CMBR, Dark Matter: Unraveling What it Means

[alw34]

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?

 

[alw34]

Is this a reference to the integrated Sachs-Wolfe effect of CMBR redshifting?

 

[Chalnoth]

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]

Did that happen before the three types described here...is this a separate phenomena:

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

thank you.

 

[Chalnoth]

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.

 

[alw34]

Is there any name for this phenomena, or can you recommend a source where I can read more about it.

 

[Chalnoth]

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