Final answer:
The discovery of cosmic background radiation is compelling evidence for the Big Bang Theory, as it is the predicted afterglow from the early universe and its properties match theoretical calculations with high precision.
Step-by-step explanation:
The discovery of cosmic background radiation was decisive in confirming the Big Bang Theory because it represents the afterglow of the early universe, known as the Cosmic Microwave Background (CMB). This radiation was a prediction of the Big Bang model and its detection provided evidence that the universe was once in a hot, dense state. The CMB is isotropic, meaning it's uniform in all directions, and has a perfect blackbody spectrum temperature of 2.725 K, which substantiates the theory of the universe's origin from an incredibly energetic explosion followed by expansion and cooling.
The work done by Penzias and Wilson, for which they were awarded the Nobel Prize, alongside other subsequent experiments, matched the Big Bang's predictions with remarkable precision. Furthermore, the Wilkinson Microwave Anisotropy Probe (WMAP) has given estimates of matter density that align with Big Bang model predictions, providing additional convincing evidence. The discovery of the CMB was a cornerstone for cosmology, essentially validating the Big Bang theory by observing conditions that directly correspond to theoretical predictions from the early universe.