[HN Gopher] Olbers' Paradox
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Olbers' Paradox
Author : luu
Score : 67 points
Date : 2022-03-27 21:19 UTC (1 days ago)
(HTM) web link (en.wikipedia.org)
(TXT) w3m dump (en.wikipedia.org)
| BoardsOfCanada wrote:
| Assuming an infinite amount of anything seems to be a good way to
| end up with a paradox.
| pazimzadeh wrote:
| What about time?
| IntrepidWorm wrote:
| One cannot even assume time is infinite- there are theories
| postulating infinite time and finite time, as well as
| theories postulating that time may not really exist at all.
| nusaru wrote:
| So basically, we don't exactly know why the night sky is mostly
| dark? (At least, that's what I got out of this)
| bckr wrote:
| > The darkness of the night sky is one of the pieces of
| evidence for a dynamic universe, such as the Big Bang model.
| That model explains the observed non-uniformity of brightness
| by invoking spacetime's expansion, which lengthens the light
| originating from the Big Bang to microwave levels via a process
| known as redshift; this microwave radiation background has
| wavelengths much longer than those of visible light, and so
| appears dark to the naked eye.
| nusaru wrote:
| > The redshift hypothesised in the Big Bang model [...]
|
| Doesn't sound to me like it's more than a hypothesis, but I
| could be wrong.
| AnotherGoodName wrote:
| Well redshift alone explains the paradox. You don't even
| need to accept the big bang theory. You can just say "stars
| that are further away are more redshifted" and that alone
| explains this. Which we know is true. That's not a
| hypothesis, redshift can be observed directly. And that
| fact alone explains the apparent paradox. The most distant
| stars are redshifted away.
|
| To quote the above article
|
| >The redshift hypothesised in the Big Bang model would by
| itself explain the darkness of the night sky even if the
| universe were infinitely old.
|
| Redshift is what explains this. There could be different
| explanations for the redshift but redshift is 100% an
| observed phenomena.
| Aengeuad wrote:
| The paradox itself is that if the universe is both infinite and
| eternal then we shouldn't have a dark sky. This is 'trivially'
| solved by demonstrating that either of those conditions aren't
| true. The article opts to use the explanation given by Edgar
| Allan Poe to demonstrate this, which is that the universe has a
| finite age and the speed of light is finite so there's only a
| finite amount of observable universe, which gives us a universe
| which was darker in the past and one that will only get
| brighter in the future as more of the universe becomes
| observable. This has some problems of course and the model Poe
| would have been working with would have been one of a cyclic
| universe of eternal growth and decay. This leads us to the Big
| Bang theory.
|
| >> The redshift hypothesised in the Big Bang model [...]
|
| >Doesn't sound to me like it's more than a hypothesis, but I
| could be wrong.
|
| The way the Big Bang theory resolves the paradox is similar to
| that of how Poe resolved it, with a finite cap on the age of
| the universe there's only a finite amount of observable
| universe, and similar to Poe's explanation it presents a
| problem in that a younger universe would have been immensely
| bright. However this new issue is resolved through the
| explanation of the expansion of space which can be observed
| through the redshift of distant galaxies.
|
| As we _do_ observe a dark sky we know the hypothesis that led
| to the paradox can 't be true, namely that the universe is both
| infinite and eternal, so the question is less about why we have
| a dark sky and more about what possible alternate hypotheses
| resolve the paradox. While the Big Bang theory is just a theory
| it's important to remember that proof is reserved for maths, a
| theory is a hypothesis backed up by observational data. General
| relativity led to the hypothesis of an expanding universe and
| this was something that was later observed from redshift
| measurements and from it we derive the Hubble-Lemaitre law,
| that galaxies are moving away from earth with speeds
| proportional to their distance, in some cases faster than the
| speed of light, this alone fully resolves the paradox and
| crucially the Big Bang theory is not incompatible with this
| observation.
| mabbo wrote:
| What we know is that an infinitly old and large universe is not
| possible given our understanding of physics and the sky being
| dark at night.
|
| That's all.
| SamLeBarbare wrote:
| Constant speed of light is the main problem...
|
| More infos:
| https://en.m.wikipedia.org/wiki/Variable_speed_of_light
|
| Paper:
| http://www.januscosmologicalmodel.com/pdf/1988-ModPhysLettA-...
| sebow wrote:
| When I first encountered this idea it quickly jumped to me that
| the universe does not appear to be "static", or at least our
| observation of it is constantly changing. This also plays into
| homogeneity, and the last assumption of being "infinite" is
| somewhat irrelevant, because if the first 2 aspects don't hold,
| being infinite still won't make the sky appear 'filled', at least
| past the point of our observation.
|
| Of course there's still the theory of an imploding universe which
| could shrink back together all matter and thus maybe achieve this
| effect, but even then we still don't have a static
| characteristic, it only makes an argument for an 'infinitely
| occurring universe', maybe something akin of the theory proposed
| by Penrose.(which however still has a lot of unanswered
| questions, imo)
| [deleted]
| 015a wrote:
| Is there a good explanation as to why this paradox isn't
| explained by universal expansion? From wikipedia: "To any
| observer in the universe, it appears that all of space is
| expanding, and that all but the nearest galaxies (which are bound
| by gravity) recede at speeds that are proportional to their
| distance from the observer.";
|
| Wouldn't this imply that, there was probably a time in the
| distant past when the night sky was saturated brilliantly with
| light, but as things expand apart, we simply see less-and-less
| because those infinitely distant galaxies are so far away that
| their light cannot ever reach us?
|
| Additionally, and maybe related; we can only see 13 odd billion
| or so lightyears away. The universe could be infinite; there
| could be a point of light at every "pixel" of the night sky,
| twenty billion light years away; we simply cannot see that far,
| as their light cannot travel faster than the rate they're
| expanding away from us. Is that accurate?
