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BICEP2 Makes Waves in Cosmology: Now What?

The views expressed are those of the author and are not necessarily those of Scientific American.


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The shock waves are still reverberating from BICEP2’s bombshell announcement that they’ve discovered the holy grail of cosmology: the telltale signature of gravitational waves from inflation. But what does this discovery really mean, and what impact will it have on cosmology?

About 13.8 billion years ago, merely 400,000 years after our Big Bang, everything in our observable universe was a hot plasma not too different from the surface of the Sun. Photos of this plasma, baby pictures of our universe around its 400,000th birthday, have already revolutionized modern cosmology and triggered two Nobel prizes. Now a team of astronomers has spent three years zooming in on about 1 percent of the sky from a state-of-the-art telescope at the South Pole, taking an even sharper photo of this plasma, including its polarization, and discovered that it’s distorted in a tantalizing way.

At the press conference, I met Alan Guth and Andrei Linde, whose theory of cosmological inflation had predicted this distortion, looking even happier than in the left-hand photo above. If they instead looked distorted as in the right-hand photo, you might wonder whether someone had slipped LSD into your morning coffee. Or whether gravitational waves – distortions in the very fabric of spacetime – were passing between you and them, bending the light rays that you see. BICEP2 has shown that humongous gravitational waves close to a billion light-years long are distorting their cosmic baby picture.

LEFT: Andrei Linde (left) and Alan Guth (right) at a Swedish crayfish party, blissfully unaware that I'm photographing them and that they'll need to dress differently to collect the prestigious Gruber and Milner prizes, which recognized them as the two main architects of inflation. RIGHT: Andrei and Alan distorted by gravitational waves

Making such strong gravitational waves requires extreme violence. For example, a cataclysmic collision of two black holes squeezing more than the Sun’s mass into a volume smaller than a city can create gravitational waves that the US-based LIGO experiment hopes to detect – but these waves are only about as big as the pair of objects creating them. So what could possibly have created the vast waves BICEP2 saw, given that our universe seems to contain no objects large enough to make them?

The answer to this question explains why Alan and Andrei were smiling: inflation! Their inflation theory in its simplest form predicts that our universe was once smaller than an atom, repeatedly doubling its size every 0.00000000000000000000000000000000000001 seconds or so, and this rapid doubling was precisely violent enough to create gravitational waves of the strength and length that BICEP2 has observed! Although it sounds like this repeated doubling of the inflating substance would violate the laws of energy physics (specifically energy conservation), it actually doesn’t, thanks to a loophole in Einstein’s theory of general relativity. I explain the physics of inflation and its aftermath in detail in chapter 5 of my new book Our Mathematical Universe in case you’re curious about how this works. When the inflating substance eventually decays into ordinary matter, the resulting hot plasma eventually cooled and clumped into the galaxies, stars and planets that adorn our universe today.

So how seriously should we take inflation? Inflation had emerged as the most successful and popular theory for what happened early on even before BICEP2, as experiments gradually confirmed one of its predictions after another: that our universe should be large, expanding and approximately homogeneous, isotropic and flat, with tiny fluctuations in the cosmic baby pictures that were roughly scale invariant, “adiabatic” and “Gaussian.” To me and many of my cosmology colleagues, the gravitational waves discovered by BICEP2 provide the smoking-gun evidence that really clinches it, because we lack any other compelling explanations for them. For example, the ekpyrotic and cyclic models of the universe that had emerged as the most popular alternatives to inflation are now suddenly ruled out because they cannot explain BICEP2’s gravitational wave detection.

This means that if the BICEP2 results hold up and we take inflation seriously, then we need to understand and take seriously also everything that inflation predicts – and these predictions form quite a long list! First of all, inflationary cosmology (“IC”) radically changes the answers to key questions given in the traditional cosmology (“TC”) textbooks I once studied:

Q: What caused our Big Bang?
TC: There’s no explanation – the equations simply assume it happened.
IC: The repeated doubling in size of an explosive subatomic speck of inflating material.

Q: Did our Big Bang happen at a single point?
TC: No.
IC: Almost: it began in a region of space much smaller than an atom.

