Why is the sky dark at night?

[peadar1987]

Maybe because the universe is expanding at the same speed, or even faster than the speed of light? Just my candy wrapper in the pocket (not even 2 cent).

My understanding is that no matter how fast two things are receding from each other, light will always take the same time to travel between them, because reference frames (although it will be either redshifted or blueshifted, depending on the direction of motion)

[ZetaX]

My understanding is that no matter how fast two things are receding from each other, light will always take the same time to travel between them, because reference frames (although it will be either redshifted or blueshifted, depending on the direction of motion)

If two objects A, B are one light year apart and don't move relative to each other, then light from A will need 1 year to reach B.

If now B accelerates away from A up to a speed of c/2, then light from A will need 2 years (in A's frame of reference) to reach B. It's 1.73 years in B's frame of reference.

The problem is that the distance changes.

[YNM]

In which coordinate system?

Ours. I don't see how other frames of reference are relevant.

Edit: actually, why is it relevant¿ The infiniteness of space and the finiteness of time does not depend on an observer moving slower than c.

Reference frame =! coordinate system

Only one coordinate system follows the expansion / contraction of space - comoving coordinates. Also, you should look up friedmann equations and the history of dark energy theory and why it was needed, you'll see the infiniteness of time in an infinite, static space.

If two objects A, B are one light year apart and don't move relative to each other, then light from A will need 1 year to reach B.

If now B accelerates away from A up to a speed of c/2, then light from A will need 2 years (in A's frame of reference) to reach B. It's 1.73 years in B's frame of reference.

The problem is that the distance changes.

That's why we have coordinate distance. Proper distance can change, coordinate distance may stays the same.

[WinkAllKerb'']

yup "time" may be one of the only word in any dictionnary that isn't defined using a contrast process ... a bit old somehow ...

[PB666]

Very simple question: why is the sky dark at night?

Sky, dark and night are words. Dark means absence of visible light. visible is what are rods and cones can detect. If there is electro magnetic radiation present and we cannot see much of it, then its because we lack adequate sensitivity. Night is part of the diurnal cycle, the earths rotation places the outward facing surface oriented away from the sun, the major source of visible light in proximity to the earth. Therefore the next question is what determines luminosity on the radially facing surfaces.

1. Universe. According to one researcher the Universe was intially dark, with temperatures so high matter could not exist and light only existed momentarily. As the universe cooled it created the progenitor of the cosmic background radiation CMB during the opaque phase. This EM is ever present but we cannot see it, our rods and cones only see down to the near IR range. Our aged universe has spread matter and energy out considerably.

2. Our galaxy. There would have been a time when our galaxy would have been bright with very blue stars and gas giants made of hydrogen. These eventually exploded resulting in stars and planets. Earth is one such planet. The aging of our galaxy has significantly red shifted the wavelength of stars produces, many stars are smaller, and aged and no longer produce shorter wavelengths of light. Once we merge with andromeda the science predicts a rapid red-shifting of wavelengths due to the merging of black holes and x-ray emmisions that drive hydrogen into intergalactic space.

3. Our local group. Not very crowded, certainly not crowded with bright stars. None that are in any close proximity to our star. As alpha-centuari approaches the part of the earth will have a brighter night, but only slightly.

4. Our system. No bright planets in close proximity to earth (mars is smallish and a dull red).

5. Earths system. Earth does not have many moons, and the moon we have is not really close. Earth could have been in a polar orbit of a gas giant and thus part of earths night would have been illuminated.

6. Urban sky. Of course I see no stars at night, I could almost read by the background EM at night because of light reflected off of tiny water droplets in the night air.

7. Your house. Turn on the light.

[PB666]

what if a photon "fade"/"vanish" at some point for some reason ? ... just a though

(beside the fact it could prevent to see the next closest "universe" as we know it ? primitive soup / atom and molecule more or less related metaphorically)

Photons are what we intercept' date=' the wave form travels, wave particle duality of light. The wave does not disappear, although annihilation under certain circumstances is possible. The waveform cannot fade, but in different inertial reference frames it can shift.

