Physics question, re. why the sky is dark

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Physics question, re. why the sky is dark

Postby Dmitri on Wed May 20, 2020 6:17 am

So this seems like a nice simple summary of the current/popular view on why the sky is dark:

https://youtu.be/gxJ4M7tyLRE

My question is, why can't it be simply explained by the stars' lifecycle? As a thought experiment, forget the expansion of universe for a second (or pretend we are wrong about it which is actually a possibility, even if small) -- if an average star dies after a few hundred million years (give it take :)), could the sky simply be the actual picture of their lifecycle? I.e. there are simply that many stars that far away, we see the light while they are "alive" and no light after they "die"?
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Re: Physics question, re. why the sky is dark

Postby Steve James on Wed May 20, 2020 8:38 am

Fwiw, I think the explanation is that the light from stars that are moving away from our pov become more red-shifted to the point where the light is invisible to our eyes. So, if we could see in that spectrum (and there were no atmosphere), the sky would always look dark.

The idea that the universe is expanding has been demonstrated. The lingering question is how fast and if it will stop. Right now, they claim that it is speeding up.
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Re: Physics question, re. why the sky is dark

Postby Giles on Wed May 20, 2020 10:46 am

Nice video, thanks for posting!

Steve James wrote:Fwiw, I think the explanation is that the light from stars that are moving away from our pov become more red-shifted to the point where the light is invisible to our eyes. So, if we could see in that spectrum (and there were no atmosphere), the sky would always look dark.

The idea that the universe is expanding has been demonstrated. The lingering question is how fast and if it will stop. Right now, they claim that it is speeding up.


Hey Steve, I think you mean: "So, if we could see in that spectrum (and there were no atmosphere), the sky would always look light"

Red shift making light invisible to our own eyes is addressed in the video, too. They also mention the cosmic background radiation from the Big Bang. What they don't say clearly (very short video, but nice!) is that this radiation is so old and from so far away that the radiation (that once would have included light visible to the human eye) has shifted even further into longer frequencies: through infrared and onward into microwave radiation. "The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since the expansion of space causes their wavelength to increase over time."
https://en.wikipedia.org/wiki/Cosmic_microwave_background

And after that come radio waves, another area in which much astronomy observations are made. If our eyes could see microwaves and radio waves like we see visible light, then yes, we'd need to wear heavy-duty sunglasses on clear nights, too!
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Re: Physics question, re. why the sky is dark

Postby Steve James on Wed May 20, 2020 11:02 am

Hey Steve, I think you mean: "So, if we could see in that spectrum (and there were no atmosphere), the sky [edit; no sky outside the atmosphere] would always look light"


I meant that if we could see in infrared we would see the light coming from stars that are much farther away. It's like looking through infrared binoculars at night. Hubble's "deep field" photo was taken in infrared, for ex. I thought Dmitri's basic question (to paraphrase) was why wouldn't the black spaces we see between stars be because stars had simply lived out their lifetimes. It's a reasonable question.

How about this. Why is it that all the stars we see with our eyes appear to be white? Why aren't they all different colors --whether the universe is fixed or expanding?
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Re: Physics question, re. why the sky is dark

Postby Dmitri on Wed May 20, 2020 12:24 pm

Steve James wrote:How about this. Why is it that all the stars we see with our eyes appear to be white? Why aren't they all different colors --whether the universe is fixed or expanding?

Great question. (I'm presuming you mean why the stars' apparent color isn't a function/result of the redshift effect, not referring to actual/native star colors -- 'cause those are very different even to the naked eye, just look at, say, Antares vs. Sirius...)

No idea how to explain that with redshift. "Intuitively" it would seem that we should be seeing all those "redder" stars, just like we're hearing lower pitch on an ambulance siren when it's moving away...
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Re: Physics question, re. why the sky is dark

Postby Giles on Wed May 20, 2020 12:58 pm

Yes, nice question. I couldn't answer it either so I googled quickly and it seems that answer is that our naked human eyes mostly can't pick up the reddish elements in stars because these stars are still so dim (comparatively) that the low light levels trigger only the rod cells in our eyes, and not the cones. As you'll know, our rod-based low-light vision is pretty much monochrome - only the cones can receive and process colour. So no colour perception = stars look more or less 'white'.

