---------------------------------------------------------------------- --- Log opened Mon Jan 21 00:00:04 2008 [...] 13:59 -!- nicolas [...] has joined #cosmo 14:05 -!- You're now known as boud 14:05 -!- mode/#cosmo [+o nicolas] by boud 14:05 <@boud> nicolas salut :) 14:05 <@nicolas> bonjour 14:06 <@nicolas> (non je ne me suis pas trompé j'ai eu peur d'un quelconque décalage horaire) 14:06 <@boud> :P 14:06 <@nicolas> je t'ai envoyé un mail tout ? l'heure aussi 14:06 <@boud> je vois que bruno est venu a 11:23, sans doute pour tester, et il a quitte a 11:27 14:07 -!- Bruno [...] has joined #cosmo 14:07 < Bruno> hello there 14:07 <@boud> Bruno hi :) 14:07 <@nicolas> Hello 14:08 < Bruno> Hello Nicolas, . Gaudin, i presume? 14:08 <@nicolas> yes 14:09 < Bruno> well gentlemen, thanks for being willing to be interviewed 14:09 <@boud> alstublieft 14:09 * boud not sure of the spelling :P 14:09 < Bruno> ?t's perfect, polite version even 14:10 <@boud> :) 14:10 < Bruno> this channel is new to me, do you know if a log is automatically saved? 14:10 <@boud> i was just going to say that - i am logging 14:10 <@boud> one or both of you can log too, depending on what client you use 14:11 <@boud> we should decide if we want the log to be private among us, or should it be considered public 14:12 <@boud> bruno ? 14:12 < Bruno> anything is fine by me, but obviously I will quote (very little) parts of it in the story, OK 14:12 < Bruno> ? 14:13 <@nicolas> it is boud who decides, I just follow the discuss .. 14:13 <@boud> ok, in that case i propose that after the end of the discussion, i post a copy somewhere "public" e.g. to the http://cosmo.torun.pl/mailman/listinfo/cosmo-media mailing list 14:13 < Bruno> ok 14:14 <@boud> if someone accidentally says personal/private type info, please say so and i'll remove it from the log before posting 14:14 < Bruno> sure 14:14 <@boud> ok, i'm ready for questions... 14:15 < Bruno> well, is the universe finite or not? 14:15 <@boud> i don't know 14:15 <@boud> i would *like* to know that and we have done work towards answering that question, but that's not the same thing 14:16 < Bruno> what would you argue, after publishing/posting your last paper? 14:17 <@boud> scientifically, i would say just what is said in our paper 14:17 <@boud> i guess you want a "popular science" compressed version...? 14:19 <@boud> maybe something like "the evidence seems to be cumulating in favour of a finite (PDS) universe" 14:20 < Bruno> well, let me put it this way: the diagrams in the paper look pretty clear to me 14:20 <@boud> thanks :) 14:20 < Bruno> what would they look like if there was no PDS signal at all? 14:21 <@boud> to give a fully quantitative answer to that question would/will require a lot of work 14:22 < Bruno> but would you expect something like a pretty evenly distributed cloud of dots? 14:22 <@boud> intuitively, yes 14:22 <@boud> probably not totally evenly distributed 14:23 <@boud> but i didn't expect such a strong concentration 14:23 < Bruno> right 14:23 <@boud> however, my intuition does not always much a full, detailed calculation 14:24 < Bruno> in what essential way does this result differ from the 2004 result? 14:24 <@boud> probably several ways: 14:25 <@boud> (1) we use a much larger amount of the data (temperature fluctuations in WMAP map) and should be less sensitive to small errors 14:26 <@boud> (2) by allowing for arbitrary twist angle, we are able to use a fairly simple method of estimating the probability 14:26 <@boud> (2 continued) in the 2004 result, we didn't calculate a formal probability or other statistical statement - in this case we did 14:27 <@boud> (3) using the new method, the 2004 result is not a stable solution and leads (through the Markov Chains) to the new result 14:28 <@boud> ok i think that's the main 3 differences 14:29 <@boud> correction to a line above: "match" not "much" - however, my intuition does not always match a full, detailed calculation 14:30 < Bruno> Roger 14:32 < Bruno> could you please explain a bit on (3): has the PDS orientation moved since then? If so, how? 14:33 <@boud> the coordinates of the face centres of the fundamental domain of the PDS (the dodecahedron) are listed in the abstracts of the two articles 14:33 <@boud> l = galactic longitude, b = galactic latitude 14:33 <@boud> they are two very different sets of coordinates 14:34 <@boud> also, in the 2004 article the twist was -36 degrees; in the new article it is +39 \pm 3 degrees 14:35 <@boud> i'm not totally sure if this is what you mean by "moved" ? 