r/askscience • u/[deleted] • Jun 18 '12
Astronomy If the universe is infinite, why is the night sky black?
I hope I'm wording this right, but it's always kind of been on my mind.
If the universe is infinite (or just really, really, really, really big) shouldn't our entire field of vision above our heads at night be the color of stars/planets, and not darkness? Shouldn't every void in space at some point in infinity be filled with either a star or a planet?
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u/nicksauce Jun 18 '12
Because the universe has a finite age, only stars/galaxies within a certain distance have had enough time to have their light reach us.
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Jun 18 '12
How do we know the universe is infinite?
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Jun 19 '12
present data suggest that the curvature is very very nearly zero, and this implies an infinite size to the universe.
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u/Bhewy Jun 18 '12
Isn't the theory that everything started in the same place (Big Bang)? So if we all started in the same place, why is it we can't see everything already? Or did everything move away at faster than light speed?
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u/obsidianop Jun 18 '12
This is the right answer. The answer about us being a "baby universe" isn't exactly wrong but I don't think it really gets to the most salient point: the universe as we see it is NOT infinite, but instead has a size that's more or less its age x the speed of light.
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u/shartifartblast Jun 18 '12
Related questions, because this is bothering me every time I try to conceive of a spatially infinite but temporally finite universe.
Does the Big Bang theory as a singularity get thrown out? In other words, how do you start from 0 and end up at an infinite in a finite amount of time without infinite expansion?
If Big-Bang-As-Singularity is thrown out, is the Big Bang a local phenomenon? Would different areas of the universe, assuming some kind of fixed frame of reference, be of different ages?
If the answer to the previous question is no, how does the Big Bang, happening over an infinite distance, happen simultaneously?
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Jun 18 '12
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u/gruehunter Jun 18 '12
One more detail: Since the universe is expanding, there should be a critical distance (relative velocity) above which the light has been redshifted out of the visible spectrum. So again, in an infinite universe, you would not be able to see an infinite distance with the naked eye.
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u/Team_Braniel Jun 18 '12
Correct.
There was a point when the Cosmic Background Radiation hadn't yet red shifted into the radio band. At that point, the whole night sky would have had a faint hue.
Even more sad, as cosmic expansion continues (and continues to speed up) we will be able to see less and less distant galaxies in the night sky. In another few billion years the only things visible in the night sky will be what is left of the Milkyway and Andromeda galaxies (after merge/collision). Future civilization's astronomers will look out into a blackness with only local galaxy stars to see. They will then assume this one cluster is all there ever was and form their sciences around that observation.
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u/kirrette Jun 18 '12
How do we know this isn't already the case? If the universe has already expanded enough to put some matter beyond our visible reach, we can't know that this is the case, surely?
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Jun 18 '12
We do know this is the case. This is why you'll hear scientists refer to, "The visible universe" instead of "the universe" we can only see so much of it.
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u/shawnaroo Jun 18 '12
It's believed that the actual universe is significantly larger than the visible universe. But we're lucky enough to exist relatively early in the history of the universe, where there's still all sorts of cool stuff to see in every direction.
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u/SlutBuster Jun 18 '12
Based on the current map of the universe, an elliptical shape with Earth in the center, it's safe to assume that is exactly what has already happened - there are almost certainly objects whose light cannot reach us.
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Jun 18 '12
Why is that sad? I think it's cool that he night sky is evolving. Future civilizations would hopefully just expand on the work being done now.
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u/gillesvdo Jun 18 '12
That is if we can survive long enough to pass on our knowledge to future civilizations.
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Jun 18 '12
LOL Well if that becomes an issue, I doubt the night sky looking different will be a major concern.
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u/thegimboid Jun 18 '12
Hopefully whatever is left of humanity at that point has colonized those distant places.
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u/littlesnabes Jun 18 '12
That is not entirely true . The Super cluster the Milky Way is in will remain intact due to it being gravitationally bound . So the super luster will always be visible . Well till its respective galaxies run out of fuel .
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u/exteras Jun 18 '12
So, question. You say "as cosmic expansion continues, we'll see less and less in the night sky". I think I'm imagining this with the earth in the center of the universe, but obviously we aren't. The universe is expanding away from some point somewhere; where it started.
Isn't it true, then, that the Earth itself is expanding away alongside everything else? And we're expanding at the same rate, aren't we? If the Big Bang Theory is true, then I'd imagine that we're sort-of expanding out in a spherical shape.
In billions and billions of years, we'd loose sight of some stuff really far away. It's accelerating in a different direction. But we'd still see everything which is accelerating in the direction we are, wouldn't we? Which would, essentially, be everything along the line of radius formed between us and the center of the universe.
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u/BaggedTaco Jun 18 '12
So that means that with each passing moment in time the search for planets with life on them gets harder as the distance to travel increases?
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u/zendopeace Jun 18 '12
Well, no, not really. Our galaxy of stars is gravitationally bound beyond the energy required to overcome the expansion of the universe. Even our local group, while very slowly pulling itself apart (on average), will remain our backyard for some time. We wont run out of exploration space for quite some time (slight understatement).
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u/Eunomiac Jun 18 '12
Yeah, but only on a cosmic scale -- this process is pretty slow from a human perspective, so it really only matters if you're considering the effect this might have on the SETI efforts of civilizations developing millions of years from now.
Of course, if you do consider the lot of those poor future civilizations, and how they will be marooned in a black void that forever consigns them to ignorance and isolation, then this is another really downer point in this thread.
