Cosmo-media

cosmo-media@cosmo.torun.pl

36 discussions

blogger introduction to cosmology questions
by Boud Roukema 31 Mar '23

31 Mar '23

hi Aleksandra, [cc: cosmo-media public mailing list] > wt., 7 mar 2023 o 09:31 Boud Roukema <...> napisał(a): > > I assume this is your blog: https://medium.com/@szymula.aleksandra > > You're welcome to ask me some questions - email would probably be the > > simplest. > Yes, that is exactly the blog. Thank you very much for giving me this > opportunity. > > These are the questions I'd like to include in this interview: > 1. For a good start, what is exactly the concept of infinity? There are many definitions of infinity [1]. The two infinities that are generally used in practice in cosmology are, informally speaking, \aleph_0, the amount of natural numbers (counting numbers) [2], and 2^{\aleph_0}, the amount of real numbers [3]. In simple words, these two infinities are each the size of a certain set of things. There's an easy proof of why these two infinities are different, related to writing down numbers in the decimal system. [1] https://en.wikipedia.org/wiki/Infinity [2] https://en.wikipedia.org/wiki/Countable_set [3] https://en.wikipedia.org/wiki/Cardinality_of_the_continuum > 2. In your opinion, what is the most mind-bending idea related to the > infinite universe, and how does it challenge our understanding of the > nature of existence? 2a. The infinitely repeating identical worlds paradox is fairly mind-bending :). In a spatially infinite universe, any physical sequence of events with a tiny probability that is allowed by our known laws of physics would have a probability of 1 of occurring somewhere. That doesn't mean that it's certain, just that it's very likely. If the Universe were really infinite, then it would very likely happen somewhere that a planetary system almost identical to ours forms, with an Earth almost identical to ours, and life evolves in almost exactly the same way as it has evolved here, and the Internet evolves and the Fediverse evolves, just like these events have taken place here. In some cases, the events will diverge in a few tiny ways, and gradually diverge more and more. This must be almost certain (the probability is 1) if the Universe is really infinite. You can read some modern descriptions about this paradox infinite universe in [4] or [5]. 2b. Our understanding of the nature of existence is quite limited, so I don't see the infinitely repeating identical worlds paradox as a challenge to it. Infinities imply surprising things. It would be a lot simpler if our Universe were finite - as Ellis & Brundrit mention [4]. [4] Ellis & Brundrit 1979, https://ui.adsabs.harvard.edu/abs/1979QJRAS..20...37E [5] Garriga & Vilenkin 2001, https://ui.adsabs.harvard.edu/abs/2001PhRvD..64d3511G > 3. Is the universe infinite in terms of space and time, or is there an > ultimate boundary? We don't know if the Universe is infinite or not. So far, we have no significant evidence of it being finite. In both finite and infinite models of the Universe, we normally assume that there are no spatial boundaries. > How does this idea challenge traditional concepts of > time and space? The Newtonian cosmological model of infinite space has lots of problems. General-relativistic models make it a lot easier to have spatially finite models. > 4. Can we ever truly comprehend the vastness of the universe? Unlikely. :) > And what are some of the most interesting phenomena that suggest its > infinite nature? I don't see any evidence of the Universe being infinite. Our current standard model, if interpreted literally, is of an infinite universe, but the standard model is not really taken seriously as a complete model - it's just a fairly good model of all of our observations. > 5. Now, how do you approach the task of visualizing or representing the > infinite universe? How has technology aided in our exploration ability? Either a finite or infinite universe is represented cognitively through a lot of work using 2-dimensional analogies, drawing diagrams, making calculations, and developing symbolic mathematical skills and intuition of differential geometry and of the topology of 3-dimensional manifolds. My guess is that by "technology" you mean software. The best software for cosmology is FOSS (free and open source software), since when the software is *not* free-software-licensed, we're not allowed to understand the software or correct bugs or improve it. FOSS helps for playing around with calculations, both with numerical calculations (with numbers) and for symbolic algebra (SageMath, Maxima). > 6. There are many theories about how our universe works and all of them > sound intriguing, as well as confusing. Since there is so many concepts, > how do various theories of the universe, such as the Big Bang, the parallel > universe, the multiverse hypothesis, or the concept of fractal universe, > contribute to our understanding of its infinite nature? How do these > theories relate to each other? The Hot Big Bang model is the standard model of the Universe developed to match general relativity to three observations - the expansion of the Universe, the cosmic microwave background, and nucleosynthesis - roughly from 1917 to becoming widely accepted from the 1960s to the early 1990s. The next level of detail is the LambdaCDM model - roughly from the 1980s to becoming well accepted by the 2000s. This is currently the standard cosmological model. Parallel universes and multiverse models are speculation that are nowhere near having any observational support. A fractal structure of the spatial distribution of galaxies is to some degree a fair approximation on some scales - it's unlikely to change standard cosmology last time I checked it. > 7. I mentioned the multiverse universe theory, and I have to say that is > one of my favorite concepts. Based on that I have to ask, what is the most > compelling evidence for the multiverse theory, and how does it relate to > the infinite universe concept? I don't work on multiverse theory, so I couldn't say. Max Tegmark got quite famous for this and might want to talk about that. :) > 8. What implications does the infinite universe have on the search for > extraterrestrial life? Completely irrelevant. We're only searching for extraterrestial life in a finite part of the Universe. The most interesting would be signs of extraterrestial life within a few parsecs, since in that case two-way communication within our lifetimes could be possible. > 9. Can we ever hope to travel to other parts of the universe given its > infinite nature? We have no evidence that the Universe is spatially infinite, but even if we did, that would be irrelevant for space travel. However, space travel by humans is a low priority. The current scientific priority is handling the climate emergency [6]. If there is not too much debris in Low Earth Orbit (LEO) once we get into a stable maximums of 2 deg C post-industrial warming (I'm being optimistic), maybe in the 2050s or 2070s or so, then planning on sending some space arks on multi-decade (or multi-century) journeys to neighbouring stellar systems *might* become realistic and ethically justified. [6] https://web.archive.org/web/20230320135908/https://report.ipcc.ch/ar6syr/pd… > (10). Finally, what is the most promising avenue for exploring the infinite > universe? The most interesting thing is looking for evidence that our Universe is finite, rather than infinite. Space in the standard cosmological model has both curvature and topology: either positive curvature (like a 2-sphere) or a multiply connected topology (like a 2-torus) would be examples where our Universe would be spatially finite. One of the recently discovered methods of detecting signs of cosmic topology is topological acceleration [7][8] - by observing the movements of galaxies carefully enough, we could, in principle, detect signs of the global shape of the Universe - curvature and topology. > What do you hope to achieve throughout your research? It would be great to discover evidence of topological acceleration. :) However, that's not guaranteed. What is more likely is that the modelling work and observations and observational analyses needed to get there will lead to many related, possibly unexpected, side discoveries in the details of the formation of galaxies and the structure of the Universe. [7] https://ui.adsabs.harvard.edu/abs/2007A%2526A...463..861R [8] https://arxiv.org/abs/2201.09102 All of the above information is public - the latest observational and theoretical work can all be found on ArXiv [9]. If you see a press release about a "new" result or idea and there's no corresponding research paper about it on ArXiv, then it's unlikely to be serious research. In principle cosmology can be self-taught - i.e. without attending university lectures - there's a huge amount of open-access information. However, in practice, university education is unavoidable. Good students generally take quite a bit of time (several years) to work through the standard material. Proposing new hypotheses in cosmology requires first understanding standard cosmology. [9] https://arxiv.org/list/astro-ph.CO/recent Cheers Boud

