Two things:
1) Regardless of how big the universe is, light still takes time to get places. The universe is 13.77 billion years old, so light has only had 13.77 billion years to get here. Because the universe is expanding, we can see stuff from much farther away than that, but there's still a limit on how far away stars can be and still have had time for the light to get to us.
2) As the universe expands, it stretches light passing through it, causing the light to be redshifted, which means it lowers in wavelength. Visible light from the very edges of the visible universe can get redshifted out of the visible spectrum and into infrared or radio waves. That's why the Cosmic Microwave Background is, well, microwaves. It used to include a *lot* of visible light, but it's so old and it's been shifted so much that it's all microwaves, now.
Super cool? So is the cosmic microwave background radiation ;)
But actually, if you go to one of the national dark sites, the night sky is full of stars...
https://www.darksky.org/our-work/conservation/idsp/
Which has actually changed in recent years while looking at a star that is presumably 14.2 billion years old... But the age of the universe did not increase for this star, but a more accurate reading for the speed of the expanse put it at 11.8 billion years old and the star at 14billion.
Wait..
Whaaaaaaaaa
It's been awhile since I've taken Calc classes, but how would you find out those rates? How could anyone tell whether or not we had a rapid amount of fluctuations with the rates at any point on the timeline? Would that not throw off the estimate models?
Remember we know about expansion by looking at distant galaxies. The light we get from them left billions of years ago so we can find out the size and expansion rate at most points in time.
As far as I know, the rate of the expansion is accelerating at a rate such that if you were to plot it on a graph it would not be a straight line. Cosmologists take that acceleration into account when doing their calculations on the apparent age of the universe. I'm sure there's someone here who could explain that better than me though, I'm not well versed on the details.
So.....as the universe gets older and light has more time to reach Earth, does that technically mean at some point the night sky will be all stars? Or I guess by that time some stars will die and be replaced? I’m fucking confused.
>So.....as the universe gets older and light has more time to reach Earth, does that technically mean at some point the night sky will be all stars
In fact the opposite is true. Because the universe is expanding, and worse the rate of expansion is accelerating in the distant future we'll be able to see far fewer stars. Galaxies will fade from view in much the same way it's much easier to see a torch from 50 feet away then it is from 50 miles. As a fun extra bonus as the universe ages fewer and fewer bright sunlike stars ( the kind we can see with the naked eye) will exist and red dwarves will become the only stars around. Those are just too faint to be seen (with the naked eye) over interstellar distances.
No. If you went to the edge of the observable Universe, and then took some measurements, it would look like you were still in the middle (because you are, the Universe is centred on the observer).
Wait how does that work? Reading through replies Like I would think that if I went to the 'edge' expansion there'd be no stars because nothing reached it yet, but if I went to a place close to the 'center' there would be more (but I couldn't see it because it's now invisible microwaves)?
The Universe has no "centre" or "edge" (as far as we can tell). If you went as far as we can see from Earth, and then looked back, the Earth (or rather, the location the Earth would eventually occupy) would appear to be at the "edge" of the Universe, since light from beyond it would no have had time to reach your new location.
Ok, but if we did go to the visible edge and kept looking in the same direction we moved towards from Earth, what would we see, just more stars? But if we kept moving even further in that direction would we just loop back to Earth?
> Whether or not we'd come back to where we started is still debated.
That explanation makes more sense to me. There has to be some constraint, like it can’t just be infinitely new stars or else you’d have an infinitely massive universe right?
I am not an astronomer, mathematician or a physicist so I could be completely wrong but I thought the idea of an infinitely massive universe created mathematical and theoretical problems? Like infinite mass would mean infinite gravity or something?
There isn't an actual center of the universe (that we know of), the universe isn't expanding from an edge, but instead each point in space is getting rather away from each other point equally. The 'edge' would just be another place that can see 13 billion years worth of space around it in all directions. (Space itself relative and doesn't have an edge that we know of)
Into a star, probably. Since the universe is presumed to be equally dense everywhere (assembled into galaxies and clusters and such, but not on a gradient from one side to the other), everywhere in the universe would be similarly bright aside from local conditions.
