If there‘s all those stars, why is it dark at night?

One hundred billion galaxies, all full of millions of stars, seems like a lot but it isn’t nearly enough to make the night sky as bright as day. If there were an infinite number of stars and the universe was infinitely old, there would be a star everywhere you looked in the night sky and it would be very bright indeed.

But the universe isn’t infinitely old. It was created approximately 14 billion years ago and since the speed of light is constant, we can only see objects that are less than 14 billion light years away. This means that we are living within a spherical ‘observable universe’ which is smaller that the total universe and that the light from stars further away from us than 14 billion light years will not have had enough time to reach the Earth.

In addition, the universe is expanding and all the galaxies, and their stars, are moving away from us. Thanks to this, the light from a moving star changes colour in a similar way that sound from a moving ambulance siren changes pitch. The light that we observe from distant receding stars is more red than it would be if they were stationary – the light is ‘red shifted’. In many cases the red shift is large enough to move the light out of the visible region of the electromagnetic spectrum.

Find related websites about the Olbers Paradox theory with physics.org

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