Why is space black?

ok thanks spidergoat i was just wondering why its black and not another colour but you explained it if thats all there is to it thanks,

 

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I could be wrong, here are some answers from NASA:

So, now on to the harder part - if the Universe is full of stars, why doesn't the light from all of them add up to make the whole sky bright all the time? It turns out that if the Universe was infinitely large and infinitely old, then we would expect the night sky to be bright from the light of all those stars. Every direction you looked in space you would be looking at a star. Yet we know from experience that space is black! This paradox is known as Olbers' Paradox. It is a paradox because of the apparent contradiction between our expectation that the night sky be bright and our experience that it is black.

Many different explanations have been put forward to resolve Olbers' Paradox. The best solution at present is that the Universe is not infinitely old; it is somewhere around 15 billion years old. That means we can only see objects as far away as the distance light can travel in 15 billion years. The light from stars farther away than that has not yet had time to reach us and so can't contribute to making the sky bright.

Another reason that the sky may not be bright with the visible light of all the stars is because when a source of light is moving away from you, the wavelength of that light is made longer (which for light means more red.) This means that the light from stars that are moving away from us will become shifted towards red, and may shift so far that it is no longer visible at all. (Note: You hear the same effect when an ambulance passes you, and the pitch of the siren gets lower as the ambulance travels away from you; this effect is called the Doppler Effect).

 

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EmptyForceOfChi:

There is a substantial difference betwixt the scientific usage of the term "theory" and the theories of philosophy.

Anyway, aside from the Doppler Shift referenced, see my replies to One Raven. Also, do realize that stars release a lot of their radiation in spectrums not visible to the naked eye.

 

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Obler's analysis (150 or so years ago) was as follows.

• The universe is assumed to be infinite, with stars uniformly distributed thoughout.
  
  
• At a given distance from Earth (say D), there is a spherical shell of stars radiating at various wave lengths.
  
  
• At distance (D + d), there are [(D + d)/D]² times as many stars, resulting in more radiation from this shell. Due to the increased distance of the shell, the amount of radiation reaching Earth from this second shell is diminished by the factor: [D/(D + d)]²
  
  
• The multiplicative factors cancel, resulting in the same amount of radiation reaching Earth from each of an infinite number of spherical shells.l

The above indicates that here whould be an infinite amount of radiation reaching us. This would fry us all.

If the universe is not infinite or if it is not uniformly dense with stars, the above analysis is not valid.

I thought that non-uniform density was the main reason for the radiation not being so intense. Note that when Obler made his analysis, it was not known that stars were concentrated in galaxies and galactic clusters, rather than being uniformly distributed.

Of course the assumption of an infinite universe might not be valid either.

 

Christ almighty...when I look up at midnight, space is dark. When I look up at noon, space is light. Now go back to sleep!!!!!!!!!

 

Who is this guy Obler?

 

Olbers. Not Obler.

 

The CMB.... interesting ?

where does that come from ?

Also if your BB theory is correct, then there must be a photon wall... ie stars receding faster than the speed of light... must give out 'no light for us'...

PS I do not believe any of this...

 

Olber's analysis is both right and wrong. The part that is right is that the background radiation of the universe has a temperature that is consistent with the energy product of each star filling a certain finite volume. The sky is blazing with the light from a lot of stars. That light has simply degraded to a temperature of around 3 degrees Kelvin by all the mechanisms that we already know. The part that is wrong is the idea that there is a difference between a finite number of stars in a finite space and an infinite number of stars in an infinite space. This comes from the very first lessons about the use of "infinity" as a constant that can be used in algebra. An infinite number of stars in an infinite amount of space has a certain number of cubic lightyears per star. A finite number of stars in a finite space has a certain number of cubic lightyears per star. In either case, the finities and the infinities cancel out. It is valid to compute the average temperature of the universe by assuming that there is an average amount of energy filling the volume between the stars. One year's output by the sun fills pi*4/3* times 1 lightyear cubed with radiation. I get 3.54*10^39 cubic kilometers. The sun puts out 3.86e33 ergs per second. There are 3153600 seconds in a year. That's 1.22e41 ergs spread out among 3.45e39 cubic kilometers. That's about 35.4 ergs per cubic kilometer of space. One square centimeter of an optical device would pick up 10e-10 of a square kilometer's worth of that energy. In one second it would pick up 300,000 times 1e-10, or 3e-5 * 35.4 ergs, about 1.05e-3 ergs, about a thousandth of an erg.

That is why the sky is black at night.

 

I am going to sign off now and cry. I have not read a thread that was filled with so much confusion and misiniformation for a long time. It as if you had all been subsumed into the Vallich monster.