As I understand it CMB was once in the high energy gamma portion of the spectrum in its reference frame. However as the universe expanded and as the light moved into new inertial reference frames (can't really have inertia without matter, and the cooling universe created matter, which was traveling out from its center of expansion at very high velocities. The further the Opaque light travels from its center, the more different the inertial reference frames are and the more red shifted the light becomes, at one point it was visible light (100s of nm wavelength), somewhere a few billion years ago the outward expansion of the Universe increased, this along with the travel of CMB light eventually results in the sub infrared light seen as the cosmic microwave background radiation. Of course you can argue what about the opaque light from [i']our part of the Universe, should we not still see gamma, and the answer is that this light has moved away from our (milky way) intertial frame, farther than the furthest blue star generation 1 galaxies that we can see (obviously matured billions of years now), and because those observers are moving away from us at close to the speed of light, they only see CMB also.

Since light travels as a wave and since moving apart stretches the wave out (since light can never travel faster than the speed of light no matter how much faster we travel outward, it always travels the same speed, but for each wave peak to reach us gets stretched out from our reference point of view). There is some CMB radiation from some areas of the universe that will never reach us. Because of cosmic inflation this placed some inertial reference frames an estimated 10s of billion light years from us, though the distance from the opaque era is much smaller, it will have to travel 10s of billions of years more to reach earth. The distance of what we can see back to CMB defines our Universe in the strictest since. And since our Universe is essentially uniform and exhibits euclidian geometry in all directions, this universe neither has a center or edge. It is boundless in all directions.

In the sense that light may travel endlessly and never reach us, it vanishes, but if you placed yourself on a near light speed ship and traveled in the direction of that light it would appear.

I don't think it is possible for the spectrum to recede so far in to radio waves that it finally cannot be observed. Technically it may be possible, at that time I think the bigger concern for an observer would be how does life exist close to 0'K, since it is ambient radiation that inevitably determines temperature.

Lets make the statement that light could disappear spontaneously. OK then there would have to be a random rate, 1% per year. You would not observe other galaxies and most of the milky way would become invisible. 1% per million years you could not observe many other galaxies, 1% per billion years CMB would start to dissappear and become anisotropic. So if light could disappear it would have to be on the order of less than say 10% per billion years 1/1E15*sec. But since even over 1E15 seconds light does not age; and since anything traveling the speed of light cannot observe the progression of time, it would mean that light would have to be traveling less than the speed of light, which is, in and of itself, a contradiction. And if light were to decay, what would light decay into. This does happen, but I think that the interference that is created has to be approaching Comptons wavelength, and the differential wavelength is something rarely encountered in space at this age of the universe; and is so energetic that it can create matter and antimatter pairs.

Edited July 21, 2015 by PB666

[YNM]

But we see stars... And galaxies are stars... They're so many, why we don't see all of them fill the sky with lustre ? Surface brightness don't fall with distance (from observer), why those faraway galaxies haven't filled the sky with sun-like surface brightness ? Even if the clouds are there why they aren't heated just like in the daylight, out of the immense amount of radiation ?

(reiterating the original contemplation I know, but that's because I can and it's a good way to start some good, unwild fire due to it's age...)

[WinkAllKerb'']

i was more on the counter process in fact like almost all being we know ... time counter end then (pfffft) ... also thinking that our period of observation rescaled is pretty short to have a clear idea of the concept ... so on i m just saying like if a photon has it's own dna, as we're unable to read our own dna i won't be surprised we're unable to read a photon dna yet ... that's more or less the idea

[SAI Peregrinus]

You really shouldn't. That video is full of errors. First of all, there is no point in space from which light has not reached us, because while it's true that some parts of the universe are more than 14bly away from us, it's only due to expansion of the universe being faster than light. Light from eeach point still reaches us eventually, and since all known universe started in the same point, there isn't a part of it which we cannot see.

That's wrong. Light from any point more distant than the Hubble length will never reach us, even provided infinite time. Also, it's not known (or even widely accepted by cosmologists) that the universe started in the same point. It's a much more common view that the observable universe started as a region of a much larger (possibly infinite) universe which expanded in the big bang/inflationary period. This page explains it well.

And if the expansion of the universe is expanding (there's some evidence that it is, but it's non-conclusive) then some points which were within our past light cone at one time will move out as the space between us and the point expands. So even if you had a singularity to start you'd still get a universe larger than the observable universe, and light from some previously observable points would eventually become non-observable.

[NFUN]

yep i really like to see the universe like a living molecule slowly floating in space and photon like small living thing too. it does the tricks pretty well and sound logic and not against any known science law right ; maths specialist ?

