Why the sky is dark in the night

There seems to be no agreement on this apparently innocuous looking point...which is the best theory for this.

 

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"Theory" ??!!

No "Theory" needed...

The "sky", i.e. the atmosphere, is more accurately, CLEAR "in the night", not "dark"...

Without Sol's radiation illuminating the "sky" (atmosphere) and turning it Blue*, it remains essentially CLEAR.

*Rayleigh scattering...

The following might be of some interest/help... : http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html

 

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The sky is dark at night, because there is no sun light. The sun light falls on the other side of the Earth.

 

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I think you need, first, to supply something to support your assertion that there is no agreement about this, if only to reassure potential contributors that this is not just some wild goose chase.

 

I assume you are referring to Olbers' Paradox. It has been resolved.

Just look up Olbers' Paradox or 'why is the night sky dark' and you will get your answer

 

Heh. I missed what this thread was really about. While I still expect we're being trolled, I'd just like to say that I find this issue (it has come up before) bizarre. Olber's Paradox never was a true paradox in need of resolution, it was intended from the start as a logical argument for what the structure of the universe appeared to be; No mystery, no problem. Just a simple thought exercise.

https://en.m.wikipedia.org/wiki/Olbers'_paradox

 

It is really interesting that one of the first people to propose a solution (a finite universe) is Edger Allen Poe. Is the universe infinite and static? Nevermore....

 

[tic mode on]
And God said “Let there be light,” and there was light. God saw that the light was good, and he separated the light from the darkness. God called the light “day,” and the darkness he called “night.” And there was evening, and there was morning the first day.


Is this what you are driving at?

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I've seen that, but I have a feeling it is apocryphal. The potential solutions and more to the point, what it potentially meant for the prevailing view of the universe were too obvious to not have been discussed. Poe was just a prolific/famous writer and at the same time, I'm unable to find references to Olbers' actual paper. Olbers may have been the one to best articulate it, but the basic problem (and at least some solutions) were known since at least as far back as Kepler.

 

You are right, it is about olbers' paradox...the wiki link and few other related works give an impression that solution is more of a proof for Big Bang......there are few alternatives also in the link and all are refuted, only the Big bang and the red shift survives...

There are issues and questions with it.....

1. In the early universe ( as per Big Bang) there should have been tremendous brightness, this got fainted as the expansion (inflation onwards) continued....the point it raises is that there should have been an epoc after which slowly the sky darkness would have started, the brightness started diminishing...ignoring the birth time of Earth, what was that time or alternatively what was CMBR value (present around 2.7 K) at which darkness was not there ?

2. There is no red shift between andromeda (and few other Galaxies) with respect to us, there are billions of stars in our own Galaxy and add billions from other non red-shifted stars from andromeda galaxy, still darkness ?

3. Ultimately it boils down to the amount of photon energy we receive from these stars, will we get desired darkness level if we calculate the energy received from stars ? Technically before we declare Big bang / Red shift as the solution of Paradox, should we not find out the energy received from our Galaxy stars (sun excluded) and andromeda stars and prove that they cannot cause any significant brightness in the night..

4. Are there planets, which are bright always, receiving significant light from multiple/two stars throughout their spin.

 

Ah, that's it, thanks. Indeed, it seems only to be a paradox if one assumes the universe is infinite. Infinities do seem to have a tendency to lead to absurdities.

 

rajesh played similar games: Starting with asking questions with no intention of accepting any answer.

No

For the first 380,000 years before temperatures had dropped sufficiently for electrons to couple with atomic nuclei, the Universe was opaque with just plasma and photons unable to travel.
After the recombination the light [brightness] was released for want of a better word] and we still see that today as the CMBR at a temperature of 2.7K

Not really sure, unless you are after the answer given at post 8.

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Andromeda is part of the local group of galaxies and all are gravitationally bound so that the large scale expansion is overcome...eg: Picture a fish swimming upstream at 10kms and hour [universal expansion] against a current of 12 kms an hour [gravity]
Remember, Olber lived around the 1800's when we were unaware the Universe was expanding.........
http://math.ucr.edu/home/baez/physics/Relativity/GR/olbers.html
Olbers' Paradox
Why isn't the night sky uniformly at least as bright as the surface of the Sun? If the Universe has infinitely many stars, then presumably it should be. After all, if you move the Sun twice as far away from us, we will intercept one quarter as many photons, but the Sun's angular area against the sky background will also have now dropped to a quarter of what it was. So its areal intensity remains constant. With infinitely many stars, every element of the sky background should have a star, and the entire heavens should be at least as bright as an average star like the Sun.

