Of Particular Significance

A Primer On Today’s Events

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 03/17/2014

The obvious questions and their brief answers, for those wanting to know what’s going on today. If you already know roughly what’s going on and want the bottom line, read the answer to the last question.

You may want to start by reading my History of the Universe articles, or at least having them available for reference.

The expectation is that today we’re going to hear from the BICEP2 experiment.

  • What is BICEP2?

BICEP2, located at the South Pole, is an experiment that looks out into the sky to study the polarization of the electromagnetic waves that are the echo of the Hot Big Bang; these waves are called the “cosmic microwave background”.

  • What are electromagnetic waves?

Electromagnetic waves are waves in the electric and magnetic fields that are present everywhere in space.  Visible light is an electromagnetic wave, as are X-rays, radio waves, and microwaves; the only difference between these types of electromagnetic waves is how fast they wiggle and how long the distance is from one wave crest to the next.  

  • What is the cosmic microwave background [CMB for short]?

The glow leftover from the Hot Big Bang.  The part of the universe we can observe today (the “observable patch”; the universe as a whole may be much larger) was once very hot, during the Hot Big Bang.  When it was hotter than about a few thousand degrees, it was opaque to light.  But once it cooled sufficiently for atoms to form, it became transparent, and any light, and any other forms of electromagnetic waves, that was still being emitted at that time was then free to stream across the universe forever.  The “glow” from that hot period thus is still present in the universe.  As the observable patch expanded, the wavelength of the electromagnetic waves increased; today most of those waves are in the range of microwaves.

In this picture (from Wikipedia commons) a snap-shot of an electromagnetic wave is shown. The wavelength of the wave is marked λ. The wave is moving to the right (in the direction marked “k”); the electric field is in blue and points up and down relative to k, and the magnetic field, in red, points in and out of the screen, perpendicular both to k and to the electric field. The polarization direction of this wave is the direction of the electric field: vertical.
  • What is polarization?

An electromagnetic wave has the property that as it heads from point A to point B, the electric field always points in a direction perpendicular to its motion.  (The magnetic field points perpendicular both to the direction of motion and to the electric field.)  The direction along which the electric field points is the “polarization” of the electromagnetic wave.

  • What are photons?

Electromagnetic waves (including visible light) are made from particles (or “quanta”) called “photons”.  Each photon is an electromagnetic wave of minimal possible intensity, and each photon’s polarization can be individually measured.

  •  What precisely is BICEP2 doing?

BICEP2 looks in each direction of the sky, and detects cosmic microwave photons from that direction.  It then determines whether the polarization of the photons from that direction is entirely random, and thus has an average of zero, or whether it has a (very tiny!) tendency for a preferential orientation.

If the photons coming from a single small region of the sky have a random polarization direction, then the cosmic microwave background from that region is “unpolarized”.  But if they have a slightly preferred direction for their polarization, then the cosmic microwave background from that region of the sky is “polarized”, and the amount and orientation of the preference is the “polarization” that BICEP2 is measuring.

  • Why is BICEP2 making this measurement?

Both the average energy (“temperature”) of the photons coming from a particular direction and their average polarization tell us something about what the observable patch of the universe was like a long time ago: 380,000 years after the Hot Big Bang, which is about when the observable patch became transparent.

  • Why do we care about that?

Because what the universe was like 380,000 years after the Hot Big Bang can in turn be used to learn about the very beginning of the Hot Big Bang, and what preceded it — which may have been a period of cosmic inflation.

  • What does the temperature of the photons from the cosmic microwave background [CMB] tell us?

The temperature of the CMB photons is, amazingly, almost completely uniform across the sky.  Once the motion of the Milky Way is accounted for, the temperature varies only at one part in 100,000.  The pattern of temperature variation across the sky — first measured by the COBE satellite, and with increasingly precision since then, most recently by the Planck satellite — tells us how non-uniform the universe was 380,000 years after the Hot Big Bang began.  From these non-uniformities, working both forward in time through the era of galaxies that we live in, and backward in time through the early Big Bang, scientists have been able to infer many properties of the universe with unprecedented precision, determining how long it has been since the Hot Big Bang, and the make-up of the observable patch (i.e. how much dark energy, dark matter and ordinary matter it contains), to a spectacular degree.  These measurements could have ruled out the possibility of cosmic inflation, but instead they are (so far) quite consistent with that possibility.

