Interstellar dust and BICEP 2

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jsmaje
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Interstellar dust and BICEP 2

Post by jsmaje » Sun Mar 01, 2015 12:29 pm

As summarised in this article amongst many, it seems that interstellar (intragalactic) dust may well have confounded the CMB polarisation signal detected by the Antarctic BICEP 2 observatory, which at first sight was thought to provide evidence for gravitational waves, as predicted to have originated during the theorised ‘inflationary’ period of the very early universe.

Given what the team have gleaned about the few putative IS dust particles so far, can someone please explain to me (a) how such dust can polarise CMB radiation (something to do with them spinning and/or their alignment to magnetic fields, as mentioned in the article, and/or something thing to do with their crystal composition?), and (b) what (if any) constraints the findings so far from SD@H may place on such an effect. I’ve never quite understood polarisation anyway!

I’m sorry not to be actively dusting myself any more (my vision is not what it used to be), but am keen to keep up with developments.
John

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Re: Interstellar dust and BICEP 2

Post by DanZ » Tue Mar 10, 2015 6:25 pm

Quite the question John! Do you mind if we answer it during our live Telecon next week on Thursday, March 19th? I believe the time for you would be 6:00pm, and you can find the toll-free number to join the call by entering the pass code 6422969 at the website here.

Dan

jsmaje
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Re: Interstellar dust and BICEP 2

Post by jsmaje » Wed Mar 11, 2015 12:54 pm

Dan, you are welcome to discuss my questions on the Telecon, but I'll not be able to participate, since I'll be driving on my way to a meeting in Cambridge at that time (and I don't have any of those various i-whatsits).

If they can be posed by proxy, and there are any useful replies, perhaps you could report them here, so that I (and other non-participants) can learn from them.

Thanks, John

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Re: Interstellar dust and BICEP 2

Post by DanZ » Tue Mar 17, 2015 5:17 pm

Will do John! Dan

SmithES
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Re: Interstellar dust - Discovery mag

Post by SmithES » Wed Mar 18, 2015 4:07 pm

Hi, guys, In the April 2015 issue of Discover Magazine, I was delighted to read of NASA mission Stardust! On page 74, titled, "20 Things You Didn't Know About...DUST," there is a paragraph, "A NASA mission called Stardust launched in 1999 to capture some of this dust. To collect speeding particles without damaging them, the spacecraft carried aerogel, a spongy, silicon-based material that's 99.8 percent empty space. Researchers posted microscopic scans of the aerogel online and enlisted citizen scientists to help search for cone-shaped cosmic dust tracks. In 2014, they announced the result: seven likely particles of interstellar dust. Careful with those.

I was so pleased to see our work cited in this magazine! Looking forward to our live chat tomorrow (3-19-15)

Evelyn, (ERSTRS, SmithES)

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Re: Interstellar dust and BICEP 2

Post by DanZ » Thu Mar 19, 2015 11:34 am

Wow, thanks for sharing Evelyn, we didn't know. Always great to see some press on everyone's great work! I'll be sure to let the Berkeley team know. Dan

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Re: Interstellar dust and BICEP 2

Post by DanZ » Fri Mar 20, 2015 1:25 pm

jsmaje wrote:As summarised in this article amongst many, it seems that interstellar (intragalactic) dust may well have confounded the CMB polarisation signal detected by the Antarctic BICEP 2 observatory, which at first sight was thought to provide evidence for gravitational waves, as predicted to have originated during the theorised ‘inflationary’ period of the very early universe.

Given what the team have gleaned about the few putative IS dust particles so far, can someone please explain to me (a) how such dust can polarise CMB radiation (something to do with them spinning and/or their alignment to magnetic fields, as mentioned in the article, and/or something thing to do with their crystal composition?), and (b) what (if any) constraints the findings so far from SD@H may place on such an effect. I’ve never quite understood polarisation anyway!

I’m sorry not to be actively dusting myself any more (my vision is not what it used to be), but am keen to keep up with developments.
John
Hi John. Long story short, Dr. Westphal says your question is really beyond his level of expertise. I'm going to run this by someone in the Astronomy Department and see if I can get an answer for you there. Standby. Dan

jsmaje
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Re: Interstellar dust and BICEP 2

Post by jsmaje » Sun Mar 22, 2015 12:18 pm

DanZ wrote: I'm going to run this by someone in the Astronomy Department and see if I can get an answer for you there. Standby. Dan
Roger, wilco. John

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Re: Interstellar dust and BICEP 2

Post by DanZ » Tue Mar 24, 2015 5:31 pm

Here's the answer I received from my good friend/frequent collaborator Dr. Steve Croft. However, I think you wanted to understand better why the dust is a problem in the first place, and thus this answer doesn't cut it, or does it? Please let me know. Dan
The physics is well understood (spinning dust grains in magnetic fields) but it just requires a very precise measurement of the foreground signal from dust to see the faint background signal from the CMB. Imagine trying to see a very faint pattern on the tiles at the bottom of a swimming pool, and there are a bunch of things floating on the surface of the pool. Unless you know exactly what's between you and what you are interested in, especially when the foreground signal is so much brighter than the CMB, then you can't subtract it off perfectly precisely. The BICEP2 team attempted to do the foreground subtraction by scraping data from a Powerpoint slide presented by the Planck team. Unfortunately this turned out not to be good enough. When the Planck team did the analysis properly, the signal claimed by the Planck team went away.

Although Stardust has enabled us to directly study interstellar dust in the lab, we know a lot about dust and its properties from astronomical observations.

