Spergel and BICEP2

David Spergel (Princeton) came into town and talked about CMB cosmology. He spent a good fraction of his talk throwing doubt on the new, hot BICEP2 / Keck result on B-mode polarization on large scales. He is concerned that the experiments don't seem to satisfy the standard null tests. He laid out a set of requests, which are easy to satisfy. He noted that six new experiments can confirm the result if it is real, so we will know extremely soon. One amusing thing about Spergel's talk was the repeated point (obvious, but often overlooked) that because all CMB experiments are observing the same, single sky, they ought to agree to better than one-sigma, especially on large scales where cosmic variance dominates.