Why is cosmic background radiation from the big bang still measurable?
Some of it does travel away from us, but the idea is that the universe is immersed in this radiation travelling in all directions equally. Don’t think of it like a flash of light which will just send out photons in a radial direction- instead, think of us being in a uniformly luminated room. There are photons we can see, which arrive at our retinas, telling us the colour of the room, but there are also photons, but there are photons missing us, photons coming from behind us, and photons going everywhere and anywhere in the room. The CMBR wasn’t formed by the actual “bang” of the big bang. What we have is a load of hot matter 1 second after the big bang, and it expands, and it cools. The temperature of this is about a billion Kelvin. After 350,000 years, the temperature hits a critical point- below this temperature, atoms can form. Above this temperature you have electrons freely running round, below it, they get themselves stuck into an atomic orbital, and all the photons just spread o
