Why sky is Black at nights?

[alinoroozi]

hi

first sry for my bad English,

Why sky is black at nights ? I mean if we have millions of stars and all of them are shining then why

night's sky is not like a giant lamp?

I heard that its because of black holes, but I don't know if that's true.

 

[swansont]

That's a good question, and one that was thought about some time ago and is known as Olbers' Paradox. The answer is not black holes, though.

 

It would be true of an infinite-sized universe that is infinitely old (a steady-state universe). That we don't see that is an argument against a steady-state universe, and indeed, the universe has a finite age and is expanding.

 

 

http://hyperphysics.phy-astr.gsu.edu/hbase/astro/olbers.html

[Griffon]

Interesting how such a simple observation is telling us something fundamental about the universe.

[swansont]

Interesting how such a simple observation is telling us something fundamental about the universe.

Yes, indeed. It's a decent example of the basic process of science — the feedback you get from data, used to modify an hypothesis.

[daniton]

it's very bright.you just can't see it because it's full of infrared light.

[swansont]

it's very bright.you just can't see it because it's full of infrared light.

It's still not bright everywhere. It's bright where you know there are stars, plus some more, but not all over.

 

http://www.ipac.caltech.edu/outreach/Gallery/IRAS/psc_allsky.html

[Enthalpy]

Hi Alinoroozi, welcome here!

 

Don't worry for the language: English isn't my native one neither - but I'm happy that we share a language.

 

The main reason is that, due to the finite age of the Universe (some 14 thousand million years), we see only a finite part of it (14 thousand million light-years radius), from which light has had enough time to arrive to us. The rest is too far, and its light hasn't reached us up to now.

 

An other important contribution is that, because distant galaxies fly away from us, their light shifts to the red and infrared, and loses power in that shift.

[swansont]

Hi Alinoroozi, welcome here!

 

Don't worry for the language: English isn't my native one neither - but I'm happy that we share a language.

 

The main reason is that, due to the finite age of the Universe (some 14 thousand million years), we see only a finite part of it (14 thousand million light-years radius), from which light has had enough time to arrive to us. The rest is too far, and its light hasn't reached us up to now.

 

An other important contribution is that, because distant galaxies fly away from us, their light shifts to the red and infrared, and loses power in that shift.

We see farther than that, because of the expansion

http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN

https://en.wikipedia.org/wiki/Observable_universe

[Airbrush]

Besides the finite age of the universe and it is not steady state, I think the main reason the sky is not bright from an infinte number of stars is simply because of the vast distances to stars and galaxies and the feebleness of the light which cannot be seen with the naked eye. If you fix the most powerful telescope on a "black" area of space, you will find faint galaxies are there.

 

75% of stars in our galaxy are red dwarfs and NONE of them are visible to the naked eye.

Edited July 22, 2013 by Airbrush

[John Cuthber]

" If you fix the most powerful telescope on a "black" area of space, you will find faint galaxies are there."

Yes, you will, and in those galaxies, between the stars, there are dark bits.

If the universe were infinite in space and time, then whatever line you pointed your telescope along, there would be a star.

The dark bits prove that the universe is, in some sense, finite (or that there's something very odd about the local bit of it. It could be infinitely big and infinitely old- if there were only stars in our local bit of it and all of the whole of it's infinite expanse was empty. I rather doubt that is how it works)

[MigL]

The universe is bright in all directions with brightness differences of less than one part in a thousand, however it is not in the infrared but in the microwave region of the EM spectrum. Speciflcally 2.7 Deg.K above absolute zero.

 

As swansont has pointed out this s because of the finite history of the universe and the infinite path ( the universe could be finite and unbounded ) and finite speed of lght. This microwave radiation is then, the faint 'echo' of the big bang. More exactly the period shortly after when all matter in the universe was ionized and not transparent.

[BearOfNH]

Nobody seems to have mentioned it yet, but there's a lot of dust in the interstellar medium.

 

I don't know how much this contributes to the blackness, relative to the other points raised already. But it seems worth a mention.

[John Cuthber]

The dust doesn't affect the original outcome.

The dust particle should be hot enough to glow because it would be surrounded in all directions by stars.

Given infinite time and exposure to infinitely many stars, it would be as hot as the surface of a star.

[BearOfNH]

Given infinite time and exposure to infinitely many stars, it would be as hot as the surface of a star.

 

But dust particles have only been around a few billion years, and there aren't an infinite number of stars.

 

I speculate oops, suggest, the temperature of a dust particle in interstellar space tends to absolute zero, in which case you get a darkening effect. Isn't this what we observe, especially when observing distant galaxies?

[casrip1@gmx.com]

i think this is a lovely video that you will enjoy: http://www.youtube.com/watch?v=gxJ4M7tyLRE

 

it answers exactly what you ask

Edited July 26, 2013 by casrip1@gmx.com

[swansont]

But dust particles have only been around a few billion years, and there aren't an infinite number of stars.

Yes, that's the point.

 

The question of dust is in the context of that being a reason for darkness in an infinite, steady-state universe. It doesn't work as a reason. In a finite universe, dust isn't necessary to explain the darkness. It's there and amplifies the effect, but isn't the cause of it. In either case, dust isn't the answer.

[J.C.MacSwell]

 

Yes, that's the point.

 

The question of dust is in the context of that being a reason for darkness in an infinite, steady-state universe. It doesn't work as a reason. In a finite universe, dust isn't necessary to explain the darkness. It's there and amplifies the effect, but isn't the cause of it. In either case, dust isn't the answer.

According to Hoyle...

 

Hoyle's expanding steady state universe, complete with creation field to hold the steady state in spite of expansion, had no need for dust to explain Olber's Paradox. Light from distant stars was cooled and redshifted same as the Big Bang. The problem with the Steady State model was in other areas, particularly in explaining the CMBR.