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Gzk paradox and expanding universe

  1. Apr 18, 2004 #1
    If the universe were divided into two mass zones, and one zone
    repelled the other,supernovae would not only accelerate away from us
    but the zone in which we are living would accelerate away from the
    supernovae.Also cosmic rays could be repelled by one zone and pushed
    through another.Could this explain the GZK paradox where highly
    energetic cosmic rays that should have been stopped from reaching the
    Earth by the microwave background have not been stopped?
     
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  3. Apr 18, 2004 #2

    Nereid

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    I'm not sure I understand what you're saying here; perhaps a few numbers might help.

    First, re supernovae 'accelerating away from us': the 'high-z' supernovae (SN) which provide evidence for the 'acceleration' are ~5 to 10 billion light years away (well, more accurately, the light which we now see from them has travelled for 5 to 10 billion years to get to us). Closer SN do not really show a deviation from the Hubble expansion rate (~71 km/s/Mpc), though the 'acceleration' surely affects what we see of them too (there are also plenty of SN which seem to be further, or closer, to us than the Hubble relation would suggest; however, these deviations are well understood as the result of the motion of the host galaxies in the gravitational field of the cluster - or super-cluster - in which they reside).

    Then the GZK limit: based on the density of CMBR photons (~400 per cubic centimetre), cosmic ray protons with an energy of ~>50 EeV (1 EeV = 1018 eV) should only be able to travel ~<100 Mly before they collide with a CMBR photon and produce lots of pions.

    You can see that these distances are quite different, so it's hard to see how the acceleration of distant SN and the GZK limit they would be connected.
     
  4. Apr 20, 2004 #3
    see reply below
     
    Last edited: Apr 20, 2004
  5. Apr 20, 2004 #4
    You can see that these distances are quite different, so it's hard to see how the acceleration of distant SN and the GZK limit they would be connected

    I agree with what you have said. Suppose the quarks in cosmic ray protons have a size i.e
    if they consisted of spheres of charge, and by special relativity length contraction these spheres got small at high speeds and energies, this would reduce the probability of a collision with a CMBR photon and allow the proton to reach the Earth.
     
  6. Apr 20, 2004 #5

    Nereid

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    Stecker has a good discussion of the GZK effect (see Section 2.2, starting on p7). It also contains references to the original papers by Griesen, Zatsepin, and Kuz'min, as well as several others on the topic.

    I think you will find that relativity effects were taken into account in making the calculations, particulary wrt the "energy dependence of the photomeson production cross sections and inelasticities", i.e. how likely it is that a proton of energy x will collide with a CMBR photon, for different kinds of collision.
     
  7. Apr 22, 2004 #6
    Thanks for the reference.It's full of useful reading.
    think you will find that relativity effects were taken into account in making the calculations, particulary wrt the "energy dependence of the photomeson production cross sections and inelasticities", i.e. how likely it is that a proton of energy x will collide with a CMBR photon, for different kinds of collision.



    I've done a calculation that shows that a sphere of charge in a quark is 10^-18 m when the quark has no speed.This is one thousand times smaller than a proton at rest.So relativistic length contraction would make the quark even smaller and less likely to interact with the CMBR than expected using the normal proton size.
     
    Last edited: Apr 22, 2004
  8. Apr 24, 2004 #7

    Nereid

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    The same considerations would surely also need to have been made re high energy protons colliding with particles such as oxygen nuclei, leading to different behaviour for air showers (for example) than what has been observed. The same effects would also occur - at much lower energies - in the LHC and its predecessors.

    There's certainly some interesting physics in the >100 GeV regime, such as the formation of a new state of matter, quark-gluon plasma, but AFAIK studies of these phenomena haven't lead to big changes in estimates of collision cross-sections.
     
  9. Apr 24, 2004 #8
    gzk paradox

    The same considerations would surely also need to have been made re high energy protons colliding with particles such as oxygen nuclei, leading to different behaviour for air showers (for example) than what has been observed. The same effects would also occur - at much lower energies - in the LHC and its predecessors.

    But the size of the quarks only determines the chance of a collision - not the outcome of the collision - assuming the number of oxygen quarks is similar to the number of microwaves a cosmic ray proton encounters on its journey.
     
    Last edited: Apr 24, 2004
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