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Proposed response to Steven Weinberg's not-quite-accurate page 9 of "Cosmology", 2008

  • The book: "Cosmology", Oxford University Press, ISBN13: 9780198526827, Hardback, 544 pages, March 2008
  • The problem: page 9 of this book is not quite accurate regarding what we know about cosmic topology.

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Proposed text

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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 imply that K=-1 and K=+1 simply connected spaces are "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 effective acceleration effect (Roukema et al. 2007).

Best regards,
Boud Roukema, ...

PS: You are welcome to reply publicly to our cosmic topology discussion list http://cosmo.torun.pl/mailman/listinfo/cos-top if you wish to. However, please do not reply to the public mailing list address if you do not wish your reply to (automatically and nearly instantly) become public.


SWeinbergCosmologyTCfAErrata - for printing
Topic revision: r3 - 12 Jun 2013, BoudRoukema
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