Why does the CMB support the Big Bang picture?
The basic point is that the spectrum of the CMB is remarkably close to the theoretical spectrum of what is known as a “blackbody”, which means an object in “thermal equilibrium”. Thermal equilibrium means that the object has had long enough to settle down to its natural state. Your average piece of hot, glowing coal, for example, is not in very good thermal equlibrium, and a “blackbody” spectrum is only a crude approximation for the spectrum of glowing embers. But it turns out that the early Universe was in very good thermal equilibrium (basically because the timescale for settling down was very much shorter than the expansion timescale for the Universe). And hence radiation from those very early times should have a spectrum very close to that of a blackbody. The observed CMB spectrum is in fact better than the best blackbody spectrum we can make in a laboratory! So it is very hard to imagine that the CMB comes from emission from any normal “stuff” (since if you try to make “stuff” at