Q: Where in space did our Big Bang explosion happen?
TC: It happened everywhere, at an infinite number of points, all at once, with no explanation for the synchronization.
IC: In that tiny region – but inflation stretched it out to about the size of a grapefruit growing so fast that the subsequent expansion made it larger than all the space that we see today.

Q: How could an infinite space get created in a finite time?
TC: There’s no explanation — the equations simply assume that as soon as there was any space at all, it was infinite in size.
IC: By exploiting a clever loophole in Einstein’s general relativity theory, inflation produces an infinite number of galaxies by continuing forever, and an observer in one of these galaxies will view space and time differently, perceiving space as having been infinite already when inflation ended.

Q: How big is space?
TC: There’s no prediction.
IC: Probably infinite.

Because of this last prediction, the BICEP2 discovery should cause dismay among multiverse skeptics – at least in this particular universe. This is because Alex Vilenkin, Andrei Linde, Alan Guth and others have shown that the space that inflation generically creates is not merely infinite, but uniformly filled with matter that forms infinitely many galaxies. This in turn means that no matter how unlikely it is that a galaxy will be indistinguishable from ours, containing someone whose life has so far been identical to yours, the probability is not zero since it clearly happened here. Which means that there must be duplicate copies of you far away in space, and indeed also similar versions of you living out countless variations of your life. Now it’s harder for skeptics to dismiss this by saying “inflation is just a theory” – first they need to come up with another compelling explanation for BICEP2’s gravitational waves.

I think that if the BICEP2 discovery holds up, it will go down as one of the greatest discoveries in the history of science. It has pushed our knowledge frontier back 38 orders of magnitude in time in a single giant leap, from the creation of Helium seconds after our Big Bang to inflation during the first few trillionths of a trillionth of a quadrillionth of that first second. This teaches us about physics at energies a trillion times greater than those produced in the Large Hadron Collider, of great relevance to string theory and other quests to unify the four fundamental forces into a single consistent theory.

Moreover, it’s a sensational breakthrough involving not only our cosmic origins, but also the nature of space: by producing the first-ever detection of Hawking/Unruh radiation (the process by which inflation’s rapid doubling generates these gravitational waves), the BICEP2 team has found the first experimental evidence for quantum gravity.

So what lies ahead? Once the celebrations are over, we’ll look forward to seeing whether today’s announcement stands the test of time. The wait won’t be long since many other experiments have been racing against BICEP2 and will soon have the data to confirm or refute their findings. Many questions should get cleared up already in October, when the Planck satellite experiment is due to release its first polarized images of the cosmic plasma. Planck will produce the first good maps of polarized dust and establish whether its contaminating effect was as small as BICEP2 assumed. It will also produce an improved estimate of a parameter known as “tau” related to when the first stars ionized our universe. My personal guess is that they’ll find these stars to have formed later than currently assumed, which (for complex but well-understood reasons) will lower Plank’s estimate of how clumpy our universe is and will not only help bring the Planck constraints on gravitational waves up into better agreement with BICEP2’s detection, but also bring Planck’s predictions for the number of galaxy clusters down into agreement with what we observe.

If BICEP2 is proven correct, it should lead to at least one Nobel Prize. Further down the road, I bet that a first-ever satellite designed specifically to measure inflationary gravitational waves will get funded (now that we know there’s a signal for it to measure), which can determine how the cosmic doubling rate during inflation changed with time. This provides a great way to distinguish between specific inflation models and also to test any inflation competitors that may have gained credibility by then (for example, string gas models predict an increase whereas all inflation models predict a decrease).

But first, let’s celebrate one of the most exciting moments in the history of science! Above all, this feels like a great triumph for Occam’s razor: although countless complicated models for inflation emerged over the years, the BICEP2 data is beautifully fit by simple “classic’’ inflation, known in geek-speak as a single slow-rolling scalar field. And Andrei Linde looked particularly happy at the press conference, perhaps because two numbers have now been measured that act as a sort of fingerprint of inflation models: n=0.9608±0.0054 (reported by Planck, quantifying the ratio of small to large spots in the baby pictures) and r=0.16±0.05 (reported by BICEP2, quantifying the gravitational wave amplitude after correcting for galactic radio noise). These measurements agree tantalizingly well with the specific predictions of what’s arguably the simplest model of all: Andrei’s own favorite, whose potential energy curve is a simple parabola, which predicts n=0.96 and r=0.15. I think William of Occam would have been impressed!

