Olbers' Paradox tells us that the night sky shouldn't be dark—it should be as bright as the surface of the sun in all directions! Why isn't it?
Why is the night sky dark? This seemingly simple question has some significant implications.
We take for granted that the night sky is dark. At the day’s end, the closest and brightest star in our sky, the sun, sinks below the horizon. It leaves behind the vast blackness of space, which is dotted with the light from distant stars, sometimes the Moon, and maybe, if you’re lucky or in the southern hemisphere, a look at the rest of our Milky Way galaxy and our extragalactic neighbors, the Small and Large Magellanic Clouds. So why should we expect anything different?
Should the night sky be dark?
In the 1820s, the astronomer Heinrich Olbers pointed out that, based on our understanding of the universe at the time, the night sky should not be dark at all. We considered the universe to be steady and unchanging. And this is a very reasonable assumption. We can be sure that at the start of every day, the sun will rise and that the stars in the sky will move in predictable patterns. We also considered the universe to be uniform and infinite in extent. The idea that there could be a Truman-show-esque edge beyond which the universe ceased to exist seemed ridiculous.
Olbers’ Paradox tells us that the entire night sky should not only not be dark, but it should be about as bright as the surface of the sun in all directions.
But if that were all true, then looking out into the night sky into an infinite universe should mean that in any direction, we will eventually see a star. Imagine looking into a dense forest. Although there's space between the trees, you see what appears to be a wall of trees. That's because a tree always falls within your line of sight. In our infinite universe, you may have to look back increasingly far to see a star in a given direction, which could mean that star is too faint to see.
But, Olbers’ Paradox tells us that if the universe is infinite and unchanging, it must also be infinitely old. Even light from the most distant stars has had time to reach our eyes as they look out into the night sky. The entire night sky should not only not be dark, but it should be about as bright as the surface of the sun in all directions.
This seeming contradiction became known as Olbers’ Paradox, even though Olbers was far from the first person to bring up the problem. We've all been there—a few different people may have a similar idea, but whoever says it the loudest and with the most confidence gets the credit. The trouble with the night sky being dark had been mentioned before by other astronomers, including Kepler in the 1600s and even by a Greek monk living in Alexandria in the sixth century. The poet and author Edgar Allan Poe discussed the issue in his 1848 prose poem, "Eureka", where he says:
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.
So even poets were puzzled by the darkness of the night sky.
Possible explanations for the dark night sky
Olbers and others suggested that perhaps interstellar dust was the problem. We know that dust between us, the observers, and a star can obscure that star by absorbing its light. If the windows on your car or house get dusty, it’s harder to see through them. So, could it be that there's simply a lot of dust blocking all this light?
Astronomers quickly realized that this couldn’t be the solution. While dust does absorb starlight, it then heats up and re-emits that energy, which means we’d still be seeing it here on Earth. Also, there simply isn't enough dust to cause the phenomenon.
There have been other proposed solutions to Olbers’ Paradox. Could the distribution of stars not be uniform? Even if there are an infinite number of stars, perhaps they clump together and hide behind one another. This solution could be partially correct. There may be stars stacked behind each other in a sort of filamentary structure hidden from our view.
The most likely explanation—our changing and expanding universe
But the more likely explanation became apparent when astronomer Edwin Hubble discovered that the universe must be expanding and evidence began to grow for the Big Bang explanation for the beginning of our universe. These discoveries showed us that the universe is not unchanging—it's constantly growing. Not only is the universe changing and stretching as you read this, but everything must have been closer together in the past. The Big Bang theory also tells us the universe had a beginning, something that kicked off this expansion. That means it's not infinitely old.
This changing, non-steady universe has two major consequences for the night sky.
- The expanding universe causes light waves trying to reach our eyes from distant stars to stretch. Stretching a light wave effectively changes its wavelength, moving it into redder and redder parts of the electromagnetic spectrum, an effect known as redshifting. The light that is stretched, or redshifted, so that it's too far-red for our eyes to see is known as infrared light. So, the expansion of the universe means that the visible light emitted by stars can eventually be stretched into light we can no longer see, giving us a dark night sky.
- Perhaps more importantly, in a universe that is not infinitely old, there has not been enough time for light from all of the distant corners of the universe to reach us. Light travels at a finite speed, but we normally don’t encounter this finite speed on scales relevant to our everyday lives. When light moves from the light in my bedroom to my eyes, for example, the distance is so short that the journey appears to happen instantaneously.
But on universe scales, those distances start to matter. It takes light eight minutes to travel from the Sun to our faces here on Earth. So if an evil supervillain were to throw a blanket over the Sun, we would be blissfully unaware for eight full minutes. It takes light four years, however, to reach the next nearest star after our Sun, a star called Proxima Centauri. The journey to Andromeda, the nearest massive galaxy beyond our own? That trip will take light 2.5 million years. So as the universe gets larger, these timescales get longer and we eventually hit the cap of 13.8 billion years, the known age of the universe.
If a star is distant enough that it would take light more than 13.8 billion years to reach us here on Earth, then that light might be on its way, but not enough time has passed in the universe’s existence for it to reach it. We're still waiting.
It takes light four years to reach the next nearest star after our Sun, Proxima Centauri.
Of all the astronomers, scientists, and philosophers who puzzled over the darkness of the night sky, Poe is often credited with posing the first workable solution. Again in "Eureka" he noted:
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 the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.
The next time you take a look up at the seemingly calm and steady night sky, keep in mind that you're surrounded by evidence that our universe is constantly changing.