Dust Thou Art, BICEP2

© Matt Strassler [February 6, 2015]

Unfortunately, though to no one’s surprise after seeing the data from the Planck satellite in the last few months, the BICEP2 experiment’s claim of a discovery of gravitational waves from cosmic inflation has blown away in the interstellar wind. [For my previous posts on BICEP2, including a great deal of background information, click here.] The BICEP2 scientists and the Planck satellite scientists have worked together to come to this conclusion, and written a joint paper on the subject.  Their conclusion is that the potentially exciting effect that BICEP2 observed (“B-mode polarization of the cosmic microwave background on large scales”; these terms are explained here) was due, completely or in large part, to polarized dust in our galaxy (the Milky Way). The story of how they came to this conclusion is interesting, and my goal here is to explain it to non-experts.

What Happened, and Why?

How did it happen that the BICEP2 scientists, after having done a spectacular experimental measurement, came to the wrong conclusion about what it meant? The issue of polarized dust within our galaxy [more accurately, “spinning dust which gives rise to B-mode-polarized microwaves”] was something that everyone, including BICEP2, knew had to be accounted for, because its presence could mimic the same effect in their experiment as gravitational waves from cosmic inflation. Unfortunately the amount of polarized dust in the region of the sky where BICEP2 was looking was quite a bit larger than BICEP2’s original analysis suggested. What was their mistake?

It’s quite simple really. BICEP2 didn’t, and couldn’t, actually know how much dust was in the region of the sky that they were studying, or how it was distributed. It wasn’t something they could measure directly. They did know, from other people’s measurements, that they were looking in a patch of sky where the amount of dust (polarized or not) was very small. They had some estimates from past studies of how much polarization was typical. They also got ahold of some unpublished data from the Planck satellite, which had been shown in public. They put this information together in various ways, and somehow managed to convince themselves that any effect from dust on their measurement had to be very small indeed. This made them confident that their discovery of B-mode polarization meant they’d seen gravitational waves from cosmic inflation. And that’s why they made a big deal when they made their announcement last spring.

But their confidence was not justified.  All of their techniques for estimating the dust were problematic, and gave them overly small estimates.

Over the last eight months, results from the Planck satellite have been gradually released, indicating that in regions of the sky with very little dust, the degree of polarization of that dust is quite a bit larger than BICEP2 (and many previous experts) assumed. And now, finally, after combining their efforts, the BICEP2 and Planck teams can conclude that although the amount of dust in the BICEP2 portion of the sky is really, really small — which is why it was hard for the Planck team to make believable measurements in that region until now — it’s big enough, and polarized enough, to have generated the BICEP2 signal.

So now the questions: Why did BICEP2 and Planck have to combine their efforts to settle the issue? And how did they do it?

Why Planck and BICEP2 Joined Forces

The story has everything to do with

  • the strengths and weaknesses of BICEP2 (and its successor Keck, built by the same team) and of Planck, and
  • the details of the polarized microwaves (which is what these experiments measure) that are generated by dust and gravitational waves.

[Microwaves are electromagnetic waves like visible light, X-rays, and radio waves; they have a longer wavelength, and lower frequency, than visible light, but have shorter wavelength and higher frequency than radio waves. Electromagnetic waves, as they move through space, are waving perpendicular to their motion; if such a wave is heading toward you, it could be wiggling left-to-right, or up-and-down, or somewhere in between.  If the wiggling of waves from a certain patch of sky has a random orientation, we call the waves “unpolarized”, while if the wiggling tends to be in a specific direction, we call the waves “polarized”.]

For the first point, it is a question of breadth versus depth. Planck is highly versatile, and can do many different types of measurements. BICEP2 and Keck are much more restricted, but for their specialized measurement, they are extremely powerful. BICEP2 and Keck only can detect microwaves in a narrow frequency band, while Planck can detect a wide range of frequencies. But BICEP2 and Keck are much more sensitive detectors than Planck, so they can detect much smaller signals. (It’s also worth knowing, though it isn’t critical in today’s story, that Planck can look across the whole sky, while BICEP2 and Keck just focus on a small patch of sky which is known to have a small amount of dust.)

BICEP2 and Keck are looking at microwaves that are vibrating with a relatively low frequency (150 GHz, i.e. 150 billion cycles per second) while Planck looks at many frequencies between 30 and 353 GHz. For today’s purposes, the only one that matters is 353 GHz, a relatively high frequency. For the rest of this post, I’ll refer to 150 GHz and 353 GHz as the “low” and “high” frequencies.

And here’s the scientific bottom line. Dust radiates microwaves much more readily at the high frequency than at the low frequency. Conversely, gravitational waves would give microwaves just as readily at the low frequency as at the high frequency. BICEP2 measured a tiny but significant amount of B-mode polarization of microwaves at the low frequency. Therefore, if BICEP2’s signal is due to gravitational waves, we expect only a tiny amount of B-mode-polarized microwaves at the high frequency, while if the signal is due to dust, we expect a much larger amount (though still quite small, and hard for Planck to measure!)

An Analogy: Is the Sky Clear or Overcast?

Here’s an analogy to help make this clear. Suppose you were to look at the sky through rose-colored glasses — glasses that filter out blue light and let red light through — and you measure how much light you are seeing. Fine, you can tell how bright the sky is in red light. But could you tell if the sky is clear (and blue) or overcast (and white?) It might be quite hard, because either way the sky looks red when seen through your glasses.

However, imagine your friend has blue-colored glasses — glasses that filter out red light and let blue light through. Your friend also can’t tell if the sky is clear or overcast, because the sky will look blue just because his glasses are blue. He can only tell how much blue light is coming in.

But now compare your measurements. If the sky is white, you and your friend will see a similar amount of light coming through your glasses, because white light contains a moderate amount of both blue light and red light. But if the sky is blue, your friend will see much, much more light through his glasses than you see coming through your red glasses. Thus, by combining your experiences (Figure 1), you and your friend can determine whether the sky is clear or overcast.

Fig. 1:

Fig. 1: Two friends are looking at the world through colored glasses; the friend wearing red glasses sees the sky as red, while the friend wearing blue glasses always sees it as blue. On an overcast day, both of them will see a moderately bright sky. But on a clear day, the friend wearing blue glasses will see a much brighter sky than the one wearing red glasses. By comparing the amount of light coming through their glasses, they can tell whether the sky is blue or white.

Back to Planck and BICEP2

What Planck and BICEP2/Keck are doing is similar, though with a few crucial differences (they don’t measure the total amount of microwave radiation, but rather the tiny degree to which it is polarized in B-modes), and with an additional, crucial twist (to be described a little later).  And for a similar reason, the Planck and BICEP2 measurements need each other; neither can draw a firm conclusion alone.  If the BICEP2 measurement at the low frequency had been due to gravitational waves, the amount of B-mode polarization at the high frequency would have been too small for Planck (which isn’t as sharp-eyed as BICEP2) to measure. But in fact Planck did observe B-mode polarization at the high frequency, with a power hundreds of times higher than the power found at the low frequency by BICEP2.

