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What caused the cooling of the universe?

  1. Feb 15, 2008 #1
    What caused the universe, specifically the Earth, to cool after the big bang? Was it because of the space expansion and thus the relationship between wavelength and energy (as wavelength increases E drops?) or what? A basic answer is good =] Thanks.
    Last edited: Feb 15, 2008
  2. jcsd
  3. Feb 15, 2008 #2


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    Yes! good reasoning.

    in the early universe matter and radiation would be in approximate equilibrium
    (if the matter was colder then the radiation would heat it up, and if the matter was hotter it would radiate off the extra)

    so you can just look at the bath of radiation.
    and what you say is right. As the scale-factor doubles, the temp drops by a factor of two!
    Because the wavelengths are all elongated by a factor of two.

    there are lots of more subtle ways to discuss how the cooling occurs with expansion.
    one can talk about the energy density. as the scalefactor doubles, the radiation energy density goes down by a factor of 16.
    (because there are only 1/8 as many photons as before, and each photon is only 1/2 as energetic as before)

    and this is consistent with the temperature going down by half, because of the energydensity form of the Stefan Boltzmann fourth power law.

    but all that is unnecessary elaboration on what you said at first, which is right----expanding by two cuts the temp in half.
    Last edited: Feb 15, 2008
  4. Feb 15, 2008 #3
    Seriously :biggrin:?
  5. Feb 15, 2008 #4


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    well, yes seriously, unless I misunderstood you. what you said sounded right to me.

    Maybe someone else will correct us on this, or raise some other point about it.
  6. Feb 15, 2008 #5
    Haha I just feel accomplished about finally getting something right in one of these posts, the maths is (basically) simple enough for me to get the gist of it (I'm in 9th grade Alg.II), and most of my posts end up with someone giving me an explanation in quantum physics :cry:. Thanks for the explanations, it helps colour in the spaces a bit more =], but I would love to hear more from anyone else who can contribute.

    Right now I have to right this paper for Biology of the history of the Earth. So far I have: Previous Universe collapses, starting another Big bang, causing space to expand, causing the wavelength E relationship to occur, while expanding clumps of matter would stick together because of the effects of gravity they have on one another, suns developed one way or another (left that bit out =P), matter would collide with enough energy to melt and sort of combine into planets, gases released in volcanoes, as cooling happens as space expands clouds can form, lightning can supply energy for gases to make organic compounds, as further cooling gas can condense creating oceans and bringing down organic compounds. Sound about right so far? Anyway thats how I got this question, when I was trying to figure out why cooling even happened, and this seemed most logical =]

    *some kids just put God created the earth -.-
    **how should the earth end ^_^
    Last edited: Feb 15, 2008
  7. Feb 15, 2008 #6
    Also would you be able to make any sort of proof along similar lines as this as why high densities correlate with high temperatures ( like a compressed cup or mercury), or is this too different =P, I think I might have just ruined my good reasoning I just earned :frown:.
  8. Feb 15, 2008 #7
    Well, for starters, if your paper is about the history of the Earth for a biology class, then getting into previous universes and the Big Bang is a little off-topic. Why don't you just start with the formation of our solar system...or even just stick to the topic (Earth)?

    If this is for a biology class, I would think the intention is for you to write about the history of LIFE on Earth, and not cosmology.

    ...just a suggestion :)
  9. Feb 15, 2008 #8
    Oh come on, I think it's wonderful that some as young as him is putting so much thought behind a paper. If it was me, I know I wouldn't have. Keep up the good work.
  10. Feb 15, 2008 #9


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    I can't offer advice about how to write your paper about the history of the earth because I don't know the teacher or class or situation, and even if I did I might not give very good advice.

    On the other hand you seem to be asking good physics and cosmology questions and I can respond to those. (this may not be the best help for writing your paper because it leads to overemphasis on astrophysics and not enough attention to geology and biology)

    But i am not going to worry about that. That is your responsibility.

    Here you are basically asking about how does a young planet cool?

    that's a good question. It cools by radiating energy off into space.

    Someone else will correct me if I am too far off on the numbers, but I think one can say that spatial EXPANSION only drove cooling for the first few million years since it started.

    The solar system formed much later, around 9 BILLION years. By that time background radiation was very cold----space was cold and dark.

    At that time, the important cooling mechanism is thermal radiation-----like infrared, radiant heat.

    As soon as the sky itself is cold, then material objects can begin dumping their heat radiantly into the sky.

    And material objects, like planets, as they gradually collect, coagulate, condense---the little bits glomming on to make larger bits and the larger bits crashing together to make even larger lumps----they actually generate a lot of heat! A meteorite impact can melt rock. So a fair amount of that heat had to be somehow gotten rid of. And it got radiated off.

    I'm sure you have thought about this.

    Maybe you should not waste time learning about it now, but there is a law called the Fourth Power Radiation Law, or else the "Stefan-Boltzmann Law" which describes the brightness with which warm objects glow.

    the power density (watts per square meter surface) is proportional to T4, the fourth power of the temperature. this is absolute temp, in kelvin.

    So if you double the temperature you increase the watts per square meter by 16 fold.

    And there is the Planck Black Body radiation law that shows how the wavelengths that a warm body radiates depend on the temperature. Higher temp means shorter wavelengths (higher frequency more energetic photons) and so on.

