  We have combined optical data from the 2dF-SDSS Luminous Red Galaxy and QSO
(2SLAQ) redshift survey with radio measurements from the 1.4 GHz VLA FIRST and
NVSS surveys to identify a volume-limited sample of 391 radio galaxies at
redshift 0.4<z<0.7. By determining an accurate radio luminosity function for
early-type galaxies in this redshift range, we can investigate the cosmic
evolution of the radio-galaxy population over a wide range in radio luminosity.
  The low-power radio galaxies in our LRG sample (those with 1.4 GHz radio
luminosities in the range 10^{24} to 10^{25} W/Hz, corresponding to FR I radio
galaxies in the local universe) undergo significant cosmic evolution over the
redshift range 0<z<0.7, consistent with pure luminosity evolution of the form
(1+z)^k where k=2.0+/-0.3. Our results appear to rule out (at the 6-7 sigma
level) models in which low-power radio galaxies undergo no cosmic evolution.
The most powerful radio galaxies in our sample (with radio luminosities above
10^{26} W/Hz) may undergo more rapid evolution over the same redshift range.
  The evolution seen in the low-power radio-galaxy population implies that the
total energy input into massive early-type galaxies from AGN heating increases
with redshift, and was roughly 50% higher at z~0.55 (the median redshift of the
2SLAQ LRG sample) than in the local universe.