1 1

mirror matter and the Hubble constant - Cyr-Racine et al
by Boud Roukema 14 Jun '22

14 Jun '22

Dzień dobry, [cc cosmo-media mailing list] Napiszę po angielsku, ale możemy zmienić między językami jak będzie potrzebny. Apparently you are interested in the article Cyr-Racine et al 2022: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.201301. The article presents a rather speculative, but interesting, hypothesis about mirror matter, offering a possible explanation of the Hubble constant puzzle. The article starts with a mathematical rescaling aimed at understanding the relation between the standard cosmological and observations, and proposes that this purely mathematical exercise could have a physical interpretation in terms of mirror matter. The first paragraph on page 201301-3 gives a brief description of "mirror matter", and how (if it exists) it would relate to ordinary matter. Figure 1 shows that the authors' mirror matter model would be consistent with the statistics of Planck observations of the cosmic microwave background. Figure 2 shows that the model could provide a high value of the Hubble constant, of about 73 km/s/Mpc, and what the values of other parameters would have to be for the model to be observationally viable. * Figure 2 bottom-left and bottom-right show values of parameters of the model that are most likely not observable. * Figure 2 top-right shows the value of the helium abundance Y_p that would be required, and the text (left-hand column of page 201301-4) gives the numbers involved. The helium abundance is well-constrained observationally, and the text acknowledges that a bit more speculation regarding properties of the model is needed in order for the model to become observationally viable - this is discussed more on page 201301-4. In other words, the model does not quite work, and the authors suggest future calculations that might help. Personally, I find a more likely explanation of the Hubble constant puzzle to be much simpler, related to the fact that the Universe is inhomogeneous. In terms of real observations, the local part of the Universe that we observe is necessarily smaller than more distant parts of the Universe; as a first approximation, this is a simple geometric effect. Deviations from the average are generally higher in smaller spatial regions. Our observations (made within our lifetime) necessarily come from light that passes through our local Universe - which is not quite the same as an "average" part of the Universe. So deviations on small scales are not so surprising. The difficulty is doing the modelling correctly, which is an ongoing research task. You could look at these some of the following articles by our cosmology group for more details of this class of explanations for dark energy and for the Hubble constant puzzle, although this work is still research in progress: * https://ui.adsabs.harvard.edu/abs/2013JCAP...10..043R * https://ui.adsabs.harvard.edu/abs/2013IJMPD..2241018R * https://ui.adsabs.harvard.edu/abs/2015CQGra..32u5021B * https://ui.adsabs.harvard.edu/abs/2016IJMPD..2530007B Cheers Boud

1 0

'Science et Vie' : prospectifs Planck/topologie cosmique
by Boud Roukema 11 Jan '12

