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Why we have night

  1. Feb 13, 2009 #1
    Some scientists ask why we have night.

    I have read about some.

    Some says it is because the universe's expanding rate is faster than light speed. So that the visible light cannot come to the Earth.

    Do you have any other opinions? Or can you explain the above statement with more explanations?

    It is such an interesting question
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  3. Feb 13, 2009 #2


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    I think you're talking about Olber's paradox. Expansion plays some role, but the main thing to keep in mind is that the universe is thought to have only a finite age, so light from stars too far away just won't have had time to reach us.
  4. Feb 13, 2009 #3


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    Maybe because of the "dark matter" (95% of matter in the Universe) that is believed to absorb light, and maybe because of the enormous distances to other light sources in the Universe. The light, the same as gravity, decreases with the square of the distance...
    Last edited: Feb 13, 2009
  5. Feb 15, 2009 #4
    The part about light decreasing with distance is not an explanation.

    Light intensity from a particular star decreases with the square of the distance, true, but the number of stars in any viewing angle increases with the square of the distance. This means we should be getting the same amount of light from any distance (on average).

    Other explanations (dark matter and simply the age of the universe) seem to be better. The further we look, the older things become, until we reach the distance where we actually see the big bang. Behind that, we can't see anything yet because light is still on its way to us.
  6. Feb 15, 2009 #5


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    Actually, the oldest thing that we can see is the cosmic microwave background radiation. Before that the universe was opaque so the light is not on its way to us but was already absorbed.
  7. Feb 15, 2009 #6


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    The main thing is that the universe is finite in extent(or at least the star filled part is)
  8. Feb 15, 2009 #7


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    In the standard cosmo model, matter and space are co-extensive. Stars/galaxies are more or less uniformly distributed throughout.
    And we do not know if space is finite or infinite---that is under investigation.

    So I disagree with Mgb about the finiteness of the star-filled volume of space being a cause of darkness. IMHO we cannot say that the universe is spatially finite. And such presumed finiteness cannot be the reason the night sky is dark.

    Jesse seems to downplay the role of expansion.
    The CMB light was emitted around year 380,000 when space was filled with hot partially ionized gas ----around 3000 kelvin.
    Space has expanded 1000-fold since then, so the temperature has gone down to around 3 kelvin.
    If space had not expanded since that time, the sky would be glowing hot. It would be like the surface of a star somewhat more reddish than the sun. Kind of orange. Like the inside of a furnace, if you ever looked in.
    Last edited: Feb 15, 2009
  9. Feb 15, 2009 #8


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    Is thought by whom? A lot of the research going on in modern cosmology extends the models back before the big bang.

    For an up-to-date popular account have a look at Einstein Online. The link is in my signature, in small print. Better not to rely on older stuff, like before 2006.

    Current work makes computer models of the universe that can be run back before bang and fit the post-bang data about as well as the usual older model (which breaks down and can't be run back past that point.)

    So there is currently no scientific reason to assume that the universe has a finite age.


    If you are only talking about STAR light---why isn't the night sky filled with starlight---then finite age is a key factor. The finite age of the stars.
    So I guess I would agree with Jesse. Just want to say that if the question is why is the night sky dark has to do with expansion, because without expansion the CMB light, and other early universe stuff, would cook us to a crisp. No darkness.
    But if the question is about starlight only (not other sources of light) then I agree the finite age of stars is a major factor.
    Last edited: Feb 15, 2009
  10. Feb 18, 2009 #9
    How can they figure out what happened before the big bang? I have always read that physics in our universe breaks down before the big bang (because of singularity?). Also, can anything that happened before the big bang affect us today?

    The universe is finite so there are not an infinite number of stars (not an infinite amount of light). The redshifting probably helps with night as well.
    I hadn't thought about the age of the universe. It would make sense that there is still light that may reach us in the future.
  11. Feb 18, 2009 #10

    Not really true...The cosmological horizon is finite...no one knows about the entire universe...

    again, not really true....its the cosmological horizon that limits what reaches us and what we can observe.

    Try Wikipedia: http://en.wikipedia.org/wiki/Cosmological_horizon
    and also OBSERVABLE UNIVERSE at the bottom

    As eons pass, we will be able to "see" (observe) less and less as expasnion of the universe accelerates and distant stars and galaxies disappear in the distance...eventually day will be night...all black as we approach absolute zero temperatures...
  12. Feb 18, 2009 #11
    No-one seems to have addressed one of the OP's key issues:
    Is the universe expanding at faster than c?

    Not exactly it would seem, since it would be dark in the middle of the day as well if that were the case.

    However, the further away from us something is, the faster it is receding from us. How far away does something have to be to be receding as fast as c?

    "[URL [Broken] constant[/URL] is: H0 = 70.1 ± 1.3 (km/s)/Mpc - kilometres per second per megaparsec

    The velocity at which a distant object is moving away from us is given by:


    where D is the comoving distance.

    To get our figure we plug in c,



    D=4285 Mpc

    Since a http://en.wikipedia.org/wiki/Megaparsec#Megaparsecs_and_gigaparsecs"is 3.262 light-years:

    D=14000000000 light years (or 14x109 light years).

    The http://en.wikipedia.org/wiki/Age_of_the_universe" [Broken] is 13.7x109 years (...ish, give or take some error).

    Therefore , on the face of it, it seems that there are grounds for saying that the "edge of the universe" is expanding away from us at the speed of light, if we define the edge of the universe to be "that bounding everything that can or ever could have an impact on us" - it's not necessarily a real thing. Since the edge of the universe is 14 billion light years and the universe is suspiciously close to 14 billion years old.

