Solving The Оlbers Paradox: Why Is Space So Dark If It Has So Many Stars?
17th Oct 2024
Prologue
Paradox (ancient Greek) — a judgment, opinion, or principle that contradicts generally accepted statements or norms, seems illogical, absurd, but is confirmed in practice.
Our world is full of paradoxes. For example, “The more we learn, the more we realise how little we know.” Let’s agree, it sounds contradictory, but that’s how it is.
Paradoxes occur in science (most often, in logic) and in various forms of art, such as painting and music. But today, we will talk about a paradox in cosmology, known as the Olbers paradox or photometric one (photometry is a field of astronomy that deals with measuring the electromagnetic radiation of stars. Author’s note).
What is Olbers paradox, why did it arise, and have scientists been able to solve it? Let’s get to the bottom of it.
What Is Olbers Paradox In Simple Terms?

The photometric paradox implies that if the Universe is infinite and filled with myriads of stars, the brightness of the sky (including the night sky) should be approximately equal to the brightness of the solar disk.
Indeed, our Sun is bright enough to give us daylight. According to scientists, there are about 2 septillion stars (that’s a number with 24 zeros) in the part accessible to us for observation. Many of them are much brighter than our Sun. Based on this, a seemingly logical question arises: why, with such an abundance of light sources, does space look so impenetrably dark?
Paradox History
The photometric paradox was first formulated by the Swiss astronomer Jean-Philippe Loys de Cheseaux in 1744, although a similar question had been asked earlier by Edmond Halley and Johannes Kepler. However, it was the German astronomer Heinrich Olbers who really popularised this problem in the early 19th century, which is why the paradox was named after him (sometimes it is called Cheseaux-Olbers).
What Assumptions Are Made In Olber’s Paradox?

The main assumption Olbers’ paradox states was based on following:
- The universe is infinite: it extends infinitely in all directions.
- The universe is static: it neither expands nor contracts.
- The universe is homogeneous: the stars in it are evenly distributed.
- The universe is eternal: It has always existed and will exist forever.
Some of these statements may seem surprising now, but in the 19th century, scientists wholeheartedly believed in them. To understand how these cosmological principles relate to the Olbers’ paradox, here is a simple explanation.

Imagine a large forest, densely planted with trees. If you stand in such a forest and look in any direction, your line of sight will sooner or later hit a tree. This means that you will not be able to see further than the nearest tree, since every direction of your view will be blocked by trees.
Now, let’s apply this analogy to the universe. If it is infinite, uniform, and filled with stars, as scientists believed until the 20th century, then no matter which direction you look at, your line of sight should eventually hit a star. This means that the night sky should be as bright as the surface of a star since every point in the sky should be occupied by a star. However, the darkness of space contradicts this, creating a paradox.
How Do We Resolve Olbers Paradox?
What are the possible solutions to the Olbers paradox? There are several, but not all of them proved to be correct. Which scientists got it wrong, in what, and why?
Light Absorption By Cosmic Dust

Cheseaux and Olbers attempted to resolve the problem by suggesting that the light of distant stars is absorbed by cosmic dust. However, this version was ruled out by John Herschel in 1848. Although starlight is indeed absorbed by cosmic dust, this does not resolve the photometric paradox.
According to the first law of thermodynamics, energy cannot appear out of nothing or disappear without a trace. It can only change from one state to another. When cosmic dust absorbs light, it heats up and then emits this energy as infrared radiation. That is, the light energy does not disappear, it simply changes form.
Fractal Structure Of The Universe

According to fractal cosmology, the infinite Universe has a hierarchical structure, like a stacking doll. Each star with planets forms a first-level system; then, these stars unite into a second-level system — galaxies; then — into clusters, superclusters, etc. As the scale of objects increases, the density of light radiation decreases, which should explain the night sky darkness. The hypothesis of a fractal system was mathematically substantiated by Carl Charlier in 1908. However, the discovery of relic radiation in 1965 does not allow to completely explain Olbers’ paradox by fractal cosmology. Relic radiation is isotropic, that is, it has the same intensity in all directions, which refutes the anisotropic distribution of matter in a fractal system on a large scale.
What Is The Correct Answer To The Olbers Paradox?
Cheseaux, Olbers, and Charlier each proposed different solutions to the problem, but they turned out to be erroneous since they were based on pre-relativistic cosmology. At the beginning of the 20th century, Einstein’s theory of relativity and the Big Bang theory turned the scientific world’s understanding of the Universe upside down, allowing them to get to the correct Olbers paradox solution.
How Does Olbers Paradox Support The Big Bang?

There are three provisions of the Big Bang theory that categorically resolve the photometric paradox.
Universe Age Is Finite
The Big Bang theory states that the universe has a finite age, about 13.8 billion years. This means that the light from the most distant stars that we can observe takes about 13 billion years to reach us. This eliminates the basic premise of the Olbers’ paradox — that stars are located at any distance from us. In fact, the universe, observed at very large distances, is so young that stars in it have not yet had the time to form.
The Universe Is Expanding
According to the Big Bang theory, the universe is constantly expanding. This means that galaxies are moving away from each other. This expansion causes a Doppler redshift of light from stars. The further away the star, the more pronounced this effect is, and the less visible the star is to the human eye.
In addition, stars do not last forever; they die and go out, but the speed of light travels relatively slowly on a universal scale. So, we may simply not know about this and continue to see the light of a star that has long gone out.
The Amount Of Matter In The Universe Is Limited
The Big Bang theory suggests that the universe contains a finite amount of matter and energy that arose from a singularity. According to the law of conservation of mass and energy, the total amount of matter and energy remains constant. This means that the amount of matter formed at the moment of the Big Bang cannot increase. Therefore, the amount of matter for the formation of stars is limited, which means that there cannot be an unlimited number of stars. In addition, the amount of visible matter is about 5% of the total mass and energy. This means that the ratio of visible stars in the sky to dark space is approximately the same.
Who First Solved The Olbers Paradox?

You may be surprised to learn that the first person to find the correct Olbers paradox solution was not a physicist or astronomer, but an American poet and writer Edgar Allan Poe. Shortly before his death in 1848, he published a fantastic prose poem, Eureka: An Essay on the Material and Spiritual Universe. It contains the following words (quote):
“Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy–since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing that the distance o the invisible background [is] so immense that no ray from it has yet been able to reach us at all.”
Let’s agree, this is a surprisingly simple and elegant explanation of the Olbers paradox, for a person who lived 100 years before the Big Bang theory and has nothing to do with astronomy. Isn’t that paradoxical?)
References and Additional Information:
- Olber’s Paradox: Why Is The Sky Dark at Night? https://www.amnh.org/exhibitions/journey-to-the-stars/educator-resources/stars/olbers-paradox
- Оlbers paradox https://www.e-education.psu.edu/astro801/content/l10_p2.html
- The Solution to Olbers’ Paradox https://www.youtube.com/watch?v=EU87w9JF_vg
- Olbers Paradox https://jimal-khalili.com/blog/olbers-paradox/
- What is the Big Bang Theory? by Matt Williams, Universe Today. https://phys.org/news/2015-12-big-theory.html
- Did Edgar Allan Poe solve Olbers’ Paradox? https://cultureofcuriosity.wordpress.com/2012/11/28/olbersparadox/
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