Max Tegmark About the Author: Known as “Mad Max” for his unorthodox ideas and passion for adventure, Max Tegmark's scientific interests range from precision cosmology to the ultimate nature of reality, all explored in his new popular book, “Our Mathematical Universe." He is an MIT physics professor with more than 200 technical papers credit, and he has been featured in dozens of science documentaries. His work with the SDSS collaboration on galaxy clustering shared the first prize in Science magazine’s “Breakthrough of the Year: 2003.”

The views expressed are those of the author and are not necessarily those of Scientific American.






Comments 25 Comments

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  1. 1. CiaranJ 1:56 pm 03/18/2014

    Just finished Max’s book recently, and now this!
    It is wonderful to be living through what is a golden age for cosmology.

    Link to this
  2. 2. hsemab 2:09 pm 03/18/2014

    One step closer to the unknown?what is there beyond the singularity?

    Link to this
  3. 3. ButchBoyd 2:37 pm 03/18/2014

    And I can easily remember when the joke was: “let’s do some cosmology; I’ll bring the beer!”

    Link to this
  4. 4. jtdwyer 3:03 pm 03/18/2014

    Max,
    Thanks for the declarative answers, but some additional explanation would have been more helpful.
    “By exploiting a clever loophole in Einstein’s general relativity theory, inflation produces an infinite number of galaxies by continuing forever, and an observer in one of these galaxies will view space and time differently, perceiving space as having been infinite already when inflation ended.”
    It’s difficult to consider that the physically developing cosmos exploited a clever loophole…

    Link to this
  5. 5. rloldershaw 4:11 pm 03/18/2014

    Before we start arguing over who gets the Nobel prize for this and how it revolutionizes our understanding of the universe, perhaps we should wait and see what the Planck mission (and other research groups) come up with in further testing of B-mode polarization later this year.

    Some have prematurely gone into physics ecstasy over the results of what is predominantly the results of ONE experiment by one experimental group.

    Remember “faster-then-light” neutrinos?
    Remember “cold fusion”?
    Remember those scores of “hints” of “WIMP” detections?

    I share the excitement at the potential for learning new things about the Big Bang event and its immediate aftermath, but I side with cooler heads who are urging that there not be a rush to conclusions before the results have been confirmed by other experiments and verified.

    Robert L. Oldershaw
    http://www3.amherst.edu/~rloldershaw
    Discrete Scale Relativity/Fractal Cosmology

    Link to this
  6. 6. tegmark 6:20 pm 03/18/2014

    Thanks for these interesting comments!

    hsemab: we still don’t know what, if anything, preceded inflation, so it’s still premature to assume that there was a singularity (as opposed to some sort of quantum fuzz that we don’t yet understand).

    jtdwyer: Perhaps an analogy with Darwinian evolution makes it easier to consider: if there’s a way for something to reproduce rapidly, nature has a tendency to find it.

    Robert: I fully agree that we need to wait for independent confirmation before getting intoxicated with champagne – the year ahead should be exciting!

    Link to this
  7. 7. jtdwyer 6:56 pm 03/18/2014

    tegmark,
    Thanks for the analogy, but IMO it was clever physicists who exploited a loophole in our description of spacetime properties to propose perpetual inflation, an infinite number galaxies and for that matter multiverses.
    - IMO, that an inflationary period enables consideration of these possibilities does not infer their physical manifestation – the universe still tends to apply Occam’s razor…

    Link to this
  8. 8. tegmark 8:27 pm 03/18/2014

    Hi jtdwyer:
    Scientists didn’t propose an infinite number galaxies or a multiverse. Scientists merely proposed inflation – and we can’t opt out of its predictions. If you reject inflation, then what explanation of the BICEP2 results do *you* propose?

