Of Particular Significance

If It Holds Up, What Might BICEP2’s Discovery Mean?

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 03/18/2014

Well, yesterday was quite a day, and I’m still sifting through the consequences.

First things first.  As with all major claims of discovery, considerable caution is advised until the BICEP2 measurement has been verified by some other experiment.   Moreover, even if the measurement is correct, one should not assume that the interpretation in terms of gravitational waves and inflation is correct; this requires more study and further confirmation.

The media is assuming BICEP2’s measurement is correct, and that the interpretation in terms of inflation is correct, but leading scientists are not so quick to rush to judgment, and are thinking things through carefully.  Scientists are cautious not just because they’re trained to be thoughtful and careful but also because they’ve seen many claims of discovery withdrawn or discredited; discoveries are made when humans go where no one has previously gone, with technology that no one has previously used — and surprises, mistakes, and misinterpretations happen often.

But in this post, I’m going to assume assume assume that BICEP2’s results are correct, or essentially correct, and are being correctly interpreted.  Let’s assume that [here’s a primer on yesterday’s result that defines these terms]

  • they really have detected “B-mode polarization” in the “CMB” [Cosmic Microwave Background, the photons (particles of light) that are the ancient, cool glow leftover from the Hot Big Bang]
  • that this B-mode polarization really is a sign of gravitational waves generated during a brief but dramatic period of cosmic inflation that immediately preceded the Hot Big Bang,

Then — IF BICEP2’s results were basically right and were being correctly interpreted concerning inflation — what would be the implications?

Well… Wow…  They’d really be quite amazing.

Would this story be bigger than the discovery of the Higgs particle?  Certainly at least as big.  To try to compare the two gets us into silly discussions; you can’t know the long-term implications of a discovery at or near the time it is made.  But the immediate list of implications would certainly be longer.

Some Definitions

Before I start: we’ll need some definitions, given in this article, of:

  • A unit of energy called GeV (roughly the mass-energy of a hydrogen atom)
  • The Planck energy, Planck mass and Planck length (processes involving the Planck energy or mass in a region the size of the Planck length will generally involve both quantum mechanics and gravity [“quantum gravity”])
  • The energy scale associated with dark energy (which is not quite the amount of dark energy — for one thing, despite the name, dark `energy’ is actually an energy per unit volume.)

If any of these is unfamiliar to you, you may want to read that article first and to have it handy, in another tab or window, to serve as a glossary.  You may also want to have my History of the Universe or my BICEP2 Discovery Primer hand.

Implications of BICEP2’s Discovery — IF … IF …

IF IF IF BICEP2’s measurement is correct, at least roughly, and IF IF IF it is being correctly interpreted, at least roughly, then there’s a long list of broad implications that I can think of, or that others of my colleagues have pointed out to me in conversations.  Some of them are vague — areas where the implications are likely to be important but are not yet very clear.  Some of these things were already somewhat implied by the previous experimental successes of the theory of inflation, while some stem directly from BICEP2.

IF … IF …

A puzzle that bothered scientists for decades, as to how the observable patch of the universe (i.e. the part that we can actually observe today; the universe may be much, much larger than this — see here) could be so uniform, would indeed be firmly solved, by a period of cosmic inflation.  The extremely flat geometry of the universe would also now be firmly explained.

We would also have confirmation about how the universe became hot — about how the Hot Big Bang got started.  The picture would be this: a large amount of dark energy first makes the universe big, via inflation, and then the dark energy turns into energetic particles, making the universe hot (and still expanding, albeit more and more slowly [until relatively recently]).  Some people like to say that inflation puts the “Bang” in “Big Bang”, but remember that it also makes the universe flat and uniform and huge (typically much larger than the observable patch) before it heats it up.

The existence of cosmic inflation would itself be another feather in the cap of Einstein’s theory of gravity — since it is Einstein’s theory that predicts that the presence of a positive cosmological “constant” [not necessarily constant], also known as “dark `energy’ ” [not really energy, but energy density and negative pressure in just the right combination] actually causes the universe to expand, rather than (as we’d naively expect from gravity) contract.

And (IF… IF…) the confirmation of cosmic inflation would mean that those who pointed it out and its advantages, and developed the idea — people like Starobinsky, Guth, Steinhardt, Linde — ought to be able to celebrate (though not all will do so, because they abandoned the idea…)

Also celebrating would be those who pointed out the possibility of a detectable signal from gravitational waves in the polarization of the CMB, which is what BICEP2 has apparently observed.  I believe these would be Kamionkowsi, Kosowsky, Stebbins, Seljak and Zaldarriaga (but the cosmologists should correct me if I’m unfair here.)

Were there any doubt left concerning Einstein’s prediction that gravitational waves exist — perhaps some leftover worry about the 1993-Nobel-Prize-winning indirect detection of energy carried being off by these waves, via precision measurements of a pulsar, one of a pair of closely-orbiting neutron stars discovered by Hulse and Taylor — it would be over.

And if anyone still wasn’t sure that gravity is due to a spin-two field, not  a spin-zero field, the BICEP2 result would settle the matter; you can’t get B-mode polarization of the CMB without combining the spin-two polarization structure of the gravitational waves with Thomson scattering.

If there were any doubt that gravity is controlled, just like everything else, by quantum physics, it would erased; BICEP2’s observation would imply that just like other fields, which are subject to quantum jitter (i.e., random “fluctuations“), space and time (somewhat more precisely, the metric that determines distances) undergoes the same type of quantum fluctuations as other fields, fluctuations that any quantum version of Einstein’s theory of gravity would predict.  No details about quantum gravity are needed for this conclusion.

The amount of dark energy required during inflation, in order that BICEP2 could observe this gravitational wave signal, would (IF… IF…) be enormous.  The energy scale of dark energy [defined here] would be about 1016 GeV, just about 100 times less than the absolute maximum possible, which is the (reduced) Planck scale.  This need not have been the case!   This energy scale  could have been trillions of times smaller, and yet still given acceptable amounts of inflation of the universe and an acceptable Hot Big BAng.  Amazingly, the dark energy during inflation, if BICEP2 is right, is much larger than it needed to be, and almost as large as it could possibly be!

Moreover, if the energy scale of dark energy [defined here] had been just a few times weaker than BICEP2 observes — 1000 times less than the absolute maximum — then the gravitational wave signal would have been so faint that it would have been completely drowned out by another process (lensing of E-mode polarization, the solid red curve on the first figure here.)  So most of us thought, with this wide range of possibilities, that it was a real long shot that BICEP2, or any future similar experiment, would ever see any signal due to gravitational waves from inflation.  That the gravitational waves would turn out to be large and powerful enough to be observed would be an amazing piece of luck for scientists wanting to understand the universe.  (IF… IF…)

[The original version of this paragraph overstated what we’d know; thanks to my colleagues for pointing out a blunder concerning the heating after inflation.]  Also, this would mean we would now have an estimate for how hot the observable patch of the universe could have become after inflation ended and the region containing the observable patch became very hot — the start of the Hot Big Bang.  The energy scale of the dark energy during inflation determines the maximum possible temperature at the start of the Hot Big Bang, more or less.  To say it another way: after inflation with dark energy of scale 1016 GeV ended, the universe would have become hot, potentially so hot that the average particle rushing around in the hot dense soup of particles had a motion-energy of 1016 GeV, though perhaps quite a bit less than this.  That’s a maximum temperature of as much as 1029 degrees [yes, that’s a 1 with 29 zeroes after it!!!]

The Large Hadron Collider’s data provide direct insights into physics at the energy scale of around 1000 GeV or so.  [The mass-energy mc² of the Higgs particle is 125 GeV.]  The BICEP2 measurement would arguably be our first direct evidence (IF… IF…) concerning physics at higher energy scales than the LHC (though one could argue we have a little information from the existence of neutrino masses.)  And not just a little higher!  Since the scale of the dark energy at inflation is at 1016 GeV, we’re talking 10,000,000,000,000 times higher!!!  We’ve been trying for years, using various methods, to find evidence concerning how physics works at or near the Planck energy and Planck length.  All previous efforts have come up with nothing; proton decay might have given us insight, but it is too rare if it happens at all; neutrino oscillations haven’t given a clear pictures; cosmology might have revealed big surprises concerning the Planck energy, but none have previously shown up.  But if BICEP2 is right, then, for the first time, we are seeing phenomena which occurred near this energy scale!