  • What does the period at and just before the Hot Big Bang have to do with small non-uniformities in an otherwise uniform temperature 380,000 years later?

Whatever non-uniformities were present when the Hot Big Bang started would have persisted and changed in a way that scientists can calculate, leading to non-uniformities of a predictable size at 380,000 years.  In other words, from knowing what the non-uniformities are at 380,000 years, scientists can work backwards, and determine what the non-uniformities were when the Hot Big Bang began.

  • Why would there have been non-uniformities in the temperature of the Hot Big Bang?

Why not?  In fact, the first question you should ask is why they’re so small!

  • Why are they so small?

Well, we’re not sure, but that’s where the idea of inflation comes in.   By causing the observable patch of the universe and its surrounding regions to blow up almost instantly into a vastly larger size, inflation would have pushed all material and all structure far, far away, making the observable patch incredibly uniform.

  • But then why are there any measurable non-uniformities at all

Ah — because of how quantum mechanics combines with inflation.  (This isn’t simple, and it deserves an article in future.) Inflation is caused by the presence of a substantial amount of dark energy.  That dark energy is associated with a field (or fields), called the inflaton (or inflatons), about which we currently know almost nothing.  But we do know this:In quantum mechanics, no field or object is ever truly constant or stationary. There is always a sort of quantum jitter which causes it to be a bit uncertain.  The inflaton field’s value undergoes this quantum jitter.  As a consequence, the dark energy that is present during inflation is not exactly constant throughout space; and that, finally, means that there are small non-uniformities in the expansion of space, which in turn lead to small non-uniformities in the Hot Big Bang, which eventually become visible as non-uniformities in the temperature of the CMB photons.

  • What’s the point of looking at CMB polarization?

The pattern of non-uniformities in the polarization can tell us some things about the universe that are different from what the non-uniformities in the temperature have already taught us.

  • Why is there any polarization at all?  Why aren’t the CMB photons completely random?

Just as the universe is becoming transparent about 380,000 years after the Hot Big Bang, and electrons are all being captured by atomic nuclei as atoms form, the last photons to hit something often scatter off electrons, a process called “Thomson scattering”.  Thomson scattering has the property that if a bunch of unpolarized photons come in and hit an electron, but there are more of them coming from above and below than from the left and from the right, then the photons which emerge forward will be somewhat polarized.  In short, Thomson scattering can convert a non-uniformity in the unpolarized photons that are present as the universe becomes transparent to a polarization effect that experiments like BICEP2 can observe.

  • Since the non-uniformities in the CMB photons are so small, doesn’t that mean that polarization effect that BICEP2 has to measure will be extremely tiny?

You bet!  BICEP2 is making a very difficult measurement.  An unprecedented one!   But of course, that’s why they might make a discovery!

  • What will the polarization tell us that the temperature non-uniformities haven’t already told us?

Well, there are two types of polarization in the polarization pattern: “E-mode” and “B-mode”.  E-mode was measured some time ago, and doesn’t tell us that much new (though it would have if its features had been surprising; they weren’t.)  B-mode, however, can tell us a lot.

  • What’s all this about E-mode and B-mode polarization?

The polarization has to be separated into two classes of patterns.  If you look in one direction in the sky, and you look at the polarization of the CMB photons nearby to that direction, you may find that the pattern has a form that would look the same if you reflected it in a mirror.  That type of polarization pattern is called “E-mode”.

Or you may find that the pattern is of a form that would flip over in a mirror; that type of polarization pattern is called “B-mode”.

Typically you’ll find a mix of the two; the pattern is almost but not quite unchanged in a mirror.

But (as previous experiments have already shown) the amount of B-mode is much less than the amount of E-mode, making BICEP2’s measurement of B-mode even more difficult!

  • Why is B-mode polarization able to tell us something that neither E-mode nor temperature non-uniformities can tell us?

E-mode polarization is sensitive to the same type of effects that cause temperature non-uniformities.  But B-mode comes from two sources.

