Cosmic Microwave Background Radiation Anisotropies

  • #1

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Hi

This question may have already been answered elsewhere. If so please accept my apologies in advance.

I am confused!

The textbok(s) I am reading describe a whole bunch of different causes for there being temperature fluctuations in the CBM, so I am confused about which one(s) of these effects is the mai cause.

For example, I understand the biggest apparent difference is caused by the earth's peculiar motion relative to the coming frame of the CMB. Then looking at the graph of CMB power spectrum as a function of angular scale, am I correct in believing that the first big peaks are due to acoustic osscilations? And if so, how does the integrated Sach's-Wolf effect fit in with this? Further more I read about quantum fluctuations in the matter density distribution during inflation. Am I correct in believing that these are supposed to be responsible for the large scale structure(s) such as galaxies we find in the universe? And is there evidence of this also in the CBM.

Please excuse my ignorance and when answering try to assume with all due respect that you are talking to a very young child! I am not a young child but I do have learning difficulties


Thanks

Peter
 

Answers and Replies

  • #2
Chronos
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In the very early universe everything was very close together and quantum fluctuations randomized their motions much like Brownian motion randomizes the motion of particles suspended in a fluid. This lead to tiny inhomogenities in properties like density and temperature that were stretched to cosmic proportions by inflation.
 
  • #3
In the very early universe everything was very close together and quantum fluctuations randomized their motions much like Brownian motion randomizes the motion of particles suspended in a fluid. This lead to tiny inhomogenities in properties like density and temperature that were stretched to cosmic proportions by inflation.
Yes I got that but does this show up in the CMB and if it does how does one separate this effect from others such as acoustic peaks? I am still so confused! :)
 
  • #4
PeterDonis
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I understand the biggest apparent difference is caused by the earth's peculiar motion relative to the coming frame of the CMB.
The effect of this is a dipole anisotropy; but most sources remove this before displaying CMB observations, so the graphs you see won't include it. When this effect is removed, the CMB is, on average, isotropic: there are fluctuations in its temperature on scales of about 1 part in 100,000 and smaller, but they average to zero over over the whole sky (once the dipole due to earth's peculiar motion is removed).

All of the other effects you are talking about appear as tiny fluctuations in the temperature of the CMB from one part of the sky to another. The only way to tell what caused various fluctuations is to compare the data with predictions of what fluctuations would be observed according to various models of things like quantum fluctuations being magnified by inflation, acoustic oscillations, the Sachs-Wolfe effect, etc. By trying to add up the model predictions for all of these effects, with different weights, and comparing the result to the observed data, cosmologists can try to figure out the relative magnitude of the various possible causes.
 
  • #5
image.png
image.png
The effect of this is a dipole anisotropy; but most sources remove this before displaying CMB observations, so the graphs you see won't include it. When this effect is removed, the CMB is, on average, isotropic: there are fluctuations in its temperature on scales of about 1 part in 100,000 and smaller, but they average to zero over over the whole sky (once the dipole due to earth's peculiar motion is removed).

All of the other effects you are talking about appear as tiny fluctuations in the temperature of the CMB from one part of the sky to another. The only way to tell what caused various fluctuations is to compare the data with predictions of what fluctuations would be observed according to various models of things like quantum fluctuations being magnified by inflation, acoustic oscillations, the Sachs-Wolfe effect, etc. By trying to add up the model predictions for all of these effects, with different weights, and comparing the result to the observed data, cosmologists can try to figure out the relative magnitude of the various possible causes.
Thanks Peter - that help to clear up
some of my confusion.

But then what causes the shape of the angular power spectrum? Is this the result of acoustic peaks, or is it mainly due to the acoustic peaks but also due to other things? Thanks
 
  • #6
PeterDonis
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what causes the shape of the angular power spectrum? Is this the result of acoustic peaks, or is it mainly due to the acoustic peaks but also due to other things?
That's what cosmologists are trying to figure out by comparing model predictions for various effects with the actual data. Different types of effects will predict a different shape of the angular power spectrum; cosmologists try to find a set of weights that can be used to give a weighted average of the different predictions that looks like the actual data. In the graphs you posted, the red curve with the gray (at least it looks gray to me) region around it is the weighted average of the model predictions, and the points with error bars are the actual data. You would have to look at the source of the graph to see what sorts of effects were included in the weighted average of predictions and with what weights.
 
  • #7
bapowell
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The gray region is the cosmic variance envelope. The red curve is the best-fit model.
 
  • #8
Thanks Peter and Bapowell. I am seeing a new tutor today, so hopefully she will be able to add some more.

Cheers
 
  • #9
bapowell
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Thanks Peter and Bapowell. I am seeing a new tutor today, so hopefully she will be able to add some more.

Cheers
Sure thing. I realize my response was terse: if you'd like to understand what "cosmic variance" is and/or what the "best-fit model" is or how it's generated, feel free to ask more questions!
 

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