Similarly, data from cosmology has made us very confident that we understand physics in the early universe back to the time when the first atomic nuclei formed during the Hot Big Bang — a few minutes after the Hot Big Bang started, when the temperature was such that the typical particles had energy of about 0.001 GeV.  And we have had reasonable confidence that we have a decent understanding, using what we’ve learned about particle physics recently, back to times of a billionth of a second, and temperatures corresponding to an energy of a hundred GeV or so.  But now our understanding may be taking (IF… IF…) an enormous leap, back to the very start of the Hot Big Bang, at a temperature corrresponding to an energy of as much as 1016 GeV, and even earlier, into the frigid inflating universe.

The success of the details of the equations that form the heart of inflationary theory suggests that not only did Einstein’s theory of gravity describe physics at very early times and very high energy scales, the basic principles of quantum field theory worked back then too.  It could easily have been imagined — and many have imagined, with concrete ideas backed up with equations — that there might be important principles that we are unaware of, or modifications of the ones we know, that would give very different predictions from standard inflationary theory.  So far, there’s no sign of that at all.  BICEP2’s result (IF… IF…) would be yet another sign that despite having done all our particle physics and gravity measurements at much lower energy scales and longer distances, those accessible to the LHC and to our previous experiments, we’ve actually understood the principles that govern the behavior of phenomena at much, much higher energy scales and much shorter distances!

The energy scale inferred from BICEP2’s measurement, 100 times smaller than the Planck energy — 1016 GeV or so — has appeared in particle physics before!  If you take the three non-gravitational forces of nature — the strong nuclear force, the weak nuclear force and the electromagnetic force — and you first consider how they are rearranged when the Higgs field is turned off, into strong nuclear, weak isospin, and hypercharge forces — and then you look at how the strengths of the forces drift as you study them at shorter and shorter distances and higher and higher energies, you find the forces all become about the same strength at an energy scale of about 1016 GeV or so.  This is called “unification of coupling constants” (i.e. force strengths) — or, originally and more ambitiously, “grand unification”, a grander notion that the three non-gravitational forces are actually manifestations of just one type of force.  [Unification of that unified force with gravity is yet another question; that’s what string theory might do, though string theory does not require grand unification be a separate process from the unification with gravity.]  Originally, back around 1980, inflation was imagined to be associated with grand unification, and if so, would have an energy scale of about … yes, 1016 GeV or so.  But that idea died out long ago as people learned more about both unification and inflation.  Yet now we must wonder: could that part of the original idea, in some vague way, have actually been right?

Over the years, scientists have invented a plethora of variations on how inflation might have, in detail, taken place.  BICEP2’s observation of a gravitational wave signature would sweep away most of them, leaving just a few.  (More will be invented in coming months, though!)  It also would sweep away some alternatives to inflation and many speculative ideas for how the universe might behave, or might at least have behaved at earlier times.  Despite “tension” (meaning mild disagreement) between BICPS2s current data and the Planck satellite’s data, a rather simple observable patch of universe, with rather simple laws of nature and a rather simple form of inflation, are consistent with what we know.

One of the variants of inflation that would be excluded by BICEP2’s data is the notion (admittedly long-shot anyway) that actually the Higgs field could play the role of the inflaton after all.   It turns out that no observably large gravitational wave signal would be expected if that were true.

BICEP2’s result would represent the first time that, without any theoretical speculation about quantum gravity, an experiment has forced us to consider processes involving physics at the Planck scale, where quantum gravity is important.  Specifically, for a variant of inflation to give such a large gravitational wave signal relative to the size of the other non-uniformities in the cosmic microwave background, the Planck energy scale becomes important.  The inflaton field, which (by definition) is the field whose stored “potential energyis the dark energy, must change by an amount close to or a bit larger than the Planck energy scale. [Sorry for the necessary technical-speak here. This whole business is inherently confusing for non-experts.  The dark energy may change or may not change at all while the inflaton field is changing; that’s a separate question.  What it means for a field like an inflaton to change by the Planck energy scale is that if a particle interacted with the inflaton field as strongly as it possibly could (the way the electron interacts with the Higgs field, but stronger), then as the inflaton field changed by an amount comparable to the Planck energy scale, the particle’s mass would change by an amount comparable to the Planck mass.]

An extremely simple possibility for inflation that would still be consistent with all the data (IF… IF…) is just to have a spin-zero field (a bit like the Higgs field, but with important differences — no weak nuclear force effects, for instance) which has a mass and no substantial interactions with any other fields.  This is a model introduced by Andrei Linde; it’s amazingly simple.  Could it really be correct?

Among the other very simple ideas that are likely to feature prominently in discussions of the near future are ones involving a “pseudo-Nambu-Goldstone boson”, of a type often called an “axion”.  [I wrote a little about this in the paragraph just below the figure within this post about the Planck satellite and what it means for variants of inflation.]  One of the variants of inflation that is most consistent with both the Planck satellite and BICEP2  (IF… IF…) is an idea called “natural inflation”, from 1990, due to Katherine Freese, Joshua Frieman and Angela Olinto.  [Here’s a pdf of the paper.] Such a field has the feature that it is periodic (i.e., if you change it by enough, it comes back to itself, the way an angle that can only range from 0 to 2π.)  It also has the feature that its interactions with all other fields are rather weak.  When this field varies a lot, the dark energy associated with it varies by quite a bit less… so the field could vary by something approaching the Planck scale yet the energy scale of the dark energy could stay rather constant, at one percent of the Planck energy.

In order for this idea to really be consistent with quantum gravity, it might require some modification, of the sort was introduced in 2008 by Liam McAllister, Eva Silverstein, and Alexander Westphal, in which the axion is periodic with a larger range than you’d naively expect.  [They did this work in the context of string theory, but I don’t believe that string theory specifically is necessary for the idea to work; it might work in other consistent quantum gravity theories, if there are any.]  Expect to hear much more about axions.  [By the way, it’s long been suggested that dark matter itself could be made from a type of axion… but presumably not the same one…]

One last one which is the weakest of the set… In a recent article, I pointed out that if the Higgs particle discovered at the LHC turns out to be of the simplest possible type — a “Standard Model Higgs” — then because this is “unnatural(in the sense of “highly non-generic”) it might call into question fundamental conceptual issues, perhaps even the whole framework of quantum field theory.  But BICEP2’s measurement would seem to support the framework of quantum field theory in a world of three smooth spatial dimensions up to very high scales and very short distances, so that line of thinking would be disfavored. (IF… IF…) Of course, measurements of the Higgs particle are still in the early days — at this point we can only say the Higgs particle is roughly “Standard-Model-like in type —  and maybe the Higgs will turn out to be more complex, or other new particles will show up at the LHC, possibly rendering the discussion moot.  But (for reasons I outlined in my Santa Barbara talk two weeks ago) having only the Standard Model, with the Higgs field turned on just a little, plus quantum gravity and an inflaton field (and plus something to explain dark matter and neutrino masses) would pose many grave conceptual problems, ones that anthropic reasoning would not address.  [Experts: Anthropic reasoning can be used to argue why the cosmological constant might be small; it can be used to argue why there is a hierarchy between the Planck mass and the proton and electron masses; but it cannot easily be used, without huge and problematic assumptions, to argue why one would find just the Standard Model at the LHC, with one light scalar Higgs field and nothing else new.] So I would tend to see the BICEP2 result (IF… IF…) as a (rather weak!!) argument slightly in favor of supersymmetry and grand unification, with supersymmetry just a little out of reach of the LHC for some reason, or modified in some way to make it harder to observe than expected.  [Experts: “mini-split” supersymmetry, which preserves quantum field theory, with unification of coupling constants at the grand unified energy scale, starts to look better than something more radical.]

Colleagues: what have I left out?  Eventually this post will be stored on this site as a reference article, so I’d like to make it complete.

Final Remarks

One more time I must remind you that we’re still some time away from trusting the BICEP2 result, and quite a long time from trusting the interpretation of the result in terms of inflation.   All of the implications I’ve mentioned are therefore provisional.   But the list is impressive, and there are probably more I have forgotten to mention, or are known to others but not to me, along with still others that haven’t yet been noticed by anyone.  So I don’t yet know if BICEP2’s measurement is right, or if inflation occurred and if it created the signal they observe, but I do know this: there’s a lot at stake.

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181 Responses

  1. Just wanted to comment now that BICEP2 is mostly overturned how much I appreciated your caveating in this article, and how often I wish other headline science discussion came well sprinkled with (IF… IF…).

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  3. “… Inflation is better thought of as a Cold Little Swoosh, because at that time our universe was not that hot (getting a thousand times hotter once inflation ended), not that big (less massive than an apple and less than a billionth of the size of a proton) and not much of a bang (expanding a trillion trillion times slower than after inflation). …”

    … Max Tegmark (is a physics professor at M.I.T. and reported in the NY Times, April 11, 2014)

    I can accept the mass being infinitesimal, even zero, since one would expect bosonic fields “were created” first leading to the resonances of fermion fields as matter. This would also explain the “zero 3D space” since quarks (fermions) were not yet created.