11 Jan '12

Bonjour Mathieu, (1) L'image à l'URL suivant montre, sur une carte tout-ciel (projection azimutale équivalente de Lambert), la meilleure solution en position céleste des six axes du modèle dodecaèdral selon les données de WMAP : http://cosmo.torun.pl/foswiki/pub/Cosmo/CosmoProjects/RBSG08_CCBYSA_GPL_GFD… Tel qu'indiqué dans son nom, l'image est republiable sous une licence au choix - GPL, GFDL ou CC-BY-SA [1]. Elle est refaite indépendamment des figures de nos articles scientifiques, afin d'éviter des ennuis de complications des droits d'auteur. Elle est correspond à la Fig. 5a de RBSG08 (Astronomy & Astrophysics 486 (2008) 55, http://arxiv.org/abs/0801.0006), où nous utilisions une nouvelle méthode - celle des corrélations croisées. Dans un sens, l'image est une prédiction de ce que l'on attend d'une analyse semblable des cartes de Planck, car c'est ce qui sort des données de WMAP. Si c'est le bon modèle, Planck devrait le confirmer. (2) La signification statistique de notre deuxième analyse avec la méthode des corrélations croisées (RBG08, Astronomy & Astrophysics 492 (2008) 673, http://arxiv.org/abs/0807.4260) est indiquée dans l'abstract : "We infer that for an infinite, flat, cosmic concordance model with Gaussian random fluctuations, the chance of finding _both_ (a) a large scale auto-correlation as weak as that observed, _and_ (b) a PDS-like signal similar to that observed is less than about 0.015% to 1.25%." (3) Le rôle le plus important de Planck serait probablement de donner plus de confiance dans l'analyse et dans la soustraction de l'avant-plan galactique surtout sur les grandes échelles, et de confirmer - ou réfuter - le manque de structures aux échelles au-dessus de 10 Gpc/h (voir les analyses de Copi et al 2007, 2009, 2011, citées dans RK11: Astronomy & Astrophysics 533 (2011) A11, http://arXiv.org/abs/1106.0727) ou la Fig. 1 de RBG08. Le manque de structures sur les plus grandes échelles a été prédit par Starobinsky et Stevens et al en 1993 (références dans RBSG08) et c'est ce qui est observé - si nous acceptons les données CMB de WMAP. Il y a nombreuses incertitudes qui compliquent l'analyse des données du fond diffus pour l'analyse de la forme de l'Univers. D'après notre analyse la plus récente (RK11), les toutes premières galaxies les plus rares devraient être topologiquement lentillées - dans certaines positions - aux redshifts de z = 106 \pm 18 environ. L'existence (avec étoiles allumées) de telles galaxies rares est actuellement à la limite théorique, et leur détection observationnelle dans les ondes sous-millimétriques est bien hypothétique. Pourtant, le lentillement topologique des objets effondrés gravitationnellement serait moins assujetti au doute que l'analyse du fond diffus. Bien cordialement Boud Roukema [1] http://creativecommons.org/licenses/by-sa/3.0/

1 0

PROPOSAL for a reply
by Kristen Eisenberg 19 Oct '11

19 Oct '11

hi everyone, i've tried to come up with a proposal compatible with the summary here: http://cosmo.torun.pl/pipermail/cosmo-torun/2006-February/000353.html and Andrzej's suggestion here: http://cosmo.torun.pl/pipermail/cosmo-torun/2006-February/000354.html The proposed text is between ######### - i'm not sure if Bartek or Andrzej want to sign, or even if this is compatible with Andrzej's arguments, though i think it should be, because all the material is protected (well, open) under GFDL copyright, but it's not especially linked to any of our names - there are typically anywhere between 5 and 100 authors on the wikipedia page articles. The LambdaCDM article presently says nothing about "bringing down the concept of flat and infinite universe" - which IMHO is rather premature anyway - IMHO that wikipedia page would be a good place to discuss that - in the wikipedia context - *if* anyone really wants to have a "public, pedagogical" debate. Kristen Eisenberg Billige Flüge Marketing GmbH Emanuelstr. 3, 10317 Berlin Deutschland Telefon: +49 (33) 5310967 Email: utebachmeier at gmail.com Site: http://flug.airego.de - Billige Flüge vergleichen

