People have always wondered why space is darkness even though it’s full of stars. The answer has a very specific name: Olbers’ paradox. Astronomers estimate that there are about 200 billion trillion stars in the observable universe. Many of these stars are as bright as, or even brighter than, our Sun. So why isn’t space illuminated?
One possible answer is that many stars are very far away from Earth. Naturally, it’s true that the farther a star is, the fainter it appears. A star 10 times farther away appears 100 times dimmer. But this answer is incomplete.
Imagine a bubble
Let’s suppose, for a moment, that the universe is so old that the light from the farthest stars has had time to reach Earth. Let’s imagine ourselves in a large bubble with us at the center. If the bubble had a diameter of about 10 light-years, it would contain roughly a dozen stars. Naturally, at several light-years away, many of these stars would appear rather dim from Earth.
If we expand the bubble to 1,000 light-years, 1 million light-years, and then to 1 billion light-years, the farthest stars in the bubble would always seem fainter. At the same time, there would be more and more stars inside the ever-expanding bubble, all contributing to the light. Even though the farthest stars appear increasingly faint, there would be many more, and the entire night sky should appear very bright.
Age matters
The universe is about 13 billion years old, a surprisingly long time in human terms but short in astronomical terms. Light from stars farther than about 13 billion light-years may not have reached Earth yet.
So the real bubble around Earth, containing all the stars we can see, extends only to about 13 billion light-years. But then why, when we observe a space with so many stars, do we see darkness in some directions? Simply because there aren’t enough stars in every possible line of sight.
Naturally, in some directions of the sky, we can see the stars. If we look in other parts of the sky, we won’t see any. In those dark areas, the stars are so far away that their light has not yet reached Earth. Over time, the light from these increasingly distant stars will have time to reach us.
Redshift
So if we wait indefinitely, will the night sky eventually be fully illuminated? Answer is yes, but only if the stars we observe were stationary relative to our point of observation. The problem is that this condition will never occur. Universe is actually expanding, with the farthest galaxies moving away from Earth at nearly the speed of light.
As galaxies move away so quickly, the light from their stars is shifted to colors that the human eye cannot see. This effect is called Redshift or Doppler shift. So, even if the farthest light had enough time to reach us, we still wouldn’t be able to see the farthest stars. Thus, the night sky will never be fully illuminated.
Furthermore, waiting indefinitely has an additional downside. Sooner or later, all the stars will burn out. Astronomers hypothesize that in the distant future, the universe will become dark, inhabited only by stellar remnants such as white dwarfs and black holes.
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