Undergrad Sachs-Wolfe effect and Superclusters/Supervoids

[Arman777]

I am reading Barbara Ryden- Introduction to Cosmology, my question is related to chapter 9.4

She first separates the type of fluctuations into the two parts, Large fluctuations ($\theta>\theta_H$) and small ones, ($\theta<\theta_H$).

where $\theta_H≈1°$
For the large fluctuations, she states that the cause of these fluctuations is the dark matter since at the time of the last scattering the energy density of the dark matter is higher than the radiation or from the baryonic matter. Then she explains the gravitational potential wells Due to this potential wells some of the photons are in the well and when they try to get out they get redshifted and some of them are on the hill of the potential (potential maximum) so when they lose potential they become blueshifted.

Hence the redshifted spots on the CMBR correspond to the potential minima's while the blueshifted spots correspond to the potential maxima's.

For the ($\theta<\theta_H$) part, she introduces a photon-baryon fluid and defines the term called acoustic oscillations.

Also, we know that Dark matter shaped the current galaxies, In this sense can we say that , by looking at the CMBR, potential minimas corresponds to the supervoids and potential maxima's superclusters or vica verse?

 

[kimbyd]

Hence the redshifted spots on the CMBR correspond to the potential minima's while the blueshifted spots correspond to the potential maxima's.

Not quite. The vast majority of the CMB signal does not come from the ISW effect. This effect only impacts the CMB to a noticeable degree on large scales, so that all of the hot and cold spots you see on the CMB are hot and cold spots in the primordial plasma that emitted the signal.

On small scales, there are so many overdense regions and voids between us and the CMB that the effect cancels out entirely.

On large scales, nearby overdense regions and voids do impact the result. This relationship isn't something you'd ever notice by just looking at the CMB map. But it does leave a statistical imprint that is detectable, and it's been shown that nearby large scale structure does correlate to CMB temperature.

 

[Arman777]

Not quite. The vast majority of the CMB signal does not come from the ISW effect. This effect only impacts the CMB to a noticeable degree on large scales, so that all of the hot and cold spots you see on the CMB are hot and cold spots in the primordial plasma that emitted the signal.

On large scales, nearby overdense regions and voids do impact the result. This relationship isn't something you'd ever notice by just looking at the CMB map. But it does leave a statistical imprint that is detectable, and it's been shown that nearby large scale structure does correlate to CMB temperature.

I tried to understand but I :/. So are you saying that the supercluster/supervoid structure happens the way I described but it only explains the structures that are near to us? ( I didn't understand the term nearby in this content)

On small scales, there are so many overdense regions and voids between us and the CMB that the effect cancels out entirely.

Hmm, maybe that's why she said acoustic oscillations.

 

[kimbyd]

I tried to understand but I :/. So are you saying that the supercluster/supervoid structure happens the way I described but it only explains the structures that are near to us? ( I didn't understand the term nearby in this content)

It happens for all structures. But when you look far away, there are lots and lots of voids and potential wells, so the blueshifts from the potential wells cancel out the redshifts from the voids. There aren't as many nearby structures, so their effect doesn't cancel as much and becomes visible.

Also, the Sachs-Wolfe effect was zero in the early universe. It's only since dark energy became a significant fraction of the energy density that the Sachs-Wolfe effect started to occur: without dark energy, potential wells (and voids) are stable. With dark energy, gravitational potentials decay over time: both wells and voids become more shallow.

Hmm, maybe that's why she said acoustic oscillations.

No, that's a different concept entirely that should be understood separately. Acoustic oscillations are what set up the CMB anisotropies in the early universe. They were sound waves in the plasma which existed before the CMB was emitted. The residuals of these sound waves also place an imprint on the distribution of galaxies (that is, how far apart various galaxies tend to be from one another on average).

 

[Bandersnatch]

Also, the Sachs-Wolfe effect was zero in the early universe. It's only since dark energy became a significant fraction of the energy density that the Sachs-Wolfe effect started to occur: without dark energy, potential wells (and voids) are stable. With dark energy, gravitational potentials decay over time: both wells and voids become more shallow.

I think there might be some mixing of terminology you two are using. Take a look at Wikipedia's article on Sachs-Wolfe effect. I think you're talking specifically about what it deems the late-time integrated S-W effect, whereas OP might be thinking of the earlier ones.

 

[Arman777]

lso, the Sachs-Wolfe effect was zero in the early universe. It's only since dark energy became a significant fraction of the energy density that the Sachs-Wolfe effect started to occur: without dark energy, potential wells (and voids) are stable. With dark energy, gravitational potentials decay over time: both wells and voids become more shallow.

I think there might be some mixing of terminology you two are using. Take a look at Wikipedia's article on Sachs-Wolfe effect. I think you're talking specifically about what it deems the late-time integrated S-W effect, whereas OP might be thinking of the earlier ones.

In my book the reason is stated as Dark matter. and the era is last scattering.

No, that's a different concept entirely that should be understood separately. Acoustic oscillations are what set up the CMB anisotropies in the early universe. They were sound waves in the plasma which existed before the CMB was emitted. The residuals of these sound waves also place an imprint on the distribution of galaxies (that is, how far apart various galaxies tend to be from one another on average).

Thats interesting. So this baryon-photon fluid distribution describes the galaxy positions. But we cannot see them right..since they are before CMBR.

So just one last time to make things clear, The blueshifted spots represents clusters/superclusters and redshifts representes voids ?

 

[kimbyd]

I think there might be some mixing of terminology you two are using. Take a look at Wikipedia's article on Sachs-Wolfe effect. I think you're talking specifically about what it deems the late-time integrated S-W effect, whereas OP might be thinking of the earlier ones.

Ah, I think you're right. The non-integrated Sachs-Wolfe effect comes from the time that the CMB was emitted. At emission, some locations were more dense than others. The photons that traveled from the more dense locations were redshifted, while those that came from the less dense locations were blueshifted. That effect is indeed the cause of most of the observed temperature differences in the CMB.

I apologize for the mistake. Usually the Sachs-Wolfe effect is talked about in relation to the ISW effect. So most everything I said above is completely off-topic.

 

[Arman777]

Ah, I think you're right. The non-integrated Sachs-Wolfe effect comes from the time that the CMB was emitted. At emission, some locations were more dense than others. The photons that traveled from the more dense locations were redshifted, while those that came from the less dense locations were blueshifted. That effect is indeed the cause of most of the observed temperature differences in the CMB.

I apologize for the mistake. Usually the Sachs-Wolfe effect is talked about in relation to the ISW effect. So most everything I said above is completely off-topic.

I see