This is shown in Figure 2, taken from the BICEP2/Keck/Planck paper and annotated by me.  The BICEP2/Keck data, taken at the low frequency, is on the left.  Note the circled points, which describe effects on large patches of the sky.  They clearly lie above the red curve (which is the prediction of how much B-mode polarization comes from gravitational lensing of E-mode polarization from other sources — an effect I described here.)  This means that there is a new source of B-mode polarization, perhaps from gravitational radiation, perhaps from dust.

The data on the right is from Planck, measured at the high frequency.  The circled points lie above the horizontal line at zero — a rough signal of B-mode polarization at the high frequency.  Note that this signal is big (compare the vertical axes on the left and right plot to see how much bigger the Planck signal is than the BICEP2 signal).  The effect BICEP2 observed at the low frequency is observed by Planck, and is apparently much brighter, at the high frequency!

BICEPvsPlanck

Fig. 2: The amount of B-mode polarization (vertical axis) as distributed in patches of different size on the sky (on the horizontal axis, small numbers correspond to large patches, large numbers to small patches.) Left: Data from BICEP2 and Keck at the low frequency. The fact that the circled points lie above the red curve (the prediction from lensing of E-mode polarization effects) is a strong observation of a very weak signal, an indication that BICEP2 may have seen a very tiny effect of either gravitational waves or polarized dust. Right: Data from Planck at the high frequency. The weak hint of a very powerful signal (that the points lie above zero) is an indication — not entirely convincing — that Planck is seeing an effect from polarized dust.  Plots are from the BICEP2/Planck paper; ellipses added by me.

Thus, as surely as Earth’s sky is blue, we learn from this that the sky above BICEP2 shows the effect of polarized dust: the effect is dim (but measurable by BICEP2) at the low frequency, and very much brighter (just enough to be seen by Planck) at the high frequency.

Or do we?  The problem with this conclusion is that it is too quick.  As you can see from the right-hand plot in Figure 1, the Planck data by itself actually isn’t very convincing. Notice each data point [each dot] has a big vertical “uncertainty bar”, and the data points for Planck are kind of all over the place, unlike those from BICEP2, which are nice and clean and quite far from zero (the horizontal line.) So is Planck seeing a real effect from dust? Is it possible that actually there’s no effect there at all, and this data is just due to random garbage in the electronics of the Planck detector being misinterpreted as a detection of B-mode polarization?

Clinching the Case

So here’s the final, convincing bit of evidence. Suppose, first, that BICEP2, Keck and Planck are really, truly seeing effects due to dust. In that case, in the patches of sky where BICEP2 sees a signal of dust, Planck should see a signal also! The pattern of the effect of dust on the sky should have the same shape for BICEP2 as it does for Planck.

On the other hand, suppose that Planck is misinterpreting random statistical fluctuations as due to dust, and BICEP2 and Keck are really seeing effects from gravitational waves. In that case, there is no reason that Planck should see effects in the same location on the sky where BICEP2 sees them; BICEP2 is seeing a real signal, and Planck is seeing something random.  Therefore the spatial patterns they see should look different.

And so, the two experiments can check: does Planck see the same spatial patterns in the signal as BICEP2 and Keck, or not?

BICEP2ccPlanck

Fig. 3: The “cross-correlation” between Planck and BICEP2/Keck, showing that their two signals line up in the sky. This means Planck is really seeing the same effect as BICEP2, and thus Planck’s hint of a very bright signal at the high frequency is real.  BICEP2’s signal therefore cannot be from gravitational waves. Dust must be present, and detailed analysis (see Figure 4) shows it is in fact responsible for some or all of BICEP2’s signal. Plot from the BICEP2/Planck paper; ellipse added by me.

Yes, they do. That’s the clincher, shown in Figure 3, also taken from their paper; the circled points lie above the horizontal line, which means that BICEP2/Keck data and Planck data are correlated.  You see that this combination of data is much more persuasive than if we simply look at Planck’s data alone; the uncertainty bars are much smaller and the data points are closer together.  And so it is in combining the two experiments in this second manner that the case for dust is actually closed.

You could push my earlier analogy further if you want, by imagining that although your glasses are nice and clear, your friend’s blue glasses are so dirty and opaque that he can barely see any light coming through at all.  He says to you “I think I see some light!” but you’re not sure you believe him.  However, suppose the sky is a little hazy with high clouds, so the brightness of the sky is a little less in some places than others.  Well, if you see bright and dark patches in your red glasses, and he sees bright and dark patches in his blue glasses, and the patches that he sees and that you see are in the same locations, then he must really be seeing the sky, not just imagining things.  And (since his glasses are so opaque) that in turn implies that the sky must be much, much brighter in blue light than in red, meaning that the sky is clear, not overcast.

For interested readers: a couple of details I have left out. Keck sees less of an effect than BICEP2, so the combined result from the BICEP2/Keck team is smaller than it was in BICEP2’s original announcement.  This may just be due to a statistical effect, or perhaps Keck, a newer experiment, is slightly better than BICEP2.  Also, although I didn’t emphasize it, the size of the effect seen in Planck, the size of the effect seen in BICEP2/Keck, and the size of the correlation between Planck and BICEP2/Keck, as illustrated in Figures 2 and 3, are all quite consistent with what one would predict from a dust signal.  One other conclusion of the paper is that B-mode polarization seen in small patches of sky — the points not circled in Figures 2 and 3 — is definitely from gravitational lensing of E-mode polarization.

Drawing Conclusions

Now it is time for the scientists to make this all quantitative; vague statements need to be made more precise. The signal these experiments observe is, in principle, due to a mixture of dust and gravitational waves. Let’s call the amount of gravitational waves “r” (a measure of how strong the effect from cosmic inflation would be) and the amount of dust “A_d” (basically short for “amount of dust” or “amplitude of dust effect”). Now we ask, what values of r and A_d are the Planck and BICEP2 (and Keck) data consistent with?

The answer is shown in Figure 4, which is taken from the paper and annotated by me. Data are always somewhat uncertain, so we can’t use the BICEP2/Keck/Planck data to get an absolutely definite knowledge of r and A_d. [Indeed no knowledge is ever absolutely definite; there is always a (possibly extremely small) amount of uncertainty.]  Instead, the inner and outer ellipses (focus your attention on the black ones) show the most likely and somewhat likely values of r and A_d. Anything outside the outer ellipse is quite unlikely indeed. If the BICEP2 (and Keck) data were from gravitational waves, you would expect the ellipses to surround the point at the end of the violet arrow, with r between 0.1 and 0.2 and with A_d = 0. But if they were from dust, you would expect the ellipses to surround the point at the end of the red arrow, with r quite small and A_d significantly different from zero. (Incidentally, if all three experiments had detected just statistical noise, you would expect the ellipses to surround the point with r = 0 and A_d = 0.)