So thanks.
;)
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Jun 18 '12
Well, we do know that stars have lifespans. So yes, it would eventually cease sending light. A star would shine on us for the same amount of time it was alive, belated by the time it took the light to travel here.
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u/fetalbeej Jun 18 '12
right, I figured this would be a factor as well. sorry for the questions, I don't know too much about cosmology (astrology?), but how long is the typical life span of a star? how often are new stars formed? is the amount of light given off by a star more significant than it's time in its life? thanks for answering my, I'm sure, elementary questions!
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Jun 18 '12
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u/JacFloyd Jun 19 '12
And in 100 billion years, the galaxies outside the Milky Way's Local Group have disappeared beyond the practical observational limit (http://en.wikipedia.org/wiki/Timeline_of_the_far_future).
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u/CatalyticDragon Jun 18 '12
"Olbers' Paradox is pretty simple: if the universe is infinite, and is filled with an infinite number of stars, then when the sun goes down, all we should see is a sky full of light. There shouldn't be any black background for stars to twinkle against, or night vision, or shadowy alleyways full of criminals to give superhero movies a dramatic start. Darkness should be exotic, because every star in the universe should be shining down on us at all times.
Clearly, that isn't happening, but why? Because we live in a sad baby universe — it's only 14 billion or so years old, which is certainly a long time, but not long enough for an infinite amount of stars to illuminate us with an infinite amount of starshine. "
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u/nalc Jun 18 '12
OP may also enjoy this article, which is related.
http://en.wikipedia.org/wiki/Hubble_deep_field
It's an extremely long exposure of a small patch of dark sky that reveals many galaxies that were not visible in prior observations with inferior telescopes.
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u/darkslide3000 Jun 18 '12
Yeah... except this is bullcrap. Due to Hubble expansion, objects move away from earth at a speed proportional to their distance, and for objects beyond those 14 billion lightyears that speed is higher than lightspeed (this is possible because they don't really move, instead the space between them and us is expanding). This means that the light of those more distant objects will never reach us (it is too slow to outrun the expansion), and even the objects we can still see today will one day go beyond that barrier and out of our sight.
Because the universe is expanding, its density is shrinking... that means it was actually much brighter in aeons past, and will become darker and emptier as more time passes.
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u/g0rth Jun 19 '12
Yeah, that's my take on this too. Regarding that, I always found utterly depressing the fact that in a very distant future, our galaxy will end up being entirely physically isolated from everything else, with no real way of telling what's beyond that.
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u/darkslide3000 Jun 19 '12
Nah, it's not that bad: If FTL travel really is physically impossible, we would never get out of that galaxy anyway. But if it isn't, all that observable universe stuff suddenly becomes obsolete (or rather become a matter of who's got the biggest engine).
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u/candre23 Jun 18 '12
we live in a sad baby universe — it's only 14 billion or so years old, which is certainly a long time, but not long enough for an infinite amount of stars to illuminate us
Unfortunately, that will never happen. Stars don't last forever, and there is only so much hydrogen in the universe to make new stars. As stars die out (faster than new stars are born) and the stars that live on get farther and farther away, the sky in the distant future will get sparser and sparser. In 90 trillion years (give or take) the last of the stars will die out and the stelliferous era will end.
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u/Hugo_5t1gl1tz Jun 18 '12
Does that mean the universe will essentially cease to exist?
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Jun 18 '12
In a few more trillion years when everything has cooled to absolute zero, then time as we have known it will stand still.
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u/candre23 Jun 18 '12
It's going to take a lot longer than that. Even after all the stars burn out and all the protons and neutrons decay (about 1x1040 years from now, if ever), it will take something like 1x10100 years for the black holes to "evaporate". That's a really, really, really long time. Even then, when there's nothing bigger than an electron left in the universe and no energy left for it to do much of anything, time itself won't exactly stop, there just won't be any events by which to measure it or anything at all to witness its passing.
There's a fantastic timeline here, if you're interested.
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u/James-Cizuz Jun 18 '12
Time as we know it does stop. Entropic time is change, and if a universe where nothing is changing; even if things are still moving time has stopped for all intents and purposes. Mainly due to the fact the universe will be the same going forward; forever.
Now universal time would never stop, and the cool thing about universal time is it is universal and unchanging. You may be used to the idea that entropic time is changed via speed and gravity, slowed down or sped-up. Universal time would not speed up, slow down under any circumstances.
However everything in our universe is governed by entropic time, so universal time may not even be worth factoring in.
Here is a good way to visual entropic time. http://youtu.be/HHRK6ojWdtU
Essentially speed changes the path a particle must flow for it to complete one cycle, so in their example a photon clock as you move it the photon does not bounce directly between two mirrors, it now travels in a zig-zag pattern, taking a longer path, longer cycle, thus entropic time slows down.
So this changes the entropic time of the universe in anything we are used to, time or entropic time slows down, all cycles take longer to complete then before.
However you may notice that while we are governed by cycles, what governs how long it takes an object to move in any direction? Universal time, which does not change. Entropic time however since speed changes the length or path an object must flow before completing a cycle, as does gravity, you can see why entropic time slows down.
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u/exteras Jun 18 '12
I believe there was a sci-fi short-story concerning this. I can't remember what it's called, but I'm sure someone will see this comment and remind us.
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u/monkeybomb Jun 18 '12
Some above referenced an Asimov story. This is another Asimov story relevant in this context.
