Expert: Courtney Seligman - 11/29/2010

Question
it seems to me that since black is the result of all light wavelengths being absorbed, and since there is nothing in space to absorb anything that the universe should be clear.
i have heard about dark matter and dark energy, but i understand these are not visible to us and that the entire view of the universe cannot be blocked by stars because some stars are just too far away for light to have reached them. so why isnt the universe clear. i mean, if you look thru something clear, like the bottom of a wine glass, what you see is whatever colors are beyond. so when we look out into space we should see what is beyond the universe. but then if the universe is everything, there would be nothing to see, so would that part of the view be invisible?

Answer
The basic problem with your statement is that black is not the result of all light wavelengths being absorbed, unless you are talking about a black "surface", such as a painted wall. In the case of empty space, black is the absence of light passing through it, in the first place. In other words, even if space was completely "clear", unless there was something in the direction you are looking, you wouldn't see anything.

In one sense, there is something in every direction -- the so-called Cosmic Background Radiation (CBR), or "microwave" background. We do "see" radiation corresponding to a perfect radiator in all directions. It just happens that the radiation looks like it was emitted by an object at 2.7 Kelvin, or about 455 degrees below zero Fahrenheit, and at that low temperature, it is all microwave radiation, which our eyes cannot detect. If we COULD see this radiation, the sky would look more or less uniformly bright in all directions; but since we can only see visible light, we don't notice the CBR just looking at the sky with our eyes.

Now, if the Universe were infinitely old and static, and stars extended in all directions throughout the Universe, it would be uniformly bright, and in fact as bright as the surface of the Sun, in all directions. But the Universe is not very old in an astronomical sense, and it is not static -- it is expanding. Each of these factors limit us to seeing the light from stars and galaxies which happen to be fairly close to us -- within about 13 billion light years' distance. And although there are a lot of those, there aren't nearly enough to fill the sky with light, let alone bright light.

We have a number of images of the "deep" sky which show hundreds of thousands of galaxies, extending into the distance, in a very small part of the sky (as an example, see http://cseligman.com/text/galaxies/goods.htm which shows an area about a thousand times smaller than the area covered by the Moon -- so small that it would look like a single starlike point in the night sky, if you could see it). But when you look at such images, although you see faint light in some directions, and brighter light in others, most of the view is "empty", simply because there aren't enough objects, even in the most crowded part of the sky, within the field of view. (Keep in mind, in looking at the sample image, that the brightest objects shown are thousands of times fainter than the faintest thing you can see without a telescope, so that the entire region shown in the image looks completely empty, even using a small telescope.)

So the sky is black (1) because we can't see the kind of light that is most pervasive -- the CBR; (2) because what is out there is mostly so far away and so faint that you can't see it without a telescope, even if you're looking right at it; and (3) because there is just too much empty space, and not enough things in it, for their light to make a noticeable difference.