The cat thing is just a thought experiment. The original purpose was to shock people into being skeptical of the weird properties of quantum mechanics. Schrodinger wanted people to understand how weird these things he was studying were and he probably didn't like cats very much, kinda like an in-joke.

There are several reasons why you can't actually do this experiment. One is that information will leak out of the box, even when it's all closed up and taped. Even if the box was airtight, some information is leaking. If information leaks out of the box, the cat will be either alive or dead. The thing that's weird is that information (detection) determines the aliveness or deadness of the cat.

This is why only very small particles exhibit these weird effects. The information can be hidden until you have a detector with enough sensitivity.

As for a detector from the first light in the universe, the cosmic background radiation is just light in the microwave band that arrives at Earth at relatively the same intensity no matter where you look. It's similar to when you turn on an old TV and you see the static. It's that kind of signal (sort of).