How do they measure the CMB temperature?

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SUMMARY

The Cosmic Microwave Background (CMB) temperature is measured by analyzing the frequency of the peak of its spectrum, which closely follows the Planck spectrum of a black body. The CMB originates from the early universe, filled with hot ionized hydrogen and helium, where radiation was produced during the matter-antimatter annihilation epoch. As the universe expanded and cooled to approximately 30,000K, protons and electrons formed neutral hydrogen atoms, allowing the universe to become transparent. This event, known as the 'Surface of Last Scattering,' is observed today as the red-shifted CMB.

PREREQUISITES
  • Understanding of Planck's Law and black body radiation
  • Knowledge of the Big Bang nucleosynthesis process
  • Familiarity with cosmic redshift and its implications
  • Basic concepts of isotropy and homogeneity in cosmology
NEXT STEPS
  • Research Planck's Law and its application in astrophysics
  • Study the implications of the Big Bang nucleosynthesis on cosmic evolution
  • Explore the concept of cosmic redshift and its measurement techniques
  • Investigate the significance of anisotropies in the CMB and their implications for cosmology
USEFUL FOR

Astronomers, cosmologists, and physics students interested in the early universe, the properties of the Cosmic Microwave Background, and the fundamental principles of black body radiation.

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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