Fig. 3: Analysis of the BICEP2, Keck and Planck data.  The black ellipses shows the values of r (amount of gravitational waves) and A_d (amount of dust) that are most likely (inner black ellipse) and somewhat likely (outer black ellipse).  [The blue (red) ellipses show the results with Keck (BICEP2) data removed]. A purely gravitational wave signal (end of purple arrow) is excluded; a purely dust signal (end of brown arrow) is allowed. A no-dust signal (horizontal axis) is excluded, while a no-gravitational-wave signal (vertical axis) is allowed.

Fig. 4: Analysis of the BICEP2, Keck and Planck data. The black ellipses show the values of r (amount of gravitational waves) and A_d (amount of dust) that are most likely (interior of inner black ellipse) and somewhat likely (interior of outer black ellipse). [The blue (red) ellipses show the results with Keck (BICEP2) data removed]. A purely gravitational wave signal (end of purple arrow) is excluded; a purely dust signal (end of brown arrow) is allowed. A no-dust signal (horizontal axis) is excluded, while a no-gravitational-wave signal (vertical axis) is allowed. Plot from the BICEP2/Planck paper; arrows added by me.

So what we learn from this figure is

  1.  BICEP2 and Keck have detected a real signal, through a spectacular measurement.
  2. The signal definitely has a significant contribution from dust.
  3. The signal may have no contribution from gravitational waves and cannot be purely gravitational waves.

Tada! But too bad for BICEP2, and for our knowledge of the universe’s early history.

One caution. Does this result mean BICEP2 definitely did not detect gravitational waves? Strictly speaking we can’t say that — and in science, it is essential to say what you mean, and be clear about what you do and do not know. We can only say that they have no clear evidence for gravitational waves.

However, when you have a choice between interpreting an experiment as having made a mundane discovery and interpreting it as having made a stunning, Nobel-Prize-worthy discovery, consider that history favors the mundane. We know that cosmic inflation need not lead to gravitational waves that BICEP2 can detect; the gravitational wave signal can easily be ten or more times smaller. It’s therefore perfectly possible that the BICEP2 signal is entirely due to dust even if cosmic inflation did occur in the universe’s past. My personal best guess is that this is all dust. Of course, the universe doesn’t care about my best guess, and in any case, measurements looking for even smaller effects will continue into the coming decades. Each scientist will decide for him or herself whether to be hopeful or not about the future; this is a matter of opinion and hunch, not of knowledge.

It is all quite disappointing for scientists involved, not just for those on BICEP2, for whom it is a bitter end to a great measurement, but for the whole community. If BICEP2’s original interpretation had been correct, it would have had a great slew of fascinating implications and opened up a whole new field of scientific inquiry. Without it, we’re back doing what we were already doing before: still looking for evidence either for or against cosmic inflation, and more generally for clues as to why this universe of ours has so many very odd features.

Epilogue and Homily

Let me add, for those readers who might be inclined to jump to wrong conclusions, or who know others that are so inclined, that it is essential not to throw out the baby with the bathwater. The reason scientific inquiry eventually leads to correct answers, despite the fact that scientific experiments are commonly wrong, is because of the insistence on subjecting experiments to extreme scrutiny and demanding they be reproduced before taking them too seriously. It is this rigorous attention to detail that distinguishes scientific knowledge from other forms of human knowledge.

Whenever you read a claim of a new scientific discovery in the newspaper, always ask: has it been confirmed, and how? A Higgs boson of some type has been discovered; two experiments at the Large Hadron Collider, each making several separate measurements, have given strong evidence for this. The universe’s expansion is accelerating; this is now confirmed by a bevy of experiments. By contrast, no other experiment ever confirmed the OPERA experiment’s claim of faster-than-light neutrinos, and indeed an error, and a detailed study confirming the error, explained why their measurement came out as it did. Similarly, no other experiment confirmed BICEP2; in this case there is no (as yet known) error in the measurement itself, but there was certainly an error and a lack of caution in interpreting the experiment.

And so it goes in science. You should not trust scientific results — such as claims of connections between vaccines and autism, and claims about deflated footballs in cold weather — that have not been subject to heavy scrutiny and have not been confirmed by independent investigations. Conversely, do trust results that have been scrutinized and confirmed… including claims that vaccines work and save lives, that the Earth’s average temperature is gradually increasing, and that radioactivity leaked from Fukushima in 2011 is not a danger to the U.S. west coast.

Scientists, like all humans, are fallible. But the scientific process, armed with its powerful methods for identifying and reversing human mistakes, is self-correcting. The scientific method works — our technologically-based economy is clear evidence of this — and it deserves more respect than it currently receives from politicians and populist pundits in my home country.

72 responses to “Dust Thou Art, BICEP2

  1. Then why BICEP team declared what made a huge propaganda hype ? , why they did not just wait till they arrange the final result with Planck team that we see now ?….. I take it as huge puplish or perish hype !

    • As I explained: they made a mistake. They convinced themselves of something that wasn’t true, and that made them over-confident. They should certainly have been more cautious before they had more information about dust, not only from the point of view of hindsight, but also, I think, as a matter of principle. You can bet there has been plenty of criticism of their tactics by other scientists in the field.

  2. I read that the final report almost deleted the cosmic strings concept and all simple models of inflation meaning that exotic physics now became a must to get out of this dark tunnel …….
    How true is that ?

  3. Matt, this is really a nice piece. One minor note. I think it is good to stress that the paper does two things: cross-correlate BICEP2 & Planck data, and add in Keck Array data (and cross-correlate that new data with Planck). The former gives the blue curves, the latter the red ones. The black is the average of the two.

    I think this is important because had they just done the former, there would be a central value of r around 0.1. And though not statistically significant, I think the way people would have characterized the result would have been different (“Is there a signal?” instead of “It’s all dust!”). But the collaborations didn’t just do the former, they used all the data they had, giving a more definitive answer. This was the right thing to do—good science.

    I should note for your readers that I’m an Editor for Physical Review Letters and that we have provisionally accepted the paper for publication (this means they have been sent referee remarks that require some small changes but not further anonymous review).

    • Thanks, Robert. Your point is well taken (and visible in Figure 4; I mentioned this issue but did not emphasize it.) You are right that I should probably point this out more strongly; I have oversimplified in order to keep an already long article a bit shorter.

  4. Matt

    Good comment about failure in science. Too many people today (particularly in Congress) do not understand that you learn from failures. A scientific experiment that fails to prove what it was designed for, is not truly a failure. It’s a data point in our database of knowledge and may eventually point us in the right direction. Of course, some failures are still failures. A rocket launched that doesn’t make orbit is a failure despite the lessons learned from it. The public needs to learn to discern the differences.

    • Excellent points. Perhaps it’s the inability of certain people (particularly those in Congress) to learn from their OWN failures that precipitates this lack of understanding.

