Why Is The Sky Dark At Night?

For centuries, the darkness of the night sky has puzzled humanity. The reason for this phenomenon can be traced back to our perspective within the Solar System. During daylight, sunlight illuminates our atmosphere from every angle, creating a bright environment. In contrast, at night, the absence of this sunlight leads to a dark sky, except where celestial bodies like stars, planets, or the Moon provide points of light.

However, this observation prompts deeper questions. If the Universe is indeed infinite, why don't we see a star in every direction? With trillions of galaxies present, why doesn’t their combined light brighten the entire sky? This question has intrigued scientists for ages, and they have worked diligently to find answers.

This conundrum has perplexed scientists for centuries. While Earth's atmosphere allows us to see into deep space at night, one might expect to observe light emanating from every point in the sky. If the Universe is infinite, then theoretically, our line of sight should eventually intersect with a star, illuminating the void of space.

If this were the case, the night sky would not be dark; it would be filled with the light of countless stars. Yet, even in regions of space that appear empty, our most advanced telescopes reveal just a few points of light against the darkness.

Indeed, the Universe is teeming with stars and galaxies, but they are often billions of light-years away. The light from these distant objects takes a long time to reach us, and as vast as the Universe is, it is not infinite.

The scientific community remains uncertain whether the Universe is finite or infinite. However, we know that the observable portion must be finite. Despite our limited understanding of the Universe's large-scale structure until recently, an infinite observable Universe is deemed impossible.

In the 1800s, Heinrich Olbers identified a mathematical paradox regarding this issue. If the Universe were infinite and filled uniformly with stars, one would expect to see endless light in every direction. Nearby stars would be visible, and distant stars would fill the gaps between them. Regardless of distance, one should encounter a star wherever one looks.

Mathematically, if the density of stars remains constant throughout space, the total number of stars is a function of their density multiplied by the Universe's volume. As distance increases, a star appears fainter due to the inverse square law of brightness, yet the surface area of a sphere increases with the square of the distance. Thus, the total amount of light would theoretically approach infinity.

When Olbers explored this logic, he realized that the observable Universe could not be infinite, but he lacked definitive proof. Critics pointed out that his argument did not consider the presence of interstellar dust that obscures light, which is evident when observing the Milky Way.

In a finite Universe, such dust can absorb starlight, preventing it from reaching us. However, if the Universe were truly infinite, each dust particle would absorb an infinite amount of light, complicating the situation further.

This suggests that our Universe cannot be static, infinite, and filled with eternal starlight. If that were the case, the night sky would be bright from every angle. Clearly, other factors are at play.

What Olbers could not foresee was that the Universe, while potentially infinite in extent, does not extend infinitely into the past. The Universe had a beginning, known as the Big Bang, establishing a limit on the amount of matter, radiation, and light observable today.

As a result, we can only perceive stars and galaxies within a finite distance, meaning the light we receive from them cannot be infinite.

Furthermore, if the Universe was once hot and dense, remnants of that early radiation should be present. Modern observations allow us to estimate the photons from the Big Bang filling our Universe today, amounting to about 411 per cubic centimeter.

We detect this radiation constantly, albeit indirectly. For example, an old television set in deep space would pick up residual "snow" from the Big Bang, which contributes to the background radiation we observe.

The reason we do not perceive this light with our naked eyes is due to the Universe's expansion over time, shifting its wavelengths beyond our visual range.

To summarize, two key factors explain the darkness of the night sky: first, the Universe has only existed for a finite duration, limiting the observable radiation; second, our vision is restricted to a small part of the electromagnetic spectrum—visible light.

If we could see in the microwave range, the night sky would appear uniformly bright. Ironically, it is our limitations that render the night sky an intriguing mystery. Advances in technology have allowed us to measure cosmic radiation, revealing insights about the Universe that our senses alone could not provide. While the night sky may seem dark, it holds answers to profound cosmic questions.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.