Why is it dark at night? Olbers' paradox, which is named after German astronomer Heinrich Wilhelm Olbers is also known as the "dark night sky paradox".
The paradox says that a static, infinitely old universe containing an infinite number of stars dispersed across an infinitely large space should be light as opposed to dark. So why is it dark at night?
Why Is It Dark At Night?You can demonstrate this be splitting the universe into a series of concentric spheres, which could be one light year thick, for example.
So, a certain quantity of stars would be in the sphere 1,000,000,000 to 1,000,000,001 light years away. If the stars in the universe are distributed evenly, then one would expect four times the stars in a second sphere that is between 2,000,000,000 and 2,000,000,001 light years away.
Because the second sphere is double the distance away though, each star that’s a part of it would seem four times dimmer on average than the first sphere.
Therefore the amount of light received from the second sphere should be equal to the amount of light received from the first sphere.
Each sphere of a given thickness, therefore, should generate the same quantity of light, from our point of view, however far away it may be. the light from each sphere should be added to the total amount.
If the universe is infinite, then there should be infinite spheres, and therefore, bright sky at night.
It would be possible for dark clouds to obstruct the light, but if that happened, then the clouds would warm up, eventually becoming as hot as stars, which would cause them to shine just as brightly.
Why Is It Dark At Night?
In general relativity theory, it’s still allowed for the paradox to exist in a finite universe. Although the sky wouldn’t show infinite brightness, each point of light visible in the sky would still have the properties of a star’s surface.
So, why is it dark at night? To explain Olbers' paradox, you need to account for the low brightness of the night sky compared to the circle of the Sun.
The universe has a finite age and stars have only been around for some part of that time. This means that the Earth can’t receive any starlight from past a particular distance which corresponds with the length of time that the oldest stars have existed.
The Big Bang theory introduced a fresh paradox by saying that the night sky had been considerably brighter long ago.
According to the theory it was at it’s height at the culmination of the recombination era. This is when it became transparent for the first time.
Every point in the nearby sky at that time would have been brighter than the circle of our Sun. This was down to very high temperatures that existed in the universe at that time.
Most light rays in the universe are relics of the Big Bang, rather than coming from stars. The Big Bang paradox can be explained by the expansion of space which is also part of the Big Bang theory.
This causes the amount of energy in the light coming to us to reduce because of redshift. Because of the cosmic expansion there are extreme amounts of radiation left over from the Big Bang which have been reduced in wavelength 1,100 times to microwave wavelengths.
This is known as the cosmic microwave background radiation. This can explain the relatively low levels of light observable in most of the night, despite the Big Bang being such a bright event.
Redshift also has an effect on the light coming from stars and quasars that are a great distance away, although to a lesser extent. So that's the answer to the question, why is it dark at night?
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