So that's why we can't perceive the red elements of the light spectrum coming to us from the stars. What's more, (my own speculation now) red shift in the stars that are closer and thus from our perspective generally brighter will be much less or even minimal anyway. Even though the space/the universe is expanding, and thus also our galaxy, I would guess that this expansion and the accompanying red shift within our own galaxy is not all that much in cosmic terms. So the red shift we see in stars in the Milky Way will be less, and from the stars in our own backyard - a few hundred or thousand light years all around - even less still.
The visible individual stars that are further away will produce more red shift, and of course other galaxies even more so because at this level the degree and speed of 'everything racing away from us' becomes even greater. But this light will be even fainter to the naked human eye, and thus the red part of the spectrum will be perceived even less. I suppose this means that if our cone cells worked in low light levels (i.e. we could see all the colours of the rainbow printed on a card in very dim light) then many stars would not only look redder but also brighter. And some closer stars, red shift aside, would have more varied colours. And we would be able to see more stars (and other galaxies) as well, i.e. the more distant ones whose light only reaches us in the red spectrum and hence is currently too dim for us to see at all.

First aspect explained more elegantly and in more detail here: https://physics.stackexchange.com/questions/169969/why-are-stars-white (first answer in the list)
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Re: Physics question, re. why the sky is dark

Postby Dmitri on Wed May 20, 2020 1:06 pm

Giles wrote:...So the red shift we see in stars in the Milky Way will be less, and from the stars in our own backyard - a few hundred or thousand light years all around - even less still.

Good thinking; makes sense!
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Re: Physics question, re. why the sky is dark

Postby Steve James on Wed May 20, 2020 1:11 pm

it would seem that we should be seeing all those "redder" stars, just like we're hearing lower pitch on an ambulance siren when it's moving away...


We can't "see" them, but we can measure their redshift change through spectroscopic analysis.
Yes, Antares is red --and our sun is white. Stars cover the spectrum from (infrared) red to (ultraviolet) blue.

Ok, it's the redshift (light) from galaxies that is measured. But, why don't we see Antares getting redder if the universe is expanding so fast? Well, it's space that's expanding. And, afa the ambulance siren! Why isn't Sirius getting bluer? Oops, maybe it's because their redness and blueness has little to do with the Doppler effect. It's just their color --due to temperature, size, and composition.

Btw, you might like seeing pictures from the Chandra mission --that uses x-rays.
Image
https://chandra.si.edu/blog/files/images/casa-both.png
https://chandra.si.edu/blog/node/652
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Re: Physics question, re. why the sky is dark

Postby Steve James on Thu May 21, 2020 11:33 am

Here's a nice explanation of the relation of redshift to expanding space.

https://www.youtube.com/watch?v=LE_wbOw39Mk
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Re: Physics question, re. why the sky is dark

Postby Dmitri on Thu May 21, 2020 7:47 pm

Steve James wrote:Here's a nice explanation of the relation of redshift to expanding space.

https://www.youtube.com/watch?v=LE_wbOw39Mk

Thanks
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Re: Physics question, re. why the sky is dark

Postby Giles on Fri May 22, 2020 2:14 am

Yeah, thanks, Steve. So the red shift seen in/from distant galaxies isn't actually - or exclusively - due to the Doppler effect after all. So if I understood that right it means that, firstly, a redshifting very distant galaxy is (in normal terms) physically moving away from us i.e. the number of miles is increasing (miles is a ridiculous but still nicely tangible unit to use here). Hence also increasing the distance of a theoretical journey that we might make from here to there. If we have quite a few billion years to spare for the trip anyway. And also space itself is expanding, leading to further redshift but not actually increasing the number of miles if we could measure them and also not bringing a further increase in the distance/time from here to there, because at this level a 'mile' is also expanding, as are we too, because we and all our stuff are 'painted' on expanding the time-space fabric. (?)

But very possibly I'm too dumb to think this through correctly. ;D
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Re: Physics question, re. why the sky is dark

Postby Steve James on Fri May 22, 2020 6:02 am

It is mind boggling for anyone. Put it like this. Space is expanding everywhere in every direction. It's not like an explosion with a center. Everywhere is a center -- to the perspective of an observer there. We can't see it because of gravity. But, looking at galaxies (ie the light from them) far away in any direction, it's possible to measure a red shift.

So, will it go on forever? Is there some boundary to space? What is outside space? Will the universe end in a big rip or will something reverse the expansion and lead to a big crunch?
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Re: Physics question, re. why the sky is dark

Postby Trick on Fri May 22, 2020 6:03 am

Dark matter and dark energy take up 95% of the whole darn thing, no wonder it’s so dark.
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Re: Physics question, re. why the sky is dark

Postby Trick on Fri May 22, 2020 6:08 am

in space, no one can hear you scream
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Re: Physics question, re. why the sky is dark

Postby Giles on Fri May 22, 2020 6:33 am

Trick wrote:in space, no one can hear you scream


True. But they can still see you doing a facepalm... ;)
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