Temperature of the big bang radiation is a few Kelvins, noise temperature of the tuner is hundreds of Kelvins. At lower VHF band, though, there is "galactic noise" which is taken into account when figuring the power needed for broadcasting. Unfortunately, this is where man-made noise is worse also, and it drowns out the galactic noise.
see this:
http://en.wikipedia.org/wiki/Antenna_noise_temperature
for some explanation