin of the Hot Big Bang. Some people (including me) simply say: “This moment is the start of the Big Bang”. Others say that the Big Bang includes the Hot Big Bang and inflation, though this is odd, since inflation is more of a Whoosh than a Bang.  Some say that inflation is what put the “Bang” into “Big Bang”, by first making the universe large and expanding, and then making it hot.  Still others say that it includes the Hot Big Bang, inflation, and everything that came before it… but this is risky, because before inflation there might have been something that does not in any sense deserve the term “Bang” (which implies a very energetic, intense and sudden event.)

Since this terminology hasn’t settled yet, what you decide to call “The Big Bang” is kind of up to you.   It’s just important to know that you have different options, and that different scientists and websites may use different meanings for “Big Bang”.

Read more about the period before inflation or after inflation.

59 responses to “Inflation

  1. Dear Professor Strassler,
    I have a very naive (and probably uterly stupid) question : if dark energy of a sort was the driving force behind universe’s expansion during inflation, why this expansion didn’t completely stop after the “reheating” phase ?

    Let me take the occasion of this question to congratulate you for this extremely informative blog.
    VM

    • The expansion of the universe is not totally due to dark energy; older models of the universe without inflation had an initial expansion that slowed over time like a ball tossed into the air slowing due to gravity. What dark energy does (roughly) is cause an *increase* in the expansion rate. As such after inflation if dark energy became effectively zero there would sill be some expansion involved.

      Now of course it appears that dark energy is not zero and as such the expansion of the universe is increasing, much to our surprise.

      • Are you meaning the present expansion is mostly a formal consequence of initial conditions after inflation and Friedman’s equations ?

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  4. Are there serious (scientifically speaking) alternative to inflation? If so, I think it could be good to mention them, or the main idea(s) behind them…

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  8. This is exciting news! I have written an idea out ( not a theory- more like science fiction) on what caused the Big Bang. I thought you and others might like it. It can be found at Amazon here: [removed by host]

    It is called: [removed by host]

  9. With no way to (currently) know about before-inflation; us non-scientists can come up with our own crazy guesses (though lacking in maths to support anything.

    So as the (observable) universe cools it starts to expand again (though suppose it’s more accurate to say ‘as the universe expands, it cools?); I imagine there’s already a ‘guess’ that it could cool enough to undergo another stage of hyperinflation before another BBT-event creates more matter/energy in the new (observable) universe? So each ‘computer-screen’ sized piece of our current universe might one day become an entire new universe itself. (Assuming whatever defines space/time has infinite elasticity?)

    • I think it’s important to distinguish between math-less guesses and what scientists actually do.

      Scientists always ground their work on their ability to predict and check things. The problem with your words is that without equations, there’s absolutely no way we could possible test, experimentally, as to whether your words are right, or check, theoretically, as to whether there could be a consistent world with these properties.

      So that makes it cocktail party speculation, and not science. You’re always free to make your own guesses, but scientists will be making equation-based guesses.

      For example, the notion of a “multi-verse” with ever-inflating regions is based on math, not words.

    • Your initial premise if off; it is not cooling that causes expansion to increase (at least not in any theory I have seen.) and as such we expect the universe’s expansion to slow continuously. The fact that it is actually increasing came as quite a surprise, one we cannot yet explain.

      If you look into the ‘big rip’ scenario there’s the possibility of something like what you suggest (Or alternatively unceasing inflation leaving a cold, cold dark universe for all eternity.)

  10. As always, very interesting stuff!

    I do have two questions.
    Firstly, if I understand correctly, the working assumption is that at some point during the inflation, dark energy did somehow cease to contribute to further expansion and was, instead, responsible for the creation of particles? Is there any hypothesis as to why this change (if it indeed was a change as such and not a process of both co-existing) could have occured?

    As to the other question, and I am sorry should I mix two completely different things together, but does matter/antimatter annihilation of these first particles play any role in this stage, or would this only be of interest later?

    Thanks!

    • 1) Yes, there is a natural way for this to happen, but it’s not an extremely short story. I may try to tell it. The exit from inflation has been widely studied by many authors over the years.