    The thing that concerns me is that one billion years from now, the age of the universe will be 14.7 billion years (..ish) and unless the Hubble constant changes, the edge of the universe as defined above, will still be 14 billion light years.

    I like playing with numbers, obviously. Can anyone explain why these figures don't say what they seem to say (ie, the apparent age of the universe might be linked to limit on what we can observe rather than when any big bang happened)?



    Note that the value of the Hubble constant and the value for the age of the universe both come from data from the WAMP (http://en.wikipedia.org/wiki/Wilkinson_Microwave_Anisotropy_Probe). Note also that the Hubble Constant is also known as the Hubble Parameter, since it's not exactly a constant.
    Last edited by a moderator: May 4, 2017
  13. Feb 18, 2009 #12


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    Isn't dark matter believed to *not* interact with light?
  14. Feb 19, 2009 #13
    The earth has night for the reason that there is a day.
    And every day there will always be night.As the night pass by the day will then follow.

    As long as the earth is revolving and rotating their will always be night.
  15. Feb 19, 2009 #14
    I think you missed the point. The question was why it's dark at night if there is an infinite number of stars in space. All the light from this infinite number of stars together should make the night very bright instead of dark. A number of explanations were given (finite age of the universe, interstellar dust and expansion).

    By the way, when the sun burns out, there will no longer be days and nights even though the earth may still be revolving
  16. Feb 19, 2009 #15
    Thanks.The truth is I have a limited view on astronomy.

    "By the way, when the sun burns out, there will no longer be days and nights even though the earth may still be revolving "

    Are you telling that time on earth based on the location of the sun will be discarded once
    these happen.??What if everything in the universe will stop ..will time continue??
  17. Feb 19, 2009 #16


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    Of course not. "Day" and "night" are defined by sunlight. No sunlight, no distinction between day and night. You could argue that without the sun we would have no day, but permanent night. Michelcolman is certainly not asserting that "time itself" will stop- the moon would still orbit the moon. Without the suns light, we couldn't see it but that's alright, we wouldn't be here anyway!

    If everything in the universe were to stop, what would you use to measure time? And then how would you define "time"?
  18. Feb 19, 2009 #17

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    I didn't see an explanation involving dust, but don't think that it can be dust. If the night sky would be white except for dust, a grain of dust will have its temperature increase until it's in radiative equilibrium with its surroundings. In short, it will begin to glow.
  19. Feb 19, 2009 #18
    Isn't the universe finite? I thought this was somewhat proven with General Relativity. I could be wrong though.
    If the universe is finite, then there can only be a finite number of stars. Thus, there would be a finite amount of light reaching the earth.

    I still like the idea that we are only seeing light from a really really long time ago. It makes sense to me that there may still be light that has yet to reach us.

    I don't know if the universe is expanding at c or greater than c. I don't think it can according to Einstein.
  20. Feb 19, 2009 #19
    If the Hubble Constant is constant (across the universe, not necessarily for all time)

    then there is certainly a finite number of stars which can have any impact on us. There is an upper limit set by "the number of stars you can fit into a sphere with radius of 14x109 light years". In practice it is a lot less than that, since we are not particularly crowded by stars - According to a http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970115.html" chappy, the number of stars is 1021 which works out to about 30 stars per cubic parsec on average. This is 30 times higher than the density of stars where we are and about 3 times less than the density 100 parsecs out from the galactic core.

    I read on various sites that our sun is average (strong anthropic principle at work?) and that the density of the sun is 1400 kg/m3 - more than water, but less than rock. The sun's mass is about 2x1030kg and keeping with the idea that it is average, that puts the mass of all the stars in the universe at about 2x1051kg.

    This http://www.cs.umass.edu/~immerman/stanford/universe.html" [Broken] puts the mass of all visible matter at 6x1051kg. I understand that, with cosmology, being within the same order of magnitude is often accurate enough.

    The bottom line is that there does seem to be a generally accepted view that there is a finite number of stars.

    To get onto the next issue, the universe being finite presents an additional problem. An infinite universe, even with an infinite number of stars may well be more conducive to night than a finite universe - since the finite universe is usually considered to be closed. There are plenty of very good arguments as to why universe doesn't have really have an edge. The consequence of that though is that given enough time and great fortune, a photon will travel all the way around the universe and end up back where it started. If all the photons are stuck in a closed system, and there are so many stars which have been generating photons all the time (this http://www.qwantz.com/fanart/alexreachrisberry_lightcannon.pdf" [Broken] takes an admittedly humorous look but arrives at the sun producing in the order 1045 photons every second), why is the nighttime sky dark?

    If the universe were closed and the universe were not expanding, then we would probably be bathed in photons all the time - even if it were only with a miniscule proportion of all those photons which have been produced in the last 14*109 years. Even if the universe were expanding - but at a slowish rate - we would still get bathed in photons throughout the 24 hour day.

    I'd posit that the universe expands at just the right speed to prevent _any_ photons from making a round journey. Faster and we would be all alone (unless this is a special epoch in the universe's history, of course), slower and we would be toasted to a crisp by all those photons falling on us.

    What is that speed?


    Last edited by a moderator: May 4, 2017
  21. Feb 23, 2009 #20
    I looked at another post, and the wisdom is that expansion at 14 billion light years is the speed of light and expansion at lesser distances is a percentage of this (so at 1.4 billion light years distance, expansion is 10% of the speed of light).

    So, the answer to my previous question appears to be "the speed of light at 14 billion light years distance".

    Does this mean that the Hubble Constant is, rather than being a true constant, a function of time? Or is it just a coincidence that the age of the universe corresponds pretty much exactly with the distance at which universal expansion is the speed of light?

    In other words, is it purely coincidence that:

    Ho=1/(age of the universe)

    just wondering,


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