Evidence of [x] is not proof of [x], but in this specific case the claim is particularly unreasonable.
Because while a cosmic microwave background radiation is something that would be expected from a Big Bang, its actual reality is weird and contradicted previous expectations. It has to do with casual connectivity. "A" can only possibly influence "B" if A could reach B at the speed of light. But the relative homogeneity of our universe is suggestive that parts of the universe which should not be casually connected, are causally connected.
So this discovery quickly led to the invention of cosmic inflation [1] whereby the early expansion of the universe is said to have dramatically accelerated well beyond the speed of light, and then slowed down - in order to enable these regions of space to become causally connected. No possible means or mechanism have been suggested, so it remains nothing but a rather inelegant hack to try to make what we observe fit a preexisting hypothesis that it largely contradicted.
As experiments to try to provide supporting evidence for inflation have also turned up negative, it's also becoming one of those contemporary model driven hypotheses that requires ever more exotic physics to even make it possible, as each failed prediction gets assimilated into the model to make it keep fitting what we see. Just add more epicycles. [2]
Because while a cosmic microwave background radiation is something that would be expected from a Big Bang, its actual reality is weird and contradicted previous expectations. It has to do with casual connectivity. "A" can only possibly influence "B" if A could reach B at the speed of light. But the relative homogeneity of our universe is suggestive that parts of the universe which should not be casually connected, are causally connected.
So this discovery quickly led to the invention of cosmic inflation [1] whereby the early expansion of the universe is said to have dramatically accelerated well beyond the speed of light, and then slowed down - in order to enable these regions of space to become causally connected. No possible means or mechanism have been suggested, so it remains nothing but a rather inelegant hack to try to make what we observe fit a preexisting hypothesis that it largely contradicted.
As experiments to try to provide supporting evidence for inflation have also turned up negative, it's also becoming one of those contemporary model driven hypotheses that requires ever more exotic physics to even make it possible, as each failed prediction gets assimilated into the model to make it keep fitting what we see. Just add more epicycles. [2]
[1] - https://en.wikipedia.org/wiki/Inflation_(cosmology)#Motivati...
[2] - https://en.wikipedia.org/wiki/Ad_hoc_hypothesis