transparency, Padmanabhan, Ye

More discussions with various people about transparency; in particular the Tolman test, which I had not realized is a great test of transparency. It is a fundamental prediction that the luminosity distance be (1+z)2 times the angular diameter distance; any departure indicates a lack of transparency (or a modification of extremely fundamental physics; I think this emerges pretty much directly from Liouville's theorem).

Nikhil Padmanabhan (Princeton) gave a great talk about measuring large-scale structure with purely photometric (ie, not spectroscopic) data. He showed reasonable evidence for the baryon oscillations, and the power spectrum for LRGs, as measured with 1.5 (Gpc/h)3 (the largest volume ever used for such studies). During his talk I realized that we can never have an enormously significant measure of the baryon feature, since we have a few sigma in SDSS, and the SDSS, in effect, measures of order 3 percent of the entire Universe (present day). The square root of 30 is not a big number, and, more-or-less, that is about as much better as we can do.

Jun Ye (JILA) gave a great talk showing incredibly stable, incredibly precise frequency, time, and wavelength standards, using frequency combs. His viewgraphs showed frequencies like 429,228,004,229,900.0 Hz, with uncertainties less than 1 Hz. In time, this kind of work will require an update in the definition of the second (which is currently based on microwave, not optical, frequencies), and permit insane fundamental tests of physical quantities and systems.


  1. Only dogs can hear that high.

  2. Hmm I have always been worried about if the "fractal dimension 2" of the galactic maps are due to lack of transparency. Actually from a single observation point I should not expect to be able to see any fractal dimension greater than 2. So any advances in this point are welcome.