When we look at the starry sky and think about what we see – at some point almost everyone asks the same question. If the universe is infinite and there are billions of stars and galaxies in the sky – why is the sky black. Why doesn’t the whole sky shine with the radiance of the stars that are in all the directions of the sky we are looking at? It was expected to be so, but paradoxically it is not. The crystal clear sky is like coal black. This photometric paradox is called the Olbers paradox, and it is not easy to explain. There are several theories related to the Olbers paradox, and two give a perfectly acceptable answer. We all know that light has finite speed and that the universe is huge.
The age of the universe
The size of the universe (and the speed of expansion) is so large that light from distant stars has not yet reached us. In addition, both the age and the expansion of the universe affect how we see the starry sky. As the age of the universe increases, so does the number of stars whose light reaches us. The age of the universe is estimated at approximately 13 billion years, and from what we see in the sky – we can say that the universe is “very young”. If the universe were 10,000 billion years old – because of the many stars formed, the sky would shine in all directions. We see what we see.
Limited part of the universe
Since the speed of light is finite, we will always see a limited part of the universe. In the depths of space, galaxies are gaining momentum and are constantly moving away from us. This is due to the interaction of gravity, which begins to accelerate galaxies. When they reach a speed equal to the speed of light – galaxies reach the limit of our visible universe. After that, they move at a speed higher than the speed of light and become invisible to us. Of course, all those many galaxies and billions of stars still exist in them – but we don’t see them anymore. This theory has estimated that the visible universe is 13 billion years old, and that the actual size (according to some calculations) of the “visible and invisible” universe is approximately 90 billion years.
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why the starry sky is black space