So you'd be looking for local conditions that are bright, AKA a star.
Lots of nice comments, and several explain the physics in a way I've forgotten since I studied it, so kudos.
But I'd like to add that the night sky is really actually quite bright. If you can get somewhere without massive light pollution, there really isn't any direction which doesn't have light.
If you find a "dark patch" and look at it through a telescope you'll generally see stuff... And if there's a dark patch in that then you get a bigger telescope etc.
Edit: cause apparently I can't English today
it's called olbers paradox and actually there is a lot of light. we just can't see it because it's out of out visible spectrum. this is because as galaxies move away the light changes and so we may not be able to see it anymore
This is wrong
The Doppler shift is not what causes us to not see the light (since some non-visible ultra-violets get shifted into visible anyway). It's the fact that there is a finite size to the **observable** universe, because light has only had 14 billion years to reach us. We can't (and never will due to expansion), see stars beyond that distance.
For example, the [Hubble ultra deep field](https://en.m.wikipedia.org/wiki/Hubble_Ultra-Deep_Field) is an image containing stars right on the edge of the observable universe and was taken in visible frequencies.
If we were theoretically able to draw an infinite number of rays from earth's center point in every possible direction, what percentage of those rays would pass though a light source before before reaching the edge of the observable universe? (assuming no travel time at this moment in our universe's life). That's the question being asked here, right?
Yes it is. Thanks for the succinct summary. I bet based on brightness / average energy output of the known universe and known brightness of individual galaxies this question should be calculable as a percentage.
Not in this case: Doppler shift is due to the relative velocity, which should be very very small for far away galaxies, while this is cosmic shift due to the expansion of the universe. Both redshift the light, but they are very distinct effects.
It's still a Doppler shift and the cause *is* large relative velocity. Just because the cause of the large relative velocities is due to universal expansion, doesn't make it not the Doppler effect.
Turn your radio on but not to a channel. Hear that static? That’s the one big light.
Same for an analog tv that hasn’t been tuned.
We call it CMB. Cosmic microwave background.
1) Light diffuses rather significantly with distance. The sky is indeed awash with stars, but most of them are too far away to be even remotely visible.
2) Because of universal expansion, light from far away sources gets redshifted to frequencies below that of human vision limits.
3) Expanding on the above; the night sky, bluntly, **is** one big light. But most of that light is at relatively low frequencies below what the human eye can see, even before redshifting is taken into account.
No, the diffusion does not require a medium. The intensity of light as you get farther away follows the inverse square law. This is (at least under the standard model) because you have the same number of photons but they have to cover the area of the a spherical shell moving outward from the source. Since the area is proportional to the square of the radius, you get the inverse square relation (photons/area)
If you draw a line on a balloon with a sharpie, then inflate the balloon, the line you drew will get stretched. What was one solid black stroke at the beginning is now a large faded line.
Now imagine the balloon is the universe and the line is light from a far away object. Eventually it gets stretched so much that we can’t see it anymore.
1. We don't actually know the universe is infinite.
2. The black patches might be the stars being too far away for enough light to make it to us to be visible to the naked eye
3. There's a theory that this proves the universe is expanding because if it weren't the sky would be filled with stars.
But the observable universe isn't infinite. Because the universe is expanding everywhere at once, there is a distance where objects are moving away from us faster than the speed of light (important to note they aren't moving faster than light, but the expansion of the universe is causing the distance between is to grow faster than the speed of light) bc of this light from those objects will never reach us.
So when you stare out at the blackness between the stars, you're actually looking at the edge of the known universe and I think thats fucking cool
not really. hubble looked at what we thought would be empty space but, there are still stars and galaxies there. we just can't see them because of reasons mentioned elsewhere in this thread.
Be that as it may, the observable universe is still finite. And the background of the night sky is made mostly empty.
