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Coldspot in CMB?

  1. Jul 20, 2011 #1
    Whats this all about the cold spot found on the WMAP satellite's photo of cosmic microwave background radiation? To explain this there's even been a possibility of a parallel universe's gravitational effect causing this cold spot?
     
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  3. Jul 20, 2011 #2

    bcrowell

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    How about giving us the source of your information? It's been known for a long time that the CMB has hot and cold spots. Therefore there must be something more about this than what you're saying, or else it wouldn't be anything new.
     
  4. Jul 21, 2011 #3

    Chalnoth

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    Basically, the "cold spot" just isn't far enough away from the expected variations for there to be much reason to believe it's anything different than normal variation.
     
  5. Jul 21, 2011 #4

    Chronos

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    http://www.newscientist.com/article/mg19626311.400 reported
    ' "It [the CMB cold spot] is the unmistakable imprint of another universe beyond the edge of our own," says Laura Mersini-Houghton of the University of North Carolina at Chapel Hill. It is a staggering claim. If Mersini-Houghton's team is right, the giant void is the first experimental evidence for another universe.'
    This claim is countered by a number of other studies - e.g.,
    No evidence for the cold spot in the NVSS radio survey
    Authors: Kendrick M. Smith, Dragan Huterer
    (Submitted on 18 May 2008)
    Abstract: We revisit recent claims that there is a "cold spot" in both number counts and brightness of radio sources in the NVSS survey, with location coincident with the previously detected cold spot in WMAP. Such matching cold spots would be difficult if not impossible to explain in the standard LCDM cosmological model. Contrary to the claim, we find no significant evidence for the radio cold spot, after including systematic effects in NVSS, and carefully accounting for the effect of a posteriori choices when assessing statistical significance.
    Comments: 7 pages, 3 figures, MNRAS submitted
    Subjects: Astrophysics (astro-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
    Journal reference: Mon.Not.Roy.Astron.Soc. 403:2,2010
    DOI: 10.1111/j.1365-2966.2009.15732.x
    Cite as: arXiv:0805.2751v1 [astro-ph]
    .
     
  6. Jul 21, 2011 #5

    Dotini

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    So the CMB cold spot could be:

    1) Noise in the measurement process. <-- My choice.
    2) Primordial temperature fluctuation.
    3) A huge void between us and the CMB.
    4) A wormhole or Einstein-Rosen bridge to another universe.
    http://en.wikipedia.org/wiki/CMB_cold_spot

    If you fancy #4, you might enjoy this link, which multiplies one interesting mystery by another: http://www.technologyreview.com/blog/arxiv/23410/

    Respectfully submitted,
    Steve
     
    Last edited: Jul 21, 2011
  7. Jul 21, 2011 #6

    bapowell

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    Doubtful it would be noise in the instrument, since this should be isotropic Gaussian white noise. More likely it is a larger than average primordial fluctuation.
     
  8. Jul 21, 2011 #7
    Can the presence of a large body of matter block the CMBR from reaching us and thus produce a cold spot?
     
    Last edited: Jul 21, 2011
  9. Jul 21, 2011 #8

    bapowell

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    No, there are definitely photons reaching the detector from the location of the big cold spot, and their temperature is of the same order of magnitude as the other cold spots.
     
  10. Jul 21, 2011 #9
    Ok so the intensity (ie. amplitude) is constant in every direction (is this correct?), but the center frequency of the black body radiation varies by a tiny percentage. It must be difficult to measure the center frequency so accurately when the shape of the radiation has such a rounded top?
     
  11. Jul 21, 2011 #10

    Chalnoth

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    If you look at the way it works, the difference in temperature is approximately linearly-related to the amount of flux you get in any one detector. So first they calibrate the detectors off of the dipole (which is much brighter than the galaxy across most of the sky), and since the CMB dipole is a fluctuation with CMB spectral scaling, anything else which has CMB spectral scaling will also raise/lower its temperature proportional to how much it is raised/lowered by the dipole.

    Of course, the relationship isn't exactly linear, but it is quite close enough for the range in temperature deviation for the primary CMB anisotropies.
     
  12. Jul 21, 2011 #11
    I feel like I should be getting this, but I am not. It should be right up my alley, so to speak, which is a bit disconcerting.

    Forgetting the detectors for the moment, we are trying to measure the black body temperature variation in different directions of the sky. This is equivlent to variation in the frequency of the black body radiation. Is this correct?