[Exit stage right, weeping.]

 

I had that same thought some time ago, but was told that the spectrum of the cosmic microwave backround is quite different to that of red-shifted starlight.

 

Ophiolite,

Don't cry. I shall clear up the whole thing right now. Space is black because Evil is the dominant force in the universe. As we all know, White is the color of Good and black is the color of Evil. Since the word of Jesus our Lord has not been spread beyond our earth yet (by our Holy Space Missionaries) the rest of the universe is full of evil and therefore black. Please do not trouble me with ideas about black and white not being colors, strictly speaking, or the fact that stars are white. Stars are Angels of God waiting for our Holy Space Missionaries to spread the word to space and make it white. Once space is white, Jesus our Lord and the Holy Host of Angels will come and proclaim our Great Work as done. Since space is black (Evil) Jesus and the Angels can not cross it from their heavenly home on Tau Ceti 4. This is why we all must support NASA and our Space Program so that we can make space White and finally be at peace with the Universe.

Thank You.

 

space is black because black is what our eyes see when there is not light.space is not actualy black,we just perseive it that way.

black is the absense of light

white is the absense of color

 

It's never that simple. The cosmic microwave background is supposed to have been left over from the "big bang." Since the "big bang" was the most energetic event ever, for its radiation to run down to 3 degrees K it must either have redshifted to a ridiculous degree or something else is working, or a combination of the two.

The estimate or guesstimate of the temperature of starlight is intended to get us thinking exactly how much energy a given volume of space receives from all the stars near it, on the average. When you spread about 35 ergs of energy per second over a million square meters, and even when you multiply that energy by a million, you're not going to succeed in boiling a cup of liquid nitrogen.

I think that Olber's Paradox also fails to take into account that an infinite series of numbers can add up to a finite sum. This is a very easy thing to prove. What is the sum of 1 + 1/2 + 1/4 + 1/8 and so on? Xeno's Paradox defeats Olber's Paradox. You can tell just by examining it that every step of the equation covers half the distance to the number 2. With Olber's paradox the numbers in the series decrease much faster. After you get down to the 7th magnitude or so, it's not worth adding up anymore. The entire nearest galaxy, Andromeda (not counting the Magellanic Clouds) is a barely visible blob in most skies.

 

Well, it has, according to the current paradigm; it has stretched by a factor of about a thousand since the decoupling era. Incidentally that is also the amount that space has expanded since that time.

 

MetaKron: You are correct, an infinite series can have a finite sum. However, the individual terms of the Oblers series were equal.

Assuming uniform density, the number of stars in a thin spherical shell increases with the square of the distance. Thus the intrinsic brightness increases with the square of the distance. The decrease in observed brightness of an individual star decreases with the square of the distance.

Therefore, every spherical shell contributes the same observed brightness (the square and inverse square laws cancel). If the universe is infinite, there are an infinite number of shells, each contributing the same amount of radiation.

Infinite series, same finite value for each term: Ergo, infinite sum & we should be fried. Various explanations for the paradox have been posted. It was only a paradox due to the assumed properties of the universe 150 or so years ago.

 

The CMBR isn't radiated directly from the big bang event, it is radiation from the hot glowing gas (pretty much all hydrogen, I think?) that filled the Universe for 400,000 years afterwards. We don't see anything from the first 400,000 years, because the Universe was opaque at that time.

The temperature of the source of the microwave background was about 3000K when it was emitted - about half the temperature of the surface of the Sun.

 

but perhaps some day we will be able to see the neutrinos decoupled just 1 second after the Big Bang, because neutrinos can pass unaffected through very dense matter. Neutrino telescopes have been set up and perhaps they can detect them

 

He only asked why the universe is black. I think that for the sake of productivity, other threads should be opened if one wants to discuss other subjects.

One can see light only if it is reflected by an object, such as air or your house whatever. The universe is one big vaccuum meaning that there is almost nothing or it is so densely spreaded that it cannot be noticed so there is nothing to reflect the light, therefore there is no color, which is interpreted by our minds as black. It isn't really black, our mind just 'translates' that for us.

 

The issue of whether the universe has expanded from a Big Bang is crucial to an understanding of why the sky is black. The sky is not filled with starlight simply because we can only see light that comes from the observable universe; the very limit of the observable universe is the Cosmic Microwave Background, which was emitted around 400,000 years after the Bang itself.

Everything else in the universe has expanded away from us so far, and at such a velocity, that we will never receive any light from it.

Therefore the sky is black, if the current paradigm is correct.