I’ve always been fascinated by the big bang, the expansion of the universe, and cosmic time scales. One part of the story just doesn’t make logical sense to me is the fact that we can still see light from the early universe.
If light travels faster than anything else and all light and matter originated at a central point, why hasn’t this light overtaken the matter? In my mind it makes sense that the light from the big bang has moved on way way past our planet, 13.8 billion years moving in a straight line at the cosmic speed limit. If that’s so, how can we still see it and measure it? Shouldn’t the photons be billions of light-years away from us now?
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The universe is really big.
And the early universe was also really big (just a lot smaller than the current really big).
The light we’re seeing from the early universe has travelled a really long way (13.8 billion light years).
The light from our part of the early universe is billions of light years away, but the light from the parts of the early universe that were billions of light years away is only just reaching us now.
It has been red-shifted an awful lot, though, which is why it is all microwaves now.
>If light travels faster than anything else and all light and matter originated at a central point, why hasn’t this light overtaken the matter?
it didn’t originate at a central point. folks may refer to it as a singularity, but it doesn’t mean a single point. we say that the big bang happened “everywhere.” honestly i think it means our current theories breakdown at the instant the big bang begins, but based on what we currently know and can extrapolate with, the instant of the start of the big bang, the universe was an impossible-for-humans-to-fathom soup of infinite density but also still infinite expanse. it didn’t “explode” from a point, it exploded everywhere.
edit to add: more directly to your point and not just a correction. in the early moments of the universe, the universe was so thick and hot that light couldn’t actually get through. even if the universe did actually begin from a central point, for the early moments of the universe when expansion was at its fastest, light would not actually able to get through anything. only when the universe was cooled enough and large enough (roughly 300,000 years or so after the beginning) did it become transparent to photons, only then could light race away in different directions, and that’s what we see in the cosmic microwave background.
So the issue is with your second bit. “If… All light and matter originated at a central point”
It did not. There is no “central point”. You may think so because you’ve heard of cosmic expansion, but that’s not really the case.
The universe isn’t really expanding “outwardly”. Every point is in itself expanding. The space within your body is expanding, it just so slow you’d never tell on that scale.
But if you rewind that back enough, what you find is that the Big Bang didn’t happen at some particular point. It happened everywhere. The universe was just a lot smaller at the time. That’s why we can still see it.
You are correct! we dont see the light from the big bang AT ALL. The CMBR (early universe light) was emitted about 400000 years AFTER the big bang when the universe had time to expand a bit. (also, the universe isnt flying apart from a central point, new space is being added in between the stationary parts of the universe)
The universe is expanding faster than light moves, so the light from that time gets stretched out over a much longer time. As it does, it moves from visible light to lower frequencies (like microwave radiation) using a thing called the Doppler effect. And the result is that the light from the early universe (But not the big bang) will always be there, but will get lower and lower frequency over time until it is no longer detectable.
(the light came from all the matter in the universe being super hot. hot things glow, so did the universe when it was young)
The light we see from the early universe doesn’t come from the big bang itself.
After the big bang, the entire universe was filled with a hot, dense fog of protons and electrons. There was tons of light around as well, but the light couldn’t travel through the fog.
Over the next 380,000 years, this fog cooled as the universe expanded. Once it become cool enough, the protons and electrons combined together to form mostly hydrogen gas, with a little helium too. When this happened, the universe became transparent, allowing all the light that was bound up in the fog to travel freely. So we can think of this as every part of the universe emitting light in all directions in gigantic, cosmic flash.
Since then, all that light has continued to travel. And this is the earliest light that we can see.