      I think we may agree that in Matt’s analogy it was the BICEP2 team that was wearing the “rose-colored glasses”.

  5. The merit/necessity of the scientific method is currently subject of discussion betwween members of the physics community. Are those physicists who sow doubt about the value of the scientific method also populist pundits? I think they are, they should not be left out of account.

  6. Pingback: Dust Thou Art, BICEP2 | Φυσ&i...

  7. The old saying states that “History is written by the victors”.

    That may be true for politics, but it is not true for science: in science, both the winners and the losers write history.

    Just one example: the search for the “luminiferous ether”.

    James Clerk Maxwell was able to calculate the speed of electromagnetic waves from the equations of his theory. He was not completely surprised when he found out that the numeric value just about equal to the speed of light, a value was well known at the time.

    His friend and mentor Michael Faraday had already designed and run an experiment that demostrated that a strong magnetic field could clearly affect polarized light.

    All this pointed to the fact that light might be an electromagnetic wave. But if we can measure and calculate the speed of electromagnetic waves, what was the reference frame for such measurements and calculations?

    This is what gave rise to the idea of a medium that was the support for electromagnetic waves: the “luminiferous ether”.

    If light was an electromagnetic wave these waves vibrate through this medium, the ether, we could be able to design an experiment to detect the relative movement of the Earth through the ether.

    One of the first persons to think a conceptual design for such an experiment was Maxwell himself, and he wrote an article with his idea for such an experiment just before his death. One of Maxwell’s friends sent this article to Nature, as I recall.

    Even though Maxwell thought of a very ingenious experiment and it was a very important experiment to be run at the time, most scientists did not pay much attention to this article. But this article did catch the attention of one experimental physicist: Albert Michelson.

    Michelson gave it a try to Maxwell’s idea in 1881, and it did not go that well: the experiment “failed” in the sense that it was not able to determine that there was such as medium as the “luminiferous ether”.

    Michelson was so frustrated by his first experiment that he decided to design a more precise equipment, based on Maxwell’s idea, to find evidence of the elusive ether.

    In 1887, with the help of Edward Morley, they run a much improved version of the experiment, with very sensitive equipment, and again, there was no ether to be found.

    We could argue that this story has some resemblance with the BICEP2/Planck story, and in a way, it does.

    Michelson’s “failed” experiments baffled most physicists for almost 25 years, including geniuses like Lorentz and Poincare, both of whom came close to the “right” answer but both missed the mark.

    This “failed” experiment was the cue that gave Einstein the proper setting for his revolutionary ideas on Relativity.

    So, we should not worry too much about “failed” experiments, for sometimes in the history of science is this way that the truth finds its way to show up.

    Kind regards, GEN

    • Torbjörn Larsson, OM

      “The old saying states that “History is written by the victors”.

      That may be true for politics, but it is not true for science: in science, both the winners and the losers write history.”

      Well, they can. But only if they are careful, as the BICEP2 team were, their paper is still correct and instead sunk by new data.

      I’m thinking of the “Arsenic Life” debacle, which had a similar hasty press conference but which was almost immediately shown to be based on abysmal analysis. The young first author was lead astray by seniors investigating an extraordinary idea (a shadow biosphere somehow unknown until now) with erroneous methods (say, no or erroneous null samples; bad lab methods), and the paper shouldn’t have passed peer review.

      Not much gained, but much lost.

      [Disclaimer: I’m interested in astrobiology, and I find the “shadow biosphere” idea an unlikely hypothesis in the first place. Everything we now know points to a rapid emergence of life, and Darwin’s old “later attempts becomes food” analysis applies in spades.]

  8. kashyap vasavada

    Matt: I have 2 questions. (1) In some graphs we see a huge peak at l=1. Honestly I have not studied the scales in the two papers. But if this is true, can dust model explain it?
    (2) What is their model of galactic dust? Are these neutral molecules like our atmosphere or charged ions and electrons? So is the model classical like Rayleigh scattering?

    • (1) I don’t know what you are referring to. You will have to be clearer.

      (2) The story of galactic dust is very complicated. These are not molecules, they are grains. I don’t know much about the subject; you can learn something from talks by people like Doug Finkbeiner.

      • kashyap vasavada

        Thanks. Sorry I meant l=100 in the multipole analysis of CMB. That was a dumb mistake! I cannot paste that graph.

    • Galactic dust is mainly slightly charged grains of ‘primordial dust’; that is the stuff that goes into making planets. Mainly a mixture of water, ammonia and methane ices it also contains iron, carbon compounds and various silicates in lesser amounts. This makes it rather good at interacting with a lot of electromagnetic radiation in a number of ways.

      • kashyap vasavada

        Thanks. But what surprises me is that mixture of lots of different molecules polarizes microwaves and someone can calculate it confidently!

        • In the case of galactic dust it is the physical structure of the grains that is most pertinent. A mixture of molecules do interact in various ways but this is mostly absorption, emission and scattering. This ‘fogs up’ any EM radiation passing through to various extents (For example light is heavily blocked but IR less so.) but just makes measurement harder, not ‘different’. (That is it adds noise.)

          The grains’ size and structure however can add a signal, polarizing or converting certain wavelengths into others. This is the tricky part and there are still unanswered questions. (For example do the grains have a permanent electric dipole moment? Do they spin?) It is an interesting area and I wish I had more accessible information on the subject.

  9. BTW, Keck is not the only “new” experiment to go after the detection of gravitational waves of cosmic inflation through B-Mode Polarization of CMB.

    There is also the SPIDER experiment:

    http://arxiv.org/abs/0807.1548

    http://www.astro.caltech.edu/~lgg/spider/spider_front.htm

  10. Torbjörn Larsson, OM

    I like the Fig 4 analysis.

    But this, I can’t find myself like:

    “They did know, from other people’s measurements, that they were looking in a patch of sky where the amount of dust (polarized or not) was very small. They had some estimates from past studies of how much polarization was typical. They also got ahold of some unpublished data from the Planck satellite, which had been shown in public. They put this information together in various ways, and somehow managed to convince themselves that any effect from dust on their measurement had to be very small indeed. This made them confident that their discovery of B-mode polarization meant they’d seen gravitational waves from cosmic inflation. And that’s why they made a big deal when they made their announcement last spring.

    But their confidence was not justified. All of their techniques for estimating the dust were problematic, and gave them overly small estimates.”

    I’ve read the BICEP2 paper, and I don’t see how the above description is correct.

    The BICEP2 team acknowledged that they main uncertainty was “the lack of a polarized dust map.” They used 6 models, including “publicly available Planck data products”, which all agreed that the dust contribution was uncorrelated with the then signal and would have to be ~3 times higher to predict it. With the data they had, they were forced to announce a possible detection.

    Or abstain to present their data in the intended model interpretation, possibly. But what use would that be?