14:36 < Bruno> well, they didn't really move, probably, but is there an explanation why the two calculations came out so differently? 14:36 < Bruno> please replace 'probably' with 'of course' 14:36 <@boud> i think that (1) is probably the reason 14:36 < Bruno> right 14:37 < Bruno> what does alpha =~20 mean (if we suppose there is a PDS universe)? 14:38 <@boud> this is discussed in section 4.2 14:38 <@boud> it means that this method probably does not give an accurate estimate of alpha - the matched circle size 14:39 <@boud> have a look at Fig 9 14:39 < Bruno> OK 14:39 <@boud> let's suppose that the alpha in the diagram is the true alpha and that the PDS model is correct 14:40 < Bruno> yes 14:41 <@boud> the matched circle in this diagram consists (in the plane of the paper or your screen) of just two points 14:41 <@boud> the two points where the two spheres intersect 14:41 < Bruno> hm i understand 14:41 <@boud> ok so far ? 14:41 <@boud> ok 14:41 <@boud> now look at the pair of points P and P' 14:42 <@boud> they are also "fairly" close to one another 14:42 <@boud> they're not exactly at the same point, but they're close 14:43 <@boud> so, again under the assumption that we have the correct model here with the correct alpha, the temperature fluctuations at P and P' should be "nearly" the same - they should be well correlated 14:43 < Bruno> ok 14:43 < Bruno> is alpha is small 14:43 < Bruno> IF alpha is small, i mean 14:43 <@boud> yes 14:43 <@boud> if alpha is big, the argument is wrong 14:44 < Bruno> ok 14:44 <@boud> now, the whole idea of using the cross-correlation is that we will include pairs which are "close" such as P and P', not just points which are "exactly" at the same position 14:45 <@boud> now imagine shifting the left sphere a bit to the left - keep the right sphere stationary 14:45 < Bruno> ok, i thought you were matching up rings, or is that the same? 14:46 < Bruno> OK, i shifted the left sphere 14:46 <@boud> "rings" was what i gave the NS reporter at the last minute when she wanted to write "circles" 14:47 <@boud> i'll get back to "rings" in a moment... 14:47 < Bruno> these reporters...but that's what I understood from the 1999 scientific american paper by luminet as well, 14:47 <@boud> now that you have shifted the left sphere further to the left, alpha is smaller 14:47 < Bruno> OK 14:47 < Bruno> yes.. 14:48 <@boud> and P and P' are still close - in fact, they are even closer to one another than before 14:48 < Bruno> uhuh 14:48 <@boud> since our method uses all pairs of points which are "close", we will find a good correlation for this new, smaller alpha, even though it's smaller than the true alpha 14:49 < Bruno> I see 14:50 <@boud> so if the true alpha is, say, 40 degrees, then we should find high cross-correlations not only for alpha = 40 deg, but also for many smaller values of alpha 14:50 <@boud> and also for alpha values a little (but not too much) bigger 14:51 <@boud> in the figure caption we write this as "approximately matched annuli" or "approximately matched discs" - if the true alpha is small enough, then it the full discs (including points P and P') will be matched 14:51 <@boud> and that gets us to "rings" - "ring" is what i suggested to the NS reporter as a less frightening (to the lay public) word than "annuli" 14:52 <@boud> and in fact the latin root of "annulus" is "ring" (in modern French we have "anneau" for english "ring") 14:53 <@boud> we haven't made a proper model of this, so that why we say our estimate of alpha should be considered "~" as in "with a big uncertainty" 14:54 < Bruno> allright, so but to make sure I understand, does the MCMC look for matched rings/annuli or disks then? (sorry to be dim, if I am) 14:55 <@boud> (you're not dim - it's the fault of the NS reporter/method-of-journalism...) 14:56 <@boud> the MCMC is certainly *not* defined to look for matched annuli/disks 14:57 <@boud> however, to *some* extent what it really does is probably partly equivalent to looking for matched annuli/disks 14:58 <@boud> it looks for high correlations based on the set of all points which are "close" under a PDS assumption at a given orientation, circle size and twist 14:59 <@boud> "close" can be in any direction, but of course it's related to the PDS geometry/topology 15:01 < Bruno> close in the sense you just explained, right? 