    Link to this
  9. 9. jtdwyer 10:28 pm 03/18/2014

    Hi tegmark,
    I had to look to be sure – nope – *I* did not reject inflation or the BICEP2 interpretation of results – I was responding to your statement:
    “By exploiting a clever loophole in Einstein’s general relativity theory, inflation produces an infinite number of galaxies by continuing forever…”
    As I understand, the existence of a universal inflationary epoch does not necessarily produce “an infinite number of galaxies by continuing forever.”
    As I understand, only specific extensions to inflation theory predict eternal inflation and multiverses – is that not correct? I am questioning the eternal expansion and multiverse hypotheses – even confirmation of expansion does not seem to confer validation upon those extensions…

    Link to this
  10. 10. skydivephil 5:54 am 03/19/2014

    Hi Max, could you please comment on these papers that were published a few years ago that say there are others ways to get primordial gravity waves other than inflation:
    http://arxiv.org/pdf/1104.3581.pdf

    arxiv.org/pdf/1106.5059‎

    Link to this
  11. 11. tegmark 8:41 am 03/19/2014

    Thanks jtdwyer for homing in on this interesting issue, which is really the crux of the matter.
    “As I understand, only specific extensions to inflation theory predict eternal inflation and multiverses”
    That’s indeed a commonly help perception outside of the cosmology community, but it’s incorrect: no “extensions” are involved. Alex Vilenkin, Andrei Linde, Alan Guth and others have shown that although you can cook up inflation models that don’t produce an infinite space, it’s difficult and arguably highly contrived. This is what Vilenkin means when he says that inflation “generically” produces an infinite space (I explain this in great detail in chapter 5 of my new book http://mathematicaluniverse.org). For example, the simplest inflation models agreeing with the BICEP2 data, like the one where the potential energy function is a parabola, predicts an infinite space (as first proven by Andrei Linde). I hope this helps clarify things!

    Link to this
  12. 12. tegmark 8:46 am 03/19/2014

    Thanks skydivephil for bringing up the so-called string gas cosmology scenario. Andrei Linde and others have published papers arguing that it’s not currently a self-consistent theory because of internal problems. The only other serious game in town for making gravitational waves now appears to be cosmic strings, but it’s not clear whether they can explain the BICEP2 signal without violating Planck constraints, and besides, with the cosmic string scenario you’d still need inflation as well, so it’s not really an alternative to inflation.

    Link to this
  13. 13. skydivephil 11:18 am 03/19/2014

    Thanks Max ,do you agree with the papers though, that the spectrum of gravity waves needs to be red tilted for simple slow roll inflation? When might we be apexected to measure this spectral tilt? I know its already been done for temperature anistropies.
    They also talk about Horava gravity which gives a blue tilted gravity spectrum. The second paper mentions that loop quatum gravity enduced a blue titled grvaity wave spectrum,Im not sure how generic this is. Im a bit confused about this as LQC predicts slow roll inflation.

    arxiv.org/pdf/1103.2475‎

    Link to this
  14. 14. zeroid 5:26 pm 03/19/2014

    Hi tegmark,

    Can you please explain just what exactly space is expanding into?

    What is there beyond the outermost fringe of space that would support the contents of space? This is something I’ve always had difficulty understanding within the inflationary/big bang theory.

    Link to this
  15. 15. jjm319 9:16 pm 03/19/2014

    Max, why is this a good fit when Figure 9 to my eyes does not show a fit of data to theory. The dots representing measurements do not seem to follow the dashed line indicating the theory and in fact even the error bars completely miss it in part of the figure. I looked for a comment on this in the paper but i did not find it. I know its adjusted for lensing but even this adjustment does not make the measurements match.

    Link to this
  16. 16. dadster 9:39 am 03/20/2014

    Hi max , good try ! But , to me , ( not a professional scientist ) , it all appears good maths and brave attempts to find some interpretations to the results obtained by ” clever ” maths. Other theories such as oscillatory cosmos cannot be ignored . To many minds oscillatory cosmos has more credibility as a physical theory . Clever maths perhaps can validate it even more. Maths is just a modeling tool , but real physical configurations could be perplexingly complex , more than idealized maths models of it.
    Nobel prizes should not be awarded in the same year as the results , however credible they appear to be for the time being , like yearly bonuses too hastily awarded to the bankers . Nobel prizes are apt reward to a team of scientists and mathematicians than to just one individual scientist in discoveries like this which
    needs so many experiments to validate it. Its not like a theory produced along with its mathematical model by one single individual ( like the relativity theory of Einstein or , the Principia of Newton ) that needed experimental validation through experiments proposed by the theorist himself .