2 1

tytul wywiadu a karta etyczna mediow
by Boud Roukema 17 Aug '09

17 Aug '09

witam Wiedza i Życie, Marek Oramus przygotował wywiad z mną, wynikiem wielu dni wielu godzin dyskusji mejlowe i na żywo, i jeśli dobrze rozumiem, będzie dziś Was to wysyłać jako propozycję artykułu. Się cieszę, że mogłbyście publikować popularno-naukowo artykuł o Wszechświecie, ponieważ wydaje mi się, że interesowałby Wasz czytelników. Mam jednakże uwagę dotyczący zaproponowanego tytułu, ,,Wszechświat jako piłka futbolowa". Jest to problematyczny dla wielu powodów: * Tytuł powinno ściągniąć uwagi czytelniki, ale nie na kosztu błędów pedagogicznych, wprowadzających czytelniku do błędnego zrozumienia: * futbol nie daje żadna intuicja dotycząca brak granic, jednakże brak granic to pojęcie kludzowe; * futbol nie daje żadna intuicja dotycząca całości przestrzeni (prawdy pomysł) zamiast coś wewnątrz przestrzenie (błędny pomysł, dotyczący modelu Wszechświata), raczej powrotne - wszyscy mają intuicję futbolu jako obiekt, który dużo się rusze w przestrzeni. * futbol ma 32 ściany, tzn wiele więcej niż 12-ściana (domen fundamentalny modelu przestrzeni dwunaśtościanowej Poincarego ma dokładnie 12 ściany), jest zewnętrznie 2-sferycna wersja 20-ściany z obciętymi rogami; to prawda, że kilka z ścian futbolu są piątokątami (te czarnymi), ale gdyby chcielibyśmy zmienić futbol do 12-ściany, mielibyśmy kroić futbol w takim sposobie, gdzie usuniemy całych te 12 widocznych ścian piątokątymi. O tym drugim punkcie, jak na pewno dobrze wiedzcie, ponieważ publikowaliście artykuły popularno-naukowe od dawna, prezentacji Wszechświat Big-bangowy jako obiekt *wewnątrz* przestrzeń, to jeden z głównych błęd w popularizacji kosmologii. Przypominam o kartą etyczną mediów z Stowarzyszenia Dziennikarzy Polskich z 29 marca 1995r.: http://pl.wikipedia.org/wiki/Karta_Etyczna_Mediów http://www.radaetykimediow.pl/dokumenty_kmp1.html Zobacz ,,zasada prawda" a także ,,Zasada pierwszeństwa dobra odbiorcy": ,,co znaczy, że podstawowe prawa czytelników, widzów i słuchaczy są nadrzędne wobec redakcji, dziennikarzy, wydawców, producentów i nadawców." Tzn, że prawa czytelników do tytułu, który mówi uczciwie co jest w temacie artykułu, jest ,,nadrzędne wobec redakcji, dziennikarzy, wydawców, producentów i nadawców." Ja we wywiadu nigdy polecałem piłka futbolu jako pomoc dla zrozumienie o co chodzi tematu. Nie widzę, że precedens łamania praw czytelników w niektórach czasopismach w USA i UK - tzn twierdząc, że model przestrzeni dwunastościanowej Poincarego, to podobny do ,,piłka futbolu" - to dobry argument powtórzyć ten łamania karty etycznej mediów cytowana wyższej. Kilka przykładowe pomysły dla tytułu, który mogłby być etycznie zasadzone: * ,,Wszechświat skończony bez granic" * ,,Mały Wszechświat bez granic" * ,,Wszechświat bez granic" * ,,Kształt Wszechświata bez granic" * ,,Wszechświat z prostymi, zamkniętymi pętlami" ale to tylko przykłady. Pozdrawiam serdecznie Boud Roukema CC: * Marek Demianski, członek Rada Naukowa Wiedza i Życie, i ekspert naukowy w temacie kształtu Wszechświata; * cosmo-media, *PUBLICZNIE* archiwizowana, otwarta grupa dyskusyjna kosmologów toruńskich: http://adjani.astro.uni.torun.pl/mailman/listinfo/cosmo-media * Marek Oramus

1 0

l'espace de Poincare : mise au jour [was: Re: Information request]
by Boud Roukema 04 Feb '09

