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Why is the Sky Dark at Night?
A discussion of Olbers' Paradox, the Big Bang and related issues — 20th anniversary edition

Copright © 1996 - 2016, P. Lutus

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If you think the answer to this question is obvious, think again. The best minds of physics have studied this question for over a century, and the current answer may surprise you.

In 1826, the astronomer Heinrich Olbers asked, "Why is the sky dark at night?" By his time, physicists had learned enough to realize that, in a stable, infinite universe (one not expanding), with an even distribution of stars, the entire universe should gradually heat up. Think about it — if there are stars emitting energy throughout the universe (energy sources), and if there is no way ultimately to dispose of that energy (energy sinks), then all the objects in the universe must rise in temperature, eventually reaching the temperature of the stars themselves.

Scientists and physicists had to learn quite a lot about the behavior of energy before they were even prepared to ask Olbers' question. In fact, for millennia the dark night sky provided an answer to a question no one thought to ask.

In these pages you will learn the simple physics behind Olbers' question, some of the answers that have been proposed, and the currently accepted answer. You will also discover the connection between a rubber band, your refrigerator, and the universe.
20th Anniversary Note
In 2016, to celebrate this page set's 20th anniversary and to deal with the recent disabling of Java in Web pages, I replaced all the Java applets with Javascript and edited some of the technical content.

In 1996 I designed these pages as one of my first Website educational projects, and (at a time when the World Wide Web was a small fraction of its present size) it won a number of awards for educational quality. I think these pages have aged rather well, even with recent discoveries with relevance to their topic (like Dark Energy).

Thanks for visiting, and if you like my content, please visit again in 20 years to see what's changed. :)

To navigate these pages, use the arrows and drop-down lists at the top and bottom of each page.

Home | Science | Why is the Sky Dark at Night? |     Share This Page