Dear Marcelo, everyone,

The topic of Steven Weinberg's rather not-fully-accurate page 9 of his new book is IMHO rather well-suited to discussing on our public mailing list rather than privately - this has nothing to do with revealing unpublished results that we're still working on, it's about correcting some inaccurate statements regarding cosmic topology.

i also think it would be worth it that the effort we spend in explaining things to Steven Weinberg can potentially help educate others directly, i.e. those who may read the mailing list archives or get pointed there by Big Brother Google Inc. An alternative (and reasonable) place to discuss this IMHO would be http://cosmocoffee.info, which is a phpBB based forum for cosmology research in general, while this mailing list is a mailman mailing list specifically for cosmic topology.

Anyway, to people who do not have a copy of the book "Cosmology" published by Steven Weinberg a few days/weeks ago, just email one of us in private and we can send you a scan of page 9 ("for educational and research purposes only", to avoid any problems with copyright etc.).

My general opinion:

Yes, i think it would be a good idea to send a message to Steven, and a consensus message signed by several of us would be a good idea. However, it would require a bit of work, since ideally it should be:

* clear * complete * well-referenced * but also... reasonably short

This is not so easy, especially keeping it short while satisfying the first three criteria...

i've posted a suggested draft on our wiki:

http://cosmo.torun.pl/Cosmo/SWeinbergCosmologyPage9Response

and i'm also posting the same initial draft text below here. Personally, i don't see that there's a huge hurry - i don't imagine OUP issuing a second edition on a time scale of days or weeks - so i haven't suggested any time scale. My suggestion is we give people a week or so to update/correct/ complain. If we wait too long, it will soon by the northern summer break and not much point expecting anyone to respond quickly.

If you want to correct/edit my suggested text and/or sign it, please speak up! i've tried to be complete in the reference list while limiting to the precise problems in Steven's text.

cheers boud

suggested text, openly editable at http://cosmo.torun.pl/Cosmo/SWeinbergCosmologyPage9Response

---------------------------------------------------------------------- Dear Steven,

We are glad that your cosmology text published by Oxford University Press a few months ago (March 2008) is, to the best of our knowledge, the first graduate level type cosmology text in which it is clearly stated that there are many more than three possible shapes for the spatial section of our Universe, assuming that the Friedmann-Lemaitre-Robertson-Walker metric is correct apart from density perturbations. Nevertheless, some of the statements on page 9 of your book are probably not as accurate as they could be. Here are some suggestions which should hopefully clarify these issues.

1. "There is no sign of [the same patterns of the distribution of matter and radiation in opposite directions] in the observed distribution of galaxies or cosmic microwave background fluctuations, so any periodicity lengths such as |L_i| must be larger than about 10^10 light years."

* It is correct that based on cosmic microwave background constraints (WMAP), it is clear that the in-diameter of the Universe (see e.g. Fig. 10 of Luminet & Roukema 1999 for various definitions of the comoving size of the Universe) is greater than 2 h^-1 Gpc.

* However, on a scale of about 12-15 h^-1 Gpc, a large number of papers have been recently published which suggest that, in particular, a Poincare dodecahedral space (for K=+1) or an equi-length 3-torus model (for K=0) better fit the WMAP data than an infinite K=0 model. There is not yet any clear consensus on what the data tell us - some papers argue that the infinite K=0 model cannot be significantly rejected based on the WMAP data. Please see the reference list below for the main papers in this series.

2. "[Most 3-manifolds] ... seem ill-motivated. In imposing conditions of periodicity we give up the rotation (though not translation) symmetry that led to the Robertson-Walker metric in the first place, so there seems little reason to impose these periodicity conditions while limiting the local spacetime geometry to that described by the Robertson-Walker metric."

* This is a little unclear. If by "symmetry" you mean the existence of isometries in the covering space, then there are no global translation symmetries at all for K=-1 or K=+1. Translations only exist for K=0. (Clifford translations exist for some K=+1 spaces, but that's a separate issue.) If you want to retain translational isometries in 3-space, then only K=0 is possible, and your words would seem to describe both K=-1 and K=+1 simply connected spaces as "ill-motivated".

* More importantly, the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric only requires local homogeneity and isotropy, not global homogeneity and isotropy. The FLRW metric is intrinsically local, it's about the limit of what happens towards a point. To write this in terms of practical observational cosmology statistics, consider a more realistic model of the Universe, i.e. with a perturbed FLRW metric rather than a perfectly homogeneous one. In this model, "local homogeneity and isotropy" means that various n-point auto-correlation functions of structure tracers within a "neighbourhood" of a Gpc or so should give identical results to within observational uncertainties. This is what is observed, but it does not constrain global homogeneity and isotropy. So it is unclear why a multiply connected 3-manifold with an FLRW metric is in any way "limited" or "ill-motivated".