This link may be of interest:
http://www.deepspace.ucsb.edu/planck-po ... t-emission

Also technical slides here: http://moriond.in2p3.fr/J14/transparenc ... /ghosh.pdf

jsmaje
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Re: Interstellar dust and BICEP 2

Post by jsmaje » Wed Mar 25, 2015 12:58 pm

Thanks Dan, and to Dr Croft. Given the near-enough 100 pages of the links he provided, it will take me some time to digest it all before
deciding if I'm then any the wiser!
John

jsmaje
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Re: Interstellar dust and BICEP 2

Post by jsmaje » Sun Mar 29, 2015 12:59 pm

Far from there having been 100 pages of Dr Croft’s references as I mistakenly said, there were in fact only 10. I must have inadvertently instructed my printer to make 10 copies of them.

Anyhow, they did not fully answer my queries, and by further reading, for example http://arxiv.org/ftp/arxiv/papers/0902/0902.4226.pdf, it seems that the light polarisation by IS particles (or grains) isn't yet all that “well understood”. While there are many theoretical mechanisms, their contributions depend greatly on size, shape, composition (crystalline and any magnetic materials), temperature due to intensity, spectrum and directions of illuminating radiation, and the induced spin and alignment with galactic magnetic fields.

I don’t actually understand much of this at the physics level, and am merely quoting nuggets from the paper, which is essentially an impassioned plea to the astronomical community to study these complicated polarisation issues further.

Have our team been (or will be) able to investigate this aspect of the few putative IS particles so far? Would it even be possible and in any way help the current debate regarding CMB light extinction by IS dust?

I suspect that such a small sample will not be enough, particularly as much of ISD is so tiny as to be virtually molecular.
In that regard, what, if any, evidence of such tiny particles may have been left in (or on the surface of) the aerogel? For example, I remember reading that glycine amino acid molecules were found on the aluminium film between the aerogel tiles – can you point me to any further info on that?

Thanks, and sorry to be so annoyingly inquisitive!
John

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Re: Interstellar dust and BICEP 2

Post by DanZ » Tue Apr 14, 2015 12:23 pm

Hi John. Here's Dr. Croft's reply. He's a fellow Brit by the way! Actually, that really wants me to get out of the middle of this now that I think aobut it. :lol: If you want to see the plot, I'll have to post it later or email it to you, but it's as basic as they get, rapidly climbing from the 1950s to almost 10,000 in 2014. Dan

My point was that foreground subtraction was the issue with BICEP2 (the context of the original question) and that the disagreement between Planck and BICEP2 was not due to a lack of understanding of the dust particles themselves, but rather the challenges of subtracting their emission.

I maintain that light polarization by dust is in fact "well understood" (first proposed by Davis and Greenstein in 1951 and refined and developed in the 64 years since). Sure, the details depend on grain size, composition, temperature, environment, and a bunch of other factors (and how they depend on these factors is in fact again quite well understood), but more of an unknown in terms of the polarizing effects of dust, at least until the Planck results, has been the distribution of magnetic fields in the Galaxy. See http://phenomena.nationalgeographic.com ... milky-way/ for a recent news article.

I'm not sure that an impassioned plea to study polarization is necessary. A literature search for dust polarization turns up over 40,000 references, and as the attached plot of publications as a function of time shows, interest in this field continues to grow rapidly.

See http://arxiv.org/pdf/1303.4998v2.pdf for a recent review of radiative transfer modeling of dust. Fig. 6 illustrates that we are able to do a pretty good job with current models. The authors note that "Recent years have seen an impressive improvement in observational capabilities across the electromagnetic spectrum", and " The number of people actively involved in 3D dust RT is growing, and the number of new published codes has increased significantly in recent years", although they also note that " the best way to reduce this uncertainty [in underlying models] is to support the improvement of dust grain models through the use of improved laboratory and observational data". So data from Stardust could certainly have a role to play, although given that only a few samples were obtained, and these were all from the region surrounding our Solar System (not necessarily representative of the huge range of environments in the Galaxy) this must be interpreted in the light of data from telescopes too.

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Re: Interstellar dust and BICEP 2

Post by DanZ » Tue Apr 14, 2015 12:24 pm

Forgot to mention, I'll have to get back to you with Dr. Westphal's answer, though my guess is this won't be something he'll investigate, but leave to others if they want to try. Nevertheless, let me see what he says. Dan

jsmaje
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Re: Interstellar dust and BICEP 2

Post by jsmaje » Sat Apr 18, 2015 2:01 pm

Thanks to Dr Croft for the further info and references.

I clearly gave the wrong impression in my previous mail by implying that the physical mechanisms of astronomical light polarisation haven’t and weren’t still being well-enough studied, although uncertainties apparently remain according to the 2nd reference in their modelling.

I meant instead to emphasise that their respective contributions regarding any particular astronomical observation (amplified over solar and galactic scales, such as the localised area surveyed by BICEP2) were incompletely known, as already revealed by the Planck data, and which may yet itself be superceded in more detail.

I look forward to Dr Westphal’s response. Given the complex make-up of the few IS candidates so far, I’d imagine their polarizing properties could be of some interest, although being embedded in aerogel might pose a practical problem.
Not to mention what their polarizing behaviour might be when spinning in a magnetic field under whatever radiation and temperature!

No end of future PhD research projects there - just as it should be!
John

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Re: Interstellar dust and BICEP 2

Post by DanZ » Fri May 01, 2015 12:02 pm

Hi John. I'm sitting with Dr. Westphal right now, and the answer is "Great question, and could eventually be studied, but at this time we don't have instruments in our labs at this time that would be able to address the question." But he's happy to discuss this with you more if you care to join us on the Thursday Telecons. Best answer for now I'm afraid. Dan

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