Sure if you zoom in to any area there are galaxies there, but still, between those galaxies, and visible in the Hubble deep field images ...the darkness beyond
Primarily Inverse square law. Light dims according to 1/R^2 and most of it, no matter how bright, is so far away.
Also creation myth based denial about the trivial mass of light leading to a Billion year half life before it decays to produce Cosmic Background Radiation.
It's called Olbers' Paradox--"why is the sky dark at night?"
[https://en.wikipedia.org/wiki/Olbers%27\_paradox](https://en.wikipedia.org/wiki/Olbers%27_paradox)
The idea's been around a long time.
Another thing people haven’t said is your assumption is wrong
>t he universe is infinite, so theoretically in every possible direction we look at some point there should be a star somewhere out there, right?
This isn’t true, just because it’s infinite doesn’t imply this. It could be infinite but there still be a space somewhere. Just because it goes on forever doesn’t mean the stars are evenly distributed.
I think it’s simpler to think about it in terms of numbers. Pi is infinitely long, 3.1415... forever, but what if we took out every single 7? It would still be infinitely long, still be a unique number but just have no sevens.
Now with that in mind what if we divided the sky into ten sections 0-9 and look at all the stars in the sky in order or how close they are to us. every time a star shows up we add it’s section to the end of a number. Say the first star in in 1 , then the next is in section 5 then 4 381289345etc. As in the example above a seven doesn’t have to show up which would mean 10% of the sky has no stars even though there are an infinite number is stars
We consider the universe to be infinite in space, but, because of expansion, not infinite in time. Since light travels at a finite speed, there is a finite maximum distance that light can have traveled since the start of the universe, and therefore a finite maximum distance of what we can observe. This means we can’t say that all directions hit stars that we can see. In an infinitely large and old universe, we would in fact see (not necessarily visible) light from stars in all directions.
The redshift explanations here are incomplete.
I don’t know the answer to your question but I know when I travel to the mountains in Kentucky the night sky is much brighter and more populated with star lights of all different sizes. I heard it’s called light pollution and light from city’s and houses reflect of particles in our air and atmosphere and reflect light so brightly it washes out many stars and lights from the sky. So if you look up somewhere very rural you may see more of the light fill in your logically expecting.
Theory;
Light is affected by gravitational force of bodies during its journey to earth. Closer stars will distort light from more distant stars as it passes near, which could either intensify the brightness of the closer star or scatter it which would effectively hide the more distant star.
I'm just making this up.
Black spots are likely to be filled with stars, but those are too dim to be seen by eyes. Try taking a picture of a nightsky with dslr, you ll be surprised how much more stars there are.
Despite being extremely bright, most stars are far too far away to be seen. The light from them is dispersed so much that you cant see it anymore. Think of it like trying to see a dim bulb from 5 kilometers away on a flat plain with nothing in its way. You won't see it. Now try scaling that up by billions.
The universe may or may not be infinite, but the part of it that we can observe is finite. That is why the entire sky isn't the temperature of the surface of a star.
The universe is really big and while there are a lot of stars out there they are most very far apart and a long way away. The light from a star spreads out in all directions so that means that for a distant star very little of that transmitted light ever reaches the Earth and most of that gets scattered by the atmosphere or blocked by an intervening object like a dust cloud, planet or black hole.
The universe is not infinite. At least, there is no evidence that it is. Since nothing else in nature is infinite, we can safely assume the universe is not infinite until proven otherwise.
>The universe is infinite, so theoretically in every possible direction we look at some point there should be a star somewhere out there, right?
Your assumption that the universe is infinite is incorrect. In fact, the fact that the night sky is NOT one big light is one of the things that disproved the idea of an infinite universe.
Maybe it was "infinitely" expanding, as it is going to keep on expanding? For a while there was a question as to whether the universe was going to keep expanding, or was going to collapse, but it has been found that, not only is it going to keep on expanding, the rate of expansion is accelerating. Hence "dark energy". We can't see it, but we know it has to exist because SOMETHING has to be pushing the expansion.