1) In small patches of sky, B-mode polarization arises from a combination of E-mode polarization present 380,000 years ago and the gravitational lensing (i.e., bending of light) of the CMB photons by galaxies that the CMB photons have passed near on their journey to Earth.  B-mode polarization on these smaller scales, which was detected about a year ago, thus tells us something about properties of the observable patch after 380,000 years post-Hot-Big-Bang.

2) But across larger swathes of the sky, B-mode polarization arises in a novel way.  Non-uniformities in the energies of the photons as the universe was becoming transparent also were potentially due to the presence of gravitational waves — ripples in space itself.  Unlike non-uniformities merely due to there being slightly more dense and less dense regions in the Hot Big Bang, which only give rise to mirror-symmetric (E-mode) polarization after Thomson scattering, gravitational waves can give rise both to mirror-symmetric and mirror-asymmetric polarization… to both E-mode and B-mode.  Thus, B-mode polarization on large scales tells us about non-uniformities due to gravitational waves that may have been present 380,000 years after the Hot Big Bang — and a discovery of non-zero B-mode polarization on large scales would be a measurement of powerful gravitational waves present in the early universe!

  • Wow! Ripples in space and time!  Gravitational waves predicted by Einstein’s theory of gravity!  Is this the first detection of gravitational waves?

No.  This detection is indirect; we actually measure polarization of light and only infer gravitational waves are present. The 1993 Nobel Prize was for the indirect detection of gravitational waves in careful measurements of a system of two neutron stars (the Hulse-Taylor pulsar system).  Efforts toward direct detection of these waves are underway and may bear fruit in the near future, but not yet.

  • What’s so important about these gravitational waves?

Their 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.  And so their presence, if in the appropriate pattern, would be more strong evidence in favor of inflation of certain types, and evidence both against certain non-inflationary ideas and against certain variants of inflation that don’t predict large amounts of gravitational waves.

Moreover, their properties — in particular, their total power — would measure, for the first time, the amount of dark energy that was driving inflation, and therefore how rapidly inflation was occurring.  And also, because this dark energy is what eventually heats the universe and starts the Hot Big Bang, it would tell us how hot the universe became after inflation and before it began to cool.

So this is a very big deal!

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63 Responses

  1. Unquestionably consider that which you said.
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  2. Is E Mode polarization the same thing with linear polarization and B Mode polarization the same thing with circular polarization?

  3. We should realize that, even though scientists work on the restraint of their feelings when a discovery of this magnitude is published, nonetheless scientists are human just like the rest of us, and this is really a HUGE event.

    But we will all have to wait until it is validated or refuted by other teams with their own experiments.

    So, let us all play it cool and smooth for now.

  4. Pretending like I’m clever or something …
    I believe the sentence “In quantum mechanics, no field or object is every truly constant or stationary” should contain the word “ever’ instead of “every” (although I am aware that normal grammar rules rarely apply to quantum mechanics).

  5. Again, I pose the question, can B-mode polarization of the CMB photons arise from refraction through dark matter along with a combination of E-mode polarization present when the universe was only 380,000 years old and the gravitational lensing (i.e., bending of light) of the CMB photons by galaxies?

    I guess another way of asking the question would be, galaxies have a high enough density, intensity of the gravitational field, that will absorb (capture) most of the photons and bend those just outside the galaxy’s influence, but dark matter may not and probably is not dense enough to do the same, but has enough density to refract the photons and cause both E-mode and B-mode polarization? So how can you differentiate this possible cause of B-mode to that that occurred 13.7 billions years ago?
    And if there is refraction through dark matter you cannot create a proper mapping of the sky.

    One additional note, there was more dark matter 13.7 billions years ago, than today. So if there is an increase in B-mode it will merely reinforce this theory.

    Why look back 13.7 billions years for gravity, we are living in it now, it is here all around us, we are a product of photosynthesis, photons captured in a fantastic twisted and compressed maze of the gravitational field. We are mere ripples in the continuum of the gravitational field.

    1. @Oaktree:

      To be able to answer your questions, let’s take your main comments one by one:

      So far, we barely know anything about dark matter, and the first steps that make sense would be to get to know a little bit more about its main characteristics (through experiments based on what we think we already know about it) before we venture into deeper and more convoluted concepts regarding its behaviour.

