Debating the Cosmological Principle

Whether you need something to distract you from world events or are just interested in the subject I thought I’d share something cosmological today.

You may recall that I recently posted about a paper by Subir Sarkar and collaborators.  Here is the abstract and author list:

In that post I mentioned that Subir would be taking part in an online debate about this issue. Well, although I wasn’t able to watch it live there is a recording of it which is available here:

It’s rather long, but there are many interesting things in it…

2 Responses to “Debating the Cosmological Principle”

  1. Just today on the train, I read this:

    It is to be emphasized that we have not required the real universe to be one satisfying the uniformity conditions imposed above; we are merely examining the nature of the idealized nature of the real world to which the obvious and all-important inhomogeneities are ironed out. We are not imposing the uniformity as a ‘cosmological principle’ … to which the real world must adhere.

    Extra points if you know who said that and guess where I read it.

    I haven’t watched the debate yet, but plan to, so my comments are general, not related to the debate.

    Some people are confused by the ‘cosmological principle’. In the very early days, it was indeed an assumption. However, already with Hubble’s work there was evidence for spatial homogeneity above a certain scale (via observed isotropy and the Copernican principle (we are not at a special location; the Copernican principle is more of an assumption, although a relatively harmless one, and from it and observed isotropy follows homogeneity)). Of course, the perfect cosmological principle of the Steady State theory was an assumption, at least with regard to the Universe being constant in time on the large scale. Now, we know that the Universe evolves, and the evidence for isotropy and hence homogeneity is much better. To some extent, we can even observe homogeneity, i.e. by measuring the CMB temperature in various directions at large redshift (via the influence of the temperature on line strengths).

    Of course, practically no-one disputes that the Universe is homogeneous on very large scales (the fractal crowd notwithstanding); the debate is about relatively smaller inhomogeneities, though perhaps larger than often assumed.

  2. Phillip Helbig Says:

    My impression from the debate is that the point of view that the evidence for acceleration is weak depends on the null hypothesis that there is none. However, if we have no prior preference, then it is clear that there is more information in favor of acceleration than against it. So, while it might not prove acceleration, it proves deceleration even less.

    Also, the concordance model is called that for a reason. There are multiple lines of evidence for the values of each of the main cosmological parameters. While one can always find possible flaws in any one test, it is much more difficult to explain how and why several independent cosmological tests conspire to agree on wrong values.

    In other branches of physics, common wisdom is that Nature uses all degrees of freedom. If one is not used, then that implies a symmetry, conservation law, conserved quantum number, etc., and of course the burden of proof is on the one claiming that this degree of freedom is not exercised. So it seems to me that we should be really strict about evidence claiming that the cosmological constant is exactly zero, the default expectation bering that it is not. (A non-zero cosmological constant, even if positive, does not necessarily imply that the Universe is accelerating now; it doesn”t even imply that the Universe will ever accelerated.)

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