* We are only aware of one paper so far showing that global topology could have an effect (albeit small) on the FLRW metric through the addition of an additional acceleration effect (Roukema et al. 2007).

Best regards,

Boud Roukema, ...

References

o "size of the Universe":

+ Luminet, J.-P., Roukema, B. F., 1999, Topology of the Universe: Theory and Observation, NATO ASIC Proc. 541: Theoretical and Observational Cosmology , 117, http://cdsads.u-strasbg.fr/abs/1999toc..conf..117L

o WMAP: PDS and 3-torus models (and other spherical models) vs K=0 infinite model:

+ Aurich, R., Lustig, S., Steiner, F., 2005, CMB anisotropy of spherical spaces, Classical and Quantum Gravity, 22, 3443, http://cdsads.u-strasbg.fr/abs/2005CQGra..22.3443A

+ Aurich, R., Lustig, S., Steiner, F., 2005, CMB anisotropy of the PoincarĂŠ dodecahedron, Classical and Quantum Gravity, 22, 2061, http://cdsads.u-strasbg.fr/abs/2005CQGra..22.2061A

+ Aurich, R., Lustig, S., Steiner, F., 2006, The circles-in-the-sky signature for three spherical universes, Monthly Notices of the Royal Astronomical Society, 369, 240, http://cdsads.u-strasbg.fr/abs/2006MNRAS.369..240A

+ Caillerie, S., Lachièze-Rey, M., Luminet, J.-P., Lehoucq, R., Riazuelo, A., Weeks, J., 2007, A new analysis of the PoincarÊ dodecahedral space model, Astronomy and Astrophysics, 476, 691, http://cdsads.u-strasbg.fr/abs/2007A%26A...476..691C

+ Gundermann, J., 2005, Predicting the CMB power spectrum for binary polyhedral spaces, ArXiv Astrophysics e-prints, arXiv:astro-ph/0503014, http://cdsads.u-strasbg.fr/abs/2005astro.ph..3014G

+ Key, J. S., Cornish, N. J., Spergel, D. N., Starkman, G. D., 2007, Extending the WMAP bound on the size of the Universe, Physical Review D, 75, 084034, http://cdsads.u-strasbg.fr/abs/2007PhRvD..75h4034K

+ Lew, B., Roukema, B., 2008, A test of the PoincarĂŠ dodecahedral space topology hypothesis with the WMAP CMB data, Astronomy and Astrophysics, 482, 747, http://cdsads.u-strasbg.fr/abs/2008A%26A...482..747L

+ Luminet, J.-P., Weeks, J. R., Riazuelo, A., Lehoucq, R., Uzan, J.-P., 2003, Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background, Nature, 425, 593, http://cdsads.u-strasbg.fr/abs/2003Natur.425..593L

+ Niarchou, A., Jaffe, A., 2007, Imprints of Spherical Nontrivial Topologies on the Cosmic Microwave Background, Physical Review Letters, 99, 081302, http://cdsads.u-strasbg.fr/abs/2007PhRvL..99h1302N

+ Riazuelo, A., Weeks, J., Uzan, J.-P., Lehoucq, R., Luminet, J.-P., 2004, Cosmic microwave background anisotropies in multiconnected flat spaces, Physical Review D, 69, 103518, http://cdsads.u-strasbg.fr/abs/2004PhRvD..69j3518R

+ Roukema, B. F., Bulinski, Z., Szaniewska, A., Gaudin, N. E., 2008, The optimal phase of the generalised Poincare dodecahedral space hypothesis implied by the spatial cross-correlation function of the WMAP sky maps, Astronomy and Astrophysics, in press, arXiv:0801.0006, http://cdsads.u-strasbg.fr/abs/2008arXiv0801.0006R

+ Roukema, B. F., Lew, B., Cechowska, M., Marecki, A., Bajtlik, S., 2004, A hint of PoincarĂŠ dodecahedral topology in the WMAP first year sky map, Astronomy and Astrophysics, 423, 821, http://cdsads.u-strasbg.fr/abs/2004A%26A...423..821R

o effect of global topology on the metric:

+ Roukema, B. F., Bajtlik, S., Biesiada, M., Szaniewska, A., Jurkiewicz, H., 2007, A weak acceleration effect due to residual gravity in a multiply connected universe, Astronomy and Astrophysics, 463, 861, http://cdsads.u-strasbg.fr/abs/2007A%26A...463..861R

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