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u/existentialhero Jun 18 '12
Even in a classical Newtonian universe where light travels instantaneously, there's no reason to suppose that the stars must be distributed such that they fill the sky as perceived from Earth. There's nothing mathematically wrong, for example, with a universe where all the matter lies more or less on a single plane, resulting in an Earth observer seeing a single band of bright light in an otherwise normal starfield.
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u/TrainOfThought6 Jun 18 '12
There's nothing mathematically wrong, for example, with a universe where all the matter lies more or less on a single plane, resulting in an Earth observer seeing a single band of bright light in an otherwise normal starfield.
Mathematically, no, but wouldn't that cause problems cosmologically? All matter being roughly in one plane wouldn't be very uniform or isotropic.
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u/existentialhero Jun 18 '12
Most definitely. I'm not trying to argue that we don't actually live in a quasi-isotropic universe—just that Olber's so-called "Paradox" has an additional logical flaw.
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u/sikyon Jun 19 '12
Could the argument not simply be made that the universe has some sort of average density of energy, and the average energy density in space is too low to be seen? For example, any particular region of space has on average some number of stars and some number of black holes which give a cosmically average energy density.
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u/dimitrisokolov Jun 18 '12
Take a monitor that has a black background and then randomly display white pixels with a certain lifetime for each one. Then expand that monitor in all directions and keep plotting your pixels. If the expansion is fast enough, it will never be filled with completely white pixels.
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Jun 18 '12
Say there is a planet with sentient life in a very, very old universe- might their "night" sky be almost completely white and filled with starlight?
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u/Fury87 Jun 18 '12
No, because all of the galxies in the universe are moving farther and farther apart. If the universe were old enough you would only be able to see the stars in your own local galaxy (even with a telescope!).
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u/louiswins Jun 18 '12
Actually, it doesn't matter how far away other galaxies are or how fast they are moving away from us (even if they appear to be moving faster than the speed of light!), their light will eventually reach us.
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u/Fury87 Jun 18 '12
This is interesting, I'm not an expert but in this video Lawrence Krauss refutes this in his talk A Universe From Nothing at around 51 minutes. It is a great talk in general if you have the time to watch it.
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u/tsears Jun 18 '12
Correct, the expansion of the universe has been observed to be accelerating, not expanding at a constant rate (or slowing down, as was once hypothesized).
Therefore the ant-on-a-stretching-rope problem is not analogous to the expansion of the universe.
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u/NEED_A_JACKET Jun 18 '12
Is gravity limited to the speed of light too? Would gravity work in an infinite universe? I would have thought that if there's an infinite amount of force in all directions there'd be no effect, locally or not.
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u/Astrokiwi Numerical Simulations | Galaxies | ISM Jun 18 '12
Gravity is also limited to the speed of light. Our current observations are consistent with an infinite universe. But even if gravity was instantaneous, all the really long range stuff would cancel out (the universe is basically constant density on those extremely large scales), and only "short" range gravity would really matter anyway.
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u/homelessapien Jun 18 '12
This article fails to mention one of the most important causes of the black nighttime sky, which is the redshifting of the light from the majority of stars in the sky due to the expansion of the universe. Because of the expansion of the universe, all stars not in the immediate vicinity of us (cosmologically speaking) appear to be moving away from us. (In fact, they are not "moving away from us," something more subtle involving einstein's equations and the cosmological scale factor is taking place). Because of this, the wavelength of all the light headed to us from those stars is being changed. In a nutshell, the farther away stars are from us, the greater their light's wavelength appears to us and the smaller the total power of that light becomes, so the light from stars far enough away is not really visible by the time it arrives here.
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u/skztr Jun 18 '12
I don't buy this argument by itself, as it assumes the validity of "the night sky is black".
Pick the blackest point in the night sky and point a sensitive instrument towards it. What I have always heard is there is no "black" out there.
Isn't the reason, then, rather that frequencies get stretched beyond the very small spectrum of "visible" light as they go? I don't know the method by which this happens, I don't know if "stretched" is the right word, but I'd really like some clarification: Hasn't the blackness of the night sky been disproven?
I would really like an answer to this, because the Ultra Deep Field image is the most emotionally powerful thing I have ever seen, and I would really like to know if I've merely misunderstood something.
IAmNotAScientist
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Jun 18 '12
You know, I thought I had a good idea of the reason behind this, but you put it much better.
The sky is black because the universe is much bigger than you think, and light is much slower.
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u/herpderpcake Jun 18 '12
Just so I understand, the universe is too young for all the stars farther away from us' light to reach us?
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u/Islanduniverse Jun 18 '12
By its very definition, you can't really have an infinite amount of anything. Because it never stops multiplying... How can something reach infinity?
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u/AgentSmith27 Jun 18 '12
Yes, but Olber's paradox is a paradox that has been shown to be an inadequate response to this question. Its more of an argument against the Steady State Hypothesis, which also violated the second law of thermodynamics.
The inverse square law shows us that the further away you get from an EM radiating source, you will receive an increasingly small level of EM radiation with each meter of distance. Think of how much light a candle gives off. What if there were an infinite number of candles spread across the galaxy with a frequency of 1 candle per 100 million light years. The summation of all of the light from these candles (assuming they burned forever) would never be appreciably visible.
The reason is that you may have an infinite number of stars, but you also have infinite space. Energy density is probably the most important factor, along with how much energy is released and how much energy is abosrbed.
