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Background Radiation & Neutrino Questions

  1. Aug 17, 2003 #1

    Nim

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    When the cosmic background radiation first appeared 300,000 years after the Big Bang, was it x-rays and then redshifted over time to microwaves?

    And to see what the Universe looked like before 300,000 years after its birth, will we need a neutrino telescope?
     
  2. jcsd
  3. Aug 17, 2003 #2

    Labguy

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    No, we see it now as ~2.73 degree microwave radiation.
     
  4. Aug 17, 2003 #3

    jcsd

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    No, it was orginally in the lower frequency range of visible light.

    A gravitational wave telescope would be better, but yes neutrino decoupling happend after about 1 second after the big bang and lasted for about 10 seconds compared to photon decoupling which started about 300,000 years after the big bang, so with neutrinos you should be able to see much further back than with the CMBR.
     
  5. Aug 17, 2003 #4

    marcus

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    Re: Re: Background Radiation & Neutrino Questions

    A temperature of 1.9 kelvin has been predicted for the cosmic neutrino background CNB

    which is currently not observable----those neutrinos are much lower energy than the ones we can detect with today's instruments

    and the CNB is, according to Lineweaver's survey of cosmology which has a lot of useful information, in fact just as jcsd says, supposed to date from the first few seconds of expansion.


    Lineweaver's article, which has the temperature estimate of 1.9 kelvin, is online at Caltech level 5 and also at

    http://arxiv.org/astro-ph/0305179
     
  6. Aug 17, 2003 #5
    If the Neutrino temp is 1.9 degree and Neutrinos are massive then they are basically at rest. If they were tachyonic they would have circled around the universe a few times by now, assuming that it is closed.
     
  7. Aug 20, 2003 #6

    Nim

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    We see it now as 2.73 degrees kelvin? So the temperature is going down?
     
  8. Aug 20, 2003 #7

    marcus

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    Nim you really should get Lineweavers article. Download it.
    It has a lot of good stuff

    The MICROWAVE background is 2.73 kelvin and dates from 300,000 years after time zero

    The neutrino background is older---going back to the first second of expansion. It is a good question why the temperature is predicted to be 1.9 kelvin. Lineweaver explains this in section 7.4 of that article

    these are two separate cosmic backgrounds only one of which has been detected. It will be very exciting and informative when the cosmic neutrino background is finally detected and can be studied

    I will edit in some Lineweaver links

    http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver_contents.html

    this gives you the TOC, the page you want is section 7.4 "Where did the energy in the CMB come from?"

    http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_4.html

    This is the same as what you get if you download it from the LosAlamos archives and print it out and look at page 24.

    I mention the CalTech version because you can get at it without
    having to download the whole article. But it is more legible if you
    download it from

    http://arxiv.org/astro-ph/0305179
     
    Last edited: Aug 20, 2003
  9. Aug 20, 2003 #8

    marcus

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    Tyger, boltzmann's k (you are implicitly invoking this) is
    8.6E-5 eevee per kelvin

    So at 1.9 kelvin, the kT energy is 1.6E-4 eevee

    If a neutrino has mass of 1 eevee, then how fast must it go
    to have kinetic energy of 2E-4 eevee?

    I think it must be going about 1/50 of the speed of light

    What are some current upper bounds on neutrino mass? Or
    estimates, if they have them?
     
    Last edited: Aug 20, 2003
  10. Aug 20, 2003 #9

    Nim

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    I don't understand this temperature thing. When it comes to different frequencies of light, I thought that lower frequencies had a lower temperature and that higher frequencies had a higher temperature... I didn't think a frequency of light could loose heat.
     
  11. Aug 20, 2003 #10

    Hurkyl

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    The light doesn't remain the same frequency; it redshifts, curtosey of the expansion of space.
     
  12. Aug 20, 2003 #11

    marcus

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    keep asking Nim
    do you know the black body curve---the spectrum
    of the glow from a generic surface at a specific temperature?
    each temp is associated with a distinctive blend of
    frequencies of light

    if something has a thermal spectrum----is glow characteristic
    of some temp
    and then you dopplershift or redshift every photon in the blend
    what results is a thermal blend of frequencies but for a different
    temperature------just as if the radiating thing were some percentage cooler

    think about glowing plasma at 3000 kelvin
    releasing light
    and then space expands by a factor of 1000
    so all the light is redshifted (wavelengths stretched out)
    by a factor of 1000

    the light still looks like thermal glow, but off a colder object.
    It looks like it came from something that is only 3 kelvin
    in fact it is microwave background or infrared or whatever you call it

    keep asking
    this makes it more interesting for Hurkyl and me and suchlikes
     
  13. Aug 20, 2003 #12

    Nim

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    So the cosmic background radiation started out at about the red, orange, yellow part of the spectrum and redshifted down to microwaves from there? And some day it will be redshifted down to radio waves? How low of a temperature will the cosmic background radiation actually end up getting to anyways?
     