    But if BICEP2 team were not confident/overconfident* [see the comments] of their finding what I can see and left it open, as they had to, that dust could eat away some or all signal:

    “The observed B-mode power spectrum is well fit by a lensed-ΛCDM** + tensor theoretical model with tensor-to-scalar ratio r = 0.20+0.07 −0.05, with r = 0 disfavored at 7.0σ. Accounting for the contribution of foreground dust will shift this value downward by an amount which will be better constrained with upcoming datasets.”

    It is not as if this would be an OPERA release of unprecedented and unexpected signals that proceed faster than the universal speed limit. It is looking at open areas of current cosmology and dust interactions both.

    * Confidence in the technical sense, at least. I don’t know if cracking open champagne bottles and/or holding press conferences is personal confidence or matters-as-usual. =D

    ** This is the lensing signal that the new paper confirms as “detected at 7.0 σ significance”, I believe.

    • Torbjörn Larsson, OM

      I forgot to mention that BICEP2 sat on the data and its possible implications for 1 year, in a highly competitive field, before they had wrung out all they could of possible errors and variants of dust models.

      I wouldn’t interpret that behavior as confidence or lack of cautiousness. It could be either, but it is unlikely.

    • You say “They used 6 models, including “publicly available Planck data products”, which all agreed that the dust contribution was uncorrelated with the then signal and would have to be ~3 times higher to predict it. With the data they had, they were forced to announce a possible detection.”

      We all agree they detected B-mode polarization, and they had to announce that. But they gave the impression that they were confident this polarization was due to gravitational waves — hence the big publicity stunts and public talks. The problem is that although they used 6 models for dust, they (a) did not account for uncertainties in the cruder models, and instead presented central values without uncertainty bands (which I misinterpreted as upper limits! because it never occurred to me that they would only plot central values); (b) used the publicly available (but unpublished) Planck data incorrectly; (c) did not account for other facts announced by Planck, including regions with large polarization fractions. So they really didn’t have proper data on dust at all — certainly not accurately treated data. Once you realize this, you realize that they really had very little justification for claiming that they’d seen gravitational waves… only that they’d seen B-mode polarization. This is what their critics pointed out soon after… and what the Planck people said from the very beginning. So either you say (a) they made an actual, significant mistake interpreting what was known about dust, or (b) they significantly overstated their case. You can’t say less than that.

  11. Matt, an excellent explanation. Let me throw a curve-ball, though. Ideally, BICEP2, Keck, and Planck should have got together and sorted this out before going public, so the question is, why did BICEP2 not do that? My suggestion is not so much excitement, but the question of funding. One priority for a research leader is to get funding continued, and we see (a) it is getting harder, and (b) funders tend to want something measured, such as numbers of papers, citations, etc. Now, in the normal course off events, even if wrong, the number of citations increases spectacularly. Therefore the way science is currently funded may promote the chances of this happening, where everyone has to try and get what they can out there as quickly as possible.

    • BICEP2 was correct to go public when they did. They had a result — a signal of B-mode polarization — and they didn’t need anyone else for it. It would have been unacceptable for them not to publish it.

      What they did incorrectly was state that the result was best interpreted as gravitational waves. They should have said that the result could be from either dust or gravitational waves, or a combination, and that they would not be able to say which it was until they had data from other sources, such as Planck.

      This has absolutely nothing to do with funding! What do you think it does to your funding future, not to mention to your job future, to claim something that everyone in the world can see was mistaken? It does not help you. On the contrary — this had to do with wanting to be the first to say something important in science: clear evidence for inflation. Fame. History books. Not money.

  12. I agree the discovery of B-mode polarisation had to be published, but I am not so sure about their paper saying, “they would not be able to say which it was until they had data from other sources, such as Planck.” Most referees would then say, why don’t you go to Planck?

    I also agree they wanted fame, but money tends to come with fame. Let me rephrase your response: What do you think it does to your future standing and reputation to claim something that everyone in the world can see was mistaken? We won’t know what drove them to it. I hope it was just excitement and carelessness, but I still suspect the fame and funding prospects contributed.

    • If you think scientists do what they do because money is what they want, and fame comes with money, you haven’t understood scientists and their culture very well. If you want to make money and you have the smarts to be a scientist, then you go to Wall Street, Google, or some other much more effective way of generating cash.

      • I agree with Matt. Any very competent scientist in this kind of league could make a load of dough if they invested their intellectual capital in FB, or Google, or Wall Street, hands down … so, the issue is not money.

        Maybe there is something regarding the fact that they are human beings, like anybody else, and are affected by their own needs, desires and shortcomings, just like anybody else.

        Kind regards, GEN

        • Just to clarify, I was not implying the researchers were trying to get rich quick. The question was, funding, and you need funds to continue your research, and my question was solely directed towards the funding of research. If you watch my spelling, you will see I am not an American, in fact I come from New Zealand, and I spent ten years on our national funding committee for the physical sciences. This was something of an eye-opener. The underpinning problem was there were never enough funds to fund the good projects. The excellent proposals were no problem, but when you got down to the good ones, only too many had to miss out. Then what I saw were an incredible array of strategies/tactics by submitters, and other panelists, to steer funding. Now, maybe I am biased by the fact I come from a country where some good projects miss out, and maybe funds elsewhere are no problem, in which case I apologise for the suggestion, but if finds are a problem, is not it the responsibility of the team leader to do what he can to maintain them? If sitting on your backside and being “pure” leads to you having to fire so many scientists, is it that unreasonable to suggest that they might go a little careless and excited to get funding.

          When I put that question, I did not quite expect the response I got. At no point did I think of personal riches. What I hoped for was that it might open up somewhere a discussion on the problems of funding fundamental science. Maybe this wasn’t the place to do it. Maybe in the US, outside places like Harvard, there are no problems of funding fundamental research, and money flows easily all the way down to the lesser institutes. I would like to think this was the case, but I really doubt it. My argument was based on the fact that a certain number of scientists want to unravel the mysteries of nature, and to do this they need funding. Failure to get it means they lose their salary, and have to go and do something else. You may say that is tough, but . . . Fair enough, but is it unreasonable to suggest someone may cut corners a bit to avoid that?

          One last question. This now becomes a nul result. What does the scientific community make of that? The issue is, had there been a polarisation, everyone gets excited that cosmic inflation happened. Now that we have a nul result, what is required to argue it did not happen?

          • Another kiwi huh?

            There’s always the temptation to… bend the rules, even to cheat to get funding. It’s a tough world and you need to be savvy. In my (less than extensive) experience scientists tend to hate having to do this, it ruins the ‘purity’ of what is being done, it’s the kind of spin you expect from politicians and other dishonest endeavors. There is a tendency to exaggerate, to label each incremental advance as a breakthrough, each treatment as a cure.