      Regarding the very specific curling pattern that primodial gravitational waves have imprinted into the polarization of CMB radiation (that was first predicted by some theories which were the basis for the BICEP experiments to detect them), that pattern is incredibly specific and unique, and it is present in all photons from CMB radiation that come to us from all directions of the (visible) universe.

      On the other hand, the effects of gravitational lensing on travelling photons is tighly coupled to the physical configurations of energy and momentum of each group of heavenly bodies that produces it, so, the effect would likely be very different from galaxy to galaxy.

      if we could dare to calculate that probabilities that such local and diverse conditions at each galaxy or cluster could produce such a similar curling pattern on polarization so as to mimic it so precisely that we could mistake it with the current standing theory, well, I guess the number would be so small that we could discard it, at least for the time being.

      Regarding whether or not there was more dark matter then than now, I neither have verifiable info to counter or validate that assertion, nor the theoretical background to validate or refute its behaviour.

      Regarding why do we care about events that happened 14 billion years ago, well, due to Entropy we are all constrained by and bound to the “arrow of time”, and since we all want to know and predict what will happen to our universe (and us within it) in the future, to be able to predict the “trajectory of the arrow”, like with any arrow, we need to know where it came from, with what initial speed and what original angle(s), and to be able to do that, we need to know a lot more about the early stages of our universe.

      Kind regards, GEN

      1. Well, thank you for that.

        You mentioned: “… the very specific curling pattern that primodial gravitational waves have imprinted into the polarization of CMB radiation (that was first predicted by some theories which were the basis for the BICEP experiments to detect them …”

        “curling pattern” … very interesting, indeed. Do you know which papers were driving this idea? If you know off hand does this curling pattern have any links to symmetry breaking? And are the spin rates FTL?

        As for the arrow, well I am not sure it exist. Velocities ratios, or more specific, energy ratios (which includes mass ratios) are more valid than time.

  6. Alan Guth’s 1981 inflationary theory, as I understand it, is that inflation occurred faster than the speed of light starting from dimensions close to the Planck length. It gets around the speed of light as a limit by positing that space itself expanded, and/or for a very brief time, another throwback to pre-1905 absolute space and time that seems very popular in the 21st century, for reasons I can’t fathom.

    Gravity waves are also supposed to travel at the speed of light, and so I have some confusion about how finding patterns like these in the B-polarization of the CBR would demonstrate either inflation by means of gravity waves, or absolutely confirm theories of either the big bang or inflation. The Harvard researchers must be arguing that the gravity waves that caused the polarization were likewise superluminal.

    I grant that the observation of the CBR polarization is very good science. The explanation as to how it relates to either gravity waves (which have never before been directly or indirectly observed) or the big bang will need to be a little plainer.

    1. I believe the objective, and I hope the Prof. corrects me if I am wrong, to understand when and why oscillations began. Do waves, field(s), always exist and the universe is merely a cyclic spring extending and compressing and a slightly different universe (particle masses) emerges every time it explodes again. Or did all this energy sipped in (out) of an adjacent universe (manifold) only to create ours temporarily, like soap bubbles.

      Gravity is the lowest intensity and more important longest reaching, the entire universe, so understanding the gravitational will most likely give us more answers than additional questions so that we help us make progress.

      But, I would like to know how dark matter is factored out?

  7. Will detectors ( like future LIGOs ) be able to detect these primordial waves in the foreseeable future. or are they just far too weak now. ( but then they did last 380,000 years after all ).

  8. Reblogged this on The Way and commented:
    If the data holds up, this is one of the great scientific advances of all time: more indirect evidence of gravitational waves and more evidence that the universe may have inflated to near its present size in 10-45 seconds giving rise to what we understand to be the big bang.

  9. Is it possible to deduce from this detection of the primordial B-mode polarization of the CMB the intensity of the gravitational wave background at recombination? This could tell us, after adjusting for Hubble expansion, whether there is hope of, some day, observing the primordial gravitational waves directly though, say, a pulsar timing array? Maybe this possibility can be ruled out now

    1. The answer to your first question is yes. I don’t know the answer to the second question. But in any case, this gravitational wave signature, if true, is surprisingly big, not small, so a second method for detection is more likely to be ruled in than out.

  10. If I understand correctly, BICEP2 measures large-multipole B-mode polarization, which is a signal of gravitational waves present at recombination 380,000 years after the big bang. In turn, these gravitational waves are predicted by inflation.

