Why is Space Dark?

dark space question

There are billions of stars out there and yet Space – once you leave the Earth’s atmosphere – is pretty much black. Are you wondering…why? Not you alone!  

In fact, many scientists asked the same question before you, like, for example, the German astronomer Heinrich Wilhelm Matthias Olbers, who in 1823 allegedly said: 

Why, if the universe is infinitely old, infinitely big and static, and filled with an infinite (or really really big) number of bright stars that are uniformly spread out, the sky is not bright?

In other words…WHY IS SPACE DARK?

Today we call this question Olbers’ paradox and you might be surprised to find out that its solution is anything but trivial!


Looking into Space from the Earth

But before we discuss the blackness of space, let’s talk for a moment about the Earth’s atmosphere and why the sky YOU see is dark at night. Because chances are, right now you are standing on the surface of the Earth and looking at the Sun or other stars THROUGH the Earth’s atmosphere, right? On its way from the Sun to you, the sunlight bounces off the particles in the Earth’s air making the sun’s photon travel in zigzags. As a result, the sky above you is blue and everything around you looks bright and colourful. At night you can no longer see the Sun, only the light of other stars. This light – too – goes through the Earth’s atmosphere, that’s why the stars twinkle. But the night sky illuminated by these distant and faint pinpoints of light IS dark. And so is space. Despite all those millions and  billions of stars that we know are out there, there doesn’t seem to be enough to light up the whole Universe. Why is that?

Olbers’ paradox

You might think that the solution to Olbers’ paradox has to do with the fact that the starlight gets weaker and fainter the longer it travels. Maybe even to the point that it eventually fades away entirely. Well…it does make sense! Indeed the intensity of light does indeed fall with distance. But the answer to Olbers’ question lies deeper! You see, in the Universe Olber describes, the ‘weakening’ of the travelling starlight would have been cancelled out thanks to the light coming from more distant stars, the ones behind them, the ones behind them and so on… and the whole space would have been uniformly bright, not black! Something here doesn’t add up!


Expanding Space

It turns out that several of the assumptions in Olbers’ question itself are simply wrong!

As we now know, the Universe is not static, it is expanding! That means the space BETWEEN stars is expanding too. They are getting further and further away from each other while the dark spaces between them grow bigger and bigger. As the visible light of these stars travels away from its birthplace, it gets stretched into longer and longer wavelengths, such as infrared and radio waves. These forms of light are invisible to a human eye!


Age of stars

The stars – another shocking discovery – have been around for only a short, astronomically speaking, amount of time. Scientists think that the Universe is about 13.8 billion years old and that it probably took some hundred million years after its ‘birth’, The Big Bang, for the first stars to light up. 

So if we live in the Universe that was born some time ago and keeps expanding ever since, the light from faraway stars and galaxies simply has not had the time to reach us. 


Beyond the bubble

Let’s mention a few fancy terms here. Astronomers call the bubble that contains all the light – visible and invisible to our eyes – that had time to reach the Earth Observable Universe and the bubble that contains visible light Visible Universe. 

In the future more stars and galaxies will appear in our Observable Universe. But the Visible Universe will actually get darker, not brighter, because of all that stretching of the light!

Moreover, because the expansion of the Universe is ACCELERATING, the really really far-away objects move away from us faster than their light travels towards us! We will never ever get to see them, not even if we stick around for another billion years. 

So yes, the ‘visible light’ space is dark and will not get any brighter! 

But there are other forms of light astronomers can study. You can learn all about the ‘invisible light’ in our Blog post What is stellar spectroscopy and what does it tell us about stars? If you have any further questions or comments, feel free to use the comment section below or send a note to our Inflatable Planetarium team via our website wonderdome.co.uk. We would love to hear from you!

Author: Irina Vladimirova

Irina Vladimorova is a presenter at WonderDome Planetarium and a regular contributor to WonderDome Astronomy Blog. She holds a degree in Astrophysics and a special place in her heart for planetary science.