      2) There’s not much importance in matter/antimatter annihilation of any type of particle of mass M when the temperature is large compared to M c^2 / k_B , where c is the speed of light and k_B is Boltzmann’s constant. As quickly as the particles annihilate, others create them. For example, electron + positron —> photon + photon, and photon + photon –> electron + positron happen at compensating rates in the Hot Big Bang phase, until the temperature drops below 10,000,000,000 degrees or so.

  11. Hi prof, I recalled you telling me that we don’t know anything regarding the beginning of the universe and that such a speculation is simply meaningless during a discussion on Hawking’s new idea about black hole that popped up. Now it would seem that your article has explained your point of view back then and cleared some doubts of mine, Nevertheless, here are my remaining novice questions.

    1) Is it correct for us to relate the rate of inflation/expansion as being inversely proportional to the average temperate within the universe?

    2) Next, since the amount of dark energy per unit volume stays the same and does not become diluted despite the expansion of our universe, is there a chance that the amount of dark energy per unit volume will increase during inflation that results in the heating of the universe?

    3) Also, may I know some of the possible speculative causes for the mechanism behind the slowing down of inflation?

    4)Finally, in your article, dark energy is stated to be transformed into particles when the inflation slows down. May I know the quantum mechanism/process behind this phenomenon? This is fascinating for me because I have never been told by anyone before that dark energy is able to be transformed into particles, so I would love to hear your answer on this.

    • 1) During the Hot Big Bang there is a simple relation, but the relation is different during inflation and it is different after particles with mass become more important than massless particles as the universe cools. During inflation the expansion is very rapid but the universe is dead cold. See http://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/ and the articles to which it links.

      2) The dark energy per unit volume can in some cases increase, due to quantum effects. But this does not heat the universe. Dark energy contributes nothing to heat; heat will generally be associated with a soup of hot particles, but dark energy (really a combination of energy and negative pressure) is never associated with particles.

      3) The “exit from inflation” is not hard to work out in equations, but it is hard to describe in words without an article and a picture. I have to defer this to a future article or post. Sorry…

      4) Basically, the dark energy is stored in a field (the inflaton) and at the end of inflation the inflaton field starts oscillating, becoming rapidly larger and smaller by turns. This already involves the transformation of pure potential energy (as dark energy is) into part-potential part-kinetic energy (as is true for any oscillating thing.) This is like having something like a laser made from inflaton particles. The inflaton particles, however, have a large mass and are unstable. They will decay into other particles, which may in turn decay into yet others. By the time you’re done, all the energy stored in the inflaton laser-like oscillation has been turned into lower mass particles, and the motion-energy they carry quickly becomes hot temperature as these particles collide and “thermalize”.

      • Now I am confused. In your explanation on inflation you state: What happened when inflation stopped?
        The best guess as to what happened (and our equations show this is possible, but don’t tell us the details) is that all that dark energy got turned into particles …. And when this happened, the universe became very hot, and very dense — and it continued expanding, though much more slowly.
        In your reply to Kai’s query at 19:10 you say: 2) The dark energy per unit volume can in some cases increase, due to quantum effects. But this does not heat the universe. Dark energy contributes nothing to heat; heat will generally be associated with a soup of hot particles, but dark energy (really a combination of energy and negative pressure) is never associated with particles.
        The only way I think of of untangling this is that there is dark energy as part of inflation when all is dark, cold and empty and that there is some sort of phase change that happens at the beginning of the hot big bang phase that turns the dark energy into particles. Please clarify. Thanks

        • Or is the answer in reply to 4) and I was a little hasty? But then if all the space is empty where do the ‘inflatons’ appear from?

        • Your interpretation is right.

          In the simplest models:

          * During inflation the inflaton field is roughly constant and all the energy is dark energy

          * As inflation ends the inflaton field begins to swing back and forth — giant inflaton waves. All the energy has now been transferred from dark energy to the inflaton waves. You can think about this as something like a universe-filling Bose-Einstein condensate of inflaton particles.

          * The inflaton particles then decay to other, lower-mass particles, which come out with very high velocities. By the time the inflaton particles are gone, the universe is filled with these low mass particles, running around at speeds approaching that of light; and after a short time during which these particles collide with one another, this soup of particles reaches a constant high temperature.

          So that’s how dark energy during inflation becomes a hot soup of particles afterward.

