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Asked by Ruben to Lisa, Mark, Rachel, Sammie, Stephen, Tim on 13 Mar 2018.
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Sammie Buzzard answered on 13 Mar 2018:
That’s a really good question- if you think about it if the universe is full of stars, shouldn’t all the light add up and make the whole sky bright as there should be a star in any direction you look in? This question is called Olbers’ Paradox, and scientists think that the reason for this is that the universe is not infinitely old; it is somewhere around 15 billion years old. That means we can only see objects as far away as the distance light can travel in 15 billion years. The light from stars farther away than that has not yet had time to reach us and so can’t contribute to making the sky bright which is why there’s lots of black still in the night sky.
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Tim Duckenfield answered on 14 Mar 2018:
Great question! Like Sammie says, the question about “why is the night sky black” has been asked a lot through history. One ‘solution’ to this “Olber’s Paradox” is that actually, our human eyes can only see a few colours.
We can see the colours of the rainbow – Red, Orange, Yellow, Green, Blue, Indigo and Violet. But there are loads of other colours, and our telescopes can see these other ones! For example, the telescopes I use can see the colour past violet – we call it ULTRA VIOLET light, or UV. Our eyes can’t see it, but we know it’s there – we have to wear suncream to block all the UV light heating our skin! So if we use our telescopes to look at space, it isn’t black at all – it can be really bright!
Space is expanding, meaning that colours change the longer the light travels – we call this “red shift”. So light that starts off green will end up red by the time it reaches us. Through red shift, most starlight has turned to a different colour we can’t see, making space black!
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