eli5 How can we see the first light of the universe?

Question

So I understand that light travels at a constant speed and that as we look at other galaxies we see them as they were when the light left that galaxy. My question is more to the thought that as the universe expands from the big bang and galaxies are formed, wouldn’t the process of creating everything we know take longer than it takes the light to travel from there to here. So wouldn’t the limit of visible light only be as old as the time it takes to travel? How can we see “The first light after the big bang” as so many scientific groups are trying to do? I’m not sure if I explained my question well. Please give me some leeway.

In: 2

Answer 1

> So wouldn’t the limit of visible light only be as old as the time it takes to travel?

Yes – with a bunch of disclaimers.

The oldest “light” we can see is from the cosmic microwave background. This came into existence about 378,000 years after the Big Bang. That sounds like a long time, but to put that into perspective, that is 0.0027% of the current age of the universe.

Anyway; this was the time when the universe became transparent. Before that “light” couldn’t get through the universe as it was basically a giant mess of plasma. Once the universe started getting big enough for that plasma to break down into individual hydrogen atoms, light could pass through it, and we get our earliest light, which spread out from everywhere, in all directions. Any direction you look up into the sky (if you have a sensitive enough radio telescope) you can detect what is left of this light. Because this “light” came from pretty much everywhere in the universe, in all directions, we can always see it, and the time it takes to travel to us doesn’t really matter – there is always some of it there.

We can’t see any light older than that because there wasn’t really light before then (or at least, no light that could go anywhere).

Note that the first stars didn’t appear until about 400 million years after the Big Bang, about a thousand times longer than the time to “photon decoupling” or “recombination” – when the first photons came out.

We also have to be a bit careful with large-scale cosmology because while light travels at a constant speed in flat spacetime, ideas about distance, time and so on get a bit messy, so we don’t always get the answers we might expect.