What are the Cosmological monopole and dipole?

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Discussion Overview

The discussion focuses on the concepts of cosmological monopole and dipole, particularly in the context of the Cosmic Microwave Background (CMB). Participants seek to understand these terms conceptually and physically, including their implications in cosmology.

Discussion Character

  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant describes the monopole as the average temperature of the CMB, while the dipole is influenced by the observer's motion relative to the CMB, resulting in blue-shifting and red-shifting effects.
  • Another participant explains that higher multipoles arise from primordial temperature variations in the early universe.
  • A further contribution discusses the mathematical representation of functions on a sphere, noting that the CMB temperature can be decomposed into spherical harmonics, with the monopole and dipole being the first terms in this decomposition.

Areas of Agreement / Disagreement

Participants generally agree on the definitions and implications of the monopole and dipole, but the discussion does not resolve deeper questions about the higher multipoles or their origins.

Contextual Notes

The discussion does not address specific assumptions or limitations regarding the mathematical treatment of the CMB or the implications of the higher multipoles.

jordy1113
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Hi there I'm currently studying cosmology but I'm having a hard time grasping that concept of the cosmological monopole and dipole (quadrupole etc) and was wondering if someone could explain what they are conceptually and physically. Thanks : )
 
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The monopole is just the average temperature of the CMB. The dipole is dominated by our motion with respect to the CMB: the CMB gets blue-shifted in the direction of our motion, and redshifted opposite to our motion.

The higher multipoles are all from primordial variations in temperature from place to place in the early universe.
 
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Similar to the way you can write a 1d function ##f(x)## as a sum of polynomials, or Fourier decompose it and write it as a sum/integral of ##\sin## functions, you can decompose a function ##g(\theta,\phi)## defined on the surface of a sphere as an infinite sum of the spherical harmonics. The CMB temperature as a function of position on the sky is a function defined on a sphere, so can be decomposed in this way. The first terms in these sums are the monopole and dipole terms, which have the interpretations @kimbyd gave.
 
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kimbyd said:
The monopole is just the average temperature of the CMB. The dipole is dominated by our motion with respect to the CMB: the CMB gets blue-shifted in the direction of our motion, and redshifted opposite to our motion.

The higher multipoles are all from primordial variations in temperature from place to place in the early universe.
Hey thanks this actually really helps me think about it better
 

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