Its entirely possible that you could have an infinite universe, and it would not be immediately obvious. The sky would NOT light up. It would NOT be bright everywhere.
I'm a little surprised how more people did not bring this up..
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u/noxumida Jun 18 '12
The universe is young? Young compared to what? Ours is the only universe we have as a frame of reference- you really can't call it young or old because we don't have other universes to compare it to. Since it's the oldest thing that we know of, I really don't see how you can call it anything but old.
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u/Vectoor Jun 19 '12
It is young compared to the hypothetical universe which is old enough for there to be stars in the entire sky.
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u/noxumida Jun 19 '12
But we don't know that the universe will last long enough for that to happen. In fact, stars may run out of fuel before that happens. You're assuming that this will in fact happen at some later time and using that hypothetical time to say that our current universe is young. In other words, your frame of reference is imaginary.
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u/itsableeder Jun 18 '12
I realise this is speculation; mainly it's the aid my own understanding by having somebody put me right.
I'll accept that if the universe is infinite it should be filled with an infinite number of stars; but should it not also be filled with an infinite amount of emptiness as well? That would account for the darkness of space, surely?
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Jun 19 '12
I see a lot of this, but i think another interesting way to look at it is mathematically. You are making the assumption that giving a finite amount of space, (The 360 degree viewing sphere that surrounds our your current location) and a infinite amount of arbitrarily small points (The stars) you would eventually fill that space. But if you consider that the amount of space that a star takes should decrease as it gets farther away from earth, Then you might be better of asking, is there an upper bounding series which is convergent to less then the finite amount of space? If there is, then we know that the total amount of space taken up by the stars in our sky would be less then that convergent series which we contrived which is in turn less then the finite amount of area in our viewing sphere. The premise of olbers paradox does not give us any reason to think this is not possible. Also consider that the area in that viewing sphere occupied by the stars is not disjoint. If all of that stars happened to line up from our point in space in a straight line in one direction, they would take up a necessarily a very small amount of space, more specifically an amount of space less then or equal to the area of the closest star. Again the premise of olbers paradox does not diclude this. Therefore mathematically speaking, olbers paradox is no paradox at all, but simply bad reasoning.
More interestingly, if our universe is expanding, then the size of stars in our viewing sphere will decrease exponentailly with time as they get farther away. This means that the points of light created by stars in our night sky will approach Infinitesimally small as time goes to infinity. As some might already know it is possible the have a infinte amount of points that contains no area. Thus with infinite time and the universe expanding infinitly, the amount of space filled in our night sky would approach 0 percent not 100 percent. More stars may appear as time goes on an light has time to travel to our planet, but then their size would then begin to decrease. The question would be does the rate of new stars appearing outweigh the rate of stars decreasing in area. There is also that the longer a star takes to appear in our night sky the smaller its initial size would be so to me it seems doubtful that with time we would approach 100 percent converege, instead I tend to lean to the other direction, but at the moment i cannot seem to find any concrete evidnce to point to one way or another.
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u/lachlanhunt Jun 18 '12
This seems to assume that all stars in an infinite universe would exist within the horizon of the observable universe. But given that the rate of the universe's expansion, anything currently beyond that horizon will never fall within it, and lots of stuff currently within it will eventually pass beyond it.
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Jun 18 '12
Are there any estimates of the necessary age of a universe to exhibit the non-dark property? Would this have an effect on diurnal and nocturnal animals causing a population collapse?
Non-science: Assuming the dominant species is at that time is at least as smart and productive as Homo sapiens, would dome cities probably become a thing as a result?
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u/shawnaroo Jun 18 '12
Many of the other posts have indicated why our current understanding of the universe leads us to believe that the non-dark property will never occur. But if it did, the amount of light reaching the earth would pretty quickly melt the crust and kill everything on it.
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Jun 18 '12
That means the light from any star outside of a 14 billion light-year radius us hasn't had the time to travel the distance to reach us...yet. Not to mention the rate that our universe is expanding and gravity from anomalies like black holes distorting the speed that light travels as well. In theory.
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u/vl4kn0 Jun 18 '12
Clearly, that isn't happening, but why? Because we live in a sad baby universe — it's only 14 billion or so years old, which is certainly a long time, but not long enough for an infinite amount of stars to illuminate us with an infinite amount of starshine.
Does that mean that as our universe gets older, it becomes lighter and lighter?
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u/X_StreetCred_X Jun 18 '12
So does that mean as the universe gets older more stars will start to appear in the night sky?
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u/ab103630 Jun 18 '12
So if we lived in a much older universe them it would be plausible for the night sky to be as bright as it is in the day?
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u/kentgreendisco Jun 18 '12
so, in time, will light from stars in other parts of (the) universe(s) reach Earth and create more twinkly stars in the night sky? and when i mean in time i mean...a long time?
"the universe still hasn't given [light] enough time to trundle across space and make it so we don't have to turn on our porch lights at dusk" translates to me as "there are stars and planets that fill the dark voids that we see in our sky, we just need to wait for those distant starshines to reach us" I am by no means a scientist. i just happened to find this post and was fascinated and wanted to understand it
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u/gunch Jun 18 '12
I would add that while we believe the universe is infinite, we know that the speed of light is not.
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u/Honey-Badger Jun 18 '12
so in theory, for arguments sake lets say in 100 billion years the earth is still here and the sun hasn't engulfed our solar system, the night sky would be much more lit up?
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u/yeast_problem Jun 18 '12
This is what I was told as a child, but it very quickly dawned on me that in an infinite universe I might still be inside a cloud of dust that obscure the light from the far away galaxies.
