Have you ever gazed into the night sky and wondered why it's dark? There are two main explanations for this. First, let's explore something called Olber's Paradox. Some may think that space is black simply because there’s nothing between stars to emit light. In a finite universe, that’s partly true. Even in the Hubble Deep Field images, which capture extensive views of space, you can find light sources like stars and galaxies scattered across what looks like empty space. However, there are still noticeable gaps between these galaxies, and within galaxies themselves, there are immense distances between clusters of stars.
As these light sources become more distant, they appear dimmer to us due to the inverse-square law: as light spreads over a larger area with increasing distance, its brightness to the observer decreases. Now, if we lived in an infinite, unchanging universe filled endlessly with stars, every point in the sky would have a star shining from some distance. This means that, theoretically, the entire sky would be as bright as a star's surface or even brighter. This concept, known as Olber’s Paradox, suggests that an infinite, static universe would lead to a sky filled with light. If there were clouds or dust, even they would appear luminous, reflecting immense amounts of starlight.
However, this paradox actually supports the idea that our universe is neither infinite nor static but rather has a beginning and, likely, an end. Observations have shown that the universe is expanding. The Hubble Constant indicates an expansion rate of about 70 km/s for every megaparsec. Since the universe is about 13.8 billion years old and expanding, there’s a limit to what we can observe, known as the observable universe. Galaxies beyond a certain distance are moving away faster than light, meaning we can’t see them, even with the most powerful telescope.
Another interesting phenomenon about distant galaxies is that they appear redder the farther they are from us. This effect, called redshift, occurs because the expansion of the universe stretches light waves. As a result, light emitted from galaxies billions of light-years away can shift from visible wavelengths to infrared, making it undetectable to the human eye.
This brings us to the second reason why the night sky appears black, which relates to our eyes’ limitations. Human vision is adapted for daytime conditions, and although we can see in low light at night, our color perception is greatly reduced. At night, things not only appear darker but also more monochrome. If you were to use a long-exposure camera, you’d notice colors that your eyes can’t see in the dark. This is also true for the night sky—Rayleigh scattering, which makes the sky appear blue during the day, continues to occur at night, though it's too dim for us to detect.
So, in a way, the night sky is still “blue,” though it appears black to us. Understanding why the night sky looks dark (or blue) can help answer bigger questions about our universe, such as whether it’s expanding and how redshift works. Without these insights, if the sky were illuminated by every star in the cosmos, it wouldn’t matter if it was day or night—the sky would always be blazing with light.