  12. How large was our observable patch of the universe at the point between inflation and the hot big bang? The Higgs field settled on a uniform value throughout our observable patch during the hot big bang. Was it because that patch was still small enough for long enough after inflation to be causally connected, or is there some other reason?

  13. On a more serious note than my previous post; have the recent (and older findings) been useful for scientists in trying to determine/ discount the potential presence of additional dimensions (as in 5+, rather than other universes), or the shape of the universe?

    Though I understand NASA studies have shown the local universe to be almost entirely flat – would it be possible that the existance of distortions in the early expansion – and later big bang – be useful in determining how global this flatness is. Or is it entirely impossible to draw any conclusions beyond a cetain sphere around the earth?

    Anyway best stop this rambling series of (probably nonsensical to the topic of your post) questions. Just like to say thank you for all your work on this site, and in taking the time to reply to so many people.

    • Inflation should ensure that the universe as a whole is almost flat; any curvature would be diluted like the dots in Mr. Strassler’s diagrams. ‘As a whole’ meaning in this case all of our universe that experienced and then exited inflation. It is always possible that there exist vast volumes of the universe with vastly differing conditions than those we see in our local patch (See for example eternal inflation.) but in regards to our own little ‘bubble’ it is a tenet of science that our current conditions are nothing special.

  14. thetasteofscience

    Thanks for this fabulous explanation. The best by far that I’ve ever read.

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  18. Dr. Strassler: In many descriptions of cosmic inflation that I read, including those associated with yesterday’s announcement of the BICEP2 discovery, I see inflation referred to as a period when the universe expanded from something extremely small (e.g. Planck length) to something the size of what’s been variously described as a grapefruit, marble or soccer ball. In other descriptions, yours included, it is described as “computer screen expanded to the size of the observable patch of the universe”. Why are the descriptions so different? I can understand the need for the “smaller version”, I think, as described here: http://aether.lbl.gov/www/science/inflation-beginners.html, “Inflation is a general term for models of the very early Universe which involve a short period of extremely rapid (exponential) expansion, blowing the size of what is now the observable Universe up from a region far smaller than a proton to about the size of a grapefruit (or even bigger) in a small fraction of a second.”

    At first I questioned the need for even that much inflation… after all, if it only happened at sub relativistic speeds, would it make any difference to us now if it took a nanosecond or a tiny fraction of one? However the above-mentioned site explains the need like this: “Unfortunately, if a quantum bubble (about as big as the Planck length) containing all the mass-energy of the Universe (or even a star) did appear out of nothing at all, its intense gravitational field would (unless something else intervened) snuff it out of existence immediately, crushing it into a singularity. So the free lunch Universe seemed at first like an irrelevant speculation — but, as with the problems involving the extreme flatness of spacetime, and its appearance of extreme homogeneity and isotropy (most clearly indicated by the uniformity of the background radiation), the development of the inflationary scenario showed how to remove this difficulty and allow such a quantum fluctuation to expand exponentially up to macroscopic size before gravity could crush it out of existence.”

    Assuming the LBL site I quote from above is correct and you are correct, how can I resolve the conflicts between the widely varying descriptions of the size of the universe at the end of the inflationary period?

    • The descriptions are in some sense equivalent; consider the number of times you must double the volume of a proton to reach the size of a grapefruit, about 10^35 or so. A soccer ball and marble only add or subtract one or so doublings. A computer screen to the size of the observable universe requires a similar number of volume doublings except we’re starting with a larger initial volume. Recall also in Mr Strassler’s example only a part of this volume becomes our observable universe.

      In any case the actual size of the initial volume does not really concern us at present; inflation means that most of the details are diluted away and it doesn’t really matter.

      The site you quote it however not entirely correct; the initial ‘bubble’ did not contain the entire mass energy of the universe but a tiny fraction of it at best, essentially zero. The mass and energy in our universe came from the decay of ‘dark energy’ driving inflation.

      The reason we need inflation is that had the universe expanded ‘slowly’ (At say sub light speeds) then we would expect various areas of it to be ‘uneven'; one side of the universe might have all the mass while the other is a void for example. But what we see is a remarkable homogeneity, everything everywhere is about the same. The only way this could happen is if our universe was once small enough to ‘naturally’ be all the same (in equilibrium) and then blown up quickly enough to preserve this state. (Our universe is currently not doing this which is why it is so ‘clumpy’ on smaller scales such as galaxies and solar systems. Forces such as gravity act to turn our once totally homogenous universe of particles and radiation into dust and stars separated by vast voids.)

