Hi Courtney. Would you mind briefly stating the evidence for the big bang. Specifically, does the CMB truly rule out any other phenomena? I know steady state is out of favour but could that not produce a cmb?..stars/other phenomena produce radiation. Are there many 'top' physicists who think the BB is unlikely?
Thanks for your time
The Cosmic Microwave Background (CMB) was the beginning of the end for the Steady State theory, as it confirmed a key prediction of the Cosmic Fireball theory (it was only called the Big Bang later, when Hoyle dismissed it as "Just a Big Bang theory" -- accuracy of the quote not guaranteed, but basically what he said).
However, the CMB alone, though convincing to many, was not the reason for abandoning the Steady State theory. By its very nature that theory required the Universe to look about the same at all times, and therefore at all distances. But it does not look the same at very large distances as it does nearby. For one thing, quasars are far more common at large distances than nearby, meaning that they used to occur more often than now. For another, the numbers of galaxies in any given region, although more and more difficult to determine at very large distances, are definitely greater at large distances, meaning they were closer together in the distant past than they are now.
In other words, it definitely appears that
(1) there was a time when the Universe was so hot and dense that it was "opaque", and radiating black body radiation more or less uniformly (in fact, far more uniformly than originally expected) everywhere in the Universe at that very early time, originally referred to as the Cosmic Fireball.
(2) there was a time when things happened in the Universe that are no longer happening, or are happening only very rarely, so its contents seem to have "evolved".
(3) there was a time when galaxies were closer together than now, just as the Hubble expansion would predict.
So it does look like some form of expanding Universe (or "Big Bang" theory) is required to fit the observations, and as a result almost all astronomers accept one version of the theory or another (though there are always debates about the details, even when a theory is well established).
As a result, even Hoyle eventually had to admit that at least in the "observable Universe", the "Big Bang" was "locally" plausible. But rather than give up his steady-state theory, he proposed an alternative steady-state theory in which the Universe as a whole doesn't change, but some parts behave as predicted by the steady-state theory, others behave as predicted by the Big Bang theory, and others behave as predicted by "Closed-Universe" theories. So in any given region you might observe almost any kind of "Universal" behavior, but the Universe "as a whole" would stay the same. But each "different" region had to be much larger than the "observable Universe", so that whatever region you happened to be in, you couldn't make any measurements that would prove there were other, differently-behaving parts of the Universe. Which was fine as far as allowing the super-steady-state theory to be "possible", but made it impossible to prove that it was right or wrong; so although I think Hoyle continued to champion the idea until he died, he had no way to convince anyone else that he might be right, and the idea pretty much died with him.