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Anti-matter annihilation and the CMBR

  1. Jan 15, 2015 #1
    Recently I watched a lecture on Anti-matter and the Standard Model...

    ...At one point Dr. Quinn, makes the statement that the CMB is the resulting energy left over from the annihilation of the matter and anti-matter which arose during cosmic inflation (leaving behind only one part in 30 million of ordinary matter). Wouldn't there be far more radiation in the CMB if this were true? Wouldn't the temperature be much higher? I was under the impression that the energy of the cmb was considerably lower than the energy found in matter in the universe.
     
  2. jcsd
  3. Jan 16, 2015 #2

    Drakkith

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    The CMB was directly created by the thermal motion of the plasma in the early universe. During recombination, electrons combined with protons and this opaque plasma turned into transparent gas. This allowed the thermal radiation to propagate freely throughout the universe and it has, over time, been redshifted from the visible frequencies into the microwave frequencies. Perhaps Dr. Quinn means that the thermal energy of his plasma was mostly generated from the annihilation of matter and antimatter, which would have left a significant amount of energy behind as photons, which were then absorbed into the plasma and turned into heat.

    That's a good question. I know much of the energy initially in the CMB has been lost as expansion redshifted it. I'm not sure what the numbers are though.
     
  4. Jan 16, 2015 #3

    Orodruin

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    Wrote a long post, but Drakkith basically said the same. Let me just add regarding the numbers:
    Radiation energy density dilutes faster than matter by one power of the scale factor (due to redshift of the momentum in addition to number density dilution). With the CMB at redshift z ~ 1000, matter energy density has gained roughly three orders of magnitude on radiation since then. This also means radiation temperature was a factor 1e3 higher at the CMB release (temperature is related to energy per photon). With the number density decrease, the photon energy density today is a factor 1e12 lower than at CMB release.
     
  5. Jan 16, 2015 #4

    Chronos

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    The matter asymmetry of the early universe is generally agreed to be about 1 part in 10 billion based on CMB measurements. Science is based on observational evidence, not philosophy.
     
  6. Jan 16, 2015 #5

    Drakkith

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    What part of this is philosophy?
     
  7. Jan 16, 2015 #6

    Chronos

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    Apologies Drak, been reading too many threads at once.
     
  8. Jan 16, 2015 #7

    Chalnoth

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    To add another point: the temperature of the CMB at emission was set by the temperature at which the primordial plasma cooled to become a gas. It has nothing to do with how the earlier, higher temperature was generated.

    Also, I think it's easier to understand what's going on by looking how expansion impacts temperature rather than energy density: temperature of radiation is inversely proportional to the scale factor. Since things in our universe are, on average, about 1100 times further away from one another than they were when the CMB was emitted, the current CMB temperature is 1/1100th the temperature of the CMB when it was emitted.

    One final point: within the primordial plasma, photons and normal matter were interacting very strongly. Because of this, the temperature of normal matter was the same as the temperature of the photons, right up until the CMB was emitted. When the normal matter became a gas, its behavior diverged significantly from the behavior of the CMB.
     
    Last edited: Jan 16, 2015
  9. Jan 16, 2015 #8

    Orodruin

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    I think you intended to write scale factor. :)
    Also, how is this different from what I wrote?
     
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