    So there are these rather elegant rules that nature follows about how objects radiate.

    Before material objects can radiate their heat off into the sky, the sky itself has to cool. Otherwise, if the sky is glowing hot, it will radiate back in just as much as the object is radiating out, and no cooling will happen.

    So that first part was accomplished by expansion. But very soon (I am not sure how soon, there was what was called "dark ages" when the sky was good and cold and not enough stars had condensed to brighten things up.

    and then stars began to condense and galaxies collect and all that structure began to assemble itself. I think roughly speaking the first galaxies were forming at around 700 million years.

    we really need a timeline dont we?. does anyone know of a good one?
    Last edited: Feb 15, 2008
  11. Feb 16, 2008 #10
    Cool, Cool, Cool =]

    So spatial expansion started cooling, and then once space* and less dense matter was cooled sufficiently thermal energy from matter like planet could then radiate into these less dense matters and space*, thus cooling the planets?

    So did spatial expansion still cause cooling, just not to the same degree that radiation did so it's no longer important, or is there another factor where spatial cooling no longer occurs (I assume the first too be true).

    Now you say it radiated to the sky, I assume then off into space after that? Now in this sense it doesn't mean space in the same sense as expansion, but the vacuum like place, but it's not really a vacuum because it has really really small amounts of stuff like H and He in it? So then it can radiate into the H and He which would heat up, but since space is so large it would be so spread out there wouldn't be a measurable effect. Is this the gist of it?

    Now if space was hypothetically a vacuum then would planets still be able to radiate into it, since the radiation come from thermal E, and that has something to do with temperature, but space (I don't think) can have a temperature because temperature is the average KE of a body of matter and if it were a vacuum then it can't have temperature, or is it once the thermal energy is radiation, it is a different type of energy (like the thing I heard about vacuum energy)?

    That 4th power radiation law is interesting, and I like it because it is simple enough for me to understand =]. So my question is why does it increase the wattage of the surface are by that much? (like a mathematical relation of power to thermal energy, if that is nothing tedious, and then I suppose it would change because it only has a certain surface area to escape through, sort of like the amount an organism can diffuse through it's membrane as related to volume of the organism).

    Also could you please elaborate a bit more on Planck Black Body radiation law?

    your right about having a timeline haha, although I'm not interested by events and times as I am so much as the ideas behind the events and times =], but still rather useful.

    Also like I said in the last post, is it possible for some one to show me a formula connecting between density and temperature, I know this is generally explained through pressure (in the event of my mercury in a box example), but I am curious about a more direct relationship. I think this still applies to cosmology, as the density prior to the big bang and the temperature, but like I said this might be completely different and comparing the two is more of an analogy.

    Now with the paper, the teacher is stupid and I'm doing this more for being personally interested. If I wanted to I could probably get away with saying god started the universe, paraphrasing the 2 chapters in the text book, and then saying a comit hits the earth and we all die. But what's the point in that? I'm just using the paper as a tool to force myself to actually go and learn stuff my telling my self that I need all this stuff, because I am truly intrigued, but I always end up distracting myself =].

    You seem to be answering most of my questions on most of my threads marcus, and thanks for all the effort you put into answering my questions =]

    Thanks for all the replies and encouragement everyone =D
  12. Feb 16, 2008 #11
    ah so I found this

    Pressure=F/A=Fd/Ad=W/V=E/V=Energy density, and a low volume= high Energy density, so does that answer my question of temperature of density =]
  13. Feb 16, 2008 #12
    High densities don't necessarily mean high temperatures, like you mentioned, Mercury is much denser that air or water, but if you leave it out in a room for long enough, it will be at the same temperature as everything else in the room. If you start with a gas at some temperature and density and compress it, the temperature will of course rise , see


    Good luck on your paper, it's good your thinking so much about it:) Just curious, your textbook attributes god for starting the Universe?
  14. Feb 17, 2008 #13
    Thanks, yea that's what I meant compressed densities =]

    What exactly does it mean my absolute temperature and absolute pressure? Absolute means the highest you can have right?

    Nope, but she is religious and said you can credit the start of the universe to whatever you like (she meant God). The text book just says there was a big bang, and leaves it at that lol.
    Last edited: Feb 17, 2008
  15. Feb 18, 2008 #14
    If anyone could help answer some more of my questions in my last few posts that would be ace =] thanks for the help so far.
  16. Feb 20, 2008 #15
    Hm Absolute temperature means, to me, temperature on the Kevin Scale. While Absolute pressure is the total pressure on a system which is equal to the gauge pressure + atmospheric.
  17. Feb 21, 2008 #16

    Cool we just did this in chemistry =D, I can actually apply something!

    Thanks Poweriso

    Would anyone be able to answer any more of my questions in #10? Thanks =]
  18. Feb 28, 2008 #17
    What cooled the Universe?

    I like simplistc stuff. Following the "Big Bang" things were hot due to a state of compression. The Universe was formed by the expansion of the denser stuff and exspansion produces cold. The more it expands the colder it will become. This is a simple fact on Earth. Release a compressed gas and it is cold on the way out. A basic effect of our air conditioning. Compress the gas and the tank gets hot, the more compression the hoter it gets. Not as knowledgable as the other replies but it works.
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