15:01 <@boud> yes 15:02 <@boud> figs 2 and 3 show this 15:03 < Bruno> OK, i'm getting there 15:04 <@boud> i think the idea of "matched annuli/disks" is a useful idea, but there would be several complications to relate it to what we did - it could probably be the subject of a whole new article... 15:06 < Bruno> allright, so i'll probably stick with something more abstract But the idea that two intersecting spheres would yield a matching ring, even in the PDS model is OK? 15:06 <@nicolas> so boud, the corellation fonction, evaluating between two "distance" does that all points are in an annuli, no ? 15:06 <@nicolas> (I am not sure for my english, sorry) 15:09 <@boud> nicolas : i'm not totally sure - maybe, maybe not... 15:10 <@boud> bruno: i agree that two intersecting spheres for the PDS model should yield "approximately" matching rings, but there is a complication here: 15:10 <@boud> start with two spheres - these define a pair of matching circles 15:11 < Bruno> yes.. 15:11 <@boud> now we allow these circles to thicken - so they become annuli 15:11 <@boud> however... 15:12 <@boud> what we see in Fig 9 is that they thicken slowly outwards, but faster inwards, as we increase from "zero separation" to "small separation = 'close'" 15:13 <@boud> if we imagine painting some lines along the matched circles (like in the middle of a road), and then we look at the "nearly" matching annuli, we will see that the "middle of the road marker" is not in the middle of the annulus, it's closer to the outside limit of the annulus 15:14 < Bruno> ok 15:18 < Bruno> ok, it's intricate,I understand. Now a perfect match in the CMB would mean that the SLS at t=380.000 years doesn't fit in the universe dodecahedron (assuming PDS), so it 'crosses itself', yes? 15:21 <@boud> i think that's close enough to correct, except that better than "universe dodecahedron" would be "fundamental domain" or "one copy of the fundamental spherical dodecahedron", though i realise that that might confuse your non-specialist readers... 15:21 <@boud> here, "spherical" means "inside of a 3-space with positive curvature" 15:21 <@boud> it doesn't mean "like a 2-sphere" 15:22 < Bruno> yes sir, i have to cut corners somewhere, not only for myself 15:22 <@boud> you could probably say "fundamental dodecahedron" 15:22 < Bruno> I will 15:23 < Bruno> Another thing, I know it's early days, but any word from A&A? And responses from other colleagues? 15:23 <@boud> we're waiting for a referee report 15:24 <@boud> other colleagues have said they're interested and are reading our paper... 15:24 <@boud> the time scale for this is months, not days 15:25 < Bruno> i know 15:25 <@boud> (it takes time to properly analyse stuff, check calculations, recheck, etc. etc. and then write it up consistently with what you've really done...) 15:26 < Bruno> now is it a problem that you have phi=39 plm 2 instead of 36? 15:27 <@boud> not at all 15:27 <@boud> one sigma difference is reasonable - that's what uncertainty estimates are all about 15:27 < Bruno> OK 15:28 <@boud> if it turns into 39 pm 0.01 , then of course it would be a problem for the model 15:28 < Bruno> Right, that would really be something exotic. Now what's next? wait for Planck? 15:30 <@boud> well, we do have some ideas of other things we could check, but they're not published yet, and will require a bit of work... 15:30 <@boud> and of course, Planck will come up with some very useful new data 15:30 <@boud> getting back to your question about what colleagues think... 15:31 <@boud> there's a bit of discussion on cosmocoffee about a totally unrelated paper about results from the ACBAR CMB experiment: http://cosmocoffee.info/viewtopic.php?t=1038 15:31 <@boud> they don't actually mention what is one of the more interesting results from the new ACBAR paper 15:32 <@boud> you'll find it on page 14 at the end of section 8.2 of the paper: http://cosmocoffee.info/arxivref.php?file=pdf/0801.1491 15:34 <@boud> they continue to agree with previous results that the best estimate for the curvature of the Universe is a small positive curvature: they get Omega_total = 1.03 (errors: +0.06, -0.04) 15:35 <@boud> this is consistent with a flat universe, but it also seems that now for several years, people doing this sort of work continually get Omega_total in the range 1.015 to 1.02 to 1.03 15:36 < Bruno> which is consistent with PDS.. 15:36 <@boud> yes - on page 19 we discuss this 15:36 <@boud> section 5.7 15:37 <@boud> especially if the true alpha is about 40-50 degrees - which gives about 1.015 to 1.018 for Omega_total 15:38 <@boud> as we discussed earlier, in our own work we don't have a good constraint on alpha, but an alpha value somewhere near the maximum alpha values in Fig. 8 would be reasonable 15:38 <@boud> would be a reasonable interpretation 15:40 < Bruno> Also 40-50 degrees. Do you suppose we'll know for (reasonably) sure after Planck? Some other time limit? 