Have you ever wondered why the night sky is dark? At first glance, it seems obvious. Nighttime means the Sun has set, and when you look up, the sky appears black, except for the twinkling stars scattered across it.
But wait. Considering the vastness of the universe, no matter where you look, there should be stars -billions and billions of them. You might expect the entire night sky to be filled with starlight, with the more distant stars appearing fainter but still numerous enough to brighten the sky. So, why, and how, is the night sky dark?
Welcome to the Olbers’ Paradox.
Olbers’ Paradox: The Mystery of the Dark Night Sky
Olbers’s paradox, also known as the dark night paradox, is an argument in astrophysics and physical cosmology that addresses the darkness of the night sky. Named after the German astronomer Heinrich Wilhelm Matthias Olbers (1 October 1758 – 2 March 1840), who described it in 1823, the paradox suggests that this darkness conflicts with the assumption of an infinite, eternal, and static universe.
If the universe were static, homogeneous on a large scale, and populated by an infinite number of stars, any line of sight from Earth would inevitably end at the surface of a star. This would result in a night sky that is completely illuminated and exceedingly bright. However, this hypothetical scenario starkly contrasts with the observed darkness and non-uniformity of the night sky, presenting a significant cosmological puzzle.

Back when Olbers’ paradox was proposed, the Steady State Theory was the prevailing scientific consensus. This theory posited that the universe is not expanding but remains constant over time. According to the Steady State Theory, new matter is continuously created to maintain a constant density as the universe expands. This creation of matter would keep the universe looking the same at all times, thus adhering to the perfect cosmological principle, which states that the universe is homogeneous and isotropic in both space and time.
In the context of the Steady State Theory, light waves traveling from distant stars to Earth would remain at the same wavelengths as they reached us, as the theory assumes no expansion of the universe to stretch these wavelengths (a phenomenon known as redshift). Additionally, if the universe were infinitely old, the light from stars at extremely far distances would have already reached us, regardless of how far away they are. This means that even light from stars 40 billion light years away would have had ample time to reach Earth. Therefore, under these assumptions, the night sky should be uniformly bright.
Resolution of Olbers’ Paradox: The Doppler Effect
One might think that the darkness of the night sky could be explained by a boundary to the universe, similar to seeing a gap in a forest indicating an edge. However, since the sky is dark in all directions, this would imply that we are in the middle of a finite universe, which is highly improbable.
Another possible explanation is that the universe is limited in time, meaning that light from distant stars has not had enough time to reach us. While it is true that the universe has a finite age, this alone does not account for the darkness of the night sky. The Big Bang theory suggests that the sky was much brighter in the past, especially during the recombination era when the universe first became transparent. At that time, the high temperature made every point in the sky as bright as the Sun’s surface, and much of the light we see today comes from the relic radiation of the Big Bang.
The actual reason for the darkness lies in the expanding universe and the Doppler effect. Named after Austrian physicist Christian Doppler (29 November 1803 – 17 March 1853), who described the phenomenon in 1842, the Doppler effect (or Doppler shift) refers to the change in frequency of a wave in relation to an observer moving relative to the wave source.
A common example of the Doppler shift is the change in pitch heard when a vehicle sounding a horn (or the sound of the engine, think of the F1 cars) approaches and then recedes from an observer. Compared to the emitted frequency, the received frequency is higher as the vehicle approaches, identical at the moment it passes by, and lower as it moves away.
The Doppler effect occurs because when the wave source moves towards the observer, each successive wave crest is emitted from a position closer to the observer than the previous crest. This causes each wave to take slightly less time to reach the observer, reducing the time between the arrivals of successive wave crests and increasing the frequency.
As the waves travel, the distance between successive wavefronts decreases, causing them to “bunch together.” Conversely, if the source moves away from the observer, each wave is emitted from a position farther from the observer than the previous wave, increasing the time between arrivals and reducing the frequency. This results in the wavefronts “spreading out.”
Edwin Hubble’s 1929 discovery revealed that distant galaxies are moving away from us, with the farthest galaxies receding the fastest. This expansion affects how we perceive light from these distant stars. The Doppler effect shifts the light from these fast-moving galaxies to longer wavelengths, moving it from the visible spectrum to the infrared and radio waves, which are invisible to the human eye. Thus, the night sky appears dark because the light is redshifted beyond our visual capabilities.
In essence, the night sky is not truly dark but glows with the Cosmic Microwave Background radiation – it’s only dark to our eyes. Cosmic Microwave Background radiation or CMBR is the highly redshifted remnant of the Big Bang, which our eyes cannot detect due to its low energy and long wavelengths. The blackness of the night sky is direct evidence of an expanding universe. So, if you want proof of the Big Bang, all you need are your eyes and a clear, dark night.
Sources
- Olbers’s Paradox on Wikipedia
- What is Olbers’ Paradox? on the NASA website
- Olbers’ Paradox on the PennState College of Earth and Mineral Sciences website