Why is the night sky black?

A seemingly silly question about an obvious fact, yet not everyone understands it clearly.

Why is the Night Sky Black?

Surely everyone knows that the sky is always a deep black with twinkling stars, especially on clear nights. Have you ever wondered why the sky is black when night falls? If you have and still haven’t found an answer, then this will provide the explanation for this seemingly silly question.

It all begins with the Olbers’ Paradox, a theory proposed by physicist Heinrich Wilhelm Matthäus Olbers in 1823. Olbers suggested that according to Sir Isaac Newton, the universe is infinite and contains an infinite number of stars. This means that no matter where you look, you would encounter a star with brightness equivalent to that of the Sun. This situation is akin to being in a dense forest; no matter where you look, your view will always be obstructed by tree trunks. Therefore, the night should be as bright as day, yet it is pitch black. From this, Olbers concluded that the universe has its own limitations.

However, many experts later pointed out flaws in this paradox. In the context of a person standing in a forest, the closer they get to the edge, the fewer trees they will see compared to when they are deep in the woods. This applies to our case when we look up at the sky and see it dark no matter which direction we face, implying that we are in some area resembling the “center of the universe” where there exists a place in the vast sky that allows us to look up and see one half dark and one half illuminated. Subsequently, astronomers have proven that the stars in the universe are unevenly distributed – some areas are sparse, while others are densely packed with thousands of stars just a few light-years apart. Given this research, the night sky should occasionally be completely dark with no stars at all, but in reality, that is not the case.

With the emergence of the Big Bang theory, scientists continued to add information to this issue, suggesting that human understanding of the universe is still limited in time, as many stars have died billions of years ago, and their light hasn’t yet reached Earth. Furthermore, general relativity has helped researchers discover that the universe itself is expanding at a tremendous and possibly eternal rate, causing light traveling through the cosmic environment to fade. This phenomenon was demonstrated by Austrian physicist Christian Andreas Doppler as the Doppler effect.

The Doppler effect states that the frequency and wavelength of sound waves, electromagnetic waves, or waves in general change when the source of the wave is moving relative to the observer. In the Doppler effect, the frequency of the wave source does not actually change. To understand the cause of the Doppler effect and the changes in frequency, we can use the example of two people throwing a ball. Person A throws a ball to person B at a certain distance. Assuming the speed of the ball is constant and person B receives a fixed number of balls each minute. If person A slowly moves closer to person B, person B will receive more balls per minute due to the reduced distance. Therefore, it is the number of wavelengths changing that causes the change in frequency.

In 1929, astronomer Edwin Hubble began studying the Doppler effect of light emitted by galaxies and discovered that galaxies are moving further away from their initial positions. This is a consequence of the Doppler effect known as redshift, where light emitted from objects moving away from the observer appears redder. General relativity predicted the issue of redshift when photons lose energy escaping gravitational fields, causing spectral lines in visible light to shift towards the red spectrum as the electromagnetic wave frequency (of light, radio waves, etc.) from galaxies, quasars, or other distant celestial bodies decreases.

Thus, although the universe is infinite and homogeneous, its lifespan is not infinite. Along with the expansion of the universe, the light that Earth receives from distant stars is very limited, and its total intensity is insufficient to illuminate the night sky of Earth.

As a result, the night sky we observe is very dark. This may seem like a normal phenomenon, but it actually conceals a desperate truth: As the universe continues to expand, the distance between objects in the universe keeps increasing. When a star reaches a certain distance from us, the light it emits will move away from us at superluminal speeds due to the expansion of the universe, meaning that this light will never reach Earth.

Even more despairing is that the expansion of the universe continues, which will cause many celestial bodies to drift away in the future and become permanently disconnected from us. In other words, as time passes, fewer and fewer celestial bodies will be observable by humanity in the universe.

According to scientists’ predictions, when this situation reaches an extreme point, except for the celestial bodies in the local galaxy group that can be maintained together by gravity, other celestial bodies farther away will completely lose contact with us.