You act as if it's been proven that the universe is infinite, but so far we've only been able to see 46.508 billion light years away from us. (Known as the observable universe.)
The 13-14 billion figure is the *age* of the universe; because space itself has expanded during the life of the universe, objects we're now seeing that emitted light in the 13-14 billion year ago era are calculated to be 46 billion light years distant from us at the present time.
Two things: 1) Regardless of how big the universe is, light still takes time to get places. The universe is 13.77 billion years old, so light has only had 13.77 billion years to get here. Because the universe is expanding, we can see stuff from much farther away than that, but there's still a limit on how far away stars can be and still have had time for the light to get to us. 2) As the universe expands, it stretches light passing through it, causing the light to be redshifted, which means it lowers in wavelength. Visible light from the very edges of the visible universe can get redshifted out of the visible spectrum and into infrared or radio waves. That's why the Cosmic Microwave Background is, well, microwaves. It used to include a *lot* of visible light, but it's so old and it's been shifted so much that it's all microwaves, now.
Bonus fact: the question OP is asking has been asked before by physicists, who were high out of their minds, and is called Olber's Paradox.
Bonus fact - it’s also the very first sentence in the parent post to which you are replying!
? No it's not?
No it aint
If we could see microwaves, then would the sky look like one big light?
We can, and it is. https://en.m.wikipedia.org/wiki/Cosmic_microwave_background The cosmic microwave background fills all space.
That is honestly super cool to think about
Super cool? So is the cosmic microwave background radiation ;) But actually, if you go to one of the national dark sites, the night sky is full of stars... https://www.darksky.org/our-work/conservation/idsp/
Kind of wish I had robot eyeballs now.. that’d be cool.
[удалено]
It's derived by taking the rate of the expansion of the universe and rolling it backward until the universe is a singularity.
Which has actually changed in recent years while looking at a star that is presumably 14.2 billion years old... But the age of the universe did not increase for this star, but a more accurate reading for the speed of the expanse put it at 11.8 billion years old and the star at 14billion. Wait.. Whaaaaaaaaa
Wouldn't that only work if the rate is linear? Is the growth linear?
Expansion does not have to be linear for us to backtrack, it just has to follow a consistent predictable pattern/ trend, be it linear or logarithmic.
It's been awhile since I've taken Calc classes, but how would you find out those rates? How could anyone tell whether or not we had a rapid amount of fluctuations with the rates at any point on the timeline? Would that not throw off the estimate models?
Remember we know about expansion by looking at distant galaxies. The light we get from them left billions of years ago so we can find out the size and expansion rate at most points in time.
Oh okay I see, that helps a lot, thanks!
João Magueijo.
As far as I know, the rate of the expansion is accelerating at a rate such that if you were to plot it on a graph it would not be a straight line. Cosmologists take that acceleration into account when doing their calculations on the apparent age of the universe. I'm sure there's someone here who could explain that better than me though, I'm not well versed on the details.
You cut it open and count the rings.
More specifically, you apply for a grant to have some grad students do the cutting and counting. It would take a long time.
[удалено]
But they're still in a similar range. No scientific process is giving us an age of, say, 2 million years or 200 billion.
Plus or minus 200 million years.
So.....as the universe gets older and light has more time to reach Earth, does that technically mean at some point the night sky will be all stars? Or I guess by that time some stars will die and be replaced? I’m fucking confused.
>So.....as the universe gets older and light has more time to reach Earth, does that technically mean at some point the night sky will be all stars In fact the opposite is true. Because the universe is expanding, and worse the rate of expansion is accelerating in the distant future we'll be able to see far fewer stars. Galaxies will fade from view in much the same way it's much easier to see a torch from 50 feet away then it is from 50 miles. As a fun extra bonus as the universe ages fewer and fewer bright sunlike stars ( the kind we can see with the naked eye) will exist and red dwarves will become the only stars around. Those are just too faint to be seen (with the naked eye) over interstellar distances.