    Link to this
  17. 17. dadster 9:57 am 03/20/2014

    Hi max , good try ! But , to me , ( not a professional scientist ) , it all appears good maths and brave attempts to find some interpretations to the results obtained by ” clever ” maths. Other theories such as oscillatory cosmos cannot be ignored . To many minds oscillatory cosmos has more credibility as a physical theory . Clever maths perhaps can validate it even more. Maths is just a modeling tool , but real physical configurations could be perplexingly complex , more than idealized maths models of it.
    Nobel prizes should not be awarded in the same year as the results , however
    credible they appear to be for the time being , like yearly bonuses too hastily
    awarded to the bankers . Nobel prizes are apt reward to a team of scientists and mathematicians than to just one individual scientist in discoveries like this which
    needs so many experiments to validate it. Its not like a theory produced along with its mathematical model by one single individual ( like the relativity theory of Einstein or , the Principia of Newton ) that needed experimental validation
    through experiments proposed by the theorist himself .

    Link to this
  18. 18. David Cummings 1:04 pm 03/20/2014

    Max Tegmark,

    Previous to reading this post, I understood chaotic inflation to be like a loaf of rising bread (to use Brian Greene’s analogy) with individual Big Bang Universes being the airbubbles inside the larger mass of dough.

    Now it seems like you are saying there is only one air bubble (so to speak) and it takes up the space of the entire loaf (to stick to that analogy).

    Am I misunderstanding what you are saying?

    Has something changed?

    Is the concept of separate and widely-separated big bang bubbles now made implausible with this new finding?

    Thanks,

    Dave

    Link to this
  19. 19. besonian 10:09 am 03/21/2014

    Master Tegelmark has left the premises. But here is a trailing comment.

    “By exploiting a clever loophole in Einstein’s general relativity theory, inflation produces an infinite number of galaxies by continuing forever, and an observer in one of these galaxies will view space and time differently, perceiving space as having been infinite already when inflation ends”

    “Continuing forever” suggests a potential infinity (you can always add one more in time to come) rather than an actual infinity.

    (Dirac wrote in the first edition (1930) of his legendary textbook that the statevector lives in a space with as many dimensions as is sufficient, maybe infinite. An explorer´s tentative way of putting it. Later, seven years after Von Neumann in 1932 had introduced Hilbert Space with its actual infinity of dimensions, Dirac toed that line, or rather space, but not Von Neumann´s elaboration).

    Is physical infinity a copy of math´s infinity?

    Tegelmark writes “it began in a region of space much smaller than an atom” and “in that tiny region…”
    Are there other regions that expanded ‘before’ our tiny patch so that actual infinity stretches ‘before’ as well as after? An almost Hoylean version. Hoyle was modest enough (admittedly a strange epithet in relation to Hoyle) to claim a creation field that let in (from nowhere) a proton per second per billion cubic-km. The modern inflation field creates whole universes from plancksized patches. Is the number of inflated patches denumerable or not? If so, is there a first inflation, a first moment in this larger multiverse? In an article by Linde (S.A. Nov.1994) we were told that there might have been a first one in multispace, but that it hardly mattered.

    Ideas of an actual physical infinity seem to lie behind the wild claim that

    “there must be duplicate copies of you far away in space, and indeed also similar versions of you living out countless variations of your life”

    that tries to populate the age old poetical or abstract visions of infinity in a very literal way. Modern mathematical formulae beget (in some) a philosophy that fills the world´s stage with everybody at any time. Is that Occam´s razor in action? It seems to draw blood. Didn´t Wheeler reject such visions for carrying too much metaphysical baggage?

    As to the reality of countless variations of galactic life, physicists almost always emphasize the happy consequences of such a claim in their popular books or articles: Gee! In the universe next door you win the sweepstakes and may marry the lovely princess and inherit the whole kingdom. But what about the alternatives: Multiverse as a vision of repeated hell, where we and our family are put on the rack in a dictator’s private galaxy and where even we are our families’ executioners. Where Timur Lenks, Stalins and Hitlers multiply and win, again and again. If anything can happen in remnants of other inflations or behind domain walls, where the laws and/or constants of nature may transform, as has been claimed, can Christ there ascend to heaven, witches perform true miracles, superstition be real science and angles dance ten thousand a pin? Will the long overstretched versions of the Star Wars world become part of inflated reality?