    So, what were the “cause(s)” before matter (and 3D space)?

    I have a problem with Mr. Tegmark’s remark;

    “… at that time our universe was not that hot (getting a thousand times hotter once inflation ended), …”

    Does this statement contradict the conservation of energy?

  4. Great post!


    “the frigid inflating universe”.

    Terribly confusing for a layman. Wouldn’t the Bunch-Davies vacuum, or equally at early times for an observer the Gibbons-Hawking effect, give the temperature here? But that comes out as the scale of inflaton fluctuations too, or 10^-5 of the 10^16 GeV inflaton.

    Does supercooling really apply here? Linde writes on the right now preferred chaotic inflation: “By now the debate is over: no realistic versions of new inflation based on the theory of thermal phase transitions and supercooling have been proposed so far. Gradually it became clear that the idea of chaotic initial conditions is most general, and it is much easier to construct a consistent cosmological theory without making unnecessary assumptions about thermal equilibrium and high temperature phase transitions in the early universe.” [ http://arxiv.org/pdf/0705.0164v2.pdf ]

    So if there is no thermal equilibrium of the background theory, don’t I have to assume that either Gibbons-Hawking enforces a blackbody temperature, or there is no well defined temperature?

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  6. @Jorma Reinikainen:

    The theory of cosmic inflation predicted that one of the tell-tale signs of Inflation would be a certain kind of gravitational wave, with a very distinct signature.

    As this theory was further developed, it also predicted that these gravitational waves would leave a very distinct mark in the polarization of photons freed at the event of first recombination.

    So, this experiment is so important because it has been able to find those very distinct marks in the polarization of the CMB radiation.

    In short, it is inflation the source of these particular gravitational waves that this experiment has been able to “confirm”.

    We still need more validations from other teams with their own experiments to be able to consider this a proper confirmation.

  7. Great post. If the BICEP2 result holds up, it is highly prejudicial against higher dimensions or branes at energy scales below 10^(14-16) GeV. While it’s true that gravitational waves/gravitons are involved, these are generic features of any semi-classical theory of gravity. The “strongly-coupled gravity” regime is not touched.

  8. Great article. I didn’t see anything about the implications regarding the multiverse model of inflation. Would the data imply that this model is still possible? Does the possible result say anything about thee curvature of space?

  9. Dumb question but is the speculation that Higgs may relate to electo-weak breaking like inflaton may relate to GUT breaking ? So maybe Higgs and inflaton combine in some way at very high energy.

  10. Matt, thanks for the excellent summary.
    Re the history of this, you missed out one important paper by Polnarev, at <a href=http://adsabs.harvard.edu/abs/1985SvA….29..607P this link ; which was the first to show in detail that gravitational waves create polarisation in the CMB. The 1996/7 papers by various combinations of K,K,S,S,Z went on to do the E-mode/B-mode decomposition, and show that B-modes are created by gravitational waves but not by scalar density perturbations.

    (It’s worth noting the converse is not true, i.e. gravitational waves create both E- and B-modes in similar amounts, but their E-modes are not separable from the larger scalar contributions).

  11. A question: What happens to a black hole in inflating space? The space within it should be expanding faster than light; would this cause the hole to be disrupted?

    1. If the black hole is much smaller than the horizon associated with inflation then there will be no effect. If it is much larger than the horizon associated with inflation, then inflation near the black hole will be modified. I’m not expert enough to know off the top of my head how to think about that.

    2. AFAIK there was no black hole(in the usual sense) at the time of inflation. only hot soup of radiation. But I guess you could think of that as a black-hole of a sort. At least that is what is been told.

  12. Prof. Strassler: What might be a decisive test for Linde’s theory of chaotic inflation? Assume for the sake of argument that Linde’s theory of chaotic inflation is valid and also that the space roar is empirically valid. Then, so far as I know, there are 4 possible explanations for the space roar: (1) decay of dark matter particles, (2) decay of unknown particles which are not dark matter particles, (3) decay of micro black holes, & (4) brane-based collision(s) between (or among) alternate universes. Are there any more possibilities?

  13. Matt: Do you know when Planck polarization analysis will be announced? I understand they already got data for polarizations before the satellite died and it is almost a year since their previous paper was published last year. This would resolve many of the experimental uncertainties.

    1. I am told that Planck is not likely to resolve things because they’re just not as powerful at doing polarization measurements. It may actually be the follow-up to BICEP2 that is likely to be the next big news, in about a year perhaps. One experiment was delayed by the sequester.

  14. One question – I am a laywoman, but people who know physics better than me tell me (also from your writings) that this inflation of spacetime occurred only inside a region of the universe (which includes our own observable patch) which means that a giant chunk of universe was created from a tiny one almost instantaneously. So where did the tiny chunk come from/ maybe you have said it somewhere. So there was some space prior to the inflation, a kind of a small, maybe micro or dwarf universe, a patch of which got exponentially inflated while other parts or patches outside of our observable patch might have remained stagnant or had their own inflations at different rates? Like a bump or lump on the whole spacetime which represented the universe at that stage? Which means, the universe was/is not uniformly expanding? So this inflation period does not represent any beginning , except the beginning of the inflation period of that specif pre-existing patch, the gray region with dark energy which you depicted with red and green dots beforehand which had existed prior to the inflation. Then it is not point zero on a time scale? Sorry for asking these insane questions. However, I haven’t found them discussed in the newspaper articles…. Also, your own Known and suspected history-graphics of the observable patch does not talk about a mini gray space patch or tiny chunk existing prior to inflation . I do understand acceleration of expansion / inflation due to dark energy for any time AFTER that initial inflation.But I do not understand what tiny chunk or gray region with dark energy means for the initial inflation. Was this inflation from a supposed point value or an already existing tiny chunk the shape and size you don’t know. And why is it then such a big thing if it’s not the beginning of the universe?
    A brie clarification will surfice. Thanks very much in advance. Maybe some of the other readers even know the answer?

    1. @Margot: As TOMH mentions in the following comment, Linde’s talk describes his chaotic inflation model reasonably well. My understanding is that a small patch gave rise to our observable universe and other patches probably have given rise to other universes- multiverse. No body knows how that patch came into existence! Some people have speculated that it may have arisen from a previous big crunch if that is what happened. But these are all wild speculations with zero experimental support. If BICEP2 is confirmed that would be the first ever experimental support for inflation model as far as I can tell. Let us see what Matt says in his future comments.

    2. We simply don’t know the answers to your excellent questions. These are all subjects of active research by theoretical physicists with expertise in quantum gravity. Hopefully we’ll all live long enough to know some of the answers.

  15. Personally I do believe, that the B-mode anisotropy can really serve as an evidence of gravitational waves in general relativity sense, just because these waves are so giant and slow, they appear frozen in time. In 4D general relativity the gravitational wave is already 4D artifact, so it has no additional time arrow available – it cannot simply move there in the same way, like the gravitational waves observed inside of deep space.

    At the human observable scale the gravitational waves do manifest itself as a 2-spin component of photons, the CMBR photons in particular. These photons are stationary and they do behave like the noise at the water surface, introduced with spreading of much faster sound waves through underwater.

    From this perspective the finding of B-mode anisotropy is another good message for these, who still believe in holographic AdS/CFT correspondence. The distant gravitational waves are dual to the CMBR photons from this perspective, their scalar component being more specific. For more complex artifacts the AdS/CFT correspondence appears broken with the extradimensions though.

      1. The understanding why gravitational waves cannot propagate in strict relativity is easy, as Rosen has derived in 1939, they cannot carry a mass, i.e. the energy. As Eddington pointed out already before many years, gravitational waves do not have a unique speed of propagation. The speed of the alleged waves is coordinate dependent. A different set of coordinates yields a different speed of propagation and such waves would propagate like noise. In brief, the fully flat space-time has no location or speed defined. If some gravity wave appears in it, the only source of reference frame (which defines the location and speed) can serve the gravitational wave itself. So in flat space-time the gravitational wave cannot propagate.

        The tiny inhomogeneities of space-time, i.e. the CMBR noise just change this situation, because they do enable the gravitational waves travel at short distance in form of photons. In fact Eddington had a very clear idea of a co-ordinate free method of determining the modes of gravitational wave propagation (via infinitesimal diffeomorphisms of the co-ordinate systems) well before Einstein, whom only managed this 15 years later.

      2. All these connections can be illustrated with miniaturized 2D analogy of 3D Universe by the spreading of ripples along water surface. The transverse light waves correspond the Faraday capillary waves in this analogy. And the gravitational waves correspond the longitudinal waves, which do manifest itself like the quadruple Brownian noise (analogy of CMBR noise) at short distance scale and like the quadruple gravity waves (i.e. surface waves of strong longitudinal component) at the large distance scales.