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u/somnolent49 Jun 18 '12
It's also important to take into account Recombination, the point at which the universe had cooled down enough for neutral atoms to form, and photons to decouple from matter. Prior to this, the universe was opaque to light, but now it became transparent.
This occurred about 300,000 years after the Big Bang, and we can still see the afterglow of this event as the Cosmic Microwave Background, light that hits us from every single direction. The color of this light has been stretched out/"cooled down", or redshifted, over the past 13 billion years, and now corresponds to a blackbody temperature of only 2.725 Kelvins.
Before photon decoupling occurred, light constantly bounced off of free electrons and protons, a process called scattering. This prevents us from using distant light to look back before decoupling occurred.
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u/cclementi6 Jun 18 '12
So our night sky is gradually (albeit EXTREMELY gradually) getting brighter as the universe expands?
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Jun 18 '12
But if the universe has only been around for a finite amount of time (14 billion years or so) then how can it be that an infinite space is created in a finite time?
(Sorry in advance since I'm not sure if my question even makes sense)
EDIT: Also my questions probably going off the actual topic and rather talking about the possibilities of infinite space/time/etc.
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Jun 19 '12
it started as an infinite volume of space
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Jun 18 '12
So.. because the night sky currently only shows us light from stars that have just reached us from billions of miles away does that mean in a billion years the sky would be far brighter due to the countless stars that already exist, but have not yet reached us? (Hopefully my wording makes sense.)
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u/GMBeats95 Jun 18 '12
Are stars so far away that their light hasn't reached us yet in that 14 billion yrs?
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u/Vectoor Jun 19 '12
Yes, it may seem really counter intuitive (I don't get it either) but apparently there is some loophole that allows the universe to expand faster than the speed of light.
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u/JtS88 Jun 19 '12
The "loophole" is that "there is nothing that exceeds the speed of light" is not exactly true. It should be "no information can travel faster than the speed of light", so if the space in between is expanding faster than c, causality isn't violated.
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u/IIoWoII Jun 19 '12
Even theoretically this wouldn't happen.
Some infinites are bigger than others, there are less intergers between 0-infinite( number of stars in the universe with an infinite universe) than there are real number between 0-720( imaginary sphere's coordinates around the earth)
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u/Sw1tch0 Jun 19 '12
You also have to factor in the mysterious phenomenon of light decay. For some reason, light decays in space over vast distances. This is why farther stars appear to be much more dim. It's all speculation at this point, but many contribute it to dark matter.
TL;DR If dark matter didnt exist, our night sky would be full of lights, but thats an entire other topic.
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u/Scottyboy808 Jun 19 '12
So in other words our universe is too young for the stars to have reached everywhere in existence and that once that happens there will be no natural darkness in the world??
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u/a-Centauri Jun 19 '12
It's also expanding at a rate of ~( 80kms-1 )Mpc^-1 . Would that affect how much starlight we see?
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u/twersx Dec 10 '12
This is a very late reply, but does this mean that in billions of billions of years, there may be no "night sky" as we know it? the whole sky will be bright and lit up?
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u/CatalyticDragon Dec 10 '12
That's a big question and I'll start by quoting NASA's WMAP project;
"Recent measurements (c. 2001) by a number of ground-based and balloon-based experiments, including MAT/TOCO, Boomerang, Maxima, and DASI, have shown that the brightest spots are about 1 degree across. Thus the universe was known to be flat to within about 15% accuracy prior to the WMAP results. WMAP has confirmed this result with very high accuracy and precision. We now know that the universe is flat with only a 0.5% margin of error. This suggests that the Universe is infinite in extent; however, since the Universe has a finite age, we can only observe a finite volume of the Universe. All we can truly conclude is that the Universe is much larger than the volume we can directly observe."
So we might conclude that yes, in enough time we might see that.
Another important thing to consider though is the universe is expanding at an increasing rate, and objects further away are moving faster than objects closer. And while the fate of the universe isn't yet known in a quadrillion years expansion may be of such strength that it out powers the forces ripping apart all matter including even subatomic particles. Stars would then not be able to form.
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u/IBWorking Jun 18 '12
Also, the universe is not infinite. "Really, really big" is not almost infinite, and, even if the universe were older, there are probably not nearly enough stars to fill the sky brightly - stars have a limited "lifespan", and the amount of light that leaves a star and enters your eyeball (with a transit time of millions or billions of years) decreases by 1/R2, where R is the distance of the star.
So, a really, really big amount of stars really, really far away is not necessarily bright.
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Jun 19 '12
present data suggest it's infinite. with an infinite number of stars.
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u/king_of_the_universe Jun 18 '12
Even though Space might be infinite, the radius of the visible universe is calculated to be about 46 billion light years (source). That's where we then see the background radiation which was emitted shortly after the beginning of time. In the far future, we would be able to see farther, but the max distance (Again, radius.) might be 62 billion light years (source) because the Metric Expansion is practically stealing the cosmos from us. And the current cosmic "event horizon" is at a distance of about 16 billion light years: A radiation event that happens in a galaxy with that or a shorter distance could be observed by us in the far future, but current events further away would never reach us. This is untouched by the fact that the distance of that "event horizon" is changing.
