Until the early 1800s, astronomers were asking a question which might seem a little ridiculous: “Why is the night sky dark with only a few specks of light scattered here and there, and not as bright as the sky during daytime?” The question might be surprising, but it wasn’t unfounded. It was based on the belief that the universe extends infinitely in all directions with an infinite number of stars, meaning that everywhere we look, there should be a star in our direct line of sight.
The question was first posed by Kepler in 1610, but it was only in the 19th Century that German astronomer Heinrich Wilhelm Olbers popularised it as a paradox. At that time, it was believed that the universe was static, infinite in space and timeless. With this model in place, Olbers’ Paradox was quite a concern. In fact, it challenged the model of the universe itself! The paradox can now be explained by the Big Bang theory, but before that, Olbers himself tried to come up with a solution.
Olbers tried to come up with a solution to his own paradox without changing the existing static, infinite model of the universe. He proposed the concept of interstellar absorption – a hypothesis that space is not transparent, and that cosmic dust absorbs and blocks out the light coming from the more distant stars. It would explain why only a few stars were seen, but it had an issue. The dust would eventually heat up due to all the light energy it absorbs, and end up re-radiating the light, thus bringing us back to square one – the sky should be as bright at night as it is during the day. It wasn’t until the early 1900s that another astronomer came up with a new solution for why the night sky was black instead of white.
Big Bang Theory
In the 1920s, American astronomer Edwin Hubble discovered that the universe is not static at all, but constantly expanding, with galaxies constantly moving rapidly outwards in all directions. This led to the Big Bang Theory, the most prevalent theory about the origin of the universe. The theory states that the universe was once a single point in time and space, when for no discernible reason, about 13.8 billion years ago, it expanded suddenly and rapidly. This theory solves Olbers’ paradox through two explanations resulting from the Big Bang Theory:
Finite Age of the Universe
According to the widely accepted Big Bang Theory, the universe has a finite age. In fact, it is around 13.8 billion years old. Also, since the speed of light is finite, we see the stars as they were when the light was emitted; so we would, for instance, see a star that is 3000 light years away, as it was 3000 years ago.
The finite age of the universe, combined with the finite speed of light means that the light from the more distant stars has not reached us yet, because the stars are not old enough (for example, a star that is 13.9 billion light years away is not old enough for the light to have reached us). However, this theory has one gap. Until a few hundred thousand years after the Big Bang, the universe was hot and dense like the core of a star. After that, when light first escaped into space, it lit up the entire universe, making every direction as bright as a star. This light is known as cosmic background radiation, and theoretically, we should be able to see it, and the sky shouldn’t be dark. To better understand this, there’s another explanation to complete the solution to Olbers’ paradox.
2. Expansion of the Universe
The Big Bang Theory was largely based on the observation that the universe is rapidly expanding. As the universe expands, light from distant galaxies and stars gets stretched and its wavelength starts increasing. This is known as “red shift”, as the wavelength goes towards the red end of the electromagnetic spectrum. Hence, as the wavelength increases, the light becomes infrared and beyond. When the wavelength increases beyond the visible spectrum, it becomes invisible to the human eye, hence explaining the darkness of the night sky.
It’s why we can’t see the cosmic background radiation – because it is red shifted to the microwave spectrum. It’s even called the Cosmic Microwave Background radiation (CMB). The same goes for all other light sources around us in the Universe – they happen to be receding fast enough for their light to be red shifted beyond the point where it would be visible to us.
In a nutshell, Olbers’ paradox, which asked the simple question – If the universe is static, timeless and infinite, why is the night sky black?, led to several significant discoveries, directly and indirectly. The model of the universe at the time the paradox was popularised couldn’t explain it, but the currently prevalent Big Bang model does.