            But what I think happened here was simple childish glee mixed with the heady draught of achievement. When you see the surprise interview they did with the professor that predicted the waves, when you watch their interviews, what I see there is a bunch of people who worked hard to do something and succeeded. Spectacularly. Had it stood up it would have been a historic moment, a validation of several people’s careers, a vast step forward for physics, accolades and congratualtions. Who wouldn’t get carried away. CERN did very well with its ;We maybe, maybe found the Higgs… ok pretty sure we did’ arrangements, but even then there were tears and celebrations.

            And the media aren’t the best either; remember OPERA and its faster-than-light neutrinos? They basically said they suspected they were wrong… somehow but that was pretty much spread about as ‘FTL travel possible?!’

            The problem with this result is that it isn’t really a null result. That would be the case if we were expecting a strong gravitational wave signal. Instead we don’t *know* how strong a signal we’ll get, if any. It isn’t even as good as ‘We didn’t find any signal’ since THAT would be a null result for some (but not by far all) inflationary models. What we have here is an ambiguous result. Is it something? Maybe. Is it nothing? Maybe. What we need is a clear signal, a presence or an absence. And there’s nothing to do now but wait until we get it.

  13. Fair enough, but from experience I know what happens when you can’t get funding.

  14. Great article, once again. If I may say, this will motivate more engineers than physicists, not necessarily a bad thing, 🙂

    Anyway here is a question and I hope I am asking it correctly. Physicists have been trying to find the differences between EMR and gravitational waves forever, but are they really two distinct forces or just one? By this I mean, these wiggles of different shapes (including polarization) and energies are merely resultants of the same field all twisted and changing above the ZPE threshold. And if you can remove the disturbances (big bang or many smaller bangs?) from this single field you will settle to gravitational waves as the smallest possible “wiggles” to maintain a positive value of matter?

    • The exact relation between gravitational and electromagnetic fields is not important here. (And there are many who do hope to unite all fundamental forces into one, we have electromagnetic, weak and strong united so far, but gravity is a hard nut to crack.) The two effects should be unrelated in the same way that a magnet sticking to your fridge is unrelated to a lightbulb glowing. A single thing can cause multiple phenomena.

      If somehow gravitational waves were not a thing because of the relation of gravity and EM that would have some serious side effects. (It would mean you could convert light into gravitational energy somehow.)

      • “A single thing can cause multiple phenomena”

        Only valid point you made, of course, this is the nature our universe. Everything can be derived by the oscillatory behavior of energy waves, field(s). We are after that “single thing”, which would hopefully put everything in perspective.

        My question is if there is a single field, causing all these higher level mechanisms creating all the variations of interactions and hence higher level systems, then gravitation waves must be part of the cause. Yes, there are different cause/effect at various levels as you go higher in complexity but working it backwards where does gravity fit wrt to EMF? Is it the cause? Are the two cause by some other field we nothing of thus far? Is gravity part of the EMF but at one extreme of the spectrum, lowest frequency (longest wave length)?

        I would put gravity first (cause of EMF) because it could be the cause of quantum confinement and hence explain the other forces as well. Gravity pulls, and it is this negative force this necessary to have a universe we are put and exists. Without this pull, nothing can exist, hence gravity must be fundamental or must be lower than EMF, in nature’s hierarchy.

        I believe gravitational waves exist just as Dr E suggested but we have to find a very quiet and clean space, free from higher levels of the EMF if we are to find them. I wonder if we can detect them if we peer through the enormous energies released in a fission explosion capture one or a multiple of voids where everything has been removed leaving only the fundamentals of nature, maybe even at levels below the ZPE for that space?

        • One small note; you don’t get below the ZPE. It is, by definition the lowest energy level you can have. If you find something ‘below’ that then IT is the new ZPE and the ‘old’ one was a ‘false ZPE’.In many cases the ZPE is in fact zero. (Such as the lowest possible energy photon.)

          I think you may be thinking of ‘background’ which is whatever blah is happening in the space you’re making measurements. (Such as the microwave background in space.) This can mask a faint signal.

  15. Fabulous article about the new analysis. From my understanding the two points that lead to the conclusions are:
    1. If dust is responsible for the B-modes then the signal should be stronger for frequencies higher than what BICEP2 examined. Planck data at such a higher frequency shows a much stronger signal, inconsistent with gravitational waves theory of B-modes.
    2. If Planck’s data is random noise then it would be spatially uncorrelated with BICEP2’s data. But we see a clear correlation. This shows both data have a same source.

    But I see a problem here. You said “suppose that Planck is misinterpreting random statistical fluctuations as due to dust” which is the basis of point 2. above. But what if it the data is incorrect due to other reasons than statistical fluctuations? For example, there may be a bug in the electronics that multiplies the true signal by 1000. In that case it would still correlated with BICEP2 because it multiplies signals from the same source by an integer. It would also mean the strong signals at high frequency (mentioned in point 1. above) would actually be much weaker, possibly to the point that gravitational waves are favoured. Does the new analysis deal with this possibility?

    • This is a possibility that permeates all of science. Every experiment can be wrong because of a systematic flaw. It basically amounts to ‘but you could be wrong.’ and accounting for it usually adds little to any discussion. The equipment should have been tested and calibrated to the point where such an error is unlikely. If such an error has occurred and been allowed through I suspect someone would lose their job over it.

      • Indeed, but I am not saying every experiment need have systematic flaws. I stress that the correlation between BICEP2 and Planck results is irrelevant if Planck has a calibration issue somewhere (and BICEP2 has been calibrated properly). A last possible defence for gravitational waves!

        • Indeed but letting such an issue through would be unforgivable. There’s always the possibility, such was the case with the ‘faster than light’ neutrinos, but people lost their jobs over that; and that was when the team published basically saying ‘We know we’re wrong we just don’t know HOW.’ If such an issue were present it would ruin all the data, it would be utterly humiliating, a rookie mistake.

        • Guys, let’s bear in mind that modern experimental science got itself standing on its two feet by the French Revolution, thanks to the services of one mister (mmm, monsieur) Antoine de Lavoisier, who happened to be in charge of tax collection.

          Being forced to solve very practical problems (how to outgun tax evaders!), he singlehandedly founded modern experimental physics and chemistry.
          We have to realize that it doesn’t make much sense to loose the perspective of the practical and real world, no matter how interesting and fun theories much look like.

          One way or another this comment has something to do with cosmic inflation, or at least, with some of its genesis.

          After all, it all started with Henry Tye asking his pal Alan Guth if they could calculate how large would be the theoretical amount of magnetic monopoles (MPs) that would show up at the very early moments after the Big Bang.

          The theoretical calculations of the amount of MPs was not consistent with the how we know that our universe evolved, so, they should find a way to get rid of such a huge load of MPs.

          Guth could not figure out a solution for such a problem, until he went to a lecture by Steven Weinberg on the Electro-Weak theory and the Higgs field, and all of a sudden, everything fit into place for Guth, and this is how cosmic inflation got started.

          The point in all this comment is that Guth never lost perspective of the evidence and the real world.

