Asking why the night sky is dark is about as natural as asking why the sky is blue. Humans spend a lot of time looking up, after all. Of course, you might be quick to say that it is dark at night because that is when the Earth is facing away from the Sun. But is that really the case? What about all those trillions of other stars? Many are similar to our own Sun, so why don’t they illuminate our night sky more? At the very least, why it dark in space?
Well, to get to the bottom of it, we must first explore Olbers’ paradox, also called the “dark night sky paradox.”
Understanding Olbers’ Paradox
To understand Olbers’ paradox, we have to understand some early models for the universe. For a really long time, we thought the universe was static, and infinite in both age and size.
Working with these conditions, geometry tells us that if you were to draw a straight line outward from your eyes in any direction of the night sky, you’d eventually see a star. Remember that the universe is infinitely large in this scenario, and therefore there are an infinite number of stars. If the universe was static and nothing was affecting these stars, you would be able to see light from them.
If this were the case, the night sky would also be super bright. You’d always see the light from some random faraway star, no matter which direction you looked. We know, however, that it is dark at night. Which means something is off with those old models of the universe. Enter Olbers’ paradox.
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A Solution to Olbers’ Paradox
According to what we’ve just outlined, Olbers’ paradox means the universe cannot be static and infinite in both age and size. This does not mean that the universe is none of these. The logic just states that the universe cannot be all three at once.
Which of those three can we disprove then? Well, we know that the universe is not static. We actually know more than that; we know the universe is expanding. You can thank the tube-shaped Hubble Space Telescope for those findings. It’s actually the expansion of the universe that gives us the Big Bang theory. The Big Bang theory is generally accepted among the majority of the scientific community, and tells us the universe is not only getting bigger, but also suggests the universe is finite in age and size.
Beyond just expanding, there’s also evidence thanks to Hubble that the universe is expanding even faster than in the past. Nobody really knows why, we just kind to attribute it to dark energy. If you want an existential crisis, the speeding expansion is where the Big Rip theory comes from. Basically, the universe will never stop expanding (it’ll start expanding faster), and our atoms will eventually be ripped apart. Lovely!
Why Is it Dark in Space? Why Is the Night Sky Dark?
So we know that the universe is expanding, and if you subscribe to the Big Bang theory, we know it is not infinite in age or size. The fact that the universe is of limited size means there is not an infinite number of stars. Even with trillions, there are still not enough to light up the entire sky. Furthermore, because the universe is expanding, distant stars are actually getting farther away all the time. This relates to an important property of light.
As light travels, it eventually starts losing some energy. Wavelengths are associated with that energy, and we can only see a very narrow wavelength as visible light. So as the light travels across the incomprehensibly vast distances of the universe, the light’s wavelength would get so long we just couldn’t see it anymore. This is where cosmic background radiation comes from; it’s in part light from stars we just can’t see anymore. It also helps explain why the cosmos are so dark.
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