CMBR, Dark Matter: Unraveling What it Means

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Discussion Overview

The discussion centers on the cosmic microwave background radiation (CMBR) and its relationship with dark matter, exploring theoretical implications and observational evidence. Participants examine the effects of dark matter on the CMB and seek clarification on specific phenomena related to the CMB's formation and characteristics.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • Chalnoth suggests that the CMB cannot be adequately explained without considering dark matter, highlighting the presence of matter that interacts differently with light.
  • Some participants inquire whether the discussion relates to the integrated Sachs-Wolfe effect or CMBR redshifting, indicating a search for clarity on the phenomena involved.
  • A participant explains that the oscillations in the CMB are influenced by baryons and dark matter differently, with dark matter falling into gravitational wells without experiencing pressure, affecting the CMB power spectrum.
  • There is a distinction made between the Sachs-Wolfe effect and the behavior of matter prior to the emission of the CMB, with emphasis on the timing of these events.
  • Participants express interest in identifying a specific name for the phenomena discussed and seek additional resources for further reading.
  • A suggestion is made to refer to Wayne Hu's website for animations that illustrate the impact of dark matter on the CMB.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the relationship between dark matter and the CMB, with some agreeing on the significance of dark matter's role while others seek clarification on specific effects and phenomena. No consensus is reached on a specific name for the phenomena discussed.

Contextual Notes

Participants reference different aspects of the CMB and its interaction with matter, indicating potential limitations in their understanding of the timing and nature of these interactions. The discussion does not resolve the distinctions between various effects related to the CMB.

alw34
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Chalnoth posted this in another discussion:

"The cosmic microwave background is almost impossible to explain without dark matter (there is clear evidence of a component of matter that feels pressure, and a component of matter that does not feel pressure, which can only be true if that matter does not interact with light, i.e., if it's dark)."

What does that mean?
 
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Is this a reference to the integrated Sachs-Wolfe effect of CMBR redshifting?
 
Last edited:
alw34 said:
Is this a reference to the integrated Sachs-Wolfe effect or CMBR redshifting?
Neither. It's about the oscillations on the CMB itself. The super-short version is that before the CMB was emitted, baryons felt the pressure from photons, so that when they fell into a gravitational potential well, they would bounce back out of it. But dark matter, experiencing little to no pressure, would merely fall into the potential well.

In the CMB power spectrum, the first peak (longest-wavelength) represents matter that had enough time to fall into a gravitational well. The second (shorter wavelength) peak represents matter that had time to fall in then bounce back. The third peak represents matter that fell in, bounced back, then fell in again.

Dark matter shows up on the first, third, fifth, etc. peaks, while normal matter contributes to all of the CMB peaks. Thus the signal of dark matter is having the odd-numbered peaks significantly higher than the even-numbered peaks. For Planck, this results in a measurement of the dark matter density with a standard deviation of less than 2% of the measured value.
 
alw34 said:
Did that happen before the three types described here...is this a separate phenomena:

https://en.wikipedia.org/wiki/Sachs–Wolfe_effect

thank you.
The Sachs-Wolfe effect has to do with CMB photons traveling through gravitational potential wells between us and the CMB.

The effect I described is about the behavior of matter before the CMB was emitted.
 
Is there any name for this phenomena, or can you recommend a source where I can read more about it.
 
alw34 said:
Is there any name for this phenomena, or can you recommend a source where I can read more about it.
I'm not aware of a specific name. But it's one of the central features of the CMB.

Wayne Hu has a website dedicated to describing the CMB, and this page has a nice animation showing the impact of dark matter on the CMB:
http://background.uchicago.edu/~whu/intermediate/driving2.html
 
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