How much light is there in the cosmos?
Is there always light everywhere in the universe? Can you always see at least a few stars? Or perhaps a distant galaxy or two? Way out beyond the clusters of galaxies are there places where even the galaxies are too far away to see, and everything is black? This snippet will say yes. Are these dark places common? This snippet will say YES.
The photos of the universe we see on-line or on TV, show only visible objects – stars, and galaxies. It can make the universe appear full of bodies reflecting or emitting light. But photo taken at random in the universe would mostly show nothing at all – just sheer blackness. One calculation from some decades ago said that most random photos would show only blackness, because the distances between galaxies are millions of light years – incomprehensibly large.
Would this stay true if we used modern data?
Some parts of our expanding universe are receding so fast we will never see them, so for the calculation we use only the observable universe volume, and this is 3.6 x 1071 km3. I hope you can understand numbers like that? The little superscript number (71) tells us how many zeroes are after the number 10; in this case 71!) In that volume are about 200 billion galaxies. This gives about 1.8 x 1060 km3 each on average, and we can imagine - for ease of calculation - that all are evenly spaced and in the middle of identical imaginary cubes which together I take as making up the universe volume. The separation between each galaxy would be twice the distance to the nearest face of its cube. This works out at 1.2 x 1020 km. A single light-year is 9.5 x 1012 km, so the average spacing is 12 million light years (well spaced, indeed!). Most of us know that our nearest galaxy neighbour is the Andromeda Galaxy, and that is 2.5 million light years away. We can just see this with our naked eyes from our galaxy, the Milky Way. With our naked eyes we cannot see an average galaxy 12 million light years away. There would be only blackness.
So the old calculation was right : for almost all random spots in the universe, peering in almost in any direction we would just see blackness. With a telescope it’s quite different – you’d see lots of galaxies. Lots of light.
Lots of light is where we live, in a solar system, in a galaxy, in light. We live in an exceptional place. A rare place. A miraculous place.
But even in our own well-lit solar system the darkness between the stars we can see is so dark that it surprised the researchers. It's about as dark as trying to see from earth, with the naked eye, a light in a domestic refrigerator on the moon with the door left open. Even if we could see that with the naked eye (and of course we can’t) the darkness of outer space is much darker. Don’t go there.
Little children: walk in the Light.