          Kind regards, GEN

      • kashyap vasavada

        @Kudzu: Do you know the details? How many, post docs, professors or technicians lost their jobs?

        • Int he case of the faster-than-light team? OPERA spokesperson Antonio Ereditato and experimental coordinator Dario Autiero were the first; they both resigned. Following them there’s been a steady stream of people bowing out about 9 around the time of the event. The OPERA project is still going but has forever been tainted by this. A pity really since they did everything right.

  16. Pingback: ΛCDM – und sonst nix: das Vermächtnis von Planck | Skyweek Zwei Punkt Null

  17. Richard Feynman, in his speech & essay “Cargo Cult Science”, wrote:
    ” … the first principle is that you must not fool yourself — and you are the easiest person to fool. … “

  18. For me It is very important that, unlike OPERA which reported something bizarre, the BICEPS 2 scientists were looking for something we would all like to know about.
    My question is: how easy is it to verify whether the electronics of very complicated equipment, as in OPERA or BICEPS2 are not biasing the results? (Penzias and Wilson certainly worried about where their signal was coming from!)

    • Penzias and Wilson is a very good example of how rather competent professionals are challenged by a incredibly unique set of conditions and, being as proficient as they were, found their own way out of that quandary, unscathed.

      They wondered about the origin of the signal they were getting, that is, the orientation of the signal.

      On most cases, electromagnetic signals have some orientation, so, this would be the default assumption: where is it coming from?

      But in this particular case, the conditions were at the same time, incredibly unexpected, and by the same token, incredibly (and unbelievably) unique: the signal was coming from everywhere!

  19. Many of us participating in this conversation have witnessed the long and slow process required to get to the point of making the public announcement of the discovery of the Higgs boson.

    That is an example of how to be careful and respectful of the scientific method, in a very practical way.

    The BICEP2 team was right about publishing their results at the time the did, because they had proper measurements of B-Mode polarizations from the CMB signal in that particular region of the sky that their instruments were scanning.

    But they were wrong about the process they used to analysis those measurements and how they came about an interpretation of such readings based on the selected process.

    Kind regards, GEN

    • I have doubts about the CERN discovery of the Higgs boson. I don’t think they have anything.

      My question is did both experiments, CMS and ATLAS use the same or similar trigger algorithm?

      To really confirm the existence of the Higgs you must be able to device a sensor that can actually “see” it, NOT assume it by preset constraints, emulated algorithms.

      • But what do you mean by ‘see it’? exactly? With many things in nature direct observation just isn’t going to happen.

  20. A Biochemistry Student

    A question, from someone quite naive in cosmology and particle physics.

    Is gravitational wave B-mode polarization a necessary condition for inflation models (or least, the most popular ones)? Does the maximal amount of polarization which could have been contributed from non-dust related B-mode polarization provide a strong constraint on any inflation models?

    While these negative results are interesting, and the ability to make such precise measurements is an accomplishment in itself, I guess I’m interested to know if any (if not Nobel-worthy) immediate positive results can be drawn from these measurements?

    • This raises a very important issue. Science works on the logic, if theory x is correct, then we should see observations y. So, what happens if you do not see y? For a theory to be useful, it must make testable predictions, even if they are difficult to carry out, and there should be a point where if not y, then perhaps not x. As I see it (with no expertise at all in this field) the options are that the dust masked the effect they were seeking (but if so, to what extent would it mask it), the effect of gravitational waves is a lot weaker than suspected, inflation did not polarise the CMB, inflation did not generate gravitational waves, or there was no inflation other than the regular expansion. The problem is, how do you tell? Would a repeat of BICEP2 at the North Pole (to point way from the galactic centre) be an advantage?

      • ‘If x then y’ is one of the ways science works. (Another is ‘We have y, what kind of x would be responsible?’) In general it is often better to treat science as probabilities, like gambling. A theory makes predictions. The harder these are looked for and not found the less likely the theory is correct. The most likely theory is the ‘official’ one.

        Thus we have looked for unicorns but found none. It is highly unlikely that they exist. It is more likely there is life on Mars and so on. Maybe we never see proof of inflation or maybe we suddenly find something that disproves it.

        • Another way science works is to make the proposition, if and only if x, then y. Now you can work backwards, and if you see y, you can formulate the set of all possible theories that could lead to y. If that set has one and only one element, then you actually prove that theory. The problem is to justify the “only” part. From then on, it is a matter of philosophy. You say, treat the problem in terms of probabilities, in which case you make the most probable the “official” one. That is probably the way that most scientists work.

          However, I don’t accept that, and my main problem with it is that the official line is usually chosen on a relatively minor subset of possible observations, and once chosen, it is extremely difficult to dislodge. I confess I am biased in this view. In my PhD I entered a controversy, and my results came down on the wrong side of what was adopted. My supervisor was looking for a better paying position, so he did not publish the most telling results. A review came out and settled the issue, in my view, the wrong way, and later I re-examined the issue and found the review had ignored up to 60 different types of experiments that falsified that position. (This was an issue in chemistry.) I tried to write a review much later, but it was rejected on the grounds that (a) the issue was settled, and (b) they did not publish logic analyses. I have also a number of other examples, but in principle all this falls into the way Thomas Kuhn suggested science works.

          Hopefully, physics does not have this problem, but I am not totally convinced.

          • This is an issue with humans in general. I think it’s more prevalent in chemistry and biology than physics since those two areas tend to have one and only one model at their cores. (Evolution, atoms…) Physics by contrast has big unknowns at its core and usually has a half dozen or so competing models for any given phenomena.

            This I think is why it’s best to treat science as probability; there’s usually one ‘official’ model but it’s always wise to keep in mind that it may not be by very much. Physics is definitely the realm of competing theories which is why the LHC’s reccent pruning of sypersymmetry has revived interest in other, less prominent theories. Ever may it be thus.

            And don’t despair, my work on triphenylarsane didn’t make it into the literature, but several years later someone else gave it a crack and validated me. I didn’t get credit but I have the moral highground. The truth will out.

    • Gravitational waves are not necessary for inflation. The initial results were surprising as they were, in a certain sense, very close to being the biggest possible which would have ruled a lot of models out. Likewise the absence of waves would remove a lot of models. Sadly this ‘result’ is very ambiguous meaning that pretty much nothing can be ruled out until better measurements are taken.

