Why is the Sky Dark at Night?

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Have you ever wondered why you look up and see a dark sky at night? That question is not as simple as it may sound. The obvious answer is that the sky appears dark at night because that is when our side of Earth faces away from the Sun as our planet rotates around its axis every 24 hours.

What about all those other far away suns that appear as stars in the night sky? Our own galaxy, the Milky Way, alone contains over 200 billion stars, and the entire universe probably contains over 100 billion galaxies. You might suppose that that many stars would light up the night like daytime.

German physicist Heinrich Wilhelm Olbers puts the same puzzle this way in 1823: If the universe is infinite in size, and the stars—or galaxies—are distributed throughout this infinite universe, then we are certain to eventually see a star in any direction we look. As a result, the night sky should be aglow; why isn’t it then?

The best resolution to Olbers’ paradox at present has two parts. First, even if our universe is infinitely large, it is not infinitely old; that is, it had a beginning, just as you and I had. The universe was born about 15 billion years ago in a fantastic explosion called the Big Bang; it began at a single point and has been expanding ever since. This point is critical because light travels at the finite, though very fast, speed of about 300,000 kilometers per second; we can see something only after the light it emits has had time to reach us.

In our everyday experience that time delay is minuscule; even seated in the balcony of the concert hall you will see the conductor of the symphony raise their baton less than a millionth of a second after she actually does. As distances increase, so does the time delay; for instance, astronauts on the Moon experience a 1.5-second time delay in their communications with Mission Control, due to the time it takes the radio signals, which are a form of light, to travel round-trip between Earth and the Moon. Most astronomers agree that the universe is 10–15 billion years old, which means that the maximum distance from which we can receive light is 10–15 billion light-years away. So even if there are more distant galaxies, their light will not yet have had time to reach us.

The second part of the answer lies in the fact that stars and galaxies are not infinitely long-lived; eventually, they will dim. We will see this effect sooner in nearby galaxies, thanks to the shorter light-travel time. The sum of these effects is that at no time are all of the conditions for creating a bright sky fulfilled. We can never see light from stars or galaxies at all distances at once; either the light from the most distant objects has not reached us yet, or if it has, then so much time would have had to pass that nearby objects would be burned out and dark.

References
spaceplace.nasa.gov
scientificamerican.com
en.wikipedia.org

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