04 Feb '09

Bonjour David, > From dlarousserie(a)sciences-et-avenir.com Tue Jan 20 10:43:18 2009 > Dear Boud Roukema, > > I am (always) journalist in France for Sciences et Avenir. I had contacted > you in 2003 about the shape of the universe. Oui, oui, je m'en souviens. :) > I do not know if you are still involved in such topic. But I have to write Tout à fait. > (shortly) the story of this research and I need to upgrade my information. > That's why I hope you could help me (Sorry to write in english because I do > not remember if you speak french...). En tant que Français expatrié, il serait dommage si j'oubliais totalement la langue de Molière... > Here are my questions. > How could you summarize the state of the art concerning this topic ? Les deux premières motivations empiriques pour l'hypothèse de l'espace de Poincaré - le problème des fluctuations manquantes et la courbure légèrement positive (Omega_totale = densité totale par rapport à la densité critique = environ 1.01 ou 1.02 , plus ou moins 0.01 ou 0.02 - sont devenues plutot renforcées qu'affaiblies. Or, si l'on trouvait que Omega_totale = 1.002 +- 0.001, l'espace de Poincare serait rejeté a une très haute signification statistique. Pour l'instant, nous sommes très loin de là. Aussi, Caillerie et al. (dont JP Luminet) ont fait une analyse semblable à leur première publiée dans Nature, mais avec beaucoup plus de précision (plus de modes propres, des harmoniques de l'espace de Poincaré), soutenant leur premier résultat. En 2008, le groupe torunien [1] [2] a publié les résultats d'un nouveau test de l'hypothèse de l'espace dodecaèdral de Poincaré, qui étend le principe des cercles identifiés. Un problème pratique du principe des cercles identifiés est que l'analyse est sensible aux petites erreurs d'angle sur le ciel, aux contaminations du signal sur les échelles relativement petites, ou à d'autres effets mal compris sur les « petites » échelles. De plus, le problème des fluctuations manquantes se voit sur les échelles angulaires sur le fond diffus au-dessus de 60 degrés. Il est donc raisonnable de chercher une méthode qui est moins sensible aux petites échelles. Le principe des cercles identifiés explique exactement quelles paires de points sur le ciel devraient être, chacune, une paire de deux images d'un seul point physique. L'extension de l'idée est de considérer non seulement deux points sur le ciel correspondant à un seul point physique, mais plutôt deux points sur le ciel qui correspondent à deux points proches, sans être (en général) identiques. Puisque deux points proches sont (statistiquement) fortement corrélés et deux points lointains sont (statistiquement) faiblement corrélés, l'on peut chercher des orientations et tailles du dodecaèdre qui impliquent des ensembles de paires de points, dont les deux membres de chaque paire sont apparemment distants entre eux sur le ciel mais sont fortement corrélés, comme si elles étaient réellement proches, l'une de l'autre. Cette methode prédit deux choses pour l'hypothèse de l'espace Poincaré. D'abord, il devrait exister une orientation et une taille optimales qui donnent des hautes corrélations entre certains points apparemment éloignées entre eux. Deuxièmement, en comparant les points sur un côté du ciel avec l'autre, l'espace de Poincaré oblige qu'une rotation de 36 degrés dans un sens ou l'autre soit appliquée. Or, si l'on permet que la rotation ait une valeur arbitraire, l'on pourrait attendre à ce que des corrélations par hasard favorisent un angle de rotation quelconque, avec peu de chance qu'il soit proche de 36 degrés ou -36 degres. L'analyse des données de WMAP montre que les deux prédictions sont confirmées : la meilleure position en coordonnées astronomiques impique une relativement forte corrélation entre les points très éloignés entre eux sur le ciel, et l'angle de rotation favorisé est 39 +- 2.5 degrés. L'hypothèse de l'espace de Poincaré n'a pas seulement réussi au test, mais elle est devenue plus contrainte, puisque maintenant nous argumentons que si l'hypothèse est la bonne, les coordonnées de ses six axes devraient être à quelques degrés près celles publiées dans nos articles de 2008 [1], [2]. Ces valeurs changent très peu en analysant les différentes versions de la carte de WMAP (ILC vs Tegmark et al.; avec ou sans masque pour cacher le plan de notre Galaxie; données de 3 ans versus données de 5 ans). Indépendamment, sur le côté théorique, où contrairement au lien entre la relativité générale et la courbure, il n'existe pas de théorie liant la topologie de l'Univers à une autre propriété physique, il y a eu au moins un développement inattendu. Jusqu'ici, nous disions dans la communauté de la topologie cosmique que la topologie de l'espace n'avait aucun effet sur les équations d'Einstein. Or, dans un article théorique du groupe torunien [3], nous avons montré qu'au moins dans certaines conditions et en utilisant la limite de gravité faible (newtonienne), la topologie peut bien avoir un effet sur la dynamique. Résumé grand public très court : Imaginons que tu es proche d'un grand amas de galaxies à ta « droite » qui t'attire par la force de gravité. Puisque l'Univers est multi-connexe, il y a deux copies identiques de cet amas, une lointaine à ta « gauche » et une lointaine à ta « droite ». Ces deux copies aussi t'attirent vers eux, mais très faiblement. Les distances à ces deux copies sont presque égales, parce que tu es proche de la « première » copie de l'amas. Donc, ces deux tirements faibles dans les deux directions opposées s'annulent - presque. Pourtant, ils ne s'annulent pas totalement. L'amas « à gauche » est un peu plus près parce que tu es un petit peu « à gauche » de la « première » copie. La petite force qui reste après l'annulation des attractions de ces deux copies distantes te tire, donc, vers la gauche. Si tu étais encore un peu plus éloigné de la première copie, cette petite force non-annulée serait un petit peu plus fort. Ce qui est surprenant est non seulement que cette force résiduelle existe, mais qu'elle est proportionnelle à ton déplacement par rapport à l'amas de galaxies. C'est un comportement algébriquement semblable à celui d'une constant cosmologique ! On ne peut pas pourtant en conclure que la constante cosmologique soit un effet de la topologie de l'Univers - l'effet aujourd'hui serait à peu près un milliard de fois trop faible. Mais il reste très intéressant qu'un lien physique entre la topologie et la gravité peut exister lorsque nous pensions qu'il n'y en avait pas, au moins pas de façon si simple. > When Plank will give data, will you try to test once again special topology Tout probablement. :) > or are you working on different topic ? > Do you think that this data could solve the problem ? La contrainte la plus intéressante de Planck serait à mon humble avis celle sur la courbure de l'Univers obtenue en analysant ensemble les cartes faites par Planck et les mesures des BAO (oscillations acoustiques baryoniques, voir par exemple [5], lorsque le nom BAO n'était pas encore utilisé) faites par le télescope SKA (Square Kilometre Array) et des sondages profonds par exemple par un WFMOS (Wide-Field Multi-Object Spectrograph) sur le VLT ou Subaru. Si l'on pourrait contraindre le paramètre de densité totale à, par exemple, Omega_totale = 1.002 +- 0.001, l'espace de Poincaré serait nettement réfuté, puisque avec les analyses des groupes Luminet et al. et Aurich et al., et les nôtres, ce paramètre doit forcément être au-dessus de 1.010 et plutôt serait vers 1.013 à 1.018. > Are there another alternative exept the Poincare shape ? Oui, mais les autres espaces de courbure positive valables ont des propriétés de symmétrie moins belles que celles de l'espace de Poincaré, et les espaces plats (par exemple, le tore) souffriraient de l'argument (théorique/intuitif) de fine-tuning. Les jugements subjectifs du rasoir d'Occam jouent un rôle ici - quels modèles sont les plus simples ? L'espace de Poincaré est un espace plus simple et symmétrique que plusieurs autres (dont l'espace euclidéen infini, puisque l'infini n'est pas du tout simple). > I think your last publication is compatible with such topology. Does someone > else confirm these idea ? > And on the contrary does someone criticize your work ? J'imagine que tu parles de [1] et [2]. A ma connaissance, pour l'instant personne autre que Ralf Aurich a essayé notre méthode étendant le principe des cercles identifiés. Aurich l'a appliqué pour le cas du tore, plutôt que pour l'espace de Poincaré. > Could you remember the main motivation of such work please ? Les motivations principales : (1) tester les prédictions de l'hypothèse de l'espace dodecaèdral de Poincaré (voir au-dessus); et (2) rendre l'hypothèse plus concrète en déterminant quelles devraient être ses coordonnées astronomiques sur le ciel. Bien amicalement, boud PS: Par souci de transparence médiatique, j'envois une copie de ce mél vers la liste avec archives publiques : http://cosmo.torun.pl/mailman/listinfo/cosmo-media Bibliographie: [1] Roukema et al. http://fr.arxiv.org/abs/0801.0006 2008, Astronomy & Astrophysics 486, 55 [2] Roukema et al. http://fr.arxiv.org/abs/0807.4260 2008, Astronomy & Astrophysics 492, 673 [3] Roukema et al. http://fr.arXiv.org/abs/astro-ph/0602159 2007, Astronomy & Astrophysics 463, 861 [4] Aurich http://arxiv.org/abs/0803.2130 2008, CQG, 25, 225017 [5] Roukema, Mamon, Bajtlik, http://fr.arXiv.org/abs/astro-ph/0106135 2002, Astronomy & Astrophysics 382, 387