    I find it truly fascinating to learn that astronomers (with sufficient biceps) apparently have nailed down part of reality beyond the Standard Model. That calls for celebration. May Gurth and Linde enjoy the undis-torted results of their mathematical-cum-physical imagination. But I am less than enchanted by mathematics trying to legitimate a fantasy world where another Max, a Mad Max with an Ax, may be ready, no, is ready , to terminate a worldline or two. Or even initiate one! How would a scientific investigator track him down, even indirectly? Let writers of science fiction concern themselves with such matters.

    Linde seems to stay within the confines of our scienti-fic tradition, however spectacular; Tegelmark appears to emerge from the world of science fiction and cartoons, where once you start down the road of possibilities there is no stopping. Shouldn’t we stay with physical reality? And leave alternative worlds to the great artists (not SFs) who can fill them with rich and probable life, our life?

    Link to this
  20. 20. Silkysmom 10:31 pm 03/21/2014

    “Their inflation theory in its simplest form predicts that our universe was once smaller than an atom”

    I find it hard to understand how everything in the universe was once smaller than an atom. Maybe the big bang was two galaxies colliding.

    Link to this
  21. 21. yeastbeast 7:25 pm 03/22/2014

    Max,

    Could you comment on how well the BICEP2 results agree with early theories of inflationary cosmology, in particular the work of Gliner (1965) and Starobinsky (1979).

    In a 1994 review (http://arxiv.org/pdf/hep-th/9410082.pdf), Linde cites these two as sources of the original formulations of inflation, so it it would be interesting to see how their models hold up.

    By the way, Gliner is alive and well, living in retirement in San Francisco. He is my stepmother’s uncle.

    Thanks,

    Lev

    Link to this
  22. 22. Dr. Strangelove 10:50 pm 03/23/2014

    Max

    What caused our Big Bang?
    IC: The repeated doubling in size of an explosive subatomic speck of inflating material

    And what caused inflation?

    How could an infinite space get created in a finite time?
    IC: By exploiting a clever loophole in Einstein’s general relativity theory, inflation produces an infinite number of galaxies by continuing forever, and an observer in one of these galaxies will view space and time differently, perceiving space as having been infinite already when inflation ended.

    When you say “continuing forever.” Are you referring to time? How can forever = finite time? If you are referring to space, your argument is circular reasoning – infinite space is created by continuing it forever. Spacetime is finite. Mass-energy is finite. How do you create an infinite number of galaxies?

    How big is space?
    IC: Probably infinite.

    How do you know? A flat universe does not mean it is infinite. It can be flat and bounded.

    Link to this
  23. 23. drgsrinivas 4:47 pm 03/29/2014

    Though abandoned long ago, reinstating Ether into our understanding of Nature actually explains almost every observation made by the modern physicists without resorting to the weird propositions of quantum mechanics and relativity.
    First is that it provides logical explanation for the results of double slit experiment i.e. how particles produce interference patterns and appear to travel in multiple slits simultaneously. Next is that it explains why there is something called gravity. In fact, it provides so simple and logical explanation for these two phenomena that they both can be argued as proof of existence of Ether.

    Ether theory also provides a logical explanation for the following

    -All those observations that are claimed as proof of relativity

    -Mass and inertia

    -Cosmic background radiation

    http://www.debunkingrelativity.com

    Link to this
  24. 24. schatzieD 2:41 pm 05/8/2014

    Uh, drgsrinivas – as Pauli once said about bad work, you are “not even wrong.”

    Link to this
  25. 25. MiaOSKI 1:21 pm 10/2/2014

    Max,

    What’s your opinion on the growing doubt that these results were in error and due to dust contamination? Specifically, Clara Moskowitz’s article last week (Sept 22) in Scientific American titled “Gravitational Wave Discovery Looks Doubtful in New Analysis.”

    Link to this

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