| 1. The CMB is the single strongest piece of evidence for the correctness of the big bang theory, and was a fatal blow for its rival, the steady state theory. |
| 2. Detailed study of the CMB allows us a glimpse into the Universe when it was only about a 400,000 years old. |
| 3. The present blackbody spectrum implies severe constraints on the "thermal history" of the Universe (how temperature changed with time since the big bang). |
| 4. The CMB and its small but (barely) measurable deviations from isotropy are the strongest constraints that we have on the formation of galaxies and other structure. Theories for the formation of that structure must explain how galaxies began to grow so quickly from such small perturbations in the initial distribution of visible matter after the decoupling of matter and radiation. As noted in the right panel, dark matter may play a key role in this explanation. |
| 5. The detailed fluctuations of the CMB have provided strong evidence that the Universe is flat and that it is presently dominated by dark energy in addition to dark matter. |
Considering the significance of these constraints,
we conclude that the serendipitous discovery of the cosmic background radiation must rank with the
discovery of the Hubble expansion as one of the two most important developments in
the history of cosmology.