Why Is It Dark At Night?

If Star Layer A is twice as far from Earth as Star Layer B, then the amount of light that reaches us from each star in A is only one-fourth the amount of light that reaches us from each star in B; but there are four times as many stars in A as there are in B.

The answer to this seemingly The answer to this seemingly simple question may boggle your brain. It's actually a famous cosmological problem, formally known as Olbers' Paradox. (Heinrich Olbers was a German astronomer who popularized discussion of this subject in 1826.) You might think that the question can be explained away by the effect of distance -- not so. To fully understand the perplexity, picture stars of equal brightness distributed evenly in concentric layers around Earth, like shells around a nut. The same amount oflight should reach Earth from each layer, because although the amount of light to reach us from each star decreases with distance (by 1/d^2), the number of stars in each layer increases, effectively balancing out the distance effect.

If the distance between A and B is 2 units, then each square in A is one-fourth as bright as each square in B; but there are four times as many squares in A as there are in B.

So light lost to distance does not account for the darkness of night. Obscuration by dust is not the answer, either, as any dust in the path of light would heat up and eventually reradiate. Most modern cosmologists have settled on two theories to account for the darkness. The first one states thatred shift (see Echo and Doppler Shift), which indicates that space itself is expanding, decreases the amount of light reaching us. The other explanation -- generally considered the main one -- is that the universe is not infinitely old. If it were, the sky would in fact be infinitely bright, because light from every point in the universe would have had time (eternity) to travel to every other point. As far as we know, there is no edge of the universe, only an edge of time. The finite age of the universe limits how much light we see.

One hundred billion galaxies, all full of millions of stars, seems like a lot but it isn’t nearly enough to make the night sky as bright as day. If there were an infinite number of stars and the universe was infinitely old, there would be a star everywhere you looked in the night sky and it would be very bright indeed.

The universe isn’t infinitely old. It was created approximately 14 billion years ago and since the speed of light is constant, we can only see objects that are less than 14 billion light years away. This means that we are living within a spherical ‘observable universe’ which is smaller that the total universe and that the light from stars further away from us than 14 billion light years will not have had enough time to reach the Earth.

In addition, the universe is expanding and all the galaxies, and their stars, are moving away from us. Thanks to this, the light from a moving star changes colour in a similar way that sound from a moving ambulance siren changes pitch. The light that we observe from distant receding stars is more red than it would be if they were stationary – the light is ‘red shifted’. In many cases the red shift is large enough to move the light out of the visible region of the electromagnetic spectrum.

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