Why is the night sky dark?
That the sky is dark at night is a conundrum that puzzled scientists for centuries. German physicist Heinrich Wilhelm Olbers famously put the problem this way in 1823:
If the universe is infinite in size, and 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?
Source.
This question is known as the “Olbers Paradox” or the “Dark Sky Paradox.” Another way to think about it is that if the universe were both infinitely large and infinitely old then the night sky would be bright from all the stars. “Stars should overlap each other in the sky like tree trunks in the middle of a very thick forest. But, if this were the case, the sky would be blazing with light.” Source.
The current answer to the Dark Sky Paradox is that the universe isn’t infinitely old. It’s about 10-15 billion years old and thus we can only see light from stars as far as 15 billion light-years away. Light from stars further away than that are not visible. Thus, even if the universe is infinitely big it’s not infinitely old (the jury’s still out about whether the universe is infinitely big). This leads to a profound conclusion: “The Universe we inhabit today had a beginning: a day without a yesterday.” Source. That beginning, of course, is known as the Big Bang. The darkness of the night sky is important evidence that the universe had a beginning.
In a universe that is not infinitely old a few other factors contribute to a dark night sky:
- Interstellar dust absorbs light. If the universe were both infinitely big and infinitely old there’d be enough light from stars falling on the dust that it would heat up the dust and then the dust would glow as brightly as the original starlight. In a universe that is not both infinitely old or big, dust can and does block starlight.
- Red-shifting of light. As the universe expands, stars and galaxies farther away are moving away from us faster than those closer. The fast movement of those far away stars “stretches lightwaves which increases their wavelength resulting in a shift towards the red end of the spectrum. And red light is more difficult to see.” Source.
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