  14. Aug 21, 2003 #13

    Labguy

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  15. Aug 21, 2003 #14

    jcsd

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    If you looked at at a convential table of elementary particles you'd see that the (electron, as there are actually three known types of neutrino) neutrino's mass is given as zero. There have been a couple of experiments that seem to suggest that neutinos may have mass but partly due to the fact that scientists have been unable to replicate the results the coin is still up in the air.
     
  16. Aug 21, 2003 #15

    Phobos

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    Just to clarify, the current "temperature of the universe" is 2.73K...the temperature back when the universe was only 300,000 years old was MUCH higher. (as the universe expands, it cools).

    The 300,000-year milestone is when the universe cooled enough to became transparent to light.
     
  17. Aug 21, 2003 #16

    marcus

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    expectations about the future depend on a choice of model
    the simplest model that fits observational data has a positive
    cosmological constant which, if it really is constant, implies
    unlimited expansion

    there is no lower bound on CMB temperature
    it can get as close to zero as you please
    as long as you are willing to wait long enough

    there is enough information in Lineweaver's Figure 1
    (a diagram showing assumed future of universe as well as past)
    to let someone estimate how long we have to wait for
    CMB to get down to 1.36 kelvin or half what it is today.

    http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure1.jpg

    Would you like to know how to read that from Figure 1?
    If so ask.

    If you have the article printed out (a lot more legible) then
    you will find Figure 1 on page 6. Here is a link to the PDF
    version you can print out.


    http://arxiv.org/astro-ph/0305179

    I think it is the best general audience
    survey of contemporary cosmology I have seen so far.
     
    Last edited: Aug 21, 2003
  18. Aug 22, 2003 #17

    Nim

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    Yeah that would be great.

    So if the CMB continues to redshift, at some point in time it will have a temperature of something like 0.000000000001 eventually and maybe a wavelegnth of a few miles. If the Universe never stopped expanding would the CMB finally just dissappear one day? There is probably a limit to how low a temperature can be without being absolute zero right, or a limit to just how long a wavelength could be. I would imagine a wavelength of a trillion light years or a temperature of say 1e-999 would be impossible.
     
  19. Aug 22, 2003 #18

    marcus

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    OK, figure 1 consists of three different plots of the past and future of the universe

    look at the top one, which has ordinary years (Gyr = billion years)
    on the left side

    and notice that it has the "scalefactor a(t)" on the right side

    the scalefactor is an index of the size of the universe which for convenience is normalized so that its value at the present is one
    a("now") = 1

    At some time in the future the average distance to galaxies will
    be twice what it is now, things will be twice as spread out, the CMB wavelengths will be twice as long, its temperature will be half what it is now.

    a("then") = 2

    You can look across Lineweavers graph from the size scale on the right over to the time (Gyr) scale on the left and see when that will be measured in billions of years from the big bang
     
    Last edited: Aug 22, 2003
  20. Aug 22, 2003 #19

    marcus

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    If the model is correct in predicting endless expansion then yes evenutally things will have expanded 1000-fold and instead of being measured in millimeters the CMB spectrum (power in each wavelength interval) will be measured in meters

    the signal will be (1000)4 fainter due to expansion

    eventually there is the philosophical issue---does something exist if it cannot be observed. this does not intrigue me personally but it may interest others

    what do you suppose they mean by the "temperature" of some radiation?

    I think that even if the wavelength of each CMB photon were stretched out 1000-fold that the CMB would still have a temperature as astronomers understand the word

    but it might be very hard to detect the CMB and measure its temp

    does it strike you as odd that I'm claiming the CMB signal would be (1000)4 times fainter (that is, a trillion times fainter)
    after only a thousand-fold expansion?
     
    Last edited: Aug 22, 2003
  21. Aug 22, 2003 #20

    Nim

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    Yeah, how exactly does a 1,000 fold expansion of the CMB make it a trillion times fainter? When you say the temperature of some radiation, are you talking about the incredibly small differences of temperature in the CMB?

    If a wavelength can get endlessly longer or if a temperature can get endlessly closer to absolute zero, wouldn't that break some rule of quantum mechanics? Isn't there supposed to be a smallest piece of everything, a quantum unit of an electric charge, time, length, temperature, etc.
     
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