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The observable Universe

Galaxies are the neighbourhoods of stars. They are systems of hundreds of billions of stars but how many stars are there in the Universe? How can we count them? Can we see them all? Counting the stars is not an easy task for sure! First, we need to clarify that we can only speak about the stars that we can observe. This is why we often talk about the “Observable Universe”. That is the part of the Universe we can observe. Why can’t we see the entire Universe? Because light has a finite speed! A first estimation is that the observable Universe is as big as light could travel since the beginning of the Universe, which would be around 13.8 billion years ago. That means that the observable Universe could be around 13.8 billion years in radius. Well… not exactly. There are other factors to take into consideration, like the expansion of the Universe.  A photon that was born during the big bang and has travelled 13.8 billion years to reach us, by the time we detect it, it has travelled much further away from us than where it originally was, making the observable Universe much bigger than 13.8 billion light years. So how big is the observable Universe? Around 45.7 billion light years. 

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