Cosmography of the Local Universe

At Cosmography of the Local Universe there is a video showing the distribution and motion of the galaxies in our neighborhood. There is also an accompanying paper by Courtois, Pomar, Tully, Hoffman and Courtois. In the video "distances" are represented by velocities in km/s. It's difficult to figure out distances to galaxies directly, what we can actually measure is the "redshift" of spectral lines, which can be interpreted as a velocity, which is then assumed to also indicate distances, due to the fairly uniform expansion of our local universe. Another term appearing in the video is the 'Zone of Avoidance' or ZOA. Our own galaxy, the Milky Way, blocks our view of the galaxies, due to the the dust and stars of the Milky Way being concentrated around the plane of the Milky Way's disk. So we really don't have much information about the galaxies laying in those directions.

One of the characteristic features of the distribution of galaxies is the presence of voids: huge, roughly spherical regions in which galaxies are very sparse. I have a collection of interesting links to the literature about voids here.

“On the quantum theory of radiation” by Albert Einstein

In Einstein's 1917 paper “On the quantum theory of radiation” he introduced the concepts of stimulated and spontaneous emission of radiation, the effects that make possible lasers and many other fascinating devices. He accomplished this in masterful fashion by starting with the simplest of assumptions, all but one of which were traditional classical physics. Using just one basic quantum notion - Bohr's idea of quantized molecular energy levels - his amazing powers of deduction led him to hypothesize new observable physical phenomenon as well as rederiving Planck's radiation law in a very neat way. This was apparently also the first time that anyone realized that photons should carry momentum as well as energy. This English translation of the original paper is beautifully written. Here are two nice retrospectives: Einstein as armchair detective: The case of stimulated radiation by Vasant Natarajan; Rereading Einstein on Radiation by Daniel Kleppner.
A dim recollection of freshman physics is probably enough to follow much of Einstein's train of logic.
"A Theory Should be as Simple as Possible - but not Simpler"