2 2

Re: [Cosmo-media] Matched circles
by Boud Roukema 09 Apr '08

09 Apr '08

Hi Paul, SERIOUS COMMENTS: (1) "Six pairs of matched annuli crystalised out of the microwave hiss - their orientation on the sky corresponding to the 12 faces of a dodecahedron." It certainly sounds nice and reads well. :)) However... The first sentence here is misleading: we searched for the best orientation of a PDS model, while this sentence suggests that the dodecahedral configuration was a *result* rather than an *assumption* of our method. i would suggest something like: "Their search method for the best orientation on the sky for the 12 faces of a dodecahedron homed in onto one particular, self-consistent set of directions." i haven't tried to put any "spice" back into the sentence - that's your (difficult!) task of trying to do it while retaining scientific accuracy. (2) "Roukema's team admit..." We quite overtly (including in the abstract) state the chance of our twist angle occurring by chance in a non-PDS model and the method by which we calculate it. So it's not an admission - it's part of our research results. How about "Roukema's team themselves stated/said that..." ? [The New Scientist also used the word "admit" - that's their inaccuracy, not ours.] (3) "a team ... claimed to have found 12 matched circles in the CMB" This is misleading. The paragraph i gave you previously was: http://cosmo.torun.pl/pipermail/cosmo-media/2008-February/000073.html > In our new paper we use a different (though related) method, which we > think is more robust than the matched circles method - in some sense > you could say that it uses thickened matched circles, i.e. annuli, > even though this is an *interpretation* of the method, not the > calculation method itself. Since we did find a best solution, and that > solution has the correct twist angle within the uncertainty estimate, > and since the cross-correlations for that solution are strong, you > could loosely say that yes, there really are matched annuli in the > WMAP data. This is not the same thing as "claimed to have found 12 matched circles in the CMB". If you want to be accurate, i suggest something like: "a team ... claimed to have found what could be loosely said to be 12 thickened matched circles" i don't think readers need to have the existence of subtlety hidden from them - we can avoid giving them the details, but we can't pretend that something is very simple if it's not so simple. (4) "team find new evidence that the fundamental shape of the universe is a dodecahedron" Wrong. You could put: "team find new evidence that the fundamental domain of the universe is a dodecahedron" or "team find new evidence that the shape of the universe is a Poincare dodecahedral space" or "team find new evidence that the shape of the universe is a dodecahedron that wraps around on itself" (5) Regarding the "map with matched circles" Since our method was not to search for matched circles, we don't have a map ready with matched circles shown on it. Of course, what we did is *related* to matched circles, but the relation is not one-to-one. What i suggest instead is the "best orientation" all-sky map on this page: http://cosmo.torun.pl/Cosmo/PressReleaseRBSG08 [ The image was created by me, at home, independently of the corresponding figure in our Astronomy & Astrophysics article, so you shouldn't have copyright problems. If formality is needed, then choose one of the GPL/GFDL/CC-BY-SA licences - probably you would want CC-BY-SA: http://creativecommons.org/licenses/by-sa/3.0/ ] If you want to use this, then a replacement for the text: "team ... claimed to have found 12 matched circles in the CMB - shown here - corresponding to dodecahedral topology." could be something like: "team ... claimed to have found that the best orientation of a dodecahedron in the sky - shown here - gives the required twist surprisingly well." OTHER COMMENTS: (*) "Torun Centre for Astronomy in Poland" It would be nice to put the university name as well, in at least one of the three places: "Torun Centre for Astronomy of Nicolaus Copernicus University in Poland" MINOR COMMENTS: (*) "their theory says the topology of space amounts to" This is a matter of taste whether to use the popular usage of "theory" to mean "hypothesis", or rather scientists' use of the word, which is quite different. Is gravity a theory? Yes, it's a theory. It's a very well established theory. Try to ignore it and you'll soon end up in hospital or worse. It's much more than just a hypothesis. i would put: "their evidence supports the hypothesis that the topology of space may amount to" (*) "would look like a cosmic hall of mirrors" i would put: "would look something like a cosmic hall of mirrors" (*) "was pioneered astrophysicists" missing "by" cheers boud