        If the water surface would be fully homogeneous (i.e. free of Brownian noise), then the surface ripples would propagate in it without scattering (i.e. like the Maxwell waves in special relativity). But in reality, the surface ripples are spreading in background independent way in regular circles only at the very beginning – but when they travel at the sufficient distance, they’re scattered into gravity waves, which is what we are observing by now at the cosmological distances as a B-mode of CMBR polarization.

        If we would live at the water surface like the waterstriders, we would observe its longitudinal waves at distance too, which is the reason, why I don’t talk about inflation like about temporal phenomena, but like about emergent artifact of light scattering with density fluctuations of vacuum, which manifest itself at distance.

    1. @zephirawt:

      Even though I’m not an expert in the field either (as I take it from your comments that you ain’t one too), I can attest to the fact that you are confused about many concepts.

      The arrow of time, (which is an expression that describes the fact that time behaves asymmetrically since, for instance, we can remember the past but we can’t “remember” or predict the future), is a consequence of the expansion of the universe, and it has nothing to do with the properties of space-time.

      We know from Noether’s theorems that in fact, space-time has inherent symmetries, including time symmetry, that explain the existence of certain conservation laws, and since we have both theoretical and experimental reasons to understand that Noether’s theorems are valid, we know that Entropy is not in itself only and exclusively a consequence of certain properties of space-time.

      Regarding gravitational waves, when they are formed, they have incredibly small wavelengths, which is one of the reasons we have so far not been able to detect them directly.

      But in this case, we had the luck that inflation itself gave us a very big hand by expanding the universe by such a huge scale factor, that the size of the wavelengths when the first recombination happened (which is the event captured by the CMB radiation, including its polarization) are large enough for us to detect its influence on polarization.

      Besides all this, in classical mechanics, we know that waves do not transport matter, and this is very simple to validate with an experiment:

      If you through a cork on the shore, you will see it bobbing up and down when waves pass by, but the waves will not move the cork away from the x,y position where it stands, the waves make it move up and down the z axis.

      In Quantum Field Theories, I do not know enough to comment on the fact that particles are themselves fluctuations of the field that they belong to, and particles at the very least have relativistic mass, and of course, particles are waves.

      So, I leave that to the experts.

  16. There are a few things to consider regarding the theory of Cosmic Inflation.

    In the case of Alan Guth, his first interest on exploring these ideas was started by a question from Henry Tye regarding how many magnetic monopoles would exist at the very early stages of the universe … it turned out that the prediction was: too many!

    Since there are no magnetic monopoles present today, and since they are massive and its presence in the early universe would have altered its evolution towards something very different than what we see today, Guth started to work on an idea that there was some event at the early stages of the universe that swept away and diluted these nasty relics (as well as other nasty effects).

    Regarding Andrei Linde’s own approach to Inflation, I do not have information about the main interest(s) that drove him to think about this theory.

    As this theory started to evolve, it was proven over the years to pass many tests, when validated against experimental data, and to be robust enough to stay on top of things as more data and more ideas were added to the picture of the Big Bang.

    We could argue that it still is an interim theory, or a “scaffolding” theory, but it has been able to withstand the test of time for the last 25 years, and that is really something kind of amazing.

  17. At the moment, when we have no idea how/why the inflation did happen, then every working physical analogy of it should be handled with caution. During scattering of waves at the water surface the wavelength of ripples expands fast after certain threshold, which could be considered as an analogy of inflation. it means, that the tired light model not only can explain the red shift, but the alleged inflation too. As Hubble already noted, the tired light model based on scattering of light with tiny stable particles will not work, but the quantum fluctuations of vacuum are temporal and potentially larger than the wavelength of visible light, as the CMBR noise indicates. So I presume, that the correct interpretation of inflation is right there. We already have observational evidence of it, which is explained at my blog.

    So, Mr. Stassler is quite right in his cautious stance, because I even don’t think, the B-mode polarization found at BICEP has something to do with inflationary period, because it can be attributed to the lensing of dark matter strings BETWEEN galaxies, which are indeed much younger than the alleged inflation. The wavelength dependence of B-mode supports this stance clearly – everything what we are supposed to do by now is to correlate it with DM lensing observable in IR spectrum (the DM lensing AROUND galaxies has been already substracted from data).

  18. It is amazing to see it in “real time”, as it happens, how theoretical physics and experimental physics work hand in hand to validate a theory, and to be able to do it with such level of precision.

    We still have to wait for other teams and experiments to independently validate or refute these results and interpretation (there are some teams with their own experiments already at work with this important task).

  19. Sigh… thanks, this gives us great insight into your character. You’re quoting someone who says “Lately, Big Bang Creationists have far overplayed their hand, making themselves look like fools”, which is not only snide and nasty, in referring to these scientists as “Creationists”, it’s actually the precise reverse of the situation. You should be ashamed to quote him.

    I remind you the Bang Bang model makes detailed predictions, and one after another those predictions have worked. There were dozens of opportunities for the model to fail, including the detailed peaks in the cosmic microwave spectrum — those peaks might simply have not been there are all, or formed a pattern that was completely unexpected and could not fit within the Big Bang model. This would not have surprised me at all. But the Big Bang predictions worked.

    1. Bah! Those predictions would have matched observations in *any* universe in which the Big Bang was even remotely a good model!

        1. No problem, you’re welcome. And any time you’d like the piss taken out of you again, you just let me know.

        1. All of them? It was essentially a tautological statement — the BB predictions would match in any universe in which the BB model was a good one — accompanied by an ironic dismissal of the implications of that statement(that it’s a good model for our universe). It was a riff on the classic Homer Simpson line: “Facts are meaningless. You could use facts to prove anything that’s even remotely true!”

    2. To talk of predictions that worked is also to ignore all the predictions that haven’t worked. And then there are the retrodictions where they new the answer they were looking for according to their preferred model and kept messaging the data until they got the answer they wanted. For details about this in regard to the cosmic background signal please see:

      Also, I am not at all ashamed to quote Grote Reber’s comment. There are other scientists who say the same thing:


      1. Vincent, what are you so upset about? If you’re right, someday the data will agree with you, and you’ll be famous and we’ll all agree.

        But if you’re not ashamed to quote someone like Reber, who is nasty, disrespectful and insulting to his colleagues, then I do think you have some personal issues to answer for. Reber’s attitude reflects badly on him as a person, and quoting him reflects on you.

        Finally, I have told many people that this is not an advertising site for their personal ideas. I’ve given you more time out of respect for your intelligence, but my patience is coming to an end. Please desist with the advertising, and put your arguments in the scientific community where they belong. This website is *explicitly* for communicating mainstream ideas about science to the public. It is not for giving voice to long-shot ideas that are rejected by the vast majority of scientists as being out of touch with data… even if, in the end, those ideas were to turn out (to everyone’s surprise) to be right.

  20. Is there a way (not necessarily feasible tomorrow) to pinpoint the mass of the inflaton? Which is the error associated with E~10^16? I think it’s quite crucial to say that GUT involves the inflaton field or not.

    1. I don’t know how much is feasible; this is at the limit of my expertise. That level of detail I still have to learn myself. I think the error on the total energy will soon not be that large, because the gravitational wave signal will be well measured. But going from there to an understanding of the properties of the inflaton will be much slower and more limited.

  21. This type of idea is old, boring and inconsistent with the data. It invariably requires tired light scenarios that have been ruled out and in any event is inconsistent with structure formation constraints, BBN and so forth. Already back at the turn of the century people had guessed that infinite static universes were impossible (olbers paradox).

  22. I found a much better explanation (diagram, actually) that shows clearly what all the BICEP polarization fuss is about.

    Theoretically , there are enough free electrons floating in space (since almost forever) so that any “density waves” from inflation or other expansion activity will result in sheets of free electrons between the waves being compressed, and that in turn will reflect the photons from the cosmic background radiation in such a way that certain polarizations will be preferred. I am in total agreement with this idea, and this is indeed a great find. Awesome bit of work I somehow believed had already been tried and failed some time ago.

    It is an indirect way to see the effects of gravity waves, too, sort of like a free electron tide in space. Nice!

  23. The temperature has been estimated to be around the grand unification scale. Given that part of the signal originates from before the inflation, can you estimate energy density before inflation from the data?

    I remember reading an old estimate that before inflation, instead of fantastic temperatures and densities, the observable universe was surprisingly rarefied and cold, containing only about a kilogram of matter at ~10000 K. I can’t remember who this was, but the argument was from the statistical properties (possibly entropy) of the primary CMB fluctuations.

