New Directions in Modern Cosmology, day 3

Another good day in Leiden. One highlight was work by Kunz (Geneva), who showed simple models for generating CDM-like inhomogeneities in the CMB without inflation. He gets very close, but in the details at large scale, causality requires some inflation-like activity. His arguments were very general. More was said about this by Magueijo (Imperial), who uses a variable speed of light to do inflation's work. In general these causality arguments come from the problem that it is very hard to set up the initial conditions.

Lachièze-Rey (Paris) described nice detectors for the baryon acoustic feature that don't require construction of the full correlation function or power spectrum. This led nicely into an afternoon discussion of homogeneity, where my results, Sylos Labini's (Rome), and Kazin's (NYU) were batted around. I didn't fare as well as I would have liked in part because we have not closed all the loopholes remaining for some kinds of technical inhomogeneities. I certainly think that it is well established that the Universe has a mean density, but Sylos Labini and others have a good point that if you can do your data analysis without assuming the mean density then you should, if for no reason other than that it is rarely well measured (so it adds uncertainty to your results). Don't get the wrong idea from my measured tone here: The universe is not a fractal on large scales!

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