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B Ancient EMR attenuation

  1. Dec 14, 2016 #1
    My (layman) understanding of CMBR is that it's the oldest observable EMR since the recombination epoch, with a peak wavelength of 1.063 mm. Does it make sense that the EMR from before this time would have an exponentially longer (though still possible to detect) wavelength?
  2. jcsd
  3. Dec 14, 2016 #2
    Seems like the answer is no per Wikipedia's Decoupling article:
  4. Dec 14, 2016 #3


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    Not really. The gas of photons that existed prior to the surface of last scattering neither gained nor lost energy as a result of its interactions with normal matter at the time, because the photons and the normal matter were at the same temperature.

    Perhaps you're thinking that photons of different wavelengths would penetrate the plasma at different rates? As near as I can tell, this is a minuscule effect. The main impact it would have would be in modifying the CMB anisotropies in a particular way. The surface of last scattering isn't a sharp surface: it's a thick, blurry surface that damps out small-scale fluctuations. If there were a wavelength-dependent effect, we'd expect the small-scale anisotropies to be damped at different rates at different frequencies. This doesn't happen to any noticeable degree from what I recall.
  5. Dec 14, 2016 #4
    That was my original thought, similar to how photons travel from the core of the sun. Thanks for the explanation.
  6. Dec 14, 2016 #5


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    Ahh, you mean how the very short wavelengths that exist at the core become the much longer wavelengths that the Sun emits from its surface?

    That happens because you have nuclear reactions at the center of the Sun which produce very short wavelength radiation. That radiation is extremely out of equilibrium. As it scatters off of the atoms in the Sun's plasma, it mixes and becomes a much larger number of lower-energy photons. By the time the radiation reaches the surface, it's mixed so well that the fact that it came from a nuclear furnace is irrelevant, and only the temperature matters.

    There actually was some pretty similar stuff going on in the early universe. A number of processes created high-energy photons, such as the annihilation of matter and anti-matter that happened early-on, as well as primordial nucleosynthesis. Just as with the Sun, these high-energy photons mixed and became a thermal bath whose properties are independent of the radiation's origins. The surface of last scattering then happened when the temperature of the plasma fell to the point that it became a neutral, transparent gas. That temperature is what sets the temperature of the CMB we see today, and that temperature had nothing to do with the origins of where all that thermal energy came from originally.
  7. Dec 15, 2016 #6
    Yes, just trying to think of a way data about the universe before recombination could be extracted. I spend most of my days at work pulling interesting things out of a mountain of chaotic noise, so this problem is of particular interest to me.
    With CMB originating from photon decoupling, it seems like there should be a similar background noise present from a neutrino decoupling event that we could eventually detect.
  8. Dec 15, 2016 #7
  9. Dec 15, 2016 #8


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    Yup! And I see that you found some sources on this.

    There's also (potentially) the gravitational wave background, but the size of that background is model-dependent, and it hasn't yet been detected. If it were detected, it'd be a window directly back to inflation, as the gravity waves don't interact much with non-solid matter.
    Last edited: Dec 15, 2016
  10. Dec 15, 2016 #9
    Interesting, this one might take me a while to wrap my head around.
  11. Dec 15, 2016 #10
    With CvB's release when the universe was 1 second old, and GWB's release around 10-32 seconds, it makes sense to focus on GWB if it's not terribly more difficult to detect than CvB.
    I'm reading through the following paper to learn more about gravitational waves, which seems fairly layman-friendly; are there any others you recommend?

    Observational strong gravity and quantum black hole structure
  12. Dec 18, 2016 #11


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    Both are extremely challenging. The indirect effects of gravitational waves on the cosmic microwave background can be easier to see - BICEP2 thought they saw it while actually measuring dust, but multiple experiments have a good chance to see it in the next years.

    PTOLEMY is a proposed experiment for the neutrino background, BBO is a proposed experiment to see primordial gravitational waves directly. The former is an improved version of KATRIN which just started data-taking, the latter is an improved version of LISA (with a proposed LISA launch date 2034), and both will need a lot of dedicated R&D, so don't expect those projects to be realized soon.
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