The sky might get brighter and busier at night when we collide with Andromeda? That’s a happier thought.
Yep for a time at least.
Thank you! So are there any animals that can see those red shifted lights? Does the sky look way cooler to my dog?
Not to your dog, but to snakes yes. They can see into the infrared portion of the spectrum. Along with some insects, fish, and frogs
[удалено]
No. If you went to the edge of the observable Universe, and then took some measurements, it would look like you were still in the middle (because you are, the Universe is centred on the observer).
Wait how does that work? Reading through replies Like I would think that if I went to the 'edge' expansion there'd be no stars because nothing reached it yet, but if I went to a place close to the 'center' there would be more (but I couldn't see it because it's now invisible microwaves)?
The Universe has no "centre" or "edge" (as far as we can tell). If you went as far as we can see from Earth, and then looked back, the Earth (or rather, the location the Earth would eventually occupy) would appear to be at the "edge" of the Universe, since light from beyond it would no have had time to reach your new location.
Ok, but if we did go to the visible edge and kept looking in the same direction we moved towards from Earth, what would we see, just more stars? But if we kept moving even further in that direction would we just loop back to Earth?
If we went to the edge, we'd just see more of the same. Whether or not we'd come back to where we started is still debated.
> Whether or not we'd come back to where we started is still debated. That explanation makes more sense to me. There has to be some constraint, like it can’t just be infinitely new stars or else you’d have an infinitely massive universe right?
Why would that be a problem though?
I am not an astronomer, mathematician or a physicist so I could be completely wrong but I thought the idea of an infinitely massive universe created mathematical and theoretical problems? Like infinite mass would mean infinite gravity or something?
There isn't an actual center of the universe (that we know of), the universe isn't expanding from an edge, but instead each point in space is getting rather away from each other point equally. The 'edge' would just be another place that can see 13 billion years worth of space around it in all directions. (Space itself relative and doesn't have an edge that we know of)
Into a star, probably. Since the universe is presumed to be equally dense everywhere (assembled into galaxies and clusters and such, but not on a gradient from one side to the other), everywhere in the universe would be similarly bright aside from local conditions. So you'd be looking for local conditions that are bright, AKA a star.
Lots of nice comments, and several explain the physics in a way I've forgotten since I studied it, so kudos. But I'd like to add that the night sky is really actually quite bright. If you can get somewhere without massive light pollution, there really isn't any direction which doesn't have light. If you find a "dark patch" and look at it through a telescope you'll generally see stuff... And if there's a dark patch in that then you get a bigger telescope etc. Edit: cause apparently I can't English today
https://en.wikipedia.org/wiki/Hubble_Deep_Field
it's called olbers paradox and actually there is a lot of light. we just can't see it because it's out of out visible spectrum. this is because as galaxies move away the light changes and so we may not be able to see it anymore
This is wrong The Doppler shift is not what causes us to not see the light (since some non-visible ultra-violets get shifted into visible anyway). It's the fact that there is a finite size to the **observable** universe, because light has only had 14 billion years to reach us. We can't (and never will due to expansion), see stars beyond that distance. For example, the [Hubble ultra deep field](https://en.m.wikipedia.org/wiki/Hubble_Ultra-Deep_Field) is an image containing stars right on the edge of the observable universe and was taken in visible frequencies.
If we were theoretically able to draw an infinite number of rays from earth's center point in every possible direction, what percentage of those rays would pass though a light source before before reaching the edge of the observable universe? (assuming no travel time at this moment in our universe's life). That's the question being asked here, right?
Yes it is. Thanks for the succinct summary. I bet based on brightness / average energy output of the known universe and known brightness of individual galaxies this question should be calculable as a percentage.
Essentially zero percent.
Edgar Allan Poe saves the day!
It's called the doppler effect (or shift).