      • One should also note that there are still conceptual puzzles as to how to get sufficiently even inflation to occur! So this is not a done deal, on the theoretical side. It might soon be a done deal on the experimental side, but puzzles will remain to solve.

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  20. Prof. Matt, I’m just a retired engineer that loves physics. I’ve been reading your posts trying to understand a little bit, what are the present knowledge about particle’s physics in a general concept. I am very interested in the subjects what you are explaining to us. I have a doubt considering present article. Isn’t the concept of singularity important for the hot big bang ? Can you clarify better to me since I’m not a specialist ?

    • No, the concept of singularity has no connection to the Big Bang at all. I don’t know why some physicists promulgated this idea. The singularity is most likely an artifact of Einstein’s equations of gravity being non-quantum-mechanical. It is widely expected that there will be no singularities of this type found in the theory of quantum gravity — just as it is expected that the singularities found deep in black holes will be removed in a quantum theory of gravity. Removal of certain singularities in string theory has been seen in some calculations [including two of my own.]

  21. Just wanted to add my kudos to a wonderful layman’s-level explanation. All of the recent public media attention to the BICEP gravity-wave findings keeps on repeating the idea that there was first a hot, dense, extremely minute big bang, followed a tiny instant later by a big expansion. As though the energy in that “bang” also drove the space creation that took place at the inflation event. I like your explanation better . . . i.e., if I understand more-or-less correctly, that the space expansion event came first, and the energy that drove that event somehow “precipitated” into a hot big bang plasma of high-energy particles, once it reached some sort of phase transition, whereby this energy stopped creating new space and phased into (perhaps) a grand unified energy field and particles (which then expanded and cooled and broke down into the known forces, then later emerged from the radiation era into a predominance of particles holding mass). But as you say (I think), there continued to be some residual space-creation energy left behind, which we observe today and call “dark energy”.

    Please feel free to give me an “F” on this, but I wanted to point out that the public press does continue to say to the lay readership that there was a hot big bang followed by a big expansion. You and Ethan Siegel continue to say (if I’m reading you both right) that the big cold inflation set the stage for the big hot (and assumedly unified) plasma. The popular press version creates the impression that there was some sort of miraculous singularity (involving infinities) from which the plasma emerged; your explanation eliminates this, thank goodness. KUDOS, once again.

  22. Prof. Strassler, your site is such a generous and valuable offering!

    My question: is the “almost” in “almost instantaneously” necessary to the explanation (e.g., from the math)? Did inflation have to take time?

    • Yes, inflation takes time. Not much! but it is a growth over time, and there’s a maximum possible doubling rate: the Planck time is the shortest time over which doubling can occur.

  23. Patrick M. Dennis

    Prof. Strassler, as I understand it, the energy of the inflaton “condensed” in some sense into constituents that we are more or less familiar with, namely quarks, leptons, and their associated bosons. Is our specific particle zoo contained implicitly within the inflaton equations, or could an entirely different menagerie have somehow emerged?

    • No, it’s not a “condensation” onto the particles we know. The right way to think about it described below… you might say that there’s a condensate of inflatons, and the inflatons decay into particles we know (maybe not in a single step though.)

      Any realistic equations for how inflation might have occurred in the real world (as opposed to equations that consider how inflation might occur more generally) has to have the property that much of the inflaton’s energy ends up, eventually, in particles that we’re familiar with. There are many ways to do this and so there’s nothing specific about how to do it, nor is there anything special about our particular menagerie as far as the inflaton is concerned. An inflaton particle can decay to any lightweight particles that interact with it.

      How the inflaton’s energy turns into the Hot Big Bang: In the simplest models:

      * During inflation the inflaton field is roughly constant and all the energy is dark energy

      * As inflation ends the inflaton field begins to swing back and forth — giant inflaton waves. All the energy has now been transferred from dark energy to the inflaton waves. You can think about this as something like a universe-filling Bose-Einstein condensate of inflaton particles.

      * The inflaton particles then decay to other, lower-mass particles, which come out with very high velocities. By the time the inflaton particles are gone, the universe is filled with these low mass particles, running around at speeds approaching that of light; and after a short time during which these particles collide with one another, this soup of particles reaches a constant high temperature.