(We say "at least as bright" because the stars of such a bright universe would begin to absorb heat from their neighbours, and precisely what happens when a star is heated is a technical matter for thermodynamic and nuclear theories. We don't expect such stars to cool down, but neither do we expect them to heat up indefinitely. Olbers' Paradox originated before physicists had developed the nuclear theory of how stars shine; thus, it was never concerned with how old the stars might be, and how the details of their energy transactions might affect their brightness.)

The fact that the night sky is not as bright as the Sun is called Olbers' paradox. It can be traced as far back as Kepler in 1610, and was rediscussed by Halley and Cheseaux in the eighteen century; but it was not popularized as a paradox until Olbers took up the issue in the nineteenth century.

There are many possible explanations which have been considered. Here are a few:

1. There's too much dust to see the distant stars.
2. The Universe has only a finite number of stars.
3. The distribution of stars is not uniform. So, for example, there could be an infinity of stars,
   but they hide behind one another so that only a finite angular area is subtended by them.
4. The Universe is expanding, so distant stars are red-shifted into obscurity.
5. The Universe is young. Distant light hasn't even reached us yet.

The first explanation is just plain wrong. In a black body, the dust will heat up too. It does act like a radiation shield, exponentially damping the distant starlight. But you can't put enough dust into the universe to get rid of enough starlight without also obscuring our own Sun. So this idea is bad.

The premise of the second explanation may technically be correct. But the number of stars, finite as it might be, is still large enough to light up the entire sky, i.e., the total amount of luminous matter in the Universe is too large to allow this escape. The number of stars is close enough to infinite for the purpose of lighting up the sky. The third explanation might be partially correct. We just don't know. If the stars are distributed fractally, then there could be large patches of empty space, and the sky could appear dark except in small areas.

But the final two possibilities are surely each correct and partly responsible. There are numerical arguments that suggest that the effect of the finite age of the Universe is the larger effect. We live inside a spherical shell of "Observable Universe" which has radius equal to the lifetime of the Universe. Objects more than about 13.7 thousand million years old (the latest figure) are too far away for their light ever to reach us.

Historically, after Hubble discovered that the Universe was expanding, but before the Big Bang was firmly established by the discovery of the cosmic background radiation, Olbers' paradox was presented as proof of special relativity. You needed the red shift to get rid of the starlight. This effect certainly contributes, but the finite age of the Universe is the most important effect.

 

what makes you think that finite stars, although very large number will not be able to brighten up the night sky ?

You can always arrange a finite number of stars in multiple layers in finite universe such that entire sky is fully covered...only the infinity is not the solution of this..

 

So...you are answering why andromeda is blue shifted which is away from context.....

You are also suggesting that CMBR was 2.7 K since 380000 years after BB, that is also bad ??

 

Imagine a 3*3 grid, and in each corner is a star.

Actually, imagine each of those 9 squares is a 3*3 grid, with a star in each corner.

Actually, imagine each of those 81 squares is a 3*3 grid, with a star in each corner.

Actually, imagine...

And keep on going until you have countless squares and billions of stars each as small as a point of light... They still won't be enough to make the night sky as bright as day.

 

No, slow down, you are confused.

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You raised Andromeda [M31] but did not know why it is cosmologically blueshifted: I just explained it...simple as that.
And as anyone not burdened with an agenda would realise, I am not suggesting the CMBR was 2.7K 380,000 years post BB: It was about 3500K.
I'll try and make things simpler for you in future.

 

Not being an astronomer I have to rely on to the various more knowledgable discussions of this topic that are available.

Thanks to origin I now know what Olber's Paradox, so I've learnt something, but I am not able to enter a technical discussion of this topic without putting in a lot of effort that I am too busy and/or indolent to make. So if you don't mind Ill bow out of this and watch from the sidelines. PB's recent cut and paste seems very illuminating (haha).

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You jump to conclusion...this happens because of need of you to refer to google again and again (lack of formal knowledge)...If you knew that for this paradox red shift also plays a crucial role...you would not have made this comment.

Now if CMBR was 3500 K after t = 380000 years from t = 0 (not ago as you mention), then I am sure if our solar system was to be there in present form (assume) then the night sky would have been bright at 3500 K.....so what is that CMBR temperature and time after t = 0 at which the darkness came ?

 

It will be in order to thank me too as I started this thread so that we all understand something, despite my/PB games......such an innocuous point, evidenced daily and has a wonderful history....

 

Thats true, billions of stars are not able to make the sky any bright...but why ? Thats the point.