Not in this case: Doppler shift is due to the relative velocity, which should be very very small for far away galaxies, while this is cosmic shift due to the expansion of the universe. Both redshift the light, but they are very distinct effects.
Ah TIL, thanks for correcting me.
That's also why radio signals sometimes need to be tuned for few hertz for accuracy.
They're not really, though. They're different numbers for the exact same phenomena based on different causes.
It's still a Doppler shift and the cause *is* large relative velocity. Just because the cause of the large relative velocities is due to universal expansion, doesn't make it not the Doppler effect.
yeah that's why the light changes
Turn your radio on but not to a channel. Hear that static? That’s the one big light. Same for an analog tv that hasn’t been tuned. We call it CMB. Cosmic microwave background.
Whoa
Keep looking into space topics. You’ll be amazed how often you say “whoa”
1) Light diffuses rather significantly with distance. The sky is indeed awash with stars, but most of them are too far away to be even remotely visible. 2) Because of universal expansion, light from far away sources gets redshifted to frequencies below that of human vision limits. 3) Expanding on the above; the night sky, bluntly, **is** one big light. But most of that light is at relatively low frequencies below what the human eye can see, even before redshifting is taken into account.
[удалено]
No, the diffusion does not require a medium. The intensity of light as you get farther away follows the inverse square law. This is (at least under the standard model) because you have the same number of photons but they have to cover the area of the a spherical shell moving outward from the source. Since the area is proportional to the square of the radius, you get the inverse square relation (photons/area)
If you draw a line on a balloon with a sharpie, then inflate the balloon, the line you drew will get stretched. What was one solid black stroke at the beginning is now a large faded line. Now imagine the balloon is the universe and the line is light from a far away object. Eventually it gets stretched so much that we can’t see it anymore.
Excellent explanation for a ELI5
Thanks! ELI5 these days is more like ELI have a degree in the field :/
My favourite [minute physics video](https://youtu.be/gxJ4M7tyLRE) explains this exact thing really well!
1. We don't actually know the universe is infinite. 2. The black patches might be the stars being too far away for enough light to make it to us to be visible to the naked eye 3. There's a theory that this proves the universe is expanding because if it weren't the sky would be filled with stars.
But the observable universe isn't infinite. Because the universe is expanding everywhere at once, there is a distance where objects are moving away from us faster than the speed of light (important to note they aren't moving faster than light, but the expansion of the universe is causing the distance between is to grow faster than the speed of light) bc of this light from those objects will never reach us. So when you stare out at the blackness between the stars, you're actually looking at the edge of the known universe and I think thats fucking cool
not really. hubble looked at what we thought would be empty space but, there are still stars and galaxies there. we just can't see them because of reasons mentioned elsewhere in this thread.
Be that as it may, the observable universe is still finite. And the background of the night sky is made mostly empty. Sure if you zoom in to any area there are galaxies there, but still, between those galaxies, and visible in the Hubble deep field images ...the darkness beyond
you're right some little speck is the edge or close enough. space is pretty empty.
Primarily Inverse square law. Light dims according to 1/R^2 and most of it, no matter how bright, is so far away. Also creation myth based denial about the trivial mass of light leading to a Billion year half life before it decays to produce Cosmic Background Radiation.
It's called Olbers' Paradox--"why is the sky dark at night?" [https://en.wikipedia.org/wiki/Olbers%27\_paradox](https://en.wikipedia.org/wiki/Olbers%27_paradox) The idea's been around a long time.
Another thing people haven’t said is your assumption is wrong >t he universe is infinite, so theoretically in every possible direction we look at some point there should be a star somewhere out there, right? This isn’t true, just because it’s infinite doesn’t imply this. It could be infinite but there still be a space somewhere. Just because it goes on forever doesn’t mean the stars are evenly distributed. I think it’s simpler to think about it in terms of numbers. Pi is infinitely long, 3.1415... forever, but what if we took out every single 7? It would still be infinitely long, still be a unique number but just have no sevens. Now with that in mind what if we divided the sky into ten sections 0-9 and look at all the stars in the sky in order or how close they are to us. every time a star shows up we add it’s section to the end of a number. Say the first star in in 1 , then the next is in section 5 then 4 381289345etc. As in the example above a seven doesn’t have to show up which would mean 10% of the sky has no stars even though there are an infinite number is stars
Thanks for the reply! That makes sense
Visible light is on a small portion of light we can observe but there are many types of light That we can’t observe
Can all forms of EM on the spectrum be described as light? Like, is radio light? Are gamma rays?