      So that’s how dark energy during inflation becomes a hot soup of particles afterward.

  24. In your earlier post reporting the recent BICEP2 data you wrote:

    “The black dots at the bottom of this figure, showing evidence of B-mode polarization both at small scales (“Multipole” >> 100, where it is due to gravitational lensing of E-mode polarization) and at large scales (“Multipole” << 100, where it is potentially due to gravitational waves from a period of cosmic inflation preceding the Hot Big Bang.)"

    I've read through this post (on inflation) and the questions and answers that followed. There is not much direct mention of gravity or gravitational waves. My basic question is: How do gravitational waves arise "from" (during?) cosmic inflation, if there are no particles/masses (which I thought were responsible for gravity)?

    In this post you wrote:
    "
    * During inflation the inflaton field is roughly constant and all the energy is dark energy

    * As inflation ends the inflaton field begins to swing back and forth — giant inflaton waves. All the energy has now been transferred from dark energy to the inflaton waves. You can think about this as something like a universe-filling Bose-Einstein condensate of inflaton particles.
    "
    I think I'm missing some connection between inflaton waves and gravitational waves. Any clarification you have time for would be appreciated. Or refer me to some additional sources (though a Google search search on "inflaton waves and gravity" yielded only articles that seemed either superficial or way over my head).

    Thanks

    • “How do gravitational waves arise “from” (during?) cosmic inflation, if there are no particles/masses (which I thought were responsible for gravity)?”

      First, particles and their masses are not responsible for gravity, in Einstein’s theory — or to the extent they are, it’s incidental. ENERGY is responsible for gravity (energy and momentum, really). That’s part of why dark energy (which is actually energy density and negative pressure) can have such a huge effect in Einstein’s theory, even though there are no particles around!

      So gravitational waves can actually be generated in any situation where the energy is moving around dramatically enough. Bursts of gravitational waves can actually generate other gravitational waves, with no other types of particles around (and certainly no particles with masses… you can and should think of the gravitational waves as made from [possibly many] particles called “gravitons”, just as light waves are made from photons.)

      Now — the subtle point here — which I have not yet attempted to explain, so you’re right, there’s no explanation here on this website — is that the gravitational waves were talking about at BICEP, unlike the inflaton waves I referred to in my comment, are initially a quantum effect — they are related to “quantum fluctuations”. The inflaton waves mentioned above are not.

      http://profmattstrassler.com/articles-and-posts/particle-physics-basics/quantum-fluctuations-and-their-energy/

      To really explain this properly is going to require a carefully written article. I can’t explain it in a comment, I’m sorry… in fact, it’s tricky enough that I haven’t yet figured out how I’m going to do it.

  25. Well, you may not be satisfied, but your reply has given me lots more to think about and research (eg., gravity not having to do with mass, but rather with energy/momentum) and I appreciate it. (You answer so many questions and comments I wonder how you have time for anything else!)

  26. Pingback: BICEP2 Takes a Peek at Cosmic Inflation | Whiskey…Tango…Foxtrot?

  27. Dear Prof Strassler, I think in the section “What causes inflation?” you could explain a bit better how inflation is a consequence of General Relativity. In GR, energy and pressure (and all the other components of the stress-energy tensor) cause gravity; pressure can be negative but energy cannot. A region has negative pressure if expanding it will increase its energy (as in a rubber band). Now if you have a region with a bit of energy and a lot of negative pressure, GR will give a gravitational repulsion rather than attraction, so the region expands, causing its energy to increase (because of the negative pressure). So if the initial combination of energy and negative pressure was just right, you end up with a larger region that has the same energy density and negative pressure as the original region. The process can continue, giving inflation.

  28. Tony (Racz) Rotz

    Did inflation coast to a stop or was it more like a baseball hitting a brick wall, thereby releasing energy instantaneously?

