Accidentally discovered in 1964 by Robert Wilson and Arno Penzias, FTP, what is this ubiquitous radiation field corresponding to a blackbody of 2.7 Kelvins?
This has provided the best current measurement for the Hubble constant, which may be inferred from WMAP's measurements of it anisotropy.  Other anisotropies in this field can be caused by gravitational waves or the Sunyaev-Zel'dovich effect.  It was generated at a redshift of approximately 1100, implying that the temperature of the universe at the last scattering surface was about 3000 Kelvin.  Accidentally discovered in 1964 by Robert Wilson and Arno Penzias, FTP, what is this ubiquitous radiation field corresponding to a blackbody of 2.7 Kelvins?
This has provided the best current measurement for the Hubble constant, which may be inferred from WMAP's measurements of it anisotropy.  
cosmic microwave background radiation (accept Wilkinson Microwave Anisotropy Probe before it's mentioned)