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u/tvw Astrophysics | Galactic Structure and the Interstellar Medium Jun 18 '12
Other answers here are spot on, but as a radio astronomer I would just like to add my two cents:
We live smack dab in the middle of the Milky Way (well, not exactly. We're actually rather close to the edge), yet we don't even really know what our galaxy looks like. That's because until recently, we could only see that part of our galaxy which is closest to us. In fact, we still have a hard time seeing the other side of our galaxy! Why? Because our galaxy is dusty and full of gas.
This gas is mostly hydrogen which clumps together and gets more dense as you get closer to the center of the galaxy. Optical telescopes can't see through it. Until the use of radio antennae as telescopes, we didn't even know how big our galaxy was. Now, with our radio telescopes, we can see things on the far side of the galaxy and get a better picture of what our galaxy looks like.
If we could see through this gas, the night sky would be so much brighter. The milky way would probably be brighter than the Sun!
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Jun 18 '12
The night sky isn't black. Your eyes can't see the radiation.
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u/Eclias Jun 18 '12
I was going to post this because it's nice and simple but you beat me to it. The night sky is only black in the visible spectrum. In microwave frequencies it's definitely not black. http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation Edit: depends how you define "Black" too.
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u/ReinH Jun 18 '12 edited Jun 18 '12
Three simple explanations:
Redshift causes the light from stars that are further away to shift all the way out of the visual spectrum.
Light from some stars simply hasn't gotten to us yet. They are outside of the observable universe.
Absorption and scattering from dust and other matter attenuates the light before it reaches us. This is known as extinction.
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u/MPS186282 Jun 18 '12
Hello! Astrophysics major here.
In addition to what others have pointed out about the universe only being a finite age, another principle applies. When you shine a flashlight on a surface, it's brighter when the light and the surface are closer together. The same applies to stars. Stars' light is "spread out" over distance, which is why every star in the sky isn't as bright as the sun.
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Jun 19 '12
Light dissipates in the drastically larger emptiness, and there is also a lot of opaque stuff blocking light(more gas clouds than stars.) Even more of the unseen stuff(something like 96% unseen.)
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u/imthewalrus06 Jun 18 '12
The biggest reason that most of space appears "black" is the redshifting of stars and galaxies. Because the universe is expanding, the majority of galaxies and celestial bodies are moving away from us at a very rapid velocity. Think of this like the Doppler effect: a light emitting body moving away from us will experience a "stretch" in it's frequency. The wavelength will literally shift in the red direction of the visible spectrum. For objects that are a certain distance away, this shift will be so dramatic that the EM waves we receive on Earth will no longer be in the visible part of the spectrum.
Tl;dr: universe expanding. Doppler effect on light.
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u/squee147 Jun 18 '12
Just want to point out that this red shifting will pull high frequency light being emitted by the star into the visible spectrum. Someone please correct me if I'm wrong, but I believe that black bodies will emit radiation at any wavelength at some non-zero value, so regardless of how fast a star is moving away from us the radiation it emits will contribute some (however small) amount of light to the visible spectrum when it reaches earth.
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u/patefoisgras Jun 18 '12 edited Jun 18 '12
The universe may be infinite (hypothetically), but the fact remains that the visible part of it is not (about 46 billion light-years radially). Besides, stars aren't everlasting; unless your premise claims that the universe is filled with infinitely many stars simultaneously, it's likely the case that when one star's light reaches us, it's already been thousands of years dead.
Even when both the universe and the star count are infinite, a bright night sky is not guaranteed. Infinity is a hard thing to play with. It's like an infinitely strong man pushing an immovable object, it messes with your mind.
To put things into perspective and possibly give a sense of an answer (for I do not claim to have the answer) for you, consider the number sets.
The natural number set, N, consists of whole positive numbers: 1, 2, 3, 4, 5, ... ∞ You would have to agree with me this set is infinitely large.
Yet within any pair of that set, say 1-2, there are an infinite number of real numbers: 1.01, 1.001, 1.435636565, etc...
I can't justify saying that one set is larger or smaller than the other (not on the spot anyway), but it does indeed have that sense. Read more on cardinality if you wish.
Another example, consider two mathematical functions:
y = x
z = x2
limx->∞ y = ∞
limx->∞ z = ∞
But it can be proven that limx->∞ z / y = ∞, meaning that z is infinitely larger than y given sufficiently large x, despite the fact that both y and z are infinitely large (i.e. there can be infinitely more space than stars even though there both is infinite space and are infinitely many stars).
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Jun 18 '12
I absolutely love your point but can't help but think you could have condensed it substantially. No offense and thanks!!
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u/Pupstink Jun 18 '12
This is actually something called Olbers Paradox. We also must remember, that light takes time to travel too. So if the universe is infinite, the light has not yet reached us so galaxies and stars that are beyond billions of light years away are not yet visible since their light has not reached us.
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u/squee147 Jun 18 '12
This question has always been a bit of pet peeve of mine. There are many good answers which have been provided above, but my favorite and the simplest IMO is that an infinite summation does not necessarily equal infinite or even a particularly large finite number which seems to be the assumption the question takes for granted.
Even if our pocket of the universe was infinitely old and infinitely large, the night sky would still most likely (I don't feel comfortable saying certainly without doing the math) be very dark relative to the day because the density of light sources is so low and the distances so great.
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u/TASagent Computational Physics | Biological Physics Jun 18 '12 edited Jun 19 '12
Actually, I just did some simple calculations to verify that this is not, in fact, accurate. If the observable universe was infinite [false], and if the universe was not expanding [false], and if there was no obstructing non-luminous media [false], then the entire sky would be a consistent brightness.