(yup nfun but if photon has smaller thing inside them we can't measure yet ? including some sort of life span related process thing ? )

I don't think there are any serious theories or scientists that state photons are anything but fundamental. Photons are the quanta of light, and are tiny bits of pure EM energy. They certainly do not have anything inside them, and they don't decay, being the carrier of one fundamental force (Electromagnetism) that can cause decay, and unaffected by the other (the Weak Force).

[K^2]

That's wrong. Light from any point more distant than the Hubble length will never reach us, even provided infinite time.

Which is totally irrelevant. We don't care if there is a point out there from which light emitted now will not reach us. We care about light we're seeing now. And we can see all the way back to Big Bang. In other words, every single point in known space is visible with some sort of a time delay ranging between 0 and age of the universe. There are points whose CURRENT state we'll never see. But there is no point which we cannot see at all.

No it is not. It is the point of relativity, yes, but obviously not of bright skies. It's our sky we are talking about, not any other.

And "our" sky somehow defines a coordinate system?

Arbitrary long is still finite.

You should look up definition of infinite.

Why¿ Only the past is necessarily finite, we have no assumption on the future.

That still lets me place a boundary on the universe. A solid wall in space which you cannot cross. Infinite space in context implies infinite in any direction. If you have a starting point to time, then a trivial Lorentz boost makes that also be a starting point of space. That places a definitive age.

If you're going to be picky about "infinite is infinite," then I'll just point out that having a boundary would break the homogeneity restriction.

In either case, we're back to infinite, static, homogeneous being sufficient, because they imply everything else, including infinite or cyclic time.

Edited July 21, 2015 by K^2

[YNM]

Which is totally irrelevant. We don't care if there is a point out there from which light emitted now will not reach us. We care about light we're seeing now. And we can see all the way back to Big Bang. In other words, every single point in known space is visible with some sort of a time delay ranging between 0 and age of the universe. There are points whose CURRENT state we'll never see. But there is no point which we cannot see at all.

Proper distance when light emitted =! Distance travelled by light =! Proper distance when light received, no ? I know it's pretty hard to calculate and understand !

[Brotoro]

Which is totally irrelevant. We don't care if there is a point out there from which light emitted now will not reach us. We care about light we're seeing now. And we can see all the way back to Big Bang. In other words, every single point in known space is visible with some sort of a time delay ranging between 0 and age of the universe. There are points whose CURRENT state we'll never see. But there is no point which we cannot see at all.

Except that the universe was filled with an opaque gas prior to the Era of Recombination (about 378,000 years after the Big Bang)...so that photosphere, the source of the cosmic background radiation, is as far back as we can currently observe.

[ZetaX]

That first quote was before I realized that you are not talking about frames of references but actual coordinates here.

Now I don't get the insistance on coordinates. We are assuming there is one that satisfies the assumptions of (in)finiteness; we are not necessarily assuming that each of them does. But I don't know about subtleties of that term in the context of relativity.

Please write down your actual assumptions (especially how you define homogenity, infinite space and finite time), because I doubt we agree on the meaning of "infinite space" and "finite time".

You should look up definition of infinite.

There is a relevant disctinction between "arbitrary large" and "infinitely large". For example, the finite subsets of an infinite set are the former, but not the later. As a much closer example, look at [-1,1] \times \mathbb{R}: every line through the origin, with the exception of the vertical one, is of finite length. While not entirely the same as in the universe, any choice of two lines will induce coordinates, and only those few involving the vertical one will actuially have infinite components. Or take one of those subsets of the plane that have finite volume, yet arbitrary (or even ininiftely) long segments.

Both concepts _might_ be the same in our setting, but that requires an actual argument.

The real concern I actually have with your argument is by the way that you continue throwing in more and more physics from our real universe without explicitely stating that. If we go that far, I would claim that such a universe is already impossible to begin with (how is it static¿ why are Schwartzschild-radii not a concern¿ how did stars actually shine since forever if time is infinite¿ and so on...).

[Beowolf]

Anyone read "Nightfall" by Isaac Asimov? If not, feel ashamed and go read it ASAP.

Not only "Nightfall", but I remember a related essay where he went through all the math of dark night skies step-by-step. Thinking back, it amazes me how much of who I became as an adult can be traced back to his 1960s essays. I consumed several books of these essays when I was young, and credit Dr. Asimov with teaching me what science really is, vs the useless rote memorization they called science in my rural Kentucky high school. Yes, teacher, I still remember Avogadro's number. And I have never in my life needed it when there wasn't already a reference right at my fingertips. But learning how to think, and separate what I know from what I think should be? THAT was invaluable!