    1. I’m confused; that part of the signal riginates from AFTER inflation. We have no information about what happened before inflation; there’s no data that relates to it, and on the theory side it’s all guesswork.

      1. OK, it appears I have misunderstood this. In the primer, you wrote: “[Gravitational waves’] existence is predicted by inflation; just as the inflaton has quantum jitter, so do space and time themselves, and this leads to ripples in space and time.” Before inflation, the universe was uniformly filled with inflaton field potential energy. The only nonuniformity was the quantum fluctuations in the inflaton energy and spacetime. So, when inflation blew up the universe, these were magnified. Inflaton energy fluctuations gave the primordial mass-energy anisotropy. But, was it so that the pre-inflation spacetime fluctuations survived in the form of gravitational waves? Their existence on very large angular scales would require them to be pre-inflation, since only faster than light waves could communicate across large angular scales. This is how I understood it. Did I get it wrong, and if so, where? (I am not a physicist, but I have a rudimentary understanding of thermodynamics through being an industrial organic chemist with some publications on physical chemistry.)

        The idea in the mentioned paper was basically so that you take the energy distribution implied by the CMB, and squeeze it back to an assumed pre-inflation size. Then, you can calculate the temperature, and from that, predict the expected distribution (i.e. not absolute, but relative statistics of high and low densities) of the pre-inflation anisotropy, and then (I believe through a horrible series of assumptions again) predict the post-inflation anisotropy and the CMB anisotropy. The result was rather suprisingly that the temperature was low. I do understand it’s like trying to hear a pin drop next to a jet engine, so it requires a lot of assumptions. I didn’t find the exact paper, but I see that Sean Carroll has been claiming something similar.

        1. @Jorma Reinikainen:

          Again, for full disclosure, I’m not an expert in this field, so, I could be introducing some wrong concepts and assumptions into my comments, but I will present them anyways:

          The phrase “just as the inflaton has quantum jitter, so do space and time themselves, and this leads to ripples in space and time” as far as I can tell, from a perspective of pure General Relativity, is meant to describe a comparison between quantum waves and classical waves.

          General Relativity is a classical theory in the sense that it does not include or support or describe quantum effects, so, a ripple in space-time like with gravitational waves is really a classical kind of wave, while a fluctuation of any kind of quantum field is a quantum mechanical kind of wave.

          When you describe mixed theories like quantum gravity, it is more valid to say that a “mostly GR” event like a gravitational wave could be analyzed also from a quantum perspective to include quantum fluctuations also into the picture.

          When you mention the phrase “Their existence on very large angular scales would require them to be pre-inflation, since only faster than light waves could communicate across large angular scales”, I understand that there are some mistakes here.

          From a purely GR perspective, gravitational waves cannot travel through space-time faster than the speed of light, just for themselves. In fact, they travel exactly at the speed of light, no more, no less.

          The inclusion of speeds faster than the speed of light into this picture is a consequence of the process of cosmic inflation (let’s be clear about one thing: the use of “faster than the speed of light” is confusing at the very least, because what inflation did was expanding the “metrics” of the early universe, or the “system of coordinates” of the early universe, and one thing it DID NOT DO was to “push” or “drive” matter faster than the speed of light within a given system of coordinates).

          So, when you imply that gravitational waves moving faster than the speed of light is something that happened before inflation, you might be wrong in more than one way.

          1. I’m not ignorant about the metric expansion of space, or suggesting actual faster than light. My question is more about when and what produced the gravitational waves. If they exist on scales that would imply superluminal propagation in a post-inflation universe, that leaves no options other than pre-inflation origin.

  24. Matt, your comment below is not correct:

    “The existence of cosmic inflation would itself be another feather in the cap of Einstein’s theory of gravity — since it is Einstein’s theory that predicts that the presence of a positive cosmological “constant” [not necessarily constant], also known as “dark `energy’ ” [not really energy, but energy density and negative pressure in just the right combination] actually causes the universe to expand, rather than (as we’d naively expect from gravity) contract.”

    What you described above is one of the classic myths of Big Bang cosmology. The cosmological “constant” is not integral to Einstein’s theory of gravity. It was an ad hoc addition to his theory as applied on cosmological scales and was only required to account for the small velocity of the stars as was believed at the time. Einstein believed, for mistaken reasons not having anything to do with relativity theory, that the universe was finite. His thinking was if the universe is finite something must be keeping the universe from collapsing, this being before the belief took hold of an expanding universe.

    Matt, before you tell me I’m wrong and embarrass yourself, please see:


    1. You have failed to appreciate the terminological difference between Einstein’s theory of general relativity, and Einstein’s ‘theory’ (really model) of cosmology. The latter is a solution of the former, although like many solutions has been discarded in the waste bin of history. Cosmic inflation is a *different* solution of the former and in both cases the cosmological constant has the effect of a negative pressure. So it is true that that was why Einstein put the CC in, but that does not make Matt’s statement any less true.

      1. Columbia, I haven’t failed to appreciate the differences between the two. And as far as I’m concerned both the CC and inflation models of all types will find themselves in the dustbin of history. General relativity as applied to cosmology doesn’t need a solution involving a repulsive force, negative pressure, or whatever you want to call it. The only good solution for solving the horizon and flatness issues is an infinite non expanding universe. An infinite universe can not collapse. An infinite universe is naturally homogenous, isotopic and flat as we observe at scales great than about 300 million light years. The cosmological redshift (not the same as the Doppler effect) is indicative of distance only as Edwin Hubble believed. The microwave background would be light redshifted from great distances in an infinite non expanding universe. For more on these points of view you can visit my website critical of Big Bang cosmythology by clicking on my underlined name at the beginning of each of my posted comments.

        1. Vincent,

          Your statement with regard to Olbers paradox is correct. However, you lose me with regard to the rest of your comments. For example you note that:

          “General relativity as applied to cosmology doesn’t need a solution involving a repulsive force, negative pressure, or whatever you want to call it.”

          That may have been true prior to 1998, but it is not true since then. In that year and since, observations of standard candle type IA Super Nova leads directly to an accelerated expansion of the universe against gravity. Believe me, the teams that originally made these measurements were not expecting this. In fact they set out to determine the universe’s deceleration due to gravity. Instead they found clear evidence of acceleration that kicked in roughly 5 to 7 billion years ago. By the way, the same observations also found clear evidence that the universe was indeed experiencing deceleration due to gravity when the universe was younger – eliminating alternative explanations.

          General Relativity with a cosmological constant offered a ready explanation (though not the only one) for these observations. However all involve “a repulsive force, negative pressure, or whatever you want to call it” operating against gravity or reversing gravity itself.

          On a general note: There seems to be some confusion. General Relativity with a cosmological constant may be an explanation for the ‘recent’ acceleration that kicked in 5 to 7 billion years ago – effectively a repulsive force that increases with distance F = (ᴧmc^2r)/3. It is negligible when the distance is small – as in the early universe. Thus it is not an explanation for Inflation in the early universe. Inflation is its own theory. As Matt rightly notes the new observations reveal the signature of gravitational waves – and the existence of these waves themselves are predicted by GR. The recent observations are yet another confirmation of these. But the size and nature of the signature itself is consistent with certain models of Inflation.

          1. S. Dino,

            Your comments are well informed and I appreciate your time to write what you posted here. I have a file of papers and articles on the subject that you refer to. I would be inclined to take the time to make a response but I’m not sure it would be worth it to me since I’m skating on thin ice with Matt at this time. He’s getting tired of having to deal with an iconoclast at his blog.

    2. My statement is correct. Einstein’s theory predicts that a positive cosmological constant can lead to inflation. It does not predict the existence of a cosmological constant, but I didn’t say that it does.

      Your unpleasant attitude toward me and toward other commenters is pushing the limit of my tolerance. What you’re doing right now is digging your reputation’s grave.

      1. Actually isn’t it correct to say that general relativity *does* predict the existence of *a* cosmological constant-since such a term is consistent with the symmetries of the Einstein-Hilbert action and cannot, therefore, be excluded by fiat? But that it does not predict neither its sign nor its magnitude?

        1. GR allows for a non-zero cosmological constant, but it does not predict a non-zero cosmological constant. If we accept that a zero cosmological constant is not a cosmological constant at all, then it’s correct to say GR doesn’t predict a CC.

          1. So, maybe it’s better we find variable-based modifications for example via dynamical vacuum potentials combined with Higgs mechanism perhaps… 😛

          2. What Stam means is that GR doesn’t predict 0 cosmological constant, and there’s no symmetry which forbids one; so in a quantum theory, you have to calculate it, and you expect it to be non-zero. So really it is GR in a quantum context that predicts a cosmological constant.