  21. Several points:
    (1) The signal that BICEP2 (incorrectly) originally reported was unexpectedly strong. It was higher than most predictions of the B-Mode signal strength from early gravitational waves due to inflation, strong enough that Planck should probablyhave been able to make some direct detections of it themselves. This had people scratching their heads immediately.
    (2) There were other questions about the BICEP2 original announcement almost as soon as it was made public, and in fact that original announcement version never made it past peer review. The actual published paper was much more cautious/very much toned down from the claims they originally made when all the press conferences were held.
    (3) There is nothing inherently preventing the BICEP2 and other teams from making measurements at multiple wavelengths themselves, in order to estimate contamination from dust; it was a matter experiment design (and probably funding — you put your limited time and money into accurately measuring a signal, not noise).
    (4) The mistakes that the team made were pretty basic — using unpublished, unverified data from Planck, having a junior member of the team make some elementary mistakes and no one else on the team catching them, having no way to accurately distinguish signal from noise, etc.
    (5) The assumption that the team made was that there was essentially no significant contamination from dust in the part of the sky they were observing, and that it could essentially be ignored. They then seem to have put together an analysis that justified that assumption.
    (6) The announcement was over the top: champagne, celebrations, with videos. I have no idea who is responsible for organizing such shenanigans, but I doubt that it was done just from the exuberance of scientific discovery alone. I would expect involvement of the PR departments of several organizations…

    With the exception of this last point, I don’t question the scientific intentions or integrity of the BICEP2 team. It’s just a cautionary tale of how things can go wrong even with conscientious, intelligent people trying their best. Unfortunately, it contributes to the general public perception of science (and scientists) that if you just wait long enough, they’ll change their story…

    • @WLM: Before we give up completely on the inflation evidence from BICEP2, we should find out if the models of polarization based on dust are sound.It is very difficult to understand how grains which according to my understanding contain trillions and trillions of *different* kind of molecules can give rise to polarizations. All the bloggers, I tried, say, “they are not experts!” So in the interest of science, the rival groups should challenge the dust models also. May be they have, but I do not know for sure!

      • This is an interesting topic. Basically dust grains tend to ‘align’ in some way, either by being charged (Having dipole moments, also known as ‘spinning dust’) and by being magnetized. Since dust is rather cold it emits microwaves as black body radiation, in the same way a hot iron bar emits light. Their alignment causes the microwaves they emit to also be aligned, that is, polarized.

        However, as far as I am aware (I am not up-to-date here) there still exists a number of unanswered questions such as the anomalous emission at 10-100 GHz (Also known as ‘foreground X’) This is because, as you note dust grains are not all the same and we can’t exactly walk down to the shops to get some to study. I am unsure of the uncertainty of models in this area.

  22. Excellent analysis, thanks a lot. It turned out to be rather a sorry story, but it certainly was an exciting ride!

  23. Great article Matt,

    I’ve read elsewhere that these results have ruled out the models that predict eternal inflation (https://medium.com/starts-with-a-bang/ask-ethan-74-gravitational-waves-c21e701f9bde). Is this an accurate statement or are they still viable at all? No special affinity for them, I’m only interested in the truth. But eternal/chaotic inflation has been proposed as plausible by many high brow cosmologists, and I assumed that if this were the case it would be wider news.

    Any help clearing this up would be greatly appreciated.

  24. Pingback: Planck vs BICEP2: nessuna traccia di onde gravitazionali primordiali | AstronomicaMens

  25. Dear Professor Strassler,
    I have a lingering doubt on the theory that the expansion of our universe is accelerating and hope that you can help clear my thought. The main reason of my doubt is that the ages of the observed objects are different. Although it has been proven that the more distant objects move at a faster speed away from us, it has taken the light longer time for these objects to be seen by us, therefore these objects as we see today have different ages (from the Big Bang). If we use the best available Hubble’s constant of 70.4 km/sec/Mpc (229.5 km/sec/Mly), then the most distant object 13.8 Gly away that we are seeing today has a speed of 298,012 km/s. The object is in its state of infancy, approximately 13.8Gy ago, soon after the Big Bang. If the object is 10 Gly away, its speed is 215,951 km/sec, and the object is approximately 3.8 Gy old.

    If we plot the recession speeds of the objects against their distances from us, we have a linearly increasing line. However, if we plot the recession speeds of the objects against their ages after the Big Bang, we have a linearly decreasing line. This tells me that as an object ages, its speed decreases. If we observe the same distance object some years later, we should see its speed lower than today’s. This is on contradiction to the theory that the universe is expanding at an ever faster pace. I’d appreciate your thought on this. Thank you in advance and thank you for all your articles.

    • There are two things to note; first of all when we plot the expansion rate of the universe we see, it is not a straight line OR a decreasing curve. Instead it initially decreases but then is starts to increase again. (See for example the first graph here: http://science.psu.edu/news-and-events/2012-news/Schneider11-2012 )

      Secondly what is being measured to get these results are ‘standard candles’; exploding stars. The brightness of a number of supernovae are measured and plotted against their redshift.

      As they are known to (pretty much) be the same *actual* brightness, apparent brightness thus tells us how far away they are. (Twice as far is four times as dim.) The redshift gives us the amount of expansion that has happened between the light’s emission and now. (If the universe doubled in size the spectrum’s wavelength is also doubled.)

      What was expected was that if one star was twice as close to Earth as another it would have *less* than half the redshift since its light would have traveled here during the ‘slower half’ of that expansion. Steady expansion would have made that *exactly* half the redshift. What was found was that in the early universe yes, this was the case, but more recent supernovae are more redshifted than expected meaning that the expansion must be increasing.

      • I have one question regarding this explanation, which depends on the 1A being a standard candle. Suppose it is not? For example, see Wang et al., Science, 340; 170 (2013), where two classes of 1As were found, one of which is only found in more recent galaxies, and the luminosity depends on metallicity. What we see is actually the outer envelope in our line, and that should largely comprise material deposited on the star just prior to the explosion, i.e. material from the companion star. The greater the metallicity, the greater the visible emissions, and irrespective of what goes on inside the explosion, you would expect that light to be absorbed before it was emitted. If so, the older 1As would be expected to be dimmer for the red shift, which is what is observed. This is presumably an effect that was not allowed for in the original calculations (either that or the peer reviewers of Science were asleep at the wheel) and this effect could well allow for the discrepancies we observe, especially given the wide scatter in the data.

        • You are correct, this effect was not taken into account at the time. Current models suggest that this is a small effect that shouldn’t affect present conclusions to a great extent. (An estimated 2% variance in brightness.) More tricky is ‘asymmetry’; that is 1A supernovae do not ‘ignite’ at the dead center of the core, something that affects both their outputs and apparent brightness by as much as 20 percent.

          Greater numbers of observations should remove the wide scatter due to asymmetry at which point the metallicity question will need to be addressed. Should the data resolve into something that does not support dark energy then the metallicity explanation will be by itself insufficient. It’s not at all expected that will be the case, but then again that’s what we say about dark energy.

      • Thank you Kudzu. That’s very helpful.

  26. I guess it depends on how good the models are. As you say, more and better data may help here.

  27. I read about the POLARBEAR experiment which claims to have found B-mode polarization that isn’t due to dust. Do their results stand up?

  28. Hello Professor Strassler. What do you think of this paper published in the journal Physical Review Letters:

    http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.113.191303

    It proposes a method by which to differentiate between b-modes produced by cosmic dust and b-modes produced by primordial gravitational waves.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s