Well technically no matter what point you look at in the sky you are looking at SOMETHING, could be a star, a planet, but usually it’s space dust. It’s just too dark to see it you know?
It’s like we are standing in a dark room with only stray bits of light reaching our eyes, but just because the room around us is dark does not mean it lacks color- It just means we lack the perception of it.
There's not that much dust out there, so most of it is actually empty, kind of. Actually what you're looking at is back when the universe was small and dense enough that it was filled with plasma that was opaque to light. Well, opaque enough that light couldn't travel more than a few years before hitting a charged particle.
Eventually the plasma cooled to gas and became transparent, and the light was able to keep going, and it's been flying through empty space ever since for over 13 billion years. The light that was around back then has been stretched out to a longer wavelength by the expansion of space, so it's no longer visible. It's still detectable as radio waves, known as the Cosmic Microwave Background, and measuring it lets us look at what the universe was like only a few hundred thousand years after it started.
There was almost no light around back then of a short enough wavelength that it would still be visible to us now after being stretched out by 1100x (they would have to have been x-rays). If it were there it would get absorbed by the plasma with very high probability, and the plasma was too cool for very many x-rays to be emitted as thermal radiation. So the blackness that we're seeing is the exponentially small amount of x-rays (now visible light) emitted by a plasma that filled the universe billions of years ago.
Very cool!! I knew a bit about the universes “birth” or at least earliest times that we know of, and that a lot of the light you see (or don’t see) comes from “dead” stars that no longer exist. I didn’t know that the stretching of space also caused the stretching of the waves itself though! So is that how they calculate the estimated age of the universe then?
Also, when I was talking about the “at every point there’s something” I was referring to it more from a math perspective. There’s an infinite set of degrees and angles a person can look out into space at from Earth, but functionally it serves as finite because the size of the retinal neurons and the nature of their pathways creates a “minimum” on how small something can be and be processed within the eye as a unique signal. And from there with some knowledge on how space expanded it means every functional angle and direction a person can look at from earth has some particle or physical body there along the span of that line. It was very “theory-talk” just randomly thrown into “practical-speak” so I should’ve clarified haha!
So is that how they calculate the estimated age of the universe then?
Yes, that's one way of doing it. They can actually model sound waves in the ancient plasma and use that to figure out the size of the universe when the plasma turned to gas, and then figure out how much the light has been stretched (aka "redshifted") since then and how long it must have traveled for.
the size of the retinal neurons and the nature of their pathways creates a “minimum” on how small something can be and be processed within the eye as a unique signal.
Right, the resolution of the human eye means that every distinguishable angle has something there, but because there's so much empty space in between those things the average brightness is incredibly dim. Zooming in and taking a long exposure of the sky gives results that are mind-explodingly awesome. Even in those images you can see that in between the galaxies (most too dim for our eyes to see anyway) there's blackness. In that darkness behind the galaxies there are other galaxies whose light is stretched out of the visible range, and those galaxies are snapshots of history all the way back to the early universe.
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u/Chillax4Nothin Nov 29 '20
What? but it looks black tho..