  25. a couple of things I do not understand

    (1)you said elsewhere regarding expansion ”’Let me say that again: it was expanding extremely rapidly, and it was deathly cold.”

    so is this wiki quote wrong ?

    (The exact drop is model dependent, but in the first models it was typically from 10^27K down to 10^22K.[17])

    (2)also was it deathly cold throught the expansion or only at the end ?

    (3)if the planck satellite find no polarisation would that mean inflation never occurred.?

    (4)are there any theoretically possible experiments that would detect the inflaton particle.

    1. Another quote: ” During inflation, the energy density in the inflaton field is roughly constant. However, the energy density in inhomogeneities, curvature, anisotropies and exotic particles is falling, and through sufficient inflation these become negligible. This leaves an empty, flat, and symmetric universe, which is filled with radiation when inflation ends. ”

      This Wikipedia quote is correct. The universe is EMPTY — and so what does heat mean if the universe is empty? The universe is filled with radiation when (meaning “in the moments after”) inflation ends.

      (2) during the expansion it is empty, and so, cold.

      HOWEVER — there is a big subtlety with this remark. It has to do with a quantum effect. I am deciding how to deal with this; it requires an article, but it will be, inherently, confusing. It turns out that the notion of temperature is perspective-dependent… and it looks as though I’m going to have to deal with that. I’m not going to try that this week though. So stay tuned.

      3) No polarization does not mean inflation never occurred. It means only that inflation occurred with not that much dark energy, so the gravitational wave signal is there, but too small to observe.

      4) Theoretically possible? Yes. Practical? Impossible to answer until we know more about what the inflaton might be like. And it is quite possible that we’ll never know how to approach the problem — or at least not in our lifetimes.

  26. Dear Matt, just FYI, I’ve located the answer to the question I asked earlier:
    “From what I understand, this fits extremely well with the basic chaotic inflation model given by V(Φ)=λΦ^4.
    We also know that amplitude of density fluctuations is ≈10^(−5) and energy scale of inflation is around 10^16 GeV.
    Given all this information, can we now make an educated guess for how many e-foldings happened during inflation ? Or at least a theoretical lower (upper ?) bound ?”

    The answer is that, for this model, the upper bound for number of e-foldings during inflation is around 10^7.
    For a quadratic potential, the upper bound is approx 10^13, which is what I think Linde uses.

    Well, makes me happy !
    I was beginning to find the 60 e-fold 100 billion light year wide observable universe somewhat claustrophobic. 😉 😉

    Here’s the relevant link:

  27. Firstly, thanks!

    I heard Hawking Radiation mentioned a couple of times in the context of these Results. I wonder how is it related to inflation and why do these results (if, if,..) proove their existence?

    1. I hope matt comments on this in his next article


      Hawking radiation has been part of the physics firmament for decades; it’s the best-known prediction of quantum gravity.

      “Now it seems that Hawking and Unruh were right!” said Max Tegmark, a cosmologist at M.I.T., noting that some physicists had wondered whether gravity obeyed the dice-playing quantum principles that Einstein had disdained. “Now we know that gravity is indeed quantized, involving graviton particles,” he added.

      If the Bicep2 results are confirmed, and if astronomers agree that the ripples were gravitational waves from inflation, the discovery of Hawking radiation could win a Nobel Prize for Dr. Hawking.

  28. Warp drive… you forgot Warp drive Matt. Anti gravity found in nature means that warp drive is… err… not completely impossible. Just unlikely.

  29. I have a very simple question. I’ve never heard of “B-mode” or
    “E-mode” polarization before. I’m here referring to the received signal.
    One gets an obvious hint from the letters B and E. Googling the
    terms seems to verify that. But … is E-mode just a difference
    in vertical/horizontal linear polarization intensity and B-mode just a
    difference in right and left circular intensity?

    1. Polarization is a vector, and the polarization of the CMB is a vector field that can be measured on the sky. We can decompose this vector field into a component that is the gradient of a scalar (named E modes, for reasons that are hopefully clear), and a piece that is the curl of a vector (B modes).

      The polarization field arises from anisotropies in the temperature of the CMB. Within inflationary theories, these anisotropies arise from fluctuations in the scalar field that drives inflation (the inflaton) and from fluctuations in the metric of spacetime. The spacetime fluctuations are gravitational waves. Because of the parity of the underlying fields, the scalar field can only give rise to E modes; the gravitational waves can produce both E modes and B modes. Hence when E modes are measured (and they were first measured about 12 years ago), within the context of inflationary theories we are measuring some sum of scalar and metric fluctuations. But when B modes are measured, we are only(*) measuring the influence of gravitational waves.

      (*) Well, almost only. Gravitational lensing by matter between the surface of last scattering (the moment about 300,000 years after the big bang where the CMB photons began free streaming to us) and we on Earth can make what was an E mode turn into a B mode. This effect is strong on small angular scales, and weak on scales of a degree and larger. This is why BICEP2 focused on scales close to 1 degree; they knew that IF the gravitational-wave-driven B-mode signal turned out to be strong enough, it would stand above the lensing signal in this regime.

  30. The most remarkable thing about this whole business as far as I’m concerned, is just how efficient this particular measurement happens to be at eliminating rival theories of inflation. m^2 phi ^2 textbook example inflation.. Gone! Standard Ekyprotic universe… Gone! And then the really disconcerting fit with chaotic inflation (.96, the median value between phi^4 chaotic inflation and phi^2), Its almost too perfect.

    Of course, this is probably less about the real physical parameter space of rival theories, and more about sociology, since a lot of papers were written trying to find models that didn’t reproduce Linde’s popular predictions.

  31. You mention energy scales of 10^16 GeV, this is obviously way higher energy then any known particles, but how does it relate to known structures energy densities, i.e. black holes?

    1. It’s far enough below the Planck energy that it doesn’t relate to black holes directly, no.

      It’s still a rather isolated piece of information, but we can hope that over the decades will gather more clues and start to build a picture.

  32. “And (IF… IF…) the confirmation of cosmic inflation would mean that those who pointed it out and its advantages, and developed the idea — people like Starobinsky, Guth, Steinhardt, Linde — ought to be able to celebrate (though not all will do so, because they abandoned the idea…)”

    Well, I believe they are still entitled to celebrate, even if they abandoned the idea! Didn’t Einstein consider the cosmological constant as his “biggest blunder”? And yet, everybody talks about “Einstein’s cosmological constant”, and nobody even mentions the two pioneers who were behind the idea of an expanding universe: the Russian Alexander Friedmann and the Belgian Georges Lemaître…Like all History, the History of science isn’t fair!

        1. Wilczek said that the charitable thing to do about Hawking’s latest paper is to ignore it. Perhaps he should hope that everyone extends that same charity to him.

  33. As with any great and somewhat unexpected discovery, there are winners and there are losers. I have heard a lot of talk as to the winners, namely inflation and its creators. But, being a Met fan I would also like to know who the losers are. Back in the 60’s the discovery of the Cosmic Background Radiation meant the rise of the Big Bang theory over the Steady State theory of creation. Now if (if, if, if – as Matt says) yesterday’s results hold up are we at a similar moment in time?

    Specifically, it is my understanding that one of the very trendy competitors to the Big Bang with Cosmic Inflation is the Ekpyrotic universe where the creation of the universe arose through the collision of two Branes (short for membrane – brought to you from the ever popular higher dimensional M-theory crowd). It was my understanding that the gravitational wave signature of the Brane collision theory would be smaller than that of the Big Bang with Inflation. So if, if, if yesterday’s results are confirmed, doesn’t it mean that creation through Brane collision is wrong, wrong, wrong?

    1. The ekpyrotic universe idea would be in trouble. Not sure if it is dead.

      Many, many other variations of inflation itself would be in serious trouble, borderline dead.

      Ideas that involve the universe having extra dimensions with size more than a few hundred times the Planck length would, I think, be in very serious trouble.

      The notion that particles called “axions” might form the dark matter of the universe, though not dead, will be pushed into certain corners (see a comment on this from earlier.)

        1. Good questions, but I don’t know because I haven’t done the calculations myself or read papers about it. I don’t know what such ideas actually predict for inflation.

          1. Ok. I’ve done some but i think my calculations are not professional enough. First phase was doughnut-like 1-dimensional interaction-space with time-dimension and duality-ring-dimension, then inflatory 2-dimensional space Roth time and duality, like fine structural ball of wool and finally would be 3d space like nowadays. Light speed would be varied due to dimensionality and in respect of uncertainty metrics.

            The reason why newborn universe could transit itself to next higher dimension is interaction volume “overcrowded” by uncertain areas – there were no space left for causal connections.