3 3

Re: [Cosmo-media] Cosmic topology
by Boud Roukema 24 Feb '08

24 Feb '08

Hi Paul, On Tue, 19 Feb 2008, Paul Parsons wrote: > I'm just getting in touch as I'm writing a piece for Focus magazine [to others on cosmo-media: this is BBC Focus magazine: http://en.wikipedia.org/wiki/BBC_Focus ] > about cosmic topology. Jean-Pierre Luminet sent me a press release > about some work he's involved with on Poincare Dodecahedral > Structure. In it, he cites a 2008 paper with your good self as lead i think you mean PDS = Poincare Dodecahedral Space > author - just wondering if I could run a few quick questions past you > about this... Sure. > What do you think of the idea that we live in a universe with > dodecahedral structure? What do i think of the idea that we live in a PDS universe? My general impression is that the CMB data seem to be successively pointing more and more towards the PDS model. Firstly, COBE hinted at a lack of structure on the largest scales. WMAP, with better resolution, confirmed a lack of structure on the largest scales (more than 10 Gpc/h), especially when we think in terms of three-dimensional space rather than just in terms of angles. The next clue was that WMAP in combination with various other experiments have continually indicated a positive curvature with the total density parameter Omega_total somewhere around a few percent above the density for a perfectly flat universe. Among the positive curvature models consistent with this estimate, the PDS is probably the most credible. Thirdly, there have been studies by several groups using different methods, including our latest work in Torun, which used a method of analysing the WMAP map which was most likely (if we assume that the PDS model is wrong) to give evidence *against* the PDS model, but instead, it gave a valid PDS solution. So the PDS model does seem to be the more natural interpretation of the data at the moment. However, we cannot yet say that the infinite flat model is ruled out to high significance by the data: there are analyses suggesting that the lack of structure above 10 Gpc/h is just coincidence, and the Omega_total estimates do not rule out the flat model to high significance. > What will the universe look like if our universe really does have > this dodecahedral topology? We should find that very high redshift (very distant) objects in some parts of the sky, seen with our telescopes as they looked a long time in the past, can be seen in other parts of the sky as they appeared even earlier, as regions of slightly high density which have not yet gravitationally collapsed, observed as the "cosmic microwave background". In other words, we could see a single physical region - traced by filaments of large scale structure including galaxy clusters and the most massive galaxies - in two different directions on the sky as they looked like at two different epochs: firstly at a very early epoch seen as temperature fluctuations, and secondly at an epoch a billion years or so later when some of the galaxies and galaxy clusters have just started to form and give off starlight. The PDS model is not yet well enough constrained to give precise coordinates for these matching objects. In our paper (astro-ph/0801.0006) you'll find our present best estimates together with some discussion of other groups' work, which give some estimates of the coordinates needed to calculate this, but from our present results, these are only approximate. > I understand that your own work has revealed evidence for this cosmic > topology in the WMAP data? Tell me more about that. The full answer is here: http://arXiv.org/abs/0801.0006 e.g. start from the abstract. This irc log might help too: http://cosmo.torun.pl/pipermail/cosmo-media/2008-January/000070.html http://cosmo.torun.pl/pipermail/cosmo-media/2008-January/000071.html i assume you want a short, layperson's summary. Let's try the following. Matched circles are the set of pairs of points at which two "topologically lensed" copies of a single physical point are seen in apparent space, on the surface of last scattering, observed as the cosmic microwave background (CMB), at what seem to be two different space points, but are really, physically, identical. We extend this idea by considering pairs of points where two points distant from one another on the CMB (in the apparent space[1]) are two points in space which in reality are not exactly identical, but are just physically *near* to one another (in the fundamental domain[1]). This allows us to use a larger number of data points in the maps than for a "pure" circles method, since there are many more pairs of points which are near to one another rather than exactly at the same position. We know that points near to each other should be (statistically) correlated to one another: the temperature fluctuations should (statistically) be similar to one another at a pair of close points. So although individual pairs of points with this new method should not match as well as pairs of points on matched circles, they should still match fairly well, and there are many more of them. Also, small errors in choosing the correct orientation and exact geometry of the dodecahedron should be less important this way. Using this idea, we searched for the best orientation of a PDS model which gives high cross-correlations of such would-be nearby pairs in the WMAP maps. If the PDS model is wrong, then these cross-correlations should, most of the time, be weak, since the points in a pair are *observed* in apparent space to be quite far from each other, in which case the correlations have been measured to be statistically close to zero. We also allowed for an arbitrary twist angle to be used when matching opposite faces of the dodecahedron. This meant that if our search algorithm (a Markov Chain Monte Carlo method) found any strong signal, then it would be more likely to find a wrong twist angle than a correct twist angle for the PDS - unless the PDS model is really correct. The result was that both a clear best solution was found, in which pairs of apparently distant points are in fact strongly correlated to one another for one particular orientation of the dodecahedral face centres, and the twist