    It seems very mysterious to me that gravitational waves at _recombination_ can tell us things about _inflation_, when the latter happened so much earlier than the former (although I realize that cosmologists routinely infer things about the pre-recombination era from CMB data, which originates at recombination). Did the gravitational waves we see at recombination actually start “waving” all the way back during inflation? Or is the connection more indirect? If they did originate during inflation, how did they survive? I might have expected that gravitational waves occurring in the dense conditions that existed pre-recombination would be strongly damped by the matter all around them and decay _exponentially_, leaving no chance to observe them at later times.

    1. The post-inflation Hot-Big-Bang universe is a very simple place for a very long time — almost perfectly uniform, to one part in 100,000, at the start of the Hot Big Bang (and we know this because the CMB is so uniform), and remaining very uniform until long after it becomes transparent. Gravitational waves will just move simply and smoothly through such a medium, in a way that can be reliably and precisely calculated. There’s very little damping, in fact, because gravity is such a weak force, so that the waves barely notice the particles in the hot soup, and the particles barely notice the gravitational waves, except to surf on them. The only damping comes from the expansion of the universe itself.

      To say it another way: the universe didn’t become transparent to light until atoms formed, but it has been transparent to neutrinos for quite a bit longer than that, and transparent to gravitational waves from the beginning of the Hot Big Bang.

      So yes, with a set of gravitational waves laid down during inflation, entering into the observable patch as it expands, and starting to wave slowly as the expansion continues, one can calculate what those gravitational waves will do to the photons 380,000 years later.

      It may seem amazing — and in some ways it is — but we already did the same thing for the density fluctuations at the time of the Hot Big Bang, and the data matches predictions extremely well. If you have the correct equations to work with, they don’t lie.

    1. Thank you. A kind of analogue to the light-house effect, then, rather than actual signal propagation.

  11. Many thanks for this “primer.” At least one account, in the New York Times, describes an inference of initial inflation “at many times the speed of light” (it also mentions potential “bubble”/”multiverses” beyond the borders of our universe).

    I dismiss that second claim as, well, piffle, but on the first I’m baffled concerning what could have thrown Special Relativity out the window very briefly nearly instantly after the Big Bang itself?

  12. Can dark matter cause refraction of the CMB photons? If so, how can we factor out the influence of dark matter, (since we don’t know much about it), to ensure the mapping of the universe is clear and meaningful.

    “… Non-uniformities in the energies of the photons as the universe was becoming transparent also were potentially due to the presence of gravitational waves — ripples in space itself….”

    Wow! what a powerful concept! I praise and salute you if you composed this powerful sentence on your own. Everything is there; non-uniformities, energies, photons, universe, transparent, gravitational waves, ripples, (and) space. Even the time variable is there in universe “becoming” transparent.

    Did you just describe the unification theory?

  13. I’ve heard a few people phrase this as evidence of quantum gravity, owing to the point that large-scale B-mode polarization arises from primordial quantum fluctuations in the graviton field. Do you think that’s a reasonable claim?

    1. At best it’s evidence of the trivial part of quantum gravity; that any waves in a quantum world should be quantized. It’s equivalent to my claim on this website that we’re pretty darn sure that gravitons exist, because all waves should be made from quanta in a quantum world. It doesn’t address any of the challenging aspects of quantum gravity.

      1. “pretty darn sure ” ?
        Electrons and positrons influence each other by ripples in the field(not particles) as in your
        But gravity has to have a “real” graviton. How does that unify the forces of nature. It is as if you are saying there is no such a thing as true quantum gravity, but there is a quantum for gravity and a quantum for the rest.

        But let me take a guess. These ripples under some conditions ( like the usual high end physics techniques) some parameter is adjusted by an anstatz to convert them to real particles and the problem is solved.

        1. I’m afraid you’ve deeply misunderstood.

          1a) “Electrons and positrons influence each other by ripples in the field” — almost correct. They influence themselves by disturbances in the field that AREN’T ripples. Particles ARE ripples in the fields.

          1b) But you left something out. There are electromagnetic waves in nature. And these are made from particles: photons. Those are real particles.

