How do they measure the CMB temperature?

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Discussion Overview

The discussion revolves around the measurement of the Cosmic Microwave Background (CMB) temperature, exploring its nature as radiation and its relationship to black body spectra. Participants delve into the origins of the CMB and the conditions that lead to its observed properties.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants inquire about the nature of the CMB, questioning whether it is merely radiation and how its temperature is determined.
  • One participant asserts that the CMB follows the Planck spectrum of a black body, suggesting that its temperature can be measured by the frequency of the peak of this spectrum.
  • Questions are raised regarding the identity of the black body, with one participant asking if it refers to the entire universe and how the CMB achieves a perfect black body spectrum.
  • Another participant explains that the early universe was filled with hot ionized hydrogen and helium, with radiation originating from matter-antimatter annihilation, contributing to the CMB's characteristics.
  • This participant describes the transition from a hot plasma to a transparent universe, where neutral hydrogen atoms formed, leading to the observable CMB today, which is red-shifted.
  • It is noted that while the radiation is largely homogeneous and isotropic, there are anisotropies present, which are acknowledged but not elaborated upon.

Areas of Agreement / Disagreement

Participants express curiosity and propose various ideas regarding the measurement and nature of the CMB, but no consensus is reached on the specifics of its black body characteristics or the implications of its origins.

Contextual Notes

There are unresolved questions regarding the definitions of terms like "black body" in this context and the implications of the CMB's isotropy and anisotropies.

touqra
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How do they measure the CMB temperature? Aren't they just some radiation?
 
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touqra said:
How do they measure the CMB temperature? Aren't they just some radiation?
Yes! The CMB very accurately follows to Planck spectrum of a black body at a specific temperature. That temperature is measured by the frquency of the peak of the spectrum as shown here.

Garth
 
What is the black body here? Is it the whole universe? How does it happen that CMB is a perfect black body spectrum?
 
The whole universe was originally filled with hot ionised hydrogen and helium that were the products of nucleosynthesis in the BB. The energy for the radiation is thought to have come from the epoch of matter-antimater annihilation where (108 + 1) matter particles annihilated 108 anti-matter particles leaving ~108 photons per atomic particle in the present universe.

That radiation was bounced around the very early and opaque universe until the plasma termperature fell to ~ 30000K when the protons and electrons associated to form neutral hydrogen atoms, then the universe became transparent.

We see that 'Surface of Last Scattering' all around us and now red-shifted by z ~ 1100 as the CMB.

The plasma and subsequent gas was remarkably homogeneous and isotropic and therefore its radiation follows a Planck black-body spectrum very closely.

There are anisotropies, of course, but they are another story.

Garth
 
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