Science & Technology

Olbers' Paradox Asks Why the Night Sky Isn't Blazing With Light

Olbers' Paradox asks the question: If the universe is filled with infinite stars, then why isn't the night sky as bright as the sun? If the universe really extended to infinity, then there should be infinite stars. That would mean that every point in the sky should have a star in it, so the sky should be bright. Obviously, the night sky is dark, but for centuries, the reason why was a little bit of a conundrum — in other words, a paradox.

From Light Comes Darkness

We can trace the origins of this paradox all the way back to Johannes Kepler in 1610, though it wasn't popularized until the 19th century by another German astronomer named Heinrich Olbers — thus, the name. Clearly, scientists have been pondering this question for centuries (and, spoiler alert, they know the answer — you can scroll to the bottom for that if you're impatient). Here are six possible solutions to the paradox, some more correct than others.

Distant stars are dimmer than nearby stars.

First up: Maybe the sky isn't uniformly bright because more distant stars are dimmer than those closer to us. Unfortunately, the paradox accounts for that. It says that if you take a given nearby star and look 10 times further out in the sky, you'll see 100 more stars sitting in that exact same vantage point. Even though they're dimmer, there are enough of them shining together to make up for their lack of brightness. If you're dealing with infinite stars, every point in the night sky should be the same brightness as every other point.

Dust is obscuring distant stars.

What if cosmic dust is just obstructing parts of the sky and creating the illusion of darkness here on Earth? That's a nonstarter. Cosmic dust that could block or absorb the light energy from distant stars would start to heat up and glow itself. That's why nebulae glow when they don't have any internal energy source.

There aren't an infinite number of stars.

Now you're getting somewhere. Still, it doesn't technically solve the paradox. The universe could have a finite number of stars and still have enough to leave the night sky fully illuminated. Astronomers currently estimate there are about 100 billion stars in the Milky Way alone and roughly 1024 stars in the entire universe. While that number isn't infinite, it's enough that it doesn't explain the paradox.

There isn't a uniform distribution of stars.

Maybe stars are grouping together and forming clusters that look like singular stars in the night sky. This solution could be correct. There's no reason to think that stars would be evenly distributed and maybe they do clump together, leaving large chunks of sky totally dark.

However, in the frustrating words of math teachers everywhere, that answer isn't the most correct.

The expanding universe renders the most distant stars invisible.

In 1929, Edwin Hubble discovered that the universe is expanding. That meant that it was smaller in the past, which meant that the universe is finite. Part of the way Hubble made this discovery was by measuring the redshift of distant galaxies. Just like the pitch of a siren gets lower as it moves away from you, the wavelength of light gets longer as an object moves away from you — that is, it moves into the infrared area of the electromagnetic spectrum. The furthest galaxies are moving away from us fastest, so their light has been redshifted so far that they're no longer visible to the human eye. This is one of the most correct solutions to Olbers' Paradox.

The universe is so young that distant light hasn't had a chance to reach us.

It's also probable that the universe as a whole is too young to see the most distant stars. The universe is only about 13.7 billion years old, and — to state the obvious — it takes light from an object one year to travel one light-year. That means that the light from anything further than 13.7 billion light years away wouldn't have had time to reach us yet. That 13.7 billion light-year figure is the size of the "observable universe"; anything beyond that just looks like darkness.

These two final propositions — redshift and a young universe — are the most likely solutions to Olbers' Paradox. The next time you look up at the night sky, take a moment to not only marvel at the beauty of the stars you can see, but also wonder about the beauty of those you can't.

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Written by Trevor English August 31, 2016

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