The argument goes like this: any ray traced from earth out to any angle would eventually meet a star. This star would occupy a cross-sectional area of the night sky that varies inversely with the distance. This area varies exactly opposite the observed intensity (due to the drop in intensity over the distance). In other words, the total light you receive from a star drops the farther it is away, but the area of the sky that all of its light fills up also decreases, in the end leaving the same brightness density per solid angle of sky.
In other words, if all of the assumptions were true (instead of none of them), the sky would be of a uniform brightness, ignoring some mathematical anomalies by assuming constant brightness across the surface of the star.
Edit: and ignoring the variability of the actual brightness of the stars themselves. The sky would be a star mosaic.
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u/AgentSmith27 Jun 18 '12
Its amazing how many people have missed this fact. Hasn't anyone heard of the inverse square law?
In an infinite universe, there would be infinite space to go along with those infinite stars. What would matter is the overall energy density of the universe, the rate of energy release and the rate of energy absorption.
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u/jrhallman Jun 18 '12
Don't we live in an infinite universe with finite stars and worlds? im not trying to impart any knowledge, I'm just wondering if this is a true statement?
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u/GLayne Jun 18 '12
There is a brilliant documentary based on that single question. It is called "Everything", presented by Jim Al-Khalili for the BBC. You should really watch it. Its second part, "Nothing" is even more amazing.
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u/aidrocsid Jun 18 '12
Most of it is very far away and there are a lot of empty spaces in between. When you make a really powerful telescope and zoom in on the gaps you'll find there's actually a ton of stuff there.
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u/WootangWood Jun 18 '12
so theoretically, in a few billion or trillion years the night sky will be as bright as daytime?
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Jun 18 '12
The universe is infinite to our best guess. Light waves travelling from distant stars to our humble rock lose their wavelength over time and degrade into non visible spectra, here's a radio map of the night sky http://www.sciencephoto.com/image/332321/large/R8000103-Radio_map_of_the_whole_sky_showing_Milky_Way-SPL.jpg It's quite scary how little of the universe around us we are actually perceptive of.
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u/JJ-Lo Jun 18 '12
We as humans simply don't have sensitive enough eyes to pick up the small amount of light that reaches us from distant stars and galaxies.
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u/dpierce970 Jun 18 '12
isn't the "expansion" of the universe just the furthest away object moving farther away?
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u/khthon Jun 18 '12
First, darkness is but the absence of light - the narrow spectrum humans see. But if you change that to infrared you get a more enriched picture.
And as the Universe is expanding faster than light is able to travel it, stars and galaxies are moving further and further apart from each other and ever increasingly distant from us. So much so that a time will come when our galaxy will be all that's visible to us.
The darkness in the sky is but the absence of light emitted, either faded out by distance or stellar death. The gaps between those "dots" will increase and become more sparse with time.
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Jun 19 '12
Can someone clarify my understanding of the big bang:
In the beginning of time, the universe was like a ball of dough. An infinitely large ball of dough. There seems to be some confusion in layman's talk that the universe was not only infinitely dense, but infinitely small, too.
Which is more accurate?
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Jun 19 '12
we're not sure which. We can't solve the exact mathematics of the very early universe. It could be infinitely large with very high density, or a point with infinite density. I'd bet on the former.
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u/Sw1tch0 Jun 19 '12
If I'm correct you're referring to why light decays over massive distances. It's up to speculation. Many scientists contribute it to dark matter. Theoretically, light shouldn't decay in a vacuum even over massive distances.
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Jun 19 '12
It has nothing to do with "decaying" over massive distances. We have no evidence that light does any such thing, and certainly it's not related to dark matter.
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u/Sw1tch0 Jun 19 '12
Interesting, the article in scientific american points to it. Then why do distant stars appear to be so dim?
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Jun 19 '12
Perhaps I misinterpreted you. Light dims over distance (multiply the distance by x, the dimming is like 1/x2 ). Some... pseuodscientific notions about light refer to it "decaying" which is why I wanted to draw the distinction between the two. Perhaps I might have jumped a little hard on that word specifically.
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Jun 19 '12
Even if there was an infinite number of stars visible from earth, it wouldn't guarantee that the entire night sky was bright - the sum of a decaying infinite series can be a finite number (eg: the sum from zero to infinity of 1/2n is 2) - the brightness of the sky depends not just upon the number of and size of the stars, but upon distance as well.
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u/akacoffeeguy Jun 20 '12
As far as infinity goes, a three dimensional space of infinite volume contains infinitely many two dimensional planes each containing infinitely many lines. The arrangement of the stars matters considering the basic case where all the stars could align themselves along a linear path allowing for infinitely stars (obviously not the case but a simple illustration).
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u/leaning_chicken Oct 29 '12
Just got linked here from another thread and thought this video could help.
http://www.youtube.com/watch?v=gxJ4M7tyLRE&list=PLED25F943F8D6081C&index=5&feature=plcp
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Jun 18 '12
The universe is not infinite, it is a (reasonably well) known size. However it is, as you say, "really really really really big", and any tiny piece of sky you look at, will have millions and billions of stars and galaxies in it. This was proved by the Hubble Deep Field image: http://en.wikipedia.org/wiki/Hubble_Deep_Field
However the fact remains that the light coming from these stars (if indeed light at all, not all emit in the visible spectrum) is very faint, and our eyes are not sensitive enough to pick it up. Hence the sky looks dark.