          2a) The moon and the earth also influence each other by non-ripple disturbances in the gravitational fields.

          2b) But also, there are gravitational waves in nature. And these are made from particles: gravitons. Those are real particles.

          In short: electric forces are to electromagnetic waves and photons just as gravitational forces are to gravitational waves and gravitons. Pretty darn sure.

            1. No, that’s not what I’m saying.

              Just think about electromagnetism.

              1a) there are electric forces.
              1b) there are electromagnetic waves, made from photons.

              Do you understand those two statements? If not, forget about gravity and make sure you understand the interaction of light and matter first!

          1. Matt,
            my bio
            Degrees: · · B.S. E.E. university of Wyoming 1979 · · MPHIL E.E. University of Sussex 1987

            I am sorry I did not want to make this about you or me, I just wanted to clarify for myself how darn sure are we about graviton. I have read and understood the basics of classical physics, electrodynamics, SR, relativistic electrodynamics(classical), a bit of GR/cosmolgy(not as much as others),QM, QFT,QED, EW .some QCD/GUT(basic SM) and I know about most QG theories like String,LQG,AS, CDT, and the rest.

            I was just trying to clarify the interaction picture between two particle ,one with QED and the other with gravity and show the contrast. Moreover you throw a curve with your 2a/2b, which totally lost me. I have no quarrel with the basic statement

            “In short: electric forces are to electromagnetic waves and photons just as gravitational forces are to gravitational waves and gravitons. Pretty darn sure.”
            although it sounds neat, but how, given the already stated contrast.

            1. Sorry, I too was not trying to be snide — it’s the nature of written text sometimes, especially when I’m rushing to answer many questions… and indeed your scientific background wasn’t clear. So if 1a and 1b is clear, my statement is simply that 2a and 2b are exactly the same — literally, the equations are identical with tiny changes. So I’m still confused what’s bothering you. The equations for electromagnetic fields give forces between charged objects, waves in empty space, and (when quantum mechanics is added) quanta, called photons. Einstein’s equations for gravitational fields are almost identical (especially for small amplitudes): they give forces between massive objects, waves in empty space, and (when quantum mechanics is added) quanta, called gravitons. It’s completely parallel. The only difference is that gravitons are a lot harder to detect than photons.

              So apologies again for any confusion, but have I answered your question clearly enough now?

          2. It was my fault heavily summarizing. I don’t want to clutter your thread with off topic, but I will state my problem as in the wiki article.

            “Most theories containing gravitons suffer from severe problems.”


            I think it is safe to leave strings out. Is it not you scientists who always teach us to be careful with what we say?

  14. hey! great article, unfortunately there’s one more mistake with those unlucky 380k years. in one of the last paragraphs you write:

    “Thus, B-mode polarization on large scales tells us about non-uniformities due to gravitational waves that may have been present 380,000 years ago — and a discovery of non-zero B-mode polarization on large scales would be a measurement of powerful gravitational waves present in the early universe!”

    besides that it’s great article! hope you’ll bring us more details soon!

    1. From the BICEP2 FAQ: (http://bicepkeck.org/faq.html)
      Why the South Pole?
      Water vapor in the atmosphere absorbs microwaves, making detailed studies of the CMB impossible from most places on earth. The South Pole is near the middle of the Antarctic plateau, the driest environment on the planet. At an effective altitude of over 10,000 feet (~3000 meters), stable weather patterns and winter temperatures averaging -72F (-58C), the South Pole is the closest a ground-based telescope can get to being in space. The patch of sky we study is visible from the South Pole continuously, 24h each day for the whole year.

  15. I have a few questions regarding gravitational waves, how difficult it has been so far to measure them, and how both inflation and the expansion of the universe might have helped in this case to measure gravitational waves.

    To start with, it is my understanding that part of the technical problem to measure gravitational waves it that they have such tiny wavelengths that they are within range of the uncertainty principle.

    My question is: have both inflation and the expansion of the universe given some help in this respect by “amplifying” the size of the wavelengths to values that are less affected by the uncertainty principle?

    Kind regards, GEN

  16. I think that where it says:

    “Just as the universe is becoming transparent about 380,000 years ago”,

    it should say something like

    “Just as the universe is becoming transparent about 380,000 years after the Hot Big Bang”.

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