Why Is Space Black? The Mystery of Olber's Paradox

Since there are stars and galaxies in all directions, why is space black? Shouldn't there be a star in every direction we look?

Follow us on Twitter: @universetoday

Team:
Fraser Cain - @fcain
Jason Harmer - @jasoncharmer
Susie Murph - @susiemmurph
Brian Koberlein - @briankoberlein
Kevin Gill - @kevinmgill

Created by:
Fraser Cain and Jason Harmer

Edited by:
Chad Weber

Music:
Left Spine Down - “X-Ray”

Imagine you’re in space. Just the floating part, not the peeing into a vacuum hose or eating that funky “ice cream” from foil bags part.

If you looked at the Sun, it would be bright and your retinas would crisp up. The rest of the sky would be a soothing black, decorated with tiny little less burny points of light.

If you've done your homework, you know that space is huge. It even be infinite, which is much bigger than huge.

If it is infinite you can imagine looking out into space in any direction and there being a star. Stars would litter everything. Dumb stars everywhere wrecking the view. It’s stars all the way down, people.

So, shouldn't the entire sky be as bright as a star, since there’s a star in every possible minute direction you could ever look in?

If you've ever asked yourself this question, you probably won’t be surprised to know you’re not the first. Also, at this point you can tell people you were wondering about it and they’ll never know you just watched it here and then you can sound wicked smart and impress all those dudes.

This question was famously asked by the German astronomer Heinrich Wilhelm Olbers who described it in 1823.

We now call this Olbers’ Paradox after him. Here let me give you a little coaching, you’ll start your conversation at the party with “So, the other day, I was contemplating Olbers’ Paradox… Oh what’s that? You don't know what it is… oh that’s so sweet!”.

The paradox goes like this: if the Universe is infinite, static and has existed forever, then everywhere you look should eventually hit a star.

Our experiences tell us this isn't the case. So by proposing this paradox, Olbers knew the Universe couldn't be infinite, static and timeless. It could be a couple of these, but not all three.

In the 1920s, debonair man about town, Edwin Hubble discovered that the Universe isn't static.

In fact, galaxies are speeding away from us in all directions like we have the cooties.

This led to the theory of the Big Bang, that the Universe was once gathered into a single point in time and space, and then, expanded rapidly.

Our Universe has proven to not be static or timeless.

And so, PARADOX SOLVED!

Here’s the short version. We don’t see stars in every direction because many of the stars haven’t been around long enough for their light to get to us.

Which I hope tickles your brain in the way it does mine. Not only do we have this incomprehensibly massive size of our Universe, but the scale of time we’re talking about when we do these thought experiments is absolutely boggling.

So, PARADOX SOLVED!

Well, not exactly. Shortly after the Big Bang, the entire Universe was hot and dense, like the core of a star.

A few hundred thousand years after the Big Bang, when the first light was able to leap out into space, everything, in every direction was as bright as the surface of a star.

So, in all directions, we should still be seeing the brightness of a star.. and yet we don’t.

As the Universe expanded, the wavelengths of that initial visible light were stretched out and out and dragged to the wide end of the electromagnetic spectrum until they became microwaves.

This is Cosmic Microwave Background Radiation, and you guessed it, we can detect it in every direction we can look in.

So Olbers’ instinct was right. If you look in every direction, you’re seeing a spot as bright as a star, it’s just that the expansion of the Universe stretched out the wavelengths so that the light is invisible to our eyes.

But if you could see the Universe with microwave detecting eyes, you’d see this[display the WMAP data]: brightness in every direction.

Did you come up with Olbers’ Paradox too? What other paradoxes have puzzled you?

[dianekershaw]

I remember in the 60s lying on the grass watching the stars and there were millions more than there are now, they practically filled every space in the sky, they look sparse nowadays compared to back then.

[JohnSmith-mnsq]

I would like to know why stars get dimmer when you move farther away from them. Because the only thing (that we know of at least) in space that really alters light are black holes. Other than that, there's not really a whole lot to stop light from reaching us, just as bright as it was when it left the surface of a star. So why does it dim?

[World-Ganming-News]

Space is not black, Space is not Visible, Only the matter that occupies Space.

[CoolCakesJack]

I would say that space is black because there's no atmosphere for the light to scatter off thus we don't see any brightness... That's the only reason it gets bright during the day.. Blue light wavelengths are scattered more by molecules in the atmosphere and our eyes pick them up. If earth had no atmosphere.. We would see a black sky and the sun the same way it would look if you were standing on the moon.. We'd also all be dead lol

[Inertia]

Is there light everywhere? I mean, if I was way out in inter-galactic space, and I could not see any stars, would I be able to see my hands? Or would I be is total darkness?

[johnmpjkken]

Fraser,
Since I haven't yet stated this, your videos are time consuming but thoroughly enjoyed, mind expanding and educational in the theoritical field of the universe. I also like reading some of the responses and occasionally am compelled (hope you don't mind) to respond to one or two.

[xebatansis]

Go into a cave at night and light a match. Can you see the walls?

[SniperLogic]

Sniper Logic
ic
I have a problem with the very precept of the paradox. Maybe I am missing something, but I’ve been trying to figure it out for years. If the universe was truly infinitely old, even with an infinite number of stars, wouldn’t it be dark? With an infinite amount of time comes an infinite amount of time for an infinite number of stars to have utilized all their energy and gone out. What am I missing? Why shouldn’t, the universe be dark if the initial presumption of the paradox were true? I understand the explanation of the reason for the paradox, I just don’t understand the initial presumption to start with, that there would be infinite brightness after an infinite amount of time had passed, as was believed....?

[PatrickPease]

I love how he turns with this irate look on his face

[forcemajeure]

Excellent videos, explains things really well. Thanks for uploading

[Marilia]

I woke this question in my mind!! Sun can light the earth but the space where sun be is dark.. and all star should be light and not dark place .. and I understand sun reflect light and gases and atmosphere on earth reflect the light of sun.. ok .. but how explain sometime the light of moon .. specially when is full moon .. light the night .. where came from of that light? Cause moon don’t emane light .. and so if this question look stupid but I am try understanding how work!

[EquiliMario]

Now i'm questioning. How does space stretch light exactly? It means that the density of space is a constant and with its expansion comes the stretching. But how exactly does light stretch?

[MiguelAPerez]

This was a pretty mind blowing video. Loved it.

[alexanderdaviesmorris]

What would happen if two black holes collided? Would they combine to form a bigger black hole or cancel each other out?

[ashleyhubber]

I'm really enjoying your videos, I like that they are simple, I've spent many hours reading about and contemplating space and time because of you. For the time and effort you have put in to these video's, I thank you.

[PSDamon]

i like this guy, he explains questions too complex to explain

[Playsinvain]

How can the universe be running away from only us? Are we not running away with her?

[demondik]

The work done by Halton Arp on quasars and their parent galaxies has shown that intrinsic redshift is not an accurate metric to measure distant objects. This makes the idea of an expanding universe questionable. Especially the rate of its expansion.

[tiagotiagot]

Has space ever glowed green or higher, or did the temperature never got high enough for the redshift go thru those colors?

[ifeltjim]

Maybe this question asks the beginning of space...