The first observation concerning the whole Universe can be made with only one glance at the night sky, without any telescope and with limited theoretical knowledge. At the beginning of the last century, the german astronomer Olbers posed the following question: “Why is the night sky dark?” The answer to this question appears simple at first sight: “because there are not enough stars in the sky and therefore there are gaps between the stars”. This answer is, however, wrong. There are enough stars to fill the sky and actually their total light “should” be sufficiently intense to set fire to the Earth. Let us examine a simple proof of this claim.
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