    Now the detectors appear to comparing intensity from different directions, how does this tell us the frequency and thus temperature?
     
  13. Jul 21, 2011 #12

    Chalnoth

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    Each detector only sees the flux in one particular frequency band. Increasing the temperature increases the flux an individual detector sees. And the amount it increases the flux is approximately proportional to the amount of temperature increase (as long as the temperature increase is small compared to the average temperature level....which it is).
     
  14. Jul 21, 2011 #13

    bapowell

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    I'll add that scanning the sky in several different frequency bands is vital to performing foreground subtractions, since galactic processes, for example, occur at different frequencies than the CMB.
     
  15. Jul 21, 2011 #14
    So each detector is actually a bank of several narrowband detectors, each tuned to detect the amplitude of a particularly wavelength. How many different wavelengths are being detected simultaneously?

    Is it correct that for the detector to be able to make very accurate relative measurements of different points in the sky that there are two banks of these detectors each looking in different directions simultaneously? Do they switch these rapidly into a common receiver perhaps with a double down conversion before being sampled?

    How much of the sky is each detector measuring ie. beam width? Presumably they use some kind of high gain, very highly directional horn antenna?

    We are talking about the WMAP instrument here correct?
     
  16. Jul 21, 2011 #15

    Chalnoth

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    Well, no, each detector is a single detector. They have horns which only allow photons within a certain range of wavelength to pass. Then all of the photons that are able to make it to the bottom of the horn dump their energy into the detector, which is read off as a voltage.

    Bear in mind that WMAP only detects photons in five different wave bands, so it isn't actually very good at estimating the temperature spectrum. It just depends upon the slight differences in flux at a given frequency across the sky.

    I believe that's how it works. That's the basic idea with Planck, though one detector in Planck is looking at the sky, whereas the other is looking at a 4K reference source. With WMAP, it actually has two back-to-back telescopes looking at different parts of the sky.

    It varies based upon frequency. With WMAP, the beam varies from just under a degree to around a quarter of a degree in resolution.
     
  17. Jul 22, 2011 #16
    Thanks Chalnoth,

    Re: "Then all of the photons that are able to make it to the bottom of the horn dump their energy into the detector, which is read off as a voltage."

    What I am not getting is if the 5 different wavelength photons are all summed at the same detector producing a single voltage then the frequency information is lost.


    Also it seems like I may have been under a misapprehension for a very long time.
    I thought that the CMBR image was showing different temperatures, which I thought meant different photon wavelengths.
    How can temperature be determined from the number or intensity of the photons?
     
  18. Jul 22, 2011 #17

    Chalnoth

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    Oh, I think you misunderstood me. There are different sets of detectors for different wave bands, at 23GHz, 33GHz, 41GHz, 61GHz, and 94GHz. The 23GHz detector, for instance, doesn't only measure photons at exactly 23GHz, but actually captures photons from about 19GHz to about 25GHz. That's what I've been talking about.

    See [URL [Broken] law[/url]. Temperature impacts not only the frequency of radiation, but also the amount that is emitted.

    Because previous experiments (in particular FIRAS) measured the average temperature of the CMB so incredibly accurately, we can simply use differences in flux to determine the small temperature differences at different parts of the sky.
     
    Last edited by a moderator: May 5, 2017
  19. Jul 22, 2011 #18

    bapowell

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    It is showing different temperatures. A gas of photons in equilibrium at a certain temperature T have wavelengths distributed according to a black body: http://en.wikipedia.org/wiki/Black_body" [Broken]. The CMB is the closest thing that nature has to a black body, and so even a perfectly uniform CMB with temperature T would consist of photons across a range of frequencies.
     
    Last edited by a moderator: May 5, 2017
  20. Jul 22, 2011 #19
  21. Jul 22, 2011 #20
    Ok thanks guys the WMAP detectors are alot clearer now as well as a better understanding of the way temperature is determined.

    Just to be sure, Flux is another way of saying intensity, or numbers of photons, correct?

    Flux can also be affected by other things though correct? eg. a galaxy between us and the CMBR. Could we be seeing variation due to this and falsely attributing this to temperature variation? Also wouldnt dispersion over 13.7B years of traveling tend to want to flatten out any initial photon temperature variation?




    Naty, they should be able to determine if the directions match.
     
    Last edited: Jul 22, 2011
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