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.
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I think I see your question, but I’m not 100%. Thing is: light existed *before* stars and galaxies and planets and whatnot. In fact, the earliest known light source is the big bang itself, but much of that light has been lost to us because of the inflationary period immediately following. The first detectable light is the CMB, or Cosmic Microwave Background, which we’ve already been looking at for a number of years, and if you haven’t heard the story if its’ discovery, you should look into it. All that being said, we’re actually trying to look for the first light emitted by stars and galaxies, so, good on ya for getting that. And you’re right about light traveling at a fixed speed and about there being a distance limit associated with that. There are all kinds of boundaries to what we can **ever** know, but we aren’t there yet, and that’s what we’re trying to get to. It’s actually way more complicated than a simple reddit answer can provide, but, out to about 14.5 billion light years or so, we will **eventually** be able to see those stars and galaxies, and beyond that, the expansion of the universe between us and them is occurring so fast that the light will never get to us. Now, because we’re looking both *out* AND *back*, we can see light from within that 14.5 billion light year sphere from 14.5 billion years ago (give or take), and that is the approximate age of the universe, so by looking at the farthest stars, we’re also looking at the oldest stars.
So, the light travels at 300000kps and every single part of the universe is expanding away from every other part at a known rate, such that the farther away something is, the faster it is moving away from us because of that expansion. Think of a turkey. Stick a flavor-infuser in it and watch as the two sides get farther apart as you pump the flavor in. Now do it with 3 infusers, spaced equally apart and you’ll see that the ends get farther away from one another than any parts of the middle. Same thing. Eventually, there are enough universal “flavor-infusers” that the ends are actually getting farther apart FASTER than light can travel between them. It’s not a violation of the speed of light, because the speed of light is about light crossing space and has nothing to do with the space *itself* expanding.
Hope this helps
*note 1: my numbers are only Approximate. Please don’t give me grief on their precision (or lack thereof)
No.
For hundreds of thousands of years the universe was too dense for light to travel. There wasn’t space between particles, much less atoms. The cosmic microwave background is light from when the universe first became transparent.
> 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.
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