Why is the sky dark at night?
If the Universe were infinitely old, and infinite in extent, and stars could shine forever, then every direction you looked would eventually end on the surface of a star, and the whole sky would be as bright as the surface of the Sun. This is known as Olbers’ Paradox after Heinrich Wilhelm Olbers [1757-1840] who wrote about it in 1823-1826 but it was also discussed earlier. Absorption by interstellar dust does not circumvent this paradox, since dust reradiates whatever radiation it absorbs within a few minutes, which is much less than the age of the Universe. However, the Universe is not infinitely old, and the expansion of the Universe reduces the accumulated energy radiated by distant stars. Either one of these effects acting alone would solve Olbers’ Paradox, but they both act at once. The radiation we do see after Olbers’ Paradox is solved is the Cosmic Infrared Background Radiation (CIBR). Back to top. Will the Universe expand forever or recollapse? This depends on the ratio of th
When I asked my wife why the sky is dark at night, she said, “It’s because the sun’s not shining!” Yet generations of astronomers have struggled to answer this question. It’s become known as Olbers’ Paradox. Although it’s one that few spend time thinking about, for cosmologists it’s always been important. The darkness of the night sky was one of the earliest observations that cosmologists could use as a basis for developing an understanding of the universe. In theory, they knew that even at night the sky should shine like a curtain of light. Explaining why it didn’t helped cosmology to be seen as a true science that is based on observation rather than speculation. It was one of the first observations that helped us realize that the universe is not infinite either in age or extent. To light the sky from horizon to horizon with starlight would require trillions more stars than now exist. Alternatively, we’d have to be able to look back in time longer than the universe has existed. The un
The oldest and simplest astronomical observation tells us something profound about the universe. The sky is dark at night. It isn’t obvious why this should be so. If you stand in a small grove of trees and look toward the horizon, you can see patches of sky in the distance between the tree trunks. But if you stand in a large forest, your view is everywhere blocked by a “solid wall” of tree trunks. Extending the analogy to three dimensions, if the universe of stars is large enough, your line of sight should be blocked in every direction by a “solid wall” of stars. If you could magnify that view sufficiently, the sky would everywhere look something like the image on the left. The entire sky would be about as bright, and as hot, as the surface of the Sun. The immense distance to the stars making up the “wall of light” would have no effect on the total amount of energy reaching us. We should be surrounded by a blazing oven of light. Instead the night sky is practically black. So where does
Hi! Thank you for calling Dr. Marc at The Space Place to hear what I’m thinking about this December. Our friends at the Highlands Museum and Discovery Center in Ashland, Kentucky, asked a very good question. Why is it dark in space? That question is not as simple as it may sound. You might think that space appears dark at night because that is when our side of Earth faces away from the Sun as our planet rotates on its axis every 24 hours. But what about all those other far away suns that appear as stars in the night sky? Our own Milky Way galaxy contains over 200 billion stars, and the entire universe probably contains over 100 billion galaxies. You might suppose that that many stars would light up the night like daytime! Until the 20th century, astronomers didn’t think it was even possible to count all the stars in the universe. They thought the universe went on forever. In other words, they thought the universe was infinite. Besides being very hard to imagine, the trouble with an inf
” By his time, physicists had learned enough to realize that, in a stable, infinite universe with an even distribution of stars, the entire universe should gradually heat up. Think about it — if there are stars generating energy throughout the universe (energy sources), and if there is no way ultimately to dispose of that energy (energy sinks), then all the objects in the universe must rise in temperature, in time achieving the temperature of the stars themselves. Scientists and physicists had to learn quite a lot about the behavior of energy before they were even prepared to ask Olbers’ question. In fact, for millennia the dark night sky provided an answer to a question no one thought to ask. In these pages you will learn the simple physics behind Olbers’ question, some of the answers that have been proposed, and the currently accepted answer. You will also discover the connection between a rubber band, your refrigerator, and the universe.
