Time Dilation in the Light Curve of the Distant Type Ia Supernova SN 1995K

The light curve of a distant Type Ia supernova acts like a clock that can beused to test the expansion of the Universe. SN~1995K, at a spectroscopicredshift of $z = 0.479$, provides one of the first meaningful data sets forthis test. We find that all aspects of SN~1995K resemble local supernova Iaevents when the light curve is dilated by $(1+z)$, as prescribed bycosmological expansion. In a static, non-expanding universe SN~1995K wouldrepresent a unique object with a spectrum identifying it as a regular Type Iasupernova but a light curve shape and luminosity which do not follow thewell-established relations for local events. We conclude that SN~1995K providesstrong evidence for an interpretation of cosmological redshifts as due touniversal expansion. Theories in which photons dissipate their energy duringtravel are excluded as are age-redshift dependencies.