            I think there will be a way avoid true horizons but can’t find writings just about this kind of idea…

          2. Roth = with (stupid mobphone).
            I meant the dimensional phase transition happens under Pauli’s law.

  34. Thank you for this Matt. Your explanation helped tremendously. Oh, on a side note, I like your emphasis “IFs”.

    1. No, it really doesn’t say anything one way or another about string theory. String theory is just as possible, but just as difficult to verify, as before. Eva Silverstein would probably argue that in the long run it may now be easier to check string theory, with BICEP2 opening up a window to physics at the Planck energy scale. She would probably be right, but the long run is quite a while off.

    1. I urge caution too. On the other hand, there are not many other explanations out there for B-mode polarization at large angular scales. So increasingly the onus is on those who urge caution to come up with a plausible alternative explanation of the data. [Assuming the data is right, which is reasonable but not certain either.]

  35. If this experiment is correct does it tell us any hints about the number or mass of the primordial black holes?

    1. I’m not an expert on this, so I don’t know if it tells us anything. Inflation tends to make it very difficult to make primordial black holes, but there presumably are loopholes.

  36. Sorry if I missed a post where you’ve already answered this question, but I hadn’t (in my limited reading elsewhere) learned that before the HBB the universe was very empty and cold. What leads to this conclusion?

    (And let me add to the chorus of thanks for your blog and your coverage of this report of the BICEP2 results — thank you!)

  37. “An extremely simple possibility for inflation that would still be consistent with all the data (IF… IF…) is just to have a spin-zero field (a bit like the Higgs field, but with important differences — no weak nuclear force effects, for instance) which has a mass and no substantial interactions with any other fields.”

    Doesn’t it need to have interactions with other fields to create the Standard Model particles when it collapses? Is this too small to be “substantial” here or are interactions before the field collapses distinct from after?

    1. It has to have some interactions, but they also have to be small — and how small or large is a quantitative question (i.e. how substantial is substantial enough.) I haven’t looked at the numbers.

  38. Thanks for this summary, Matt. A question: would the bicep2 results constrain preon models or other speculation about the fundamentality of the standard model? Or do they just basically rely on some quantum fields that can fluctuate?

  39. Thanks !! In future, will reach for my barrel of salt when reading about guesstimates about how much inflation occurred. 🙂

  40. An addendum and some motivation for my question above.

    I’ve seen several claims by Linde that inflation resulted in around 10^12 e-foldings.
    He had an article at Scientific American based on this number – I dont know if its a lower or upper bound – and several talks online quoting this number.

    I recall vaguely that this follows from assuming a quadratic potential and using the fact that density fluctuations have amplitude 10^ -5
    But I don’t remember the details.

    Also, a lot of Linde’s work seems highly speculative to me with little clarity on what is well established and what isn’t – so here’s my chance to get that stlled in this regard.

    1. Linde’s work is highly speculative. He also has a remarkable track record of landing on some aspects of nature. One must always take him seriously, even when he sounds crazy. Chaotic inflation sounded crazy. It’s just crazy enough to perhaps be true.

      I first formed this impression, by the way, when I took his inflation class during my graduate school years at Stanford.

  41. Dear Prof Strassler,

    First off, let me say, I immensely appreciate your effort to present the science clearly and cautiously, as opposed to the hype from media and many popular scientists.

    In your post, you mention that “inflation made the universe very large, probably much larger than the patch we observe”.

    Now my question:

    After the BICEP2 results, we now know that ns=0.96 and r=0.2.
    (IF we assume the results are correct, of course…)

    From what I understand, this fits extremely well with the basic chaotic inflation model given by V(Φ)=λΦ^4.

    We also know that amplitude of density fluctuations is ≈10^(−5) and energy scale of inflation is around 10^16 GeV.

    Given all this information, can we now make an educated guess for how many e-foldings happened during inflation ? Or at least a theoretical lower (upper ?) bound ?

    PS: I am aware that around 60 e-foldings are needed to explain the homogenity of CMB and any satisfactory model of inflation must satisfy this.
    My question is that given this extra info, can we do better ?

    1. I don’t see any reason why the number of e-foldings couldn’t be 600 or 6000. Or how we would find out. Maybe one of my colleagues can set me straight if I’m wrong…

      But soon 50 might be constrained by the data. At this point I don’t think we can put any constraints until we know which of the many models for the inflaton is correct.

  42. As another example of how this BICEP result would have major implications if true, there was a very interesting paper from today by Marsh et al that stated — IF — we take BICEP at face value and take from the result an energy scale of inflation, there are strong constraints on axion dark matter for values of the axion decay constant f_a larger than the hubble scale of inflation H_I.

    The basic idea is that if f_a > H_I then axion acts a light scalar field during inflation, so it has its own quantum fluctuations that are separate from the inflaton’s, and so the axion develops isocurvature perturbations. The amplitude of the isocurvature perturbations is proportional to the scale of inflation. Since the inferred scale on inflation inferred from BICEP is so large, the amount of isocurvature in the axion dark matter is large as well, and this can put it in tension with bounds on isocurvature modes in the CMB.

    The conclusion seems to be that for QCD axions, having f_a > H_I is only consistent with the isocurvature bound if you fine tune the initial condition for the axion during inflation to about one part in 10^8. More generally for axion like particles, if you have large axion decay constants, f_a > H_I, the fraction of dark matter in axion dark matter can only be a negligibly small value ( 10^{-28} eV.

    Reference: D. Marsh, D. Grin, R. Hlozek, P. Ferreira, “Tensor Detection Severely Constrains Axion Dark Matter,” http://arxiv.org/abs/1403.4216.

    1. Thanks for this comment. Yes, this paper has been making the rounds, and I appreciate the summary. Some of these general points, before we knew about the scale of inflation, were made in earlier papers, but I don’t know this literature very well.

  43. Matt, I’ve been reading for a long time and appreciate your blog very much. It’s very informative, and I find it a great resource to give students. Two quick points on this post:

    1) I’ve always understood that the reheating temperature is bounded by but not set by the scale of inflation. In perturbative reheating with a slowly decaying inflaton, the reheat temperature is set by the decay rate and can be much lower.

    2) The “monodromy inflation” work of Silverstein, Westphal, and McAllister should certainly be given a lot more attention now (deservedly so). But it’s more of a model than a construction. My guess is that there is a correct model of this type, but work over the last few years by Bena, Grana, and others has shown we have a ways to go before we really understand how moduli stabilization works in full.

      1. The general idea that there are monodromies that could give rise to large-field inflation feels right to me. But, even in the best cases, our understanding of moduli stabilization is a mix of 10D/11D physics and 4D effective field theory, while we’re generally interested in some higher-D physics to drive inflation. There are technical points in how that fits together that are turning out to be stubborn to resolve even in the best-understood compactifications. It’s possible models based on large numbers of axions avoid these points if the relevant potential is purely from the 4D EFT, but it’s been a long time since I’ve looked at that.

        Perhaps the best way to make my point is to say that I think these inflation models are good signposts in directing our thinking, but the models we have are really at the edge of what we understand in terms of string compactifications, and there are a number of issues that could possibly cause problems.

  44. ” The extremely flat geometry of the universe would also now be firmly explained.”

    Erm….not really. Inflation doesn’t really solve any of those problems; it just replaces one fine-tuning problem by a worse one. See for example


    Though of course this doesn’t conflict with anything else you have said: inflation almost certainly did happen, it just doesn’t explain the things it was originally claimed to explain!

    Actually this just makes the recent discovery [IF IF IF etc ] all the more exciting: if inflation really happened, we can’t make sense of it until we understand how inflation got started. So we are really forced to think about the pre-inflationary era, and that is really something.

      1. Agreed. The real advantage of inflation is that it gives us a way to work towards a solution of all these problems, not that it provides one in itself.

  45. As a lowly human I find myself rather poorly equipped to understand just what the hell is going on with stuff (thanks evolution!). This blog helps a bit. Thanks very much Matt!

  46. Prof. Strassler: In connection with the idea “… a large amount of dark energy first makes the universe big, via inflation, and then the dark energy turns into energetic particles …” I can think of two basic possibilities: (1) something generates the Higgs field from the inflaton field or (2) the Higgs field coexists with the inflaton field and something interferes with the Higgs field in the early universe. Is a there a third basic possibility concerning the Higgs field in the early universe?

    1. (2) is correct; the Higgs field coexists with the inflaton field, is on but very small during inflation (and completely irrelevant, since its value is so small), and then is kicked off during the early Hot Big Bang by the effects of high temperature, before turning back on again as the universe cools.