15:42 < Bruno> would 40-50 degrees say anything about the size of the fundamental dodecahedron? 15:42 <@boud> i'll answer the last question first: yes most definitely 15:43 <@boud> alpha is the angular radius of the matched circles 15:43 <@boud> equation (15) gives you the exact formula you need 15:44 <@boud> rSLS is close to 9.5h^{-1} Gpc (where you can put in h = 0.71 = H_0 / 100km/s/Mpc is the Hubble constant) 15:45 <@boud> it only changes slowly if we change Omega_total and/or Omega_matter 15:45 < Bruno> but would be the size at t=380 000 years, right? 15:46 <@boud> yes 15:46 <@boud> well, i'm not sure if it's exactly 380 000 years, i don't usually use a number there... 15:47 < Bruno> Ok,but it has expanded a lot since then, hasn't it? 15:47 <@boud> each local region has locally expanded by a factor of about 1100, yes 15:48 <@boud> however, all of this work is done in comoving coordinates 15:48 <@boud> this is a choice of coordinates in which the universe is static 15:48 <@boud> all the expansion is then put into a variable we call "the scale factor" - usually written a(t) 15:49 <@nicolas> and comoving coordinates = real coordinates only actually 15:50 <@boud> from equation (15), since you have rSLS and alpha, you can deduce R_C 15:50 < Bruno> am I terribly wrong in saying that the fundamental dodecahedron has grown 1100-fold (if it exists)? 15:52 <@boud> you're correct in saying that, but the same thing applies to the SLS, it too has grown something like 1100-fold since when the Universe was ~ 4 x 10^5 years old 15:52 < Bruno> OK 15:53 < Bruno> Well, I think I have probed as far as is possible/useful/bearable for you 15:53 <@boud> nicolas - i would be careful with the word "real" here - from a point of view of local physical experiments, comoving coordinates are just a theoretical construct, and "physical/local/proper" coordinates are more "real" 15:54 <@boud> well, i was just going to finish with eq. 15... 15:54 <@boud> to talk about a "size" of the fundamental dodecahedron, there are a few different ways you can define "size" 15:55 <@boud> but one way is to take the distance from one face centre to the opposite face centre 15:55 < Bruno> yes.. 15:55 <@boud> if we are thinking in R^4, then this is an angle of pi/5 (36 degrees) 15:56 <@boud> so then this "size" is pi/5 * R_C 15:56 <@boud> so then you have it in some length units (e.g. Gpc) 15:58 < Bruno> and it is... 15:59 <@boud> if i've done it right just now in octave, then for alpha = 40-50 degrees i get 13-15 h^{-1} Gpc 16:00 <@boud> if you want you can put h=0.71 in that, as some people do nowadays, but i prefer the old style of using h^{-1}Gpc as the "length unit" 16:01 <@boud> the diameter of the SLS is around 19 h^{-1} Gpc 16:01 < Bruno> You mean 19 Gpc? 16:01 <@boud> no, i mean 19 h^{-1} Gpc 16:02 <@boud> h = H_0 / 100km/s/Mpc is the Hubble constant 16:02 < Bruno> OK, I understand 16:02 <@boud> for most of the XX century, there was a very big uncertainty in the value of h 16:03 <@boud> and it cancels out of many calculations 16:03 <@boud> so in many cases, we don't actually need to use a number there 16:04 <@boud> this is \LaTeX notation h^{-1} = 1/h 16:04 < Bruno> I get it 16:06 < Bruno> Boud, thanks for your patient explanation, I will do my best to put (some of) it to good use in the piece 16:06 <@boud> ok, my pleasure :) 16:07 < Bruno> one more (slightly off-topic) thing: [off-topic thing removed from log] 16:12 < Bruno> OK, boud, I'll be in touch then, Nicolas, goodbye and good luck to you both in getting to grips with the universe 16:12 <@boud> ok, tot siens 16:13 <@nicolas> goodbye 16:13 < Bruno> tot ziens, oant sjen (Frisian) 16:14 -!- Bruno [...] has quit ----------------------------------------------------------------------