[K^2]

That first quote was before I realized that you are not talking about frames of references but actual coordinates here.

That's the same thing.

Now I don't get the insistance on coordinates. We are assuming there is one that satisfies the assumptions of (in)finiteness; we are not necessarily assuming that each of them does. But I don't know about subtleties of that term in the context of relativity.

Please write down your actual assumptions (especially how you define homogenity, infinite space and finite time), because I doubt we agree on the meaning of "infinite space" and "finite time".

For simplicity, lets leave just two coordinates and talk about flat space-time only. Consider a space-time that's infinite in spacial dimension, but does not extend back past t = 0. In that case, universe is a sub-set of R², such that t > 0. This universe is infinite in x, and has a finite age at any point in this set. Only a finite sub-set of points is visible from any one point due to light cone.

Consider a new coordinate system boosted at a Lorentz factor γ with respect to the original. Now the boundary is ct + βx > 0. Suddenly, universe has a wall. One that's moving at a speed c/β through space. If you have a definition of homogeneous or static to which this applies, please, let me know.

There is a relevant disctinction between "arbitrary large" and "infinitely large". For example, the finite subsets of an infinite set are the former, but not the later.

Sure. But if I can pick an arbitrarily large subset, it guarantees that set itself is infinite. You cannot pick an arbitrarily large subset from a finite set. I don't see how you can talk about a finite time when I can chose a coordinate system with an arbitrary time span.

The real concern I actually have with your argument is by the way that you continue throwing in more and more physics from our real universe without explicitely stating that.

On the contrary. I'm considering a more general case. I'm prepared to re-state all of the above with an arbitrary metric in arbitrary dimensions if you want. You cannot have a boundary one one coordinate (time) in one frame of reference, and not have boundaries on all other coordinates (spacial) in some other choices of frame of reference. A space without bounds requires time without bounds. Whether it's infinite or cyclical is dealer's choice.

[Camacha]

Second, it is absolutely not true that infinite, unchanging universe would result in bright sky. In fact, if this was true, our local group alone would have been sufficiently large. We should se stars in all directions. We do not, because distribution of stars is not uniform. Stars are bound in galaxies, galaxies in groups, groups in clusters, clusters in filaments. It's this fractal structure that guarantees that number of stars a given distance away from us does not rise as rapidly as brightness of stars drops due to the same distance.

[...]

I'm glad the video did not bring up another common misconception that interstellar dust is the cause of it. While it's true that we'd see a lot more light if there was no dust, if universe was, indeed, filled with stars uniformly, the interstelar dust would heat up due to starlight to the point where it itself glows same as the stars around it. So while interstelar dust makes the sky even darker, it's not the cause of it being dark to begin with.

If I understand correctly, you are saying that the lack of uniform distribution is the reason the sky is not bright. By logic, this means that some part of the sky are underpopulated, while others are overpopulated. Should this not mean that at least the overpopulated band of our galaxy should be bright? I know it is called the Milky Way for a reason, but the light is easily overpowered by even artificial light sources and is only a tiny fraction of for instance our sun. Somehow distribution does not seem to explain the lack of light at night.

[K^2]

If I understand correctly, you are saying that the lack of uniform distribution is the reason the sky is not bright. By logic, this means that some part of the sky are underpopulated, while others are overpopulated. Should this not mean that at least the overpopulated band of our galaxy should be bright? I know it is called the Milky Way for a reason, but the light is easily overpowered by even artificial light sources and is only a tiny fraction of for instance our sun. Somehow distribution does not seem to explain the lack of light at night.

Not at all. The whole point here is that we are always taking away. It's about fractals.

For any uniform distribution in an infinite universe, no matter how far apart the stars are spread, and no matter how small they are in comparison, some finite fraction of the infinite space is occupied by stars. This is not generally so if macroscopic structure of the universe is a fractal. A fractal arrangement allows finite densities of the stars here, near us, but for the total density of stars in the universe to be precisely zero. Note, that number of stars and quantity of matter in the universe is still infinite. It's just that an overall average density is zero.

A good, simple to understand example is the Serpinski triangle. We start with a large triangle, cut away 1/4 of it. Then cut away 1/4 of the remainder, and so on. What's the total area of all points that remain in the Serpinski triangle? Well, it's 1 - 1/4 - (3/4)(1/4) - (3/4)(3/4)(1/4) - ... I can re-arrange it to read 1 - (1/4) * sum((3/4)^n) for n = 0 to inf. That sum is easily evaluated as a geometric progression, and is exactly 4. (Wolfram Alpha). So the total area of Serpinski triangle is 1 - (1/4) * 4 = 0.

