Did electromagnetic radiation start the Big Bang

Not exactly, however, it did radiate a lot of it. The big bang was actually the implosion of a super massive black hole which is now your universe. Yes, we live inside a black hole and there are trees here. Although it took only the briefest of moments, that implosion took billions of years as perceived from the hyperspace (outside) from where it came. All the matter of that previous universe was converted into energy and it blasted into here from everywhere whereupon it was then converted back into fundamental matter to start the cycle all over again. This is why there is no particular direction in which we can point in order to direct someone towards that blast, it was an implosion. Alius Sage

No. The inherent dimensionless nature of a singularity "started" the Big Bang.

Electromagnetic radiation has been *present* since Time Zero, but it was not itself responsible for the expansion of the universe. The Big Bang occurred because the universe cannot possess a spatial size of zero for any nonzero duration.

While we may never know what exactly triggered the Big Bang, we can definitely rule out EM radiation. Why? Because EM is a force (it's one of the 4 basic forces WITHIN our universe), and like all forces, physical constants, matter, energy, dimensions (spatial and temporal), etc... that make up our universe, EM was unified with everything at the moment of the Big Bang.

e_M emerged as the Universe cooled due to expansion

at some degree of energy density the very atoms could not exist, the sub atomic particles including photons the modern CONCEPT for E_M radiation could not exist

at some point the expansion was enough for photons and atoms to "condense" from the energy soup and light ( E-M) would allow interaction between atoms, plasma, etc.

the earliest properties of the Universe are only extension of what we ( think) we know about the basic laws

If the LAWS of the universe changed we would have no way of knowing it

No... Electro-magnetism is a force in *this* universe; it may not exist outside it. Whatever was the cause of the big bang, EM radiation is one of the by-products - not the source.

It is thought all four known fundamental forces of nature took part (gravity,electromagnetism, weak and strong force) and possibly other unknown forces (Dark Energy). The Big Bang is where all things in the Universe were one unified force

There was no EM radiation before the Big Bang to start it. The Universe did not exist until the Big Bang created it.

Its just as good as any other atheistic theory. They all presume existence before existence.