Expansion of the Youngest Galactic Supernova Remnant G1.9+0.3
  We present a measurement of the expansion and brightening of G1.9+0.3, the
youngest Galactic supernova remnant, comparing Chandra X-ray images obtained in
2007 and 2009. A simple uniform expansion model describes the data well, giving
an expansion rate of 0.642 +/- 0.049 % yr^-1, and a flux increase of 1.7 +/-
1.0 % yr^-1. Without deceleration, the remnant age would then be 156 +/- 11 yr,
consistent with earlier results. Since deceleration must have occurred, this
age is an upper limit; we estimate an age of about 110 yr, or an explosion date
of about 1900. The flux increase is comparable to reported increases at radio
wavelengths. G1.9+0.3 is the only Galactic supernova remnant increasing in
flux, with implications for the physics of electron acceleration in shock waves
