At first glance, the answer seems very simple: the sky is dark at night because there is no sunlight. As the Earth rotates to face away from the Sun, direct light from the central star of the Solar System no longer reaches the part of the sky we observe.
However, this explanation overlooks a very important issue. Besides the Sun, the universe contains billions, even hundreds of billions, of stars in countless galaxies.
Each star emits light. So why isn't the total light from all those stars enough to illuminate the night sky?
Olbers' Paradox: If the universe is infinite, must the sky be as bright as the sun?
This is not a new question. In 1823, the German astronomer Heinrich Olbers argued that the night sky should be as bright as the surface of the Sun if the universe were infinite and filled with stars.
Olbers' argument is quite intuitive. If the universe is truly infinite and stars are scattered everywhere, no matter which direction we look in the sky, our vision will eventually encounter a star.
It's true that the farther away a star is, the weaker its light reaches Earth. But the problem is that the number of stars at great distances has also increased dramatically.
Therefore, the amount of light from countless distant stars should ideally compensate for the dimming of each individual star.
If this argument is correct, the entire night sky would have to be filled with starlight and have a brightness equivalent to that of the Sun.
But the reality is quite different. When the sun sets, the sky becomes dark, and only a small fraction of the light from distant celestial bodies can be observed with the naked eye.
So what happened to this paradox?
One commonly cited explanation for Olber's paradox is that the universe is expanding.
When light travels through an expanding universe, its wavelength is stretched. This phenomenon is called cosmic redshift.
As wavelength increases, the energy of the photon decreases, causing light from very distant galaxies to become weaker when it reaches Earth.
This is absolutely true, and the expansion of the universe is indeed contributing to a reduction in the amount of light we receive from distant celestial bodies.
However, the expansion of the universe alone is not enough to explain why the night sky is so dark.
The more important answer lies in the age of the universe itself.
The universe does not exist forever, and the stars do not shine forever.
Galaxies and the stars within them did not exist from eternity. They only came into existence after the universe formed from the Big Bang.
The first stars are thought to have formed several hundred million years after the Big Bang. This has a very important consequence for Olber's paradox: light from very distant celestial bodies could not have had enough time to reach us.
In other words, we don't see the entire light history of a hypothetical universe that exists indefinitely. We only perceive the light emitted during the time that universe has existed.
This also means that the amount of light that stars and galaxies have emitted throughout the history of the universe is finite.
This is the key to resolving Olbers' paradox.
If the universe existed forever, there would be infinite time for light from countless stars at different distances to reach us.
Then, the argument that the night sky must be brightly lit becomes more reasonable.
But the universe doesn't exist indefinitely in terms of time. It has a finite history, beginning with the Big Bang about 13.8 billion years ago.
Stars and galaxies also only shine for a finite period of time.
Therefore, instead of simply saying the night sky is dark because there is no sun, a more accurate explanation is that the night sky is dark because the universe has not existed forever.
Light from many galaxies and stars so far away is still on its journey to Earth and may never reach us.
Meanwhile, the light from the celestial bodies we observe is only the portion of light that has had enough time to travel through space to reach Earth.