      1. Could you elaborate on why the strength of the Higgs field is affected by temperature? Particularly why the high temperatures turn it off when inflation ends.

        1. The same reason why a magnet is magnetized when it is cold and becomes demagnetized when it is hot. Or why a metal is a crystal when it’s cold and a liquid when it’s hotter. Heat often destabilizes a field in such a way that it can’t stay on. The transition between these situations is called a “phase transition”.

          Note that things can actually happen the other way… where a field is on at high temperature and off at low temperature. But it’s not uncommon to have this situation.

          1. I’m not sure I follow. My understanding is that magnetization is undone when the temperature reverses the spontaneous symmetry breaking of the vector field. How does temperature destabilize a scalar field? What symmetry is being undone?

          2. I meant to ask what symmetry breaking or phase transition is being undone in the Higgs field?

            1. This is actually a little tricky.

              Naively, what’s being broken is the electroweak gauge symmetry. Many people will say these words. But any true expert in quantum field theory will tell you that gauge symmetries actually aren’t symmetries. That’s why there are no Nambu-Goldstone bosons generated in the broken phase.

              Less naively: No symmetry is broken, but there are indeed two phases. One phase has massless W and Z particles, the other phase has W and Z particles with mass. The phase transition is between these two phases.

          3. Forgive me Matt, but I’m trying to get a handle on my understanding. Your reply sounds to me like the description of EW symmetry breaking at Standard Model energies. If the field disappears at hot Big Bang temperatures, what phase transition is happening? The EW bosons are massless up to the unification scale. Is it really turning off at this point, or becoming irrelevant at 10^16 GEV and above? Perhaps I’m mixing two different things.

            1. It *is* the “electroweak symmetry breaking” phase transition [note the misnomer! no actual symmetry is broken.] I’m just saying that the temperature is WAY ABOVE this transition temperature, so the Higgs field is off. It only goes back on when the temperature drops down to where typical particles have energy below 100 GeV.

              What’s actually happening with the Electroweak bosons is actually subtle. They pick up a different type of mass, called a plasma mass, in a hot gas (the same is true for photons.) But they’re not getting mass from the Higgs field; the Higgs field is off.

              Constrast this with very high energy collisions that would occur in a collider at zero temperature. In that case, the electroweak boson masses aren’t zero, and do come from the Higgs field — yes, the masses are irrelevant because the energy of the collision is so high, but Higgs field is on.

  47. I see people saying things like “If inflation is there, then the multiverse is there. Hard to build models that have inflation and no multiverse”. If we assume the results are valid is this an implication? I’m guessing these people are exaggerating since you didn’t mention it in your post but was curious if this is correct.

    1. It is true that both Linde and Guth see it as difficult to have inflation without having a hugely complex universe with huge separated regions, which is what one may call a “multiverse” in this context. They are great scientists and one has to take them seriously. But this is something we only know about from theoretical considerations; there’s no experimental evidence for it as of now. My own understanding of the theoretical considerations involved is too weak for me to have a strong opinion, and at this point I’m inclined to take the view that the issue is not settled yet, though a “multiverse” of this type is a possibility. I plan to write about this soon.

    1. Well — dark energy is really not a material in the sense that we think of materials. You can’t slice and dice it and hold it in your hand. And the statement that space is expanding is not really the statement that objects are repelling one another. So it’s not really accurate. On the other hand, New Scientist is doing its best with very little space and time to explain things, so as compromises go, it could be a lot worse.

  48. Thanks for all the information and insight! Regarding the Higgs field and inflation, however, there’s already a paper on the arXiv arguing that, in the context of Linde’s chaotic inflation, the Standard Model Higgs field, acting as the inflaton, can produce a sufficient signal of gravitational waves as long as the kinetic term in the Lagrangian is allowed to run with the scale, so that it becomes suitably modified at high field values:

  49. Wow, thanks for this post, who’d have thought something as exciting as this would happen in Physics in 2014 – no one really foresaw this did they?

    THIS is a reason that makes me even more nervous than you about the BICEP2 results – it would be extraordinary if the observation is accurate and is confirmed by the Planck polarisation results later this year (or other experiment) – almost miraculous you might say 😉

    It will be great if it all turns out well, but, probably naively, I do wonder if people are overstating the promise of implications for particle physics at 10^16 Gev – I mean so what if inflation kicked off at that scale – how will that translate to useful observations – are we going to see signatures of supersymmetric particles in the CMB?

  50. youe recent article are superb. thanks.

    ‘ But now our understanding may be taking (IF… IF…) an enormous leap, back to the very start of the Hot Big Bang at temperatures of 1016 GeV, and even’

    typo ?

      1. Hi Matt,
        When you say Hot Big Bang, are you referring to the Big Bang after reheating? correct me if I am wrong, but there is no model-independent connection between the scale of inflation and the reheating temperature

        1. Thanks; I am indeed referring to the Big Bang after inflation stops. Andrew Frey also reminded me that this my statement is too strong. We do know the heating *could* have been this large, but not how large it actually was.

      2. as gev is energy and not temperature I thought it should say
        ‘back to the very start of the Hot Big Bang, at temperatures corresponding to an energy as much as 1016 GeV,’

  51. As usual you explain things very well – a pleasure to read.
    I am by no means an expert, but there seems to me to be one more implication (I already mentioned this on the other thread): inflation works! So the energy density in a quantum field theory does indeed couple to gravity. This rules out the speculative idea that the constant term in a QFT is somehow irrelevant and “can be ignored”. Therefore the same should apply to the dark energy we see now – it can’t be explained by ignoring quantum field theory and interpreting it as some purely classical effect.

  52. Matt: Excellent summary. Thanks. For the time being I am assuming that the experiment is correct. I have two questions. (1) Your statement : “ one should not assume that the interpretation in terms of gravitational waves and inflation is correct.” Polarizations produced by scattering from atoms and electrons is well known. But I have a feeling that there may not be any model other than Gravitational waves for this kind of B- mode polarization. Is this correct? Do you have any handy reference to understand B-mode polarization? (2) From the last scattering which happened at 380,000 years after the bang, how sure one can be that it indicates what happened in perhaps first 10^ (-35 ) sec when the energy was 10^ (16) Gev?

    1. (1) There are no particularly convincing explanations for this B-mode polarization other than gravitational waves, as far as I know. I do have a reference for B-mode polarization but I don’t have it handy… will look for it. Remind me if I forget.

      (2) You don’t know what it indicates without the equations for the Hot Big Bang and inflation. We believe those equations (Hot Big Bang because of experimental data on nucleosynthesis etc., Hot Big Bang and inflation provisionally because of their success in predicting other properties of the cosmic microwave background radiation.) In science, you’re always combining experimental data with theoretical interpretation of that experimental data, testing and prodding, until you have a consistent picture that agrees with all the known data.

  53. Thanks as usual for your combination of thoroughness and clarity. In my opinion you do the best job of explaining what all of the fuss of yesterday’s announcement was about to laypeople without watering down too much or making over generalizations.
    I do have one question. You say that the Planck energy is the largest energy that the dark energy could possibly have. Why is that? What evidence do we have that the Planck energy is the highest possible energy? I know there is certainly no experimental evidence of the highest possible energy. I also know that we don’t really know whether or not the Standard Model or any other theory is correct at such high energies. So on what basis do theorists claim that the Planck Energy is the highest possible energy? Wouldn’t we need a (mostly) complete quantum theory of gravity to know that?

    1. Quantum gravity would certainly be necessary at that point and the equations that we use for inflation, which assume quantum fields and semi-classical gravity, would certainly be wrong. Maybe the energy density could be larger, but the implications for gravitational waves would be very, very different, so we’d not expect to get a signal anything like what we see.

      In any case, the lack of a signal seen in previous experiments already put an *experimental* limit below a few percent of the Planck energy scale.

      1. I’m sorry, I think I did not state my question very clearly. I’m still not sure if I can word it correctly, but I will try again. My question was more about the Planck energy itself than the value of the dark energy. How do we know that the Planck energy is the correct upper limit? Isn’t the Planck energy determined simply by dimensional analysis on physical constants? On what theoretical or experimental grounds do we expect that there can be no energies higher than this?

        1. The Planck energy is not so much a hard limit but rather the point where one set of theories break down. IF the dark energy scale was around or greater than the Planck energy then we would need a new type of theory to explain it. This is much like how at low speeds (energies) Newtonian physics works but at higher speeds we need to take into account relativity.

          In this case we have pretty good evidence that the limit was below 10% of the Planck energy. Not because there is some intrinsic limit built into nature but because the phenomena we observe today cannot be produced by theories using energy scales above said limit.

          Similarly if these results are valid a number of theories are ruled out for being *below* a certain limit energy-wise.

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