angle for this solution is 39 plus or minus 2.5 degrees, which is surprisingly close to what is required for the PDS model. So the PDS had two quantitative predictions for our method: that a strong correlation should be found for some PDS orientation, and that the twist angle for this strongest correlation should be close to either plus or minus 36 degrees. Both were satisfied. [[1] Since you're writing a general article on cosmic topology, i presume you'll explain near the beginning what the fundamental domain and apparent space (covering space) are.] > PS - one additional question to the ones I sent before... I gather David > Spergel and Neil Cornish have already tried looking for matching circles in > the CMB, but found nothing. So how did you and your team manage to get a > positive result? How does your team's method differ? > Are there really matched circles in the WMAP data? Will you be able > to check this to a higher degree of confidence when the Planck data > are released? In our new paper we use a different (though related) method, which we think is more robust than the matched circles method - in some sense you could say that it uses thickened matched circles, i.e. annuli, even though this is an *interpretation* of the method, not the calculation method itself. Since we did find a best solution, and that solution has the correct twist angle within the uncertainty estimate, and since the cross-correlations for that solution are strong, you could loosely say that yes, there really are matched annuli in the WMAP data. Whether these annuli can be thinned down into matched circles may not be as straightforward as it was thought earlier. i think the main argument comes from the Ulm group (Aurich, Lustig & Steiner): there are problems trying to go to "high" resolution. We extended from the circles method to using "nearby" pairs of points and are starting from the moderately large scale signal rather than the small scale signals. Other groups have done some work on the very largest scales, those which have been causing the most controversy since the WMAP data and analyses first became publicly available. If the coordinates of our best estimate solution are correct to within our stated precision, then future work will eventually have to thin the annuli down to "circles". However, for several different reasons, this may not be as easy as it might seem. Planck data will certainly help, but data from many different ground telescope surveys will probably also be necessary to correct the maps for various contaminants which are generally thought to be in the maps. Our own 32m radio telescope at the Torun Centre for Astronomy is being used for the OCRA (One Centimetre Receiver Array) project together with support from Jodrell Bank, for removing foregrounds at very high resolution: galaxy clusters seen due to the Sunyaev-Zel'dovich effect. OCRA may contribute by helping to get high resolution coordinates of the PDS model after (if!) the low resolution model continues to get more observational support. > How might we find out the topology of our universe? What experiments > can we do? Again, will the Planck mission help? Will any other future > missions help, eg CMBPol? It's hard to predict which experiment will give a result of high enough significance to convince the cosmological community (assuming that the model is indeed correct). It was not easy to predict ahead of time that the supernovae Type Ia surveys would give the most convincing evidence in favour of a cosmological constant/dark energy. Planck will help, ALMA and SKA will help, at very high resolution, OCRA will help, but most probably it will be a combination of data from several different space and ground-based telescopes, along with careful, thorough analyses of the data, making as few arbitrary assumptions as possible. Possibly one particular experiment may get the most publicity as "discovering the PDS", but in terms of real science rather than media attention, my speculation is that it would most likely be a combination of many different data and analyses, just like for dark energy. > To me, the universe wrapping round on itself seems much more natural > than it being infinite. What do you think? i agree. i think it is most natural to consider the Universe as a physical object. After all, the alternative would seem to be... a spiritual object, in which case we're no longer doing science if we try to study it. Paraphrasing Janna Levin, most physical objects we know of have finite masses and sizes; so why should the Universe be an exception? > What do you think the next big thing in cosmic topology will be? Predicting the future in fundamental scientific research, especially on such a deeply fundamental question, is close to useless IMHO. Grant committees want that, but i'm not the person who'll give it to them. If we know the answers ahead of time, then it's not research. On the other hand, i think most people in the cosmic topology community would agree that any particular cosmic topology candidate is, given successively better and better data and analyses, highly falsifiable: the astronomical coordinates representing the model imply predictions, which if satisfied should lead to more precise estimates of the coordinates, which in turn should enable more precise predictions, creating a tightening cycle of constraints. If successive steps follow this cycle, then probably most of them could be considered as "big things". Hope these answers help... i'd be happy to comment on a draft of the article before it gets published. You should see this email on the mailing list archive: http://cosmo.torun.pl/pipermail/cosmo-media/2008-February/thread.html cheers boud

1 0

test only - ignore
by Boud Roukema 18 Feb '08

18 Feb '08

test only

1 0

log of irc chat with Bruno from VPRO Mon 21.01.2008
by Boud Roukema 21 Jan '08

21 Jan '08

---------------------------------------------------------------------- --- 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 ----------------------------------------------------------------------

1 1

2024

2023

2022

2021

2020

2019

2018

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008

2007

2006

2005

2004

2003

2002

Cosmo-media