|
| Additionally; space is pretty empty, but not empty. Even ignoring
| all this, its not unreasonable to think that, over truly
| universal distances, this matters. We look at the night sky and
| see nothing; we point a powerful telescope at that same place and
| see millions of objects, but still darkness between them. Is
| there an argument against the notion that, maybe, the light from
| objects even further away was just absorbed naturally before it
| could get here? Or maybe our telescopes are not sensitive enough?
|
| Additionally; the paradox makes an assumption that space is
| uniform, but even our naive observations of the universe prove
| this to be inaccurate. Its tremendously _non_ uniform; mostly
| thanks to gravity. Stellar matter is not distributed uniformly;
| it coalesces into galaxies. Galaxies are not distributed
| uniformly; they coalesce into galactic neighborhoods, fibers
| which stretch across the cosmos. There are many regions like the
| Bootes Void, which contain magnitudes fewer galaxies as we 'd
| expect. There's the Great Attractor, an abnormally massive area
| which affects the placement of galaxies around it. Maybe on an
| absolutely, truly, insanely large scale, trillions of lightyears,
| infinite lightyears, that non-uniformity averages out, but again
| it comes down to; the universe may be infinitely large, but it
| doesn't seem to be infinitely old.
| fknorangesite wrote:
| > Is there a good explanation as to why this paradox isn't
| explained by universal expansion?
|
| It _is_ - as described in the second paragraph of the linked
| article.
|
| > we can only see 13 odd billion or so lightyears away
|
| The radius of the observable universe is ~45 billion light
| years, not 13.
|
| > but even our naive observations of the universe prove this to
| be inaccurate.
|
| On the contrary; at a cosmic scale it is _very_ uniform.
| Galaxies are just blips:
| https://en.wikipedia.org/wiki/Cosmological_principle
| nathanmcrae wrote:
| I don't know how all of the factors you bring up interact, but
| as far as the last point: the universe is indeed uniform and
| isotropic at a large enough scale (~300 million ly), called,
| nicely enough, the End of Greatness (
| https://en.wikipedia.org/wiki/Observable_universe#End_of_Gre...
| ).
| [deleted]
| lproven wrote:
| Reminds me of the classic Barrington J Bayley short story "the
| Knights of the Limits"...
|
| > 'No, she's quite a way off,' Corngold said, taking a look at a
| meter. 'Roughly a googol olbers.'
|
| > 'Your gadget can see that far? But good God - how do you find a
| single object at that distance?'
| pingpongrandom wrote:
| Cool concept, but doesn't consider planets, or nebulae, or other
| space objects that can block light. I have no expertise to speak
| with authority on this, so please, weigh in on this
| consideration. But wouldn't this break the "homogeneity"
| assumption?
|
| EDIT: Thinking about this further, could you make approximation
| rules about the things that block light, too? e.g. By the nature
| of a star's mass, you can assume that some opaque object is
| likely between Earth and any given star with some likelihood?
|
| EDIT2: Thanks to everybody for the replies - this really helps
| clarify! Cheers!
| bmn__ wrote:
| > but doesn't consider planets, or nebulae, or other space
| objects that can block light
|
| This is addressed in the article. ctrl+f cloud
| [deleted]
| thechao wrote:
| There's no "blocking light": anything that "blocks light" would
| get hot from absorbing that light and, then, reradiate. By this
| time (some point along the curve of "infinitely old universe")
| everything should be radiating a lot.
| trhway wrote:
| Or the energy may get captured and stored as chemical for
| example.
| mhh__ wrote:
| If you imagine a curve of the light from a star, the dip in the
| curve from a planet passing in between us and the star is
| actually relatively small. They don't block that much light
| (and what they do block heats the planet, which in turn will
| radiate lower energy light)
| theptip wrote:
| If you read the section on "The Paradox" in the article, it's
| basically using a simplifying geometric assumption to divide
| the universe into concentric shells, each of which contributes
| the same intensity of light to the observer. If space is sparse
| enough that any light reaches us from shell N+1, then you'll
| get the same amount of light from every shell, and so you'd
| have a fully-saturated bright sky.
|
| So there are possible infinite/steady-state universes universes
| (very dense, lots of dust) where you don't see shell N+1. But
| since we can see stars from shell N+1 and other shells, we know
| that we don't live in that universe. Therefore our universe
| isn't a steady-state / infinite time one.
| mananaysiempre wrote:
| Blocking light means absorbing energy. In an eternal universe,
| you have to reach thermal equilibrium eventually, so you have
| to shed that energy somehow, and for a constant amount of
| matter just hanging around in empty space radiation is the only
| logical possibility. At the end of the day, your absorber will
| reach the same temperature as the emitter and emit the same
| amounts of light that it absorbs on frequencies where it does
| absorb, though perhaps in different directions. This is the
| insight behind Kirchhoff's law and blackbody radiation.
| AdamH12113 wrote:
| A more concise explanation can be found in the Usenet Physics
| FAQ:
|
| https://math.ucr.edu/home/baez/physics/Relativity/GR/olbers....
| WinterMount223 wrote:
| > too. It does act like a radiation shield, exponentially
| damping the distant starlight.
|
| This is wrong, damping is not exponential but to the fourth
| root.
| vikingerik wrote:
| A root is a fractional exponent; I can see that as a valid
| description.
| WinterMount223 wrote:
| Exponentially means that it grows like exp(x).
| adhesive_wombat wrote:
| Also it's an important distinction, because an
| exponential will always eventually grow (or shrink,
| depending on sign) faster than a polynomial, no matter
| how high order.
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