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B The universe is shrunk a little bit more than we thought

  1. Aug 24, 2016 #1
  2. jcsd
  3. Aug 24, 2016 #2

    Chalnoth

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    Yes, the Planck satellite has a slightly smaller estimate for the distance to the horizon.

    The claim that neutrinos may extend the observable limit out to 46 billion light years isn't right, though. Because neutrinos travel through matter very efficiently at lower energies, measuring the cosmic neutrino background would provide an image of a significantly younger universe than the CMB offers. But the dynamics of the early universe expansion are such that things emitted earlier don't necessarily come from further away. See here:
    https://arxiv.org/pdf/0907.2887.pdf

    Because massive neutrinos travel a little bit slower than the speed of light, the CNB surface is actually a little bit closer than the CMB surface.

    There are also, unfortunately, practical problems with actually observing the Cosmic Neutrino Background. Namely, the CNB is already extremely low-temperature (colder than the CMB: about 1.95K), and neutrinos pass through matter very efficiently at low energies (the lower the energy, the more easily they pass through matter). So to measure the CNB, we'd be looking at the very rare signals coming from the rare collisions of these CNB neutrinos with normal matter, and each individual collision would transfer so little energy to the normal matter that it'd be very difficult to measure at all. See here, for example:
    http://journal.frontiersin.org/article/10.3389/fphy.2014.00030/full

    Quoted from the conclusion:
     
  4. Aug 25, 2016 #3
    Thanks for the explanation about neutrinos and the articles! Now I understand it better
     
  5. Aug 25, 2016 #4

    Fervent Freyja

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  6. Aug 25, 2016 #5

    Chalnoth

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    The paper seems perfectly reasonable and non-controversial to me. My main criticism was with the neutrino comments in the article, which don't appear in the paper (though the paper does make an offhand comment about neutrinos allowing us to see further, which as I note above isn't correct).
     
  7. Aug 25, 2016 #6

    Fervent Freyja

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    Thanks!
     
  8. Aug 26, 2016 #7

    Jorrie

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    I agree, but as I understand the particle horizon (theoretical observable limit), it postulates a massless particle traveling unhindered from cosmic time zero. Using the Planck 2013 data, it puts the particle horizon at around 46.3 Glyr. With the WMAP data, the distance was 46.7 Glyr.
     
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