There is a whole lot of radiation out there from stars which can be picked up as a pretty uniform field, these cosmic rays form a lot of the "background radiation" (http://en.wikipedia.org/wiki/Background_radiation) that we see pervading everywhere. This is, if you like, the constant "light everywhere" thing you allude to, it's just not in our visible spectrum.
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Jun 19 '12
the observable universe is a reasonably well known size. There's more universe beyond it; probably an infinite amount.
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u/geotek Jun 18 '12
Stephen Hawking talked about this in the beginning of his famous book. The answer is simple, the light of those stars haven't reached us yet.
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u/MadMathematician Jun 18 '12
This problem is very famous, it is also known as Olbers' Paradox. Quick answer: Light has a finite speed. Read more at: http://en.wikipedia.org/wiki/Olbers'_paradox
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u/OmicronPersei8 Jun 18 '12
The astronomers around the turn of last century (1900) were asking themselves the same question, it was a key problem with the static universe. In short, even if it's infinitely big, it's not infinitely old. Not all the light has had a chance to reach us. Much light that's heading towards us never will reach us because of the expansion of space time.
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u/meanreus Jun 18 '12
I doubt this will even be read, but I have a question about the question. That being about the universe's infinity. If we live in a world which seems to deal with finite quantities, measurements, etc. how could the universe be infinite. I know this is perhaps more of a philosophical question, but it is hard to wrap my head around an infinite universe full of finite things.
Edit: super-pathetic spelling mistake
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Jun 19 '12
there's no reason it can't be. We just can't directly observe it to be so. But the data suggest it is.
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Jun 18 '12
This is known as Olber's Paradox, it was solved when it was discovered that the Universe is expanding.
Edit: Sentence structure.
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u/jcolman1 Jun 18 '12
I'm always confused by this question. Who has ever said the universe has an infinite number of stars? Sure, on paper if you make that assumption, the answer would be a totally lit sky, but really really big is in no way the same thing as infinite. Especially when you consider that the really really big number of stars is dwarfed by the really really big amount of empty space between them.
It doesn't matter how old the universe gets, the number of stars will always be finite, and as long as the space between stars is far greater than the number of stars in the system, there will be lines of sight that do not encounter a star: ie, darkenss in the night sky.
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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Jun 19 '12
present data suggest that the universe is infinite.
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u/brainflakes Jun 18 '12
As well as the visible universe not being infinite (we can only see things up to 14 billion light years away) the sky is also a lot fuller than you can see with the naked eye. Even seemingly empty patches are actually filled with galaxies, Hubble was pointed at one such patch and took the famous Hubble Deep Field picture.
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Jun 18 '12
Well really visibility wouldn't let us see planets that are very far away with the naked eye. But the real reason why is because of dark matter/energy. Dark matter makes up 70% of all the weight of our galaxy and dark energy is increasing the speed of the expansion evermore. As our universe ages there will be less and less stars in the sky as space pulls things farther and farther apart.
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u/iehava Jun 18 '12 edited Jun 18 '12
The observable universe is NOT infinite. We know this because of things like the fact that we can actually see the 'edge' of the universe [(CMBR)] (http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation), and redshift, etc. We know that the universe is 13.72 billion years old and scientists are very, very confident of the accuracy of that number (with very good reasons).
Now onto your question:
During the Planck Epoch, which is, in layman's basically the smallest measurement of time after the big bang, all of the matter in the universe was in a very, very small space. For the next several hundreds of thousands of years, the baryonic (normal) matter of the early universe was either in an unimaginably hot, dense soup of ionized (or doubly ionized, depending on the time after the big bang and thus the temperature), free floating atomic constituents or hydrogen actively fusing. During this time, if you were anywhere in this infant universe, you would see blinding light all around you. Problem is, the temperature would be in the thousands or millions degrees Kelvin (again, depending on exactly how long after the big bang), and would vaporize you instantly. Not to mention the unimaginable effects of gravity that would crush you instantly.
The only objects in the universe that naturally give off visible light (that is, light in the visible portion of the Electromagnetic Spectrum), are stars, active galactic nuclei, and matter that is being heated up due to friction or gravitational potential energy (such as the accretion disk of a quasar or a condensing molecular cloud). Now, technically, ALL matter gives off light. Not in the traditional sense of visible spectrum light, but somewhere along the EMR spectrum. Right now you're radiating light in the infrared part of the EM spectrum. But lets focus on visible light.
The thing to understand is that when an object radiates light, it does so in all directions at once. Because light is subject to the inverse square law, its intensity is diminished over distance. This means, that the farther you are away from an object, the fewer photons (light 'particles') will be hitting your eye (or anything else). Over the vast distances in our universe, photons from each light source get so spread out that they become hard to see. When you look up at the night sky, at any particular point, chances are there IS something there, but it is simply too faint, too far away to see with the naked eye, or even most telescopes.
If you haven't seen it, I would also suggest you take a look at some of the Hubble Deep Field images. Basically, astronomers were curious to see if there was anything out there, so they pointed the Hubble Space Telescope at a seemly black, empty part of the sky, and did a long, ten day exposure to collect as much light as possible. What they found was amazing! In a patch of sky smaller than a dime held at arm's length, there are tens of thousands of galaxies! This just reinforces my earlier statements: Light is EVERYWHERE in our universe. Just because you can't see it because it is in a non-visible spectrum, or just very, very far away doesn't mean its not there!
Edit: existentialhero and KaneHau pointed out the importance of the distinction of the observable universe