  29. Pingback: That Signal From the Beginning of Time Could Redefine Our Universe | Blog

  30. George Dellapa

    Your blog is the best thing I have read on every topic you discuss. A question: you say that during inflation the size of the universe expanded from the size of a computer screen to the size of the observable patch in less time than it takes a quark to cross from one side of a proton to the other. But if, as you say, the universe is not expanding into anything, then what does the term size signify? Is it that the distance between points within the universe or more specifically the time it takes light waves to pass from one point to another is increasing? Couldn’t you then say with equal validity that light was slowing down or that objects with dimensions were shrinking? And isn’t that as true of today’s expansion as the period of inflation. I’m just asking if that is just another way to look at the expansion so I understand what that term means.
    And you said that when talking about the size of the universe early on what we are talking about is not so much about the size of space but about time. Could you say more about that?
    I have read elsewhere that the period of inflation took the universe from a subatomic spec to the size of a basketball. But you say from a computer screen to the size of the observable universe. Is there a reason for the difference in your estimate of the pre and post inflation size?
    Why is inflation necessary to explain the uniformity in temperature across the observable universe? It is always said that this is impossible without inflation because different parts of the universe would not have ‘time’ to reach this equilibrium because they would have been separated from each other by distances greater than the speed of light and thus not able to communicate or effect their surrounding areas. But if the temperature uniformity was there from the beginning as the result of tiny quantum fluctuations why couldn’t it have just been kept during a expansion at the speed of light. Why couldn’t the initial quantum fluctuations in the infinitesimal primordial seed just expanded at the speed of light. How does this faster than light inflationary expansion help explain the uniformity in the CBR?

  31. Prof, I’m looking forward to that article on how the gravity waves that BICEP 2 hopefully observed were formed, a version for lay-people and dummies who follow modern cosmology (like myself). So, if I read you right, these gravity waves do not stem from those inflaton waves caused when the “marble rolling downhill” on the potential inflaton field energy plot sloshes around the bottom of the bowl, triggering the process of hot energy formation (“bang”) in the newly blown-up / inflated universe. (Sorry again for the crude visual analogies, they’re all I’ve got, don’t have time to learn the math). Instead it’s related to zigging and zagging quantum fluctuations during the rapid space inflation, presumably as the “marble” is dropping down the steep slope and inflating space (???). I.e., the gravity waves originate more on the (rapid) space creation side, versus the creation and initial micro-moments of the hot big bang (???? — I’ve read that the quantum effects in that hot stuff is what caused the CMB anisotrophy and thus the galaxies etc., i.e. scalar field fluctuations and not from the tensor field ???, the tensor field being where the space and curvature and biggest gravity wave effects come from ????). I’ve read that the high “r” factor of the BICEP findings indicates a lot of energy in the events going on within this inflation process, a strong inflaton field. I’ve also read that this favors the chaotic inflation model. Is the “chaos” driving this model — presumably quantum “chaos”, not related to big-world chaos theory — related to “tensor”-related (???) quantum effects causing the gravity waves? I don’t need an answer here, but I hope you might cover this in your article on just how those gravity waves originated in the inflation / big bang process. THANKS MUCH, once again!!! In all the excitement and all the popular press articles coming out on this, no one yet has taken the time yet to shed any light on just how and why and where and when those big gravity waves should result from all of this — other than to say that a lot of energy was in play, and big concentrations of energy have gravity effects, and any kind of rapid movements or changes in mass or energy can set off gravity waves.

  32. Pingback: That Signal From the Beginning of Time Could Redefine Our Universe | Science | WIRED

  33. Hi Matt. This just occurred to me reading your article – is it possible that the matter/antimatter asymmetry in the observable universe was entirely generated at the moment that inflation stopped (and so the small CP violations we have discovered experimentally so far may not be related to this at all)?

    How solid are our equations for this transition from inflation to the start of the Hot Big Bang?

  34. This article makes a reference to “the size of the observable patch of the universe”. I was wondering how this size was measured. Does it have as its radius the comoving distance from here to the surface of last scatter of the CMB (something like 46 billion light years to that region of space today) the travel distance traversed by the oldest waves we see now (ie. 13.7 billion light years) or does it refer to the maximum possible distance from here that any wave we see today could have been emitted (around 5.757 billion light years)? Or was it some other measure altogether. This may be a trivial difference in terms of describing the speed of inflation, but I was curious what the expression meant.

  35. Pingback: Did BICEP2 Detect Gravitational Waves Directly or Indirectly? | Of Particular Significance

  36. Pingback: That Signal From the Beginning of Time Could Redefine Our Universe | NEWS UP TODAY

  37. I’d like to find out more? I’d like to find out more details.

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