Despite that, near any vertex, local density of points in the Serpinski triangle can be quite high. Our Sun happens to be near such a vertex. It's a star in a galaxy, which is in a group, which is in a cluster, which is in a supercluster. Of course, it's not a perfect fractal, but in a finite universe, it doesn't have to be. Nonetheless, if galactic densities of stars persisted throughout known universe, the sky would be pretty much as bright and hot as stars themselves are. It's all the empty space between galaxies, and between groups, and between clusters and so on that keeps the sky dark. It's all the stars that aren't there.

[Camacha]

Despite that, near any vertex, local density of points in the Serpinski triangle can be quite high. Our Sun happens to be near such a vertex. It's a star in a galaxy, which is in a group, which is in a cluster, which is in a supercluster. Of course, it's not a perfect fractal, but in a finite universe, it doesn't have to be. Nonetheless, if galactic densities of stars persisted throughout known universe, the sky would be pretty much as bright and hot as stars themselves are. It's all the empty space between galaxies, and between groups, and between clusters and so on that keeps the sky dark. It's all the stars that aren't there.

Yes, but again, locally the densities are rather high. Especially when looking in the direction of the galactic core of the Milky Way, you have most (since we live in the outer edges) of a full galaxy of stars in a relatively narrow band or the sky. It seems that such a high density would mean the brightness of a sun or collection of suns, but it is not the case. Or even other galaxies. Andromeda (M31) has an apparent width of a number of Moons (or Suns, as they are almost as big in the sky). Most of what we see of it are its stars. Yet it is actually very faint, and not as bright as a disc shaped Sun.

It just does not seem to work out.

[Bill Phil]

Yes, but again, locally the densities are rather high. Especially when looking in the direction of the galactic core of the Milky Way, you have most (since we live in the outer edges) of a full galaxy of stars in a relatively narrow band or the sky. It seems that such a high density would mean the brightness of a sun or collection of suns, but it is not the case. Or even other galaxies. Andromeda (M31) has an apparent width of a number of Moons (or Suns, as they are almost as big in the sky). Most of what we see of it are its stars. Yet it is actually very faint, and not as bright as a disc shaped Sun.

It just does not seem to work out.

http://stargazerslounge.com/uploads/monthly_09_2013/post-6762-137922477212.jpg

That's due to angular size as well as distance, plus the light pollution from nearby stars.

[Camacha]

That's due to angular size as well as distance, plus the light pollution from nearby stars.

The angular size is the same or larger than that of our Sun and Moon, which both are many times brighter. Distance it itself does not change anything, only the angular size changes (which we already excluded) and maybe the possibility of getting blocked out by something else. Neither seems to be the case with M31 and even though dust is visible in the Milky Way band, most bits are unobscured. Finally, light pollution should make things brighter, not darker, so that does not seem to be it either.

I just do not see why things are different from, for instance, our own sun. Anything as bright as our Sun (or a number of suns) and as big (angularly) as our Sun should be as bright as our Sun. The only thing I can think of is that there might be some dithering going on and that what seems bright is actually mostly dark.

Edited August 8, 2015 by Camacha

[Aanker]

Well, the light also has to go through our own atmosphere. I'm sure the 'night sky' and the milky way in particular all look brighter from a spacecraft facing away from both the Earth and the Sun.

[Camacha]

Well, the light also has to go through our own atmosphere. I'm sure the 'night sky' and the milky way in particular all look brighter from a spacecraft facing away from both the Earth and the Sun.

Our sun has to penetrate the same atmosphere, yet has a rather different brightness. The atmosphere does not seem to be the cause.

[WestAir]

Like this?

It's an interesting thought game but I've got to go with K^2 on this. I will admit, however, that I personally I always (incorrectly) thought brightness decreased with distance to the point that very distant objects just weren't bright to us anymore. Thanks for the good read guys,

[Camacha]

Like this? [...] It's an interesting thought game but I've got to go with K^2 on this.

Yes, like that. Like I said, I do not see any reason why it would not work like that, other than maybe the dithering thing due to distribution.

If you think something else I am very curious what that is exactly, because I am pretty much stuck. An object the size of the Sun or more, made of stars, should produce the light of a Sun.

Edited August 8, 2015 by Camacha