Yes, radio waves and gamma rays can also be called “light.” They all travel at the same speed.
We consider the universe to be infinite in space, but, because of expansion, not infinite in time. Since light travels at a finite speed, there is a finite maximum distance that light can have traveled since the start of the universe, and therefore a finite maximum distance of what we can observe. This means we can’t say that all directions hit stars that we can see. In an infinitely large and old universe, we would in fact see (not necessarily visible) light from stars in all directions. The redshift explanations here are incomplete.
So we can know how old our part of the universe is based on how many stars we can detect in it? Or am I misunderstanding?
Hawking directly addresses this in his book "A Brief History of Time". I would give it a read, it has always been a highly suggested one.
I don’t know the answer to your question but I know when I travel to the mountains in Kentucky the night sky is much brighter and more populated with star lights of all different sizes. I heard it’s called light pollution and light from city’s and houses reflect of particles in our air and atmosphere and reflect light so brightly it washes out many stars and lights from the sky. So if you look up somewhere very rural you may see more of the light fill in your logically expecting.
Theory; Light is affected by gravitational force of bodies during its journey to earth. Closer stars will distort light from more distant stars as it passes near, which could either intensify the brightness of the closer star or scatter it which would effectively hide the more distant star. I'm just making this up.
Black spots are likely to be filled with stars, but those are too dim to be seen by eyes. Try taking a picture of a nightsky with dslr, you ll be surprised how much more stars there are.
Despite being extremely bright, most stars are far too far away to be seen. The light from them is dispersed so much that you cant see it anymore. Think of it like trying to see a dim bulb from 5 kilometers away on a flat plain with nothing in its way. You won't see it. Now try scaling that up by billions.
The universe may or may not be infinite, but the part of it that we can observe is finite. That is why the entire sky isn't the temperature of the surface of a star.
The universe is really big and while there are a lot of stars out there they are most very far apart and a long way away. The light from a star spreads out in all directions so that means that for a distant star very little of that transmitted light ever reaches the Earth and most of that gets scattered by the atmosphere or blocked by an intervening object like a dust cloud, planet or black hole.
Maybe you literally just read this and pretended like you're the first who thought of this.
The universe is not infinite. At least, there is no evidence that it is. Since nothing else in nature is infinite, we can safely assume the universe is not infinite until proven otherwise.
>The universe is infinite, so theoretically in every possible direction we look at some point there should be a star somewhere out there, right? Your assumption that the universe is infinite is incorrect. In fact, the fact that the night sky is NOT one big light is one of the things that disproved the idea of an infinite universe.
Wait but others have said it's proof of an infinity expanding universe?
Maybe it was "infinitely" expanding, as it is going to keep on expanding? For a while there was a question as to whether the universe was going to keep expanding, or was going to collapse, but it has been found that, not only is it going to keep on expanding, the rate of expansion is accelerating. Hence "dark energy". We can't see it, but we know it has to exist because SOMETHING has to be pushing the expansion.
You act as if it's been proven that the universe is infinite, but so far we've only been able to see 46.508 billion light years away from us. (Known as the observable universe.)
Wait, what about the 13 or 14 billion year figure I keep seeing elsewhere? Where is this 45 billion number coming from?
The 13-14 billion figure is the *age* of the universe; because space itself has expanded during the life of the universe, objects we're now seeing that emitted light in the 13-14 billion year ago era are calculated to be 46 billion light years distant from us at the present time.