The Olbers paradox
Why is the sky dark at night? What seems like an obvious question, actually needs a more complex explanation.
If in fact the universe, as big as it may be, has finite dimensions and the stars are evenly distributed, we should find one every which way we look. Obviously, distant stars should appear weaker than the ones that are closer to us, with a decreasing luminosity as 1/d² where d is the distance from our Planet. At the same time, though, there should be a lot more stars the farther you go; their number should increase in relation to the distance as in d². Consequently the two effects annihilate each other and at night we should be able to see an evenly bright blue vault. Instead, as the sun sets, our sky appears dark, with bright spots here and there; hence the paradox (PHOTO: 10SpSt_t). The explanation can be found in the combination of two elements: the finite age of the Universe and its expansion.
Distances in the Universe are so great that even light, which travels at the maximum speed according to the laws of physics, 300 thousand kilometres per second, takes some time to cover them, So the further away we look, the more we go back in time and see objects as they were when the light left them to reach us. It is likely that some objects are so far from us that their light hasn’t arrived yet. In fact we can only observe what is within an hypothetic sphere centred on Earth and with a 13,7 billion light year radius, the distance travelled by light within a time frame which corresponds to the beginning of the Universe’s existence according to the Big Bang theory. And this is the main reason which explains the paradox.
However, also the Universe’s expansion plays a minor but important role. Because of galaxy recession, in fact, the further an object is the more the light we receive is “stretched” increasing its wave length, shifting, that is, towards red. If the object is far enough, its light becomes invisible because it goes beyond the threshold which divides the visible spectrum from infrared and microwave such as basic cosmic radiation. This effect causes luminosity to decrease faster with respect to the inverse of the square distance; thus the contribution of the most distant stars becomes less important.