Why the night sky is dark? It sounds obvious. That’s what night is. The sun has set and when you look up at the sky, it’s black. Except where there’s a star, of course. The stars are bright and shiny.
But wait. Imagine you are deep in a forest. All around you, there are trees. Wherever you look, you are looking at a tree. Maybe a big tree close up or a bunch of small trees further away. Surely it should be the same with stars. We’re deep in the universe and whatever direction we look in, there ought to be stars there – billions and billions and billions of them. You would have thought that they’d fill the whole night sky, with the more distant ones fainter but more numerous.
This is called “Olbers’ Paradox” after a 19th-century astronomer, although the conundrum was around for a couple of centuries before him. And the answer – at least, now – is fairly clear.
The reason the night sky isn’t just a blaze of light is because the universe isn’t infinite and static. If it were, if the stars went on forever, and if they had been there forever in time, we would see a bright night sky. The fact that we don’t tells us something very fundamental about the universe we live in.
A limit to the universe may seem a natural explanation – if you were in a forest and you could see a gap in the trees, for example, you might surmise that you were near the edge. But it’s dark on all sides of us, which would mean not just that the universe is bounded, but that we’re in the middle of it, which is pretty implausible.
Alternatively, the universe could be limited in time, meaning that light from far-away stars hasn’t had time to reach us yet.
Blame the Doppler effect
But actually the explanation is neither of these. Light from the far-away stars gets fainter because the universe is expanding.
Edwin Hubble discovered in 1929 that distant galaxies and stars are travelling away from us. He also found that the furthest galaxies are travelling away from us at the fastest rate – which does make sense: over the lifespan of the universe, faster galaxies will have travelled further.
And this affects how we see them. Light from these distant, fast-moving galaxies and stars is shifted to longer wavelengths by the Doppler effect. In the case of these stars, the effect shifts visible light into invisible (to the human eye) infra-red and radio waves, essentially making them disappear. Indeed, the blackness of the night sky is direct evidence of an expanding universe.
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Video: Why is the night sky dark?
In a nutshell: the night sky isn’t actually dark, it shines as the Cosmic Microwave Background, but our eyes cannot see it, because it is highly redshifted because of the Doppler Effect.
Named after the Austrian physicist Christian Doppler (29 November 1803 – 17 March 1853), who described the phenomenon in 1842, the Doppler effect (or the Doppler shift) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.
A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by and lower during the recession.
The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency.
While they are traveling, the distance between successive wavefronts is reduced, so the waves “bunch together”. Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wavefronts is then increased, so the waves “spread out”.
- Doppler effect on Wikipedia