Why is the sky dark if light is arriving from all directions in space?

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Jon  asked the Naked Scientists:
I've often heard the argument (even on your podcasts) that the night sky is dark because of the red shift. 

The argument goes that as you go farther and farther away (and into the past), the light becomes increasingly red shifted, until there is no visible light left.

That would make sense ... except ... that the electromagnetic spectrum does not stop at blue.  As light is red shifted, the ultraviolet should become blue, and then those frequencies past ultraviolet should start shifting down into blue. 

What is the real explanation here?  Is the dark body radiation curve for stars such that there really is very little light above the visible spectrum?

Needless to say, I like your show.  I'm slowly listening to every single one.

Jon Bondy
Vermont, USA

What do you think?
« Last Edit: 23/05/2011 21:48:48 by chris »


Offline grizelda

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The starlight is filtered through the interstellar medium which is less transparent to the blue side of the spectrum. That's why the sun looks red when it is low on the horizon as it's light is filtered through more atmosphere.


Offline Phractality

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Starlight is mainly black-body radiation; the hotter the star, the bluer it appears. Even the hottest stars emit little radiation beyond UV, and the most distant galaxies are redshifted almost infinitely. The cosmic microwave background is believed to be the blackbody radiation from nearly 14 billion years ago, when space first became transparent. That was originally blackbody radiation far hotter than any star, and it is now redshifted to a color temperature of 2.7°K.
Imagination is more important than knowledge. Einstein


Offline imatfaal

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Jon Bondy
« Reply #3 on: 23/05/2011 21:51:33 »
Phract - the surface of last scattering that produced CMBR wasn't nearly that hot.  It is reckoned that temperature was around 3000 K - so cooler than our sun and considerable cooler than other suns.  For a sun to be cooler than the surface of last scattering then hydrogen would be recombining.   Also remember that blackbody radiation is a wide-spectrum radiation - but you are right that even hottest stars will have their peak in the UV (40000K eta carinae would equate to 72nm).  Redshifted almost infinitely - no, not even close; the CMBR is much more redshifted.
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