Since we can essentially always see Cosmic Microwave Background radiation, does that mean that the Big Bang and the expansion of space are still generating light/radiation? If not why are we still able to see it? Wouldn’t the photons have passed us at this point?
In: Physics
Light takes time to travel. This means that light from a very long distance away could take quite a while to reach us.
The CMB radiation we see today has been traveling since the dawn of the universe. Of course the photons from nearby have passed us, but those from very far away have not.
The amount of energy in the universe is finite, constant. We can’t inject energy into the universe or remove any, only transform it.
When the big bang occurred, space grew from an infinitely small singularity to 1,800,000,000,000,000km wide in the space of 1 second. In the initial moments after the big bang, space was so hot and energy-dense that we don’t really have proper terms to describe it.
Anyway, a couple of hundred thousand years after the big bang, space cooled enough to allow the first atoms to form, which is what finally allowed light to travel freely in the universe. This sudden shift and creation of atoms caused a colossal explosion of electromagnetic radiation [light], essentially taking a photograph of the newborn universe.
As space expands, so does the wavelength of the light travelling through it. It goes from very small and very-high-energy wavelengths like gamma and x-ray, and gets stretched over time, losing energy and eventually becoming things like ultraviolet, visible, and then eventually microwaves. We happen to exist at a point where space has expanded so much that that initial flash of light that permeated the entire universe at once has now lost so much energy that it’s now a ‘background’ blanket of microwave radiation that exists everywhere in the universe – which is where we get the full name, the Cosmic Microwave Background Radiation.
Don’t think of it as single photons moving from point A and passing past Earth at Point B, because that linear thinking doesn’t really explain why we can constantly see it at every point in space.
Think of it like an amount of gas in a container – if we squeeze that container, making it smaller and more energy-dense like the early universe was, then the gas inside will get compressed too, causing it to heat up and get denser. If we take the container and then pull on it and make it bigger, then the gas inside will get cooler, and disperse and get less energetic as it expands to fill the now larger container. The same kind of situation applies to the expanding universe and the CMBR.
Hope this helps!
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