Why is space black?

December 16, 2014 by Fraser Cain, Universe Today
GOODS field containing distant dwarf galaxies forming stars at an incredible rate. Credit: ESO

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

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

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.

Why is space black?
Big Bang Diagram

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: brightness in every .

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

Why is space black?
Cosmic microwave background. Image credit: WMAP

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35 comments

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imido
Dec 16, 2014
This comment has been removed by a moderator.
Modernmystic
4.4 / 5 (7) Dec 16, 2014
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: brightness in every direction.
The steady state Universe model would behave in similar way. But these distant galaxies should be still visible (the background will not be homogeneous) - we already have http://www2011.mp...xt.html.


If the universe were "steady state" and had no beginning then there would be no stars at all...they'd have burned out an infinite number of years ago.

One must check all the premises that a conclusion is based on...the laws of thermodynamics do not allow for an eternal single universe. This show is winding down...albeit veeeeery slowly.
imido
Dec 16, 2014
This comment has been removed by a moderator.
Modernmystic
5 / 5 (7) Dec 16, 2014
In AWT...


Well in the real world, the laws of thermodynamics apply consistently, and isotropically.
imido
Dec 16, 2014
This comment has been removed by a moderator.
Modernmystic
5 / 5 (7) Dec 16, 2014
If the physicists would really observe our world instead of extrapolating of abstract equations, they would understand it better.


How ironic :)

Physicists have observed the operation of the laws of thermodynamics more times than you or I can count. It's not theory, it's as much a fact as the Earth is round.

Show me your perpetual motion machine or energy manufacturer based on your THEORY and I'm sold...beyond that I prefer not to debate the color of the sky or play tic tac toe :)
antialias_physorg
4.3 / 5 (7) Dec 16, 2014
if the Universe is infinite, static and has existed forever, then everywhere you look should eventually hit a star.

The funny thing is that the sky can be dark even in such a universe if (and only if) the lifetime of the average star is below the average distance between stars (in light years) in any one direction.

So while there is plenty of evidence for a Big Bang scenario - Olber's Paradox isn't one.
imido
Dec 16, 2014
This comment has been removed by a moderator.
Z99
5 / 5 (10) Dec 16, 2014
I don't know what AWT is, nor do I much care. Possibly, some of the problems with your post are your grasp of English, I don't much care about that, either. You claim that particles "expand" and then "condense by their gravity". Hence their entropy decreases. Violating the Laws of Thermodynamics, LOL! If you have ANY experimental basis for claiming normal matter of a gravitationally bound body "evaporates" into dark matter, then cite it. If you are claiming that energy from fusion (photons and neutrinos) condenses, then cite the evidence for that. You do know that the scattering cross-sections for these particles contradicts your thesis, don't you? Or are these magical transformations only significant on scales of >> 10^15 years?? Actually, nevermind; just consider increasing your dose of Aripiprazole.
fourinfinities
1 / 5 (5) Dec 16, 2014
"...by proposing this paradox, Olbers knew the Universe couldn't be infinite, static and timeless."

A Creator could have seen that as well. He/She/It might have realized that if initial conditions were just right, if the expansion rate perfectly matched the gravitational force, as cosmologists have found, creation would nonetheless last a god-awful long time. And, provided enough dark matter and energy were included, interesting things would happen.

Modernmystic
5 / 5 (7) Dec 16, 2014
"...by proposing this paradox, Olbers knew the Universe couldn't be infinite, static and timeless."

A Creator could have seen that as well. He/She/It might have realized that if initial conditions were just right, if the expansion rate perfectly matched the gravitational force, as cosmologists have found, creation would nonetheless last a god-awful long time. And, provided enough dark matter and energy were included, interesting things would happen.



A gigantic ball of yarn could have been responsible too. Pisser we have no evidence for it or I could make a billion selling books to people telling them how to behave and then live forever in yarnland after they die. But no one would believe it....oh wait....
IMP-9
4 / 5 (4) Dec 16, 2014
Olber overlooked that we were born on Earth. Our eyes developed here.


You misunderstand Olber's Paradox. In the absence of extinction in a static universe if there was a sun-like star along every single line of sight then the sky would be as bright as the Sun. If there was some kind of star you still find the sky would be about a quarter as bright, still completely blinding. The way our eye's developed does not change that. Even with a telescope there is not a star along every line of sight.
DarkWingDuck
2.5 / 5 (2) Dec 16, 2014
Actually....

There are an infinite number of infinities and one infinity can actually be larger than another. Thus, the infinity of space simply has to be larger than the infinity of stars. Strange but mathematically true.
Salamander
3 / 5 (2) Dec 16, 2014
1. Blackness is a psychological thing -- put something bright next to something not bright and the not bright thing looks black.

Salamander
5 / 5 (2) Dec 16, 2014
2. Space can be finite and unbounded.
Salamander
1 / 5 (2) Dec 16, 2014
3. The argument that an infinite number of stars would necessarily be bright is wrong – consider dust and gravity.
keithbdi
1 / 5 (3) Dec 16, 2014
Thinking like this is caused by 'Eviloution' it darkens the mind.

In the beginning God created the heavens and the earth Gen 1:1
danimal82
5 / 5 (1) Dec 16, 2014
That is why there isn't a star in every point in the sky. Our eyes were not designed to see them. Fortunately, we have thumbs. And consequently telescopes. And so we know, ... there IS a star at every point in the sky.


The cosmic microwave background is not a "star." So no, there IS NOT a star at every point in the sky.
movementiseternal
Dec 16, 2014
This comment has been removed by a moderator.
Salamander
1 / 5 (4) Dec 16, 2014
4. The red shift could be due to other things -- other than speed! http://weinsteins...out.html
imido
Dec 16, 2014
This comment has been removed by a moderator.
Modernmystic
5 / 5 (4) Dec 16, 2014
You claim that particles "expand" and then "condense by their gravity". Hence their entropy decreases. Violating the Laws of Thermodynamics, LOL!
This is computer simulation of two phenomena: http://i.imgur.com/VQtAw2y.gif. Which process follows the thermodynamics and which one does violate it?


You're not getting the BASICS here. IF you have a steady state universe, then you, by NECESSITY, have to have a mechanism to reverse entropy or create matter/energy. If you don't understand this very, very, VERY simple point....then I suggest you do some reading.

I'll give you the benefit of the doubt and try to explain it in very elementary terms. Think of the universe like a clock. It started out completely wound up (minimal entropy) as it ticks it loses entropy. Eventually it will run out in a finite amount of time (maximum entropy), and part of that process is that the stars burn out. They haven't...you're wrong...
imido
Dec 16, 2014
This comment has been removed by a moderator.
vidar
1 / 5 (1) Dec 16, 2014
Many things are a matter of perception and perspective. I enjoy the comments and expressions. One thing that bothers me about the big bang folks is if everything started out as a single point who or what was on the outside looking in?
aragorn_wolf
not rated yet Dec 17, 2014
The universe is such an amazing creation. I just can't believe that everything that exists in the past, present and future of this universe was at one time gathered into a single point in time and space, and then, expanded rapidly. There is so much matter in the universe that it would probably have been a few thousand light years in size.
imido
Dec 17, 2014
This comment has been removed by a moderator.
antialias_physorg
5 / 5 (5) Dec 17, 2014
There is so much matter in the universe that it would probably have been a few thousand light years in size.

At that time it wasn't matter. Stuff was just so incredibly hot that what we now take for granted (protons, neutrons electrons, ...) didn't exist. We can create some of those conditions in particle colliders.
At those times you only have fields (as of the current standard model)

There is so much matter in the universe that it would probably have been a few thousand light years in size.

If you take all the matter in the universe and put it in one place -and somehow magically preventing it from collapsing into a black hole- at e.g. at the density of the sun then you get a cube about 1000 light years accross. (Note that the Earth is quite a bit denser than the sun so an Earth-density cube would be a bit smaller)
LeHutch
5 / 5 (1) Dec 17, 2014
The biggest contributor to the affect in this article is that the intensity of light is not reversibly proportional to the distance. It is related to a squared term of the distance. This means that a candle or light's (for the modern age) intensity viewed from 10 feet would be 1% as much 100 feet away.
Modernmystic
5 / 5 (4) Dec 17, 2014
have to have a mechanism to reverse entropy or create matter/energy
I just illustrated the way, in which entropy gets reversed in our Universe (http://i.imgur.com/VQtAw2y.gif) Regarding the creation of matter/energy from nothing, isn't this a miracle, which the Big Bang theory considered from its very beginning?


OK then, I guess our energy problems are solved. Give me your proposal for your energy production machine and I'll send you some investment money.
PawlT
not rated yet Dec 17, 2014
My paradox is why are there any stars at all? I'm surprised, not by the darkness, but the many specks of light.
OZGuy
5 / 5 (2) Dec 17, 2014
Care to join in playing spot the sockpuppet? I've spotted 3 so far, anyone care to take a guess who they belong to? Yes it's the usual suspect. We'll probably see a couple more puppets before this thread concludes.
OZGuy
5 / 5 (2) Dec 17, 2014
Make that 4...
antialias_physorg
5 / 5 (3) Dec 17, 2014
I'm counting 5 one-post-wonders (plus the 'original'). Talk about a split personality.
NATO 7_62mm X 51mm
not rated yet Dec 17, 2014
Why is space black? I can't say for sure, however I can tell you why aspirins are not black. If aspirin were black, they would not work....
Solon
1 / 5 (1) Dec 20, 2014
"My paradox is why are there any stars at all? I'm surprised, not by the darkness, but the many specks of light."

According to Neil Armstrong and Chris Hadfield, space is totally black, there